1 Characterization of reservoir spaces

The volcanic reservoir spaces are usually characterized in three steps. (1) In terms of their origins, shape, and size, a logical classification scheme is established to divide the reservoir spaces in volcanic gas reservoirs. (2) The classification scheme provides identification markers for various types of reservoir spaces; then, the types of reservoir spaces in volcanic rocks are identified, and the distribution and accumulation characteristics of various types of reservoir spaces are described based on the shape, scale, configuration, and dissolution traces, in combination with the characteristics of lithology and lithofacies, rock structure, and texture that are associated with the reservoir spaces. (3) The morphology of various types of reservoir spaces is characterized qualitatively based on an observation of cores and thin sections; the size and accumulation capacity are characterized quantitatively by means of experiments such as porosity and permeability tests, conventional mercury injection, and constant-rate mercury injection.

2 Characterization of throats

Similar to the characterization of reservoir spaces, throats in volcanic reservoirs are also characterized in three steps. (1) In terms of their origins, shape, and size, the throats in volcanic reservoirs are classified. (2) Identification markers of various types of throat are set up; then the throat types in volcanic rocks are identified and the distribution and seepage characteristics of various types of throats are described based on throat size, shape, and dissolution traces, in combination with the characteristics of lithology and lithofacies, rock texture, and structure that are associated with the throats. (3) The shape characteristics of various types of throats are characterized qualitatively using core observation and thin section examination, and the sizes and permeation capacities are characterized quantitatively by means of experiments such as porosity and permeability tests, conventional mercury injection, and constant-rate mercury injection.

3 Characterization of A-P patterns

On the basis of characterized reservoir spaces and throats, the A-P patterns in volcanic gas reservoirs are established according to the modes of pore-throat-fracture assemblage. For various A-P patterns, the accumulation-permeation capacity and production characteristics are characterized respectively and the control of the A-P pattern on production characteristics is clarified by integrated analysis of static and dynamic data, so as to guide the formulation and optimization of technical policy for development.

4 Characterization of reservoir microscopic producing capacity

The cutoff threshold of throat size in volcanic gas reservoirs is determined by various methods. On this basis, the permeability cutoff of volcanic reservoirs is determined, the classification criteria of throat and permeability are set up, the effective pore volumes and mobile fluid volumes are evaluated under the control of various throat, permeability, and displacement pressure parameters, and the characteristics of the microscale fluid-producing capacity of volcanic rocks under different controlling conditions are clarified. These criteria provide a basis for evaluating reserve producing capacity and calibrating recovery parameters for volcanic gas reservoirs.

1 Research results from previous work

Zhao and colleagues [2–4] divided the reservoir spaces in volcanic rocks into two categories: pores and fractures. They subdivided pores into 10 categories: vesicles, pores within amygdaloids, interporphyrocrystic pores, contraction pores, intermicritic pores, interhyalinocrystalline pores, intracrystal pores, dissolution pores, expanded pores, and plastic flow pores. They subdivided fractures into six categories: structural fractures, cryptoexplosive fractures, diagenetic fractures, weathering fractures, vertical joints, and columnar joints.

Ren and coworkers [5] examined in detail the categories and characteristics of reservoir spaces in volcanic reservoirs in their study on the volcanic reservoir in Well Block 609 of the Liaohe depression. As a result, these authors divided the volcanic reservoir spaces into a primary category and a secondary category; the primary pores include primary vesicles, residual vesicles, interporphyrocrystic pores, intercrystal pores, pores within amygdaloids, and contraction pores; the primary fractures include cooling contraction fractures, contraction joints, and interclast fractures. The secondary pores include porphyrocrystic dissolution pores, intraporphyrocrystic and intramatrix dissolution pores, matrix dissolution pores, devitrified dissolution pores, dissolution pores in amygdaloids, dissolution pores in altered rocks, postfilling redissolution pores, and dissolution pores in metasomatic deposits; the secondary fractures include structural fractures and weathering fractures.

Yu and colleagues [6] divided the reservoir spaces in volcanic rocks into primary and secondary categories in their study on the volcanic reservoirs of the Wucaiwan depression (Junggar basin). The primary reservoir spaces include (1) primary pores (primary vesicles, residual vesicles, intercrystal pores and intra-crystal pores); (2) primary fractures (cooling contraction fractures); (3) secondary pores, including porphyrocrystic dissolution pores and dissolution pores in amygdaloids; and (4) secondary fractures, mainly including structural fractures and weathering fractures.

Liu and coworkers [7] divided the reservoir spaces in volcanic rocks into pores and fractures based on the morphology in their study on the evolution of the volcanic reservoir space in the Yingcheng Formation of the Xujiaweizi faulted depression in the Songliao basin. These authors recognized 15 types in four categories in terms of their origins: (1) primary pores, including vesicles, amygdaloidal pores, interclast pores, and intercrystal pores; (2) secondary pores, including interclast dissolution pores, intercrystal dissolution pores, intracrystal dissolution pores, devitrified dissolution pores, and dissolution pores in altered deposits; (3) primary fractures, including contraction fractures, contraction cleavages, and grain explosion cracks; and (4) secondary fractures, including structural fractures, dissolution fractures, and weathering fractures.

As this discussion implies, the classification schemes of reservoir spaces have been proposed from different perspectives, which partly reflects the complicated origins and diverse types of reservoir spaces in volcanic reservoirs. However, the shapes and origins of reservoir spaces have been taken into account when volcanic geologists have proposed classification schemes.

2 Classification scheme used in this book

With regard to the features of reservoir spaces in volcanic gas reservoirs such as their complex origins and their highly variable shapes and scales, the reservoir spaces in volcanic rocks are divided into two categories based on shape first: pores and fractures. Pores refer to a type of reservoir space that has an approximately equidimensional morphology; fractures refer to sheetlike reservoir spaces in 3D, in which two of the dimensions are far larger than the third (i.e., the ratio is greater than 10:1). Then, in terms of their origins, pores are subdivided into three categories: vesicles, intergranular pores, and dissolution pores, and fractures are subdivided into five categories: contraction fractures, structure fractures, blast fractures, dissolution fractures, and sutured fractures. Finally, according to the scale of reservoir space, the pores of different origins are further subdivided into 11 types at four levels: cavities, macropores, small pores, and micropores; and fractures are divided into four types: macro-, medium, small, and microfractures (Table 11.1).

1 Identification of vesicles

Vesicles are the product of effusion, with different characteristics of size, shape, and configuration from other types of pores. According to their origins and characteristics, therefore, vesicles in volcanic rocks can be identified effectively by taking an integrated analysis of their characteristics in lithology, lithofacies, texture, and structure of vesicle-bearing volcanic rocks.

2) Characteristics of rock structure and texture

As the markers for lithology and lithofacies characteristics, typical rock textures and structures can serve as indirect guides for identifying reservoir spaces. The effusion of magma usually results in the directional arrangement of vesicles and magma constituents; therefore, typical rhyolitic structure and welded texture are formed, and spherulitic textures are usually formed during the cooling of magma. Hence, rhyolitic structure, spherulitic texture, and welded texture are indirect identification markers for vesicles (Figure 11.2b and c).

2 Identification of intergranular pores

Intergranular pores are the product of explosive volcanic eruption. They are affected by the process of volcanic clastic formation, accumulation, and compaction, and their size, shape, and occurrence differ from those of vesicles. Intergranular pores in volcanic rocks are identified systematically by integrating the characteristics of lithology, lithofacies, structure, and texture of the rocks associated with intergranular pore development.

2) Characteristics of rock structure and texture

Volcanic explosion breaks the magma into volcanic debris of various sizes and ejects them out of volcanic conduits, and volcanic clastic rocks and their unique intergranular pores are formed by accumulation and compaction of the ejecta. Therefore, the structure and texture of volcaniclastic rocks may be used as indirect identification markers for intergranular pores. In volcanic agglomerates, the agglomeratic textures are developed and their reservoir spaces are dominated by large interclast pores. In volcanic breccia, breccia textures are developed. In autoclastic breccia lava, autoclastic textures are developed and their reservoir spaces are dominated by intergranular pores. In tuff, tuffaceous textures are developed and matrix micropores are dominant (Figure 11.3).

3 Identification of dissolution pores

Dissolution pores are of secondary origin, formed by the dissolution of soluble components such as feldspar phenocrysts and volcanic ash in vesicles, intergranular pores, and matrix mass. Compared with vesicles and intergranular pores, the shapes of dissolution pores are fairly diverse and occur in a wide range of lithology and lithofacies. Dissolution pores can be identified systematically by integrating dissolution trace markers with features associated with changes in pore shape and size.

4 Fracture identification

Fractures refer to the sheetlike spaces in rocks, distinctly different from pores with an approximately equidimensional shape. Based on their origins, fractures in volcanic rocks can be divided into five types: contraction fractures, blast fractures, structural fractures, dissolution fractures, and sutured fractures. They have different sizes, shapes, and occurrence and are developed in different lithological and lithofacies settings. On the basis of these characteristics, various types of fractures in volcanic rocks can be effectively identified according to fracture-bearing lithology and lithofacies as well as their rock textures and structures.

2) Shape characteristics

The shapes of fractures in volcanic rocks are diverse, with each fracture type having its own distinct morphological characteristics. Contraction fractures are affected by the distribution and variation of temperature during magma cooling, and they appear mostly as concentric circular and crevasse cracks (Figure 11.5a); large columnar joints near a volcanic conduit is regularly polygonal in cross section and flat linear in longitudinal section (Figure 11.5b). The shape of blast fractures is influenced by the intensity of gas-liquid explosion and the size and shape of phenocrysts, being generally netlike, with single fractures showing a flat or slightly curved linear pattern (Figure 11.5c). Structural fractures are formed from tectonic activities and the differential in the stress resistance of rocks, appearing in a linear pattern or grouped stripes, and a single fracture shows a flat or slightly curved linear pattern with a flat and regular fracture surface (Figure 11.5d). The shape of dissolution fractures is affected by the geometry of primary fractures and the dissolution pathways and intensity, and they manifest mostly netlike, crevasse-crack, flat to highly curved linear or stripe patterns, with various widths of fracture surfaces and irregular edges (Figure 11.5e). A sutured fracture is associated with the shape of its interface with the rock bed and pressure solution surfaces, mostly having a serrated or highly curved linear pattern with uneven fracture surfaces (Figure 11.5f).

5 Distribution characteristics of reservoir spaces

The identification results of volcanic gas reservoir spaces in the XX gas field revealed the following characteristics of classified reservoir spaces in volcanic gas reservoirs (see Table 11.1). (1) Pores are predominant in volcanic reservoir spaces, with a share of matrix porosity over 95% of the total porosity; fractures are well developed but make up less than 5% of the total porosity. (2) In these gas reservoirs, volcanic lava formed by center-fissured eruption is predominant and the effect of later dissolution is limited, so vesicles are mostly well developed in volcanic rocks, accounting for about 45% (Figure 11.6); intergranular pores rank second, accounting for about 35%; and dissolution pores are poorly developed, accounting for about 20%. (3) Among fractures in volcanic rocks, structural fractures are most common, accounting for about 70% (Figure 11.7); contraction fractures account for about 12%; blast, dissolution, and sutured fractures account for about 8%, 5%, and 5%, respectively.

1 Shape characterization of reservoir spaces

Cores provide important firsthand information for understanding reservoirs in the subsurface [10]. From core observation and description, the shapes of large reservoir spaces can be identified and confirmed visually. As to the small reservoir spaces, which cannot be observed directly and visually, Scanning Electric Microscope (SEM) uses cast thin sections to observe the shape of pores. For shape characterization of reservoir spaces, therefore, the two methods described earlier are integrated, and numerous cores and cast thin sections have been examined and analyzed to explain the characteristics of shape and distribution of various types of reservoir spaces.

1) Vesicles

Normally, volatile gas is expelled from magma as bubbles, which is affected by the flow of magma. As a result, vesicles show elongation and deformation along the flowing direction of magma, mostly having sub-rounded and ovoid shapes. After vesicles are formed, an irregular rhombic morphology would be produced if matrix filling occurred or filling and replacement by secondary minerals took place during later diagenetic processes (Table 11.3). In general, the nearly rounded vesicles are distributed mostly in the middle subfacies of the effusive facies, which is rarely affected by magma flow. Ovoid vesicles are distributed mainly in the top, upper, and lower subfacies of the effusive facies, and rhombic vesicles are usually distributed in the top and lower subfacies. In the XX gas field, ovoid vesicles are most common, accounting for about 50%, sub-rounded ones account for about 30%; and rhombic vesicles account for 20%.

Table 11.3

Classification of Vesicle Shapes in Volcanic Gas Reservoirs

Reservoir Space	Origin	Shape Classification
Vesicle	Formed by volatile degassing during eruption and surface flow
		Sub-rounded	Ovoid	Rhombic

2) Intergranular pores

Normally, volcanic debris tends to accumulate in situ, and therefore they have poor roundedness and sorting, leading to irregular-shaped intergranular pores. In comparison, well-sorted volcanic debris lead to rhombic intergranular pores with granular support; for moderate sorting, a striped pattern supported by multiple grains tends to be developed in intergranular pores; poor sorting is associated with a complicated dendritic pattern jointly supported by grains and matrix (Table 11.4). The rhombic and striped intergranular pores are usually developed in volcanic breccia of the fallout subfacies and in welded breccia of the hot clastic flow subfacies, both of the explosive facies; the dendritic intergranular pores tend to be developed in the tuffaceous breccia or brecciated tuff of the fallout subfacies; and the shape of micropores is relatively simple, most common in welded tuff of the hot clastic flow subfacies, crystalline tuff of the hot base surge subfacies, and tuff of the fallout subfacies. In the XX volcanic gas reservoir, rhombic intergranular pores are predominant, accounting for about 60%; dendritic ones account for about 25%; and striped ones account for only about 15%.

Table 11.4

Shape Classification of Intergranular Pores in Volcanic Gas Reservoirs

Reservoir Space	Origin	Shape Classification
Intergranular pore	Primary pores, formed by accumulation and compaction after volcanic debris ejected onto surface
		Rhombic	Striped	Dendritic

3) Dissolution pores

The shape of dissolution pores is affected mainly by dissolution and the property of soluble components. When phenocrysts are dissolved completely, the dissolution pores assume the regular phenocryst morphology; when secondary minerals in vesicles or intergranular pores are locally dissolved or the phenocrysts and matrix are partially dissolved, most of the dissolution pores appear as dissolution embayments; when volcanic ash is partially dissolved through weak dissolution activities, the dissolution pores usually have a disseminated and complex pattern (Table 11.5). Regular phenocryst dissolution pores are mostly developed in porphyrocrystallic tuff and rhyolite; dissolution embayments are widespread in various volcanic lava and volcanic clastic rocks; disseminated dissolution pores are most common in various volcanic tuff. In the XX volcanic gas reservoir, dissolution embayments are predominant, accounting for about 50%; disseminated ones account for about 35%; and regular porphyrocrystallic ones are poorly developed, accounting for only about 15%.

Table 11.5

Shape Classification of Dissolution Pores in Volcanic Gas Reservoirs

Reservoir Space	Origin	Shape Classification
Dissolution pore	Secondary pores, formed by dissolution of soluble constituents such as feldspar phenocrysts and volcanic ash in vesicles, intergranular pores and matrix mass.
		Regular porphyrocrystallic	Embayment	Disseminated

4) Fractures

Outcrop observation, core description, and thin section examination indicate that the fracture shapes in volcanic reservoirs are closely related to their petrogenetic origins. Overall they can be divided into three types: concentric circular, crevasse cracklike, and netlike (or net shaped). Individual fractures manifest in three patterns: flat linear, slightly curved linear, and highly curved linear (or serrated) (Table 11.6). Significant differences exist among various fracture shapes. (1) The shape of contraction fractures follows the evolution pattern in crystallo-mineralogy, appearing mostly as concentric rings and crevasse cracks, whereas the large columnar joints near volcanic conduits are regular net shaped. (2) Blast fractures are formed by cryptoexplosions, manifesting as crevasse cracks and networks, and the individual fractures have a flat to slightly curved linear pattern. (3) Structural fractures exhibit flat to slightly curved linear and striped patterns mostly developed in groups. (4) The shape of dissolution fractures is complex and usually appears as crevasse cracks and networks, with individual fractures having slightly to highly curved linear or striped patterns. (5) Finally, sutured fractures are formed by compaction and pressure solution controlled by multiple factors, including rock bedding, tectonic stress field, and formation compressibility, and characterized by a highly curved linear (or serrated) pattern. In the XX volcanic gas reservoir, the predominance of structural fractures is associated with flat to slightly curved linear patterns, accounting for about 80%; the crevasse crack and netlike patterns account for about 12%; the highly curved linear and concentric circular patterns are poorly developed, accounting only for about 5% and 3%, respectively.

Table 11.6

Shape Classification of Fractures in Volcanic Gas Reservoirs

Reservoir Space	Origin	Shape Classification
Fracture	Sheetlike space, formed by fracturing of volcanic rocks due to loss of cohesion
		Concentric circular	Crevasse crack like	Netlike (net shaped)
		Flat linear pattern or striped	Slightly curved linear pattern or striped	Highly curved linear pattern (serrated) or striped

3 Characterization of the accumulation capacity in reservoir spaces

Accumulation capacity is an important indicator for evaluating the sizes of various types of reservoir space and their fluid-bearing capacity. Two parameters—porosity and the maximum mercury intrusion saturation—are usually used to characterize the accumulation capacity of reservoir spaces in volcanic rocks.

1) Accumulation capacity of pores

Porosity and permeability tests, mercury injection experiments, and log interpretation information are used to characterize the overall accumulation capacity of pores in volcanic rocks and the accumulation capacity of various types of pores.

Overall accumulation capacity of pores

Because of their complex origins and variable shapes and sizes, the accumulation capacity of pores varies significantly. In the XX volcanic gas reservoir (Table 11.10, Figures 11.11 and 11.12), the porosity as revealed by whole core experiments of volcanic rocks ranges from 0.44% to 18.8%, mainly between 3% and 8%, with an average of 7.10%. The log interpretation indicates that the maximum effective porosity is 16.6%, the average is 6.5%, and the maximum mercury intrusion saturation has total range of 11% to 99%, a main range of either 15% to 40% or 70% to 100%, and an average of 65%, suggesting a generally strong but highly variable accumulation capacity of pores.

Table 11.10

Characteristics of the Accumulation Ability of Pores in Volcanic Reservoirs (XX Gas Field)

Pore Type	Typical Rock	Logging Interpreted Effective Porosity, %	Experimental Porosity, %	Maximum Mercury Intrusion Saturation, %
Vesicle	Rhyolite	5.14	6.75	65.42
Intergranular pores	Rhyolitic volcanic breccia	4.5	5.58	60.21
Dissolution pores	Rhyolitic brecciated lava	7.45	8.4	73.42
All pores	Volcanic rocks	6.5	7.1	65

Accumulation capacity of pores with various origins

Pores with different volcanic petrogeneses show great variation in accumulation capacity because of their different shapes and sizes (see Table 11.10). Dissolution pores have the highest values in effective log porosity (average 7.45%), experimental porosity (average 8.4%), and maximum mercury intrusion saturation (average 73.42%), thus they have the strongest accumulation capacity. Vesicles have moderate values in effective log porosity (average 5.14%), experimental porosity (average 6.75%), and maximum mercury intrusion saturation (average 65.42%), hence they have a moderate accumulation capacity. Intergranular pores have the lowest values in effective log porosity (average 4.5%), experimental porosity (average 5.58%), and maximum mercury intrusion saturation (average 60.21%), and thus they have the lowest accumulation capacity.

Accumulation capacity of pores with different sizes

Pores with different sizes in volcanic rocks have different levels of accumulation capacity (Table 11.11).

Table 11.11

Statistics on the Accumulation Ability of Various Types of Pores in Volcanic Gas Reservoirs (XX Gas Field)

Pore Type		Subfacies	Effective Log Porosity, %	Experimental Porosity,%	Maximum Mercury Intrusion Saturation, %
Vesicle	Macro vesicles	Top subfacies of effusive facies Upper subfacies of effusive facies Lower subfacies of effusive facies Splash subfacies of explosive facies	5.16	7.14	65.02
	Small vesicles	Middle subfacies of effusive facies Hot clastic flow subfacies of explosive facies	3.17	5.56	51.69
Intergranular pore	Macro intergranular pores	Fallout subfacies of explosive facies Hot clastic flow subfacies of explosive facies	5.35	5.87	69.81
	Small intergranular pores	Fallout subfacies of explosive facies Hot base surge subfacies of explosive facies	3.87	5.29	46.66
Dissolution pores	Dissolution pores	Top subfacies of effusive facies Upper subfacies of effusive facies Splash subfacies of explosive facies Fallout subfacies of explosive facies	6.63	8.62	75.06
	Matrix dissolution pores	Lower subfacies of effusive facies Hot clastic flow subfacies of explosive facies Hot base surge subfacies of explosive facies Volcanic conduit facies	5.31	6.96	65.49

Vesicles

Macrovesicles (including vesicular cavities) are mostly developed in top, upper, and lower subfacies of the effusive facies and splash subfacies of the explosive facies. The complete escape of volatiles results in large sizes and well-developed vesicles, and thus favorable petrophysical properties and accumulation capacity. Most small vesicles (including micropores) are developed in the middle subfacies (effusive facies) and hot clastic flow subfacies (explosive facies). The incomplete escape of volatiles results in small sizes and poorly developed vesicles, and consequently rather poor petrophysical properties and accumulation capacity. In the XX volcanic gas reservoir, macrovesicles have relatively high values in effective log porosity, experimental porosity, and maximum mercury intrusion, at 5.16%, 7.14%, and 65.02%, respectively. Small vesicles have relatively low values in effective log porosity, experimental porosity, and maximum mercury intrusion, at 3.17%, 5.56%, and 51.69%, respectively, indicating that macrovesicles have a notably stronger accumulation capacity than small vesicles.

Intergranular pores

Macro-intergranular pores (including intergranular cavities) are mainly developed in fallout and hot clastic flow subfacies of the explosive facies. In these grain-supported volcanic breccia rocks, the intergranular pores are well developed, with a large radius, good petrophysical property, and strong accumulation capacity. Most small intergranular pores (including matrix micropores) are common in the fallout and hot base surge subfacies. Because smaller breccia grains and matrix usually provide support among breccia bodies, intergranular pores have a small radius and are poorly developed, characterized by poor petrophysical properties and a relatively weak accumulation capacity. In the XX gas field, the macro-intergranular pores have relatively high values of effective log porosity, experimental porosity, and maximum mercury intrusion, at 5.35%, 5.87%, and 69.81%, respectively. The small intergranular pores have relatively low values of effective log porosity, experimental porosity, and maximum mercury intrusion, at 3.87%, 5.29%, and 46.66%, respectively, indicating that macro-intergranular pores have a stronger accumulation capacity.

Dissolution pores

Dissolution pores with a large radius (including dissolution cavities) are formed mainly in top and upper subfacies of the effusive facies, as well as splash and fallout subfacies of the explosive facies. Because primary pores and fractures are well developed through the actions of surface water and underground water, dissolution pores tend to have good petrophysical properties and strong accumulation capacity. The matrix dissolution pores (including some dissolution pores) with a small radius are distributed mainly in the volcanic conduit facies, lower subfacies of the effusive facies, and hot clastic flow and hot base surge subfacies of the explosive facies. Because of the poorly developed primary pores and fractures, limited fluid and dissolution activity, and small radius, matrix dissolution pores tend to have relatively poor petrophysical properties low accumulation capacity. In the XX gas field, the dissolution pores in volcanic rocks have relatively high values of effective log porosity, experimental porosity, and maximum mercury intrusion, at 6.63%, 8.62%, and 75.06%, respectively. These values are lower for matrix dissolution pores, at 5.31%, 6.96%, and 65.49%, respectively. This indicates that dissolution pores with a large radius have a higher accumulation capacity.

After further comparative analysis (see Table 11.11), it can be observed that dissolution pores have the strongest accumulation capacity among the six types of pores in three categories, macrovesicles and matrix dissolution pores rank second, and the accumulation capacity of small intergranular pores is the weakest.

2) Accumulation capacity of fractures

Fracture accumulation capacity is characterized through core description and log interpretation:

1. Core description. With cores, the fracture width and length are measured to calculate the ratio of fracture area to core surface area so as to obtain the surface porosity of fractures. In the XX gas field, fracture description for over 1400 m cores shows that (Figure 11.13) the maximum surface porosity of fractures in the volcanic gas reservoirs is 2.7%, and the surface porosity of fractures ranges from 0.02% to 0.6%, with an average of 0.28%, accounting for about 3.9% of the total reservoir porosity.

2. Log interpretation. Fracture porosity can be derived from electrical imaging log analysis and the statistical models of conventional dual laterologs (see Chapter 6 for details). In the XX volcanic gas reservoirs, log interpretations based on more than 50 wells are used to determine fracture porosity. The results (Figure 11.14) indicate that the maximum fracture porosity in volcanic gas reservoirs is 1.9%, and the fracture porosity ranges from 0.01% to 0.07%, averaging 0.06%, thereby accounting for about 0.8% of the total reservoir porosity.

From the preceding discussion, one can conclude that (1) in volcanic reservoirs, the overall accumulation capacity of fractures is limited, and the subsurface accumulation ability of fractures is lower because of their low compaction resistance; (2) the fracture accumulation capacity varies greatly because of the complex origins and highly variable types and sizes, and the fracture accumulation ability varies greatly; (3) among various types of fractures, structural fractures tend to be well developed, with a large width, and they have the strongest accumulation capacity; the accumulation capacity of dissolution fractures ranks second; the accumulation capacity of contraction fractures and blast fractures are relatively weak; most sutured fractures are closed and have no effect on the accumulation-permeation.

1 Pore-contraction throats

Pore-contraction throats refer to the throats formed by pore contraction between vesicles or intergranular pores, and they are widespread in volcanic reservoirs. Throats between vesicles have two origins: (1) the throat is formed at the contracted part of a vesicle, and (2) the throat is developed within a vesicle or between vesicles when contraction occurs due to secondary mineral growth (e.g., quartz). For the pore-contraction throats between intergranular pores, they are formed from the accumulation and compaction of volcanic debris. Pore-contraction throats usually have a tubular shape. Most throats that originate from vesicular contraction are regular in shape, whereas those that originate from debris accumulation and compaction and crystal growth are more complicated. The radius and length of pore-contraction throats vary greatly; they can be divided into four types: large long, large short, small long, and small short (Figure 11.15).

2 Dissolution throats

Dissolution throats are formed by dissolution after primary throats and matrix mass. A dissolution throat is a major type between dissolution pores, with two origins: (1) the soluble components in a pore-contraction throat are dissolved and the original throat is restored; and (2) the soluble components in the matrix are dissolved and a new throat is generated. The dissolution throat usually follows the shape of its primary throat but may become irregularly shaped due to dissolution. The radius of dissolution throats tends to be larger than that of the original ones, and most are large throats. Therefore, dissolution throats mostly fall into the large and long type and large and short type based on their radius and length.

3 Fractured throats

Fractured throats refer to fractures that become pathways among vesicles, intergranular pores, and dissolution pores, playing the role of pore communication. Fractured throats are formed by rock contraction, gas-liquid explosion, tectonic activities, and dissolution, and they usually have a sheetlike configuration. Because a fracture has two dimension that are far greater than the third (with a ratio greater than 10), fractured throats are divided into two types according to their width and extension length: the large and long type, and the small and long type. Throats that originate from structural fractures, dissolution fractures, and large columnar joint fractures are mostly of the large-long type, whereas those that originate from contraction fractures and blast fractures are generally of the small-long type (see Table 11.12).

1 Identification of pore-contraction throats

Pore-contraction throats, formed variously from volcanic debris accumulation and compaction, degassing of volatiles in magma, and secondary crystal growth, are the constricted part of vesicles or intergranular pores. Therefore, these types of throats have distinct shapes and sizes and are developed in specific reservoir spaces and specific lithologies and lithofacies. Pore-contraction throats can be effectively identified based on their origin, shape, and size and by taking into account the characteristics of reservoir space, lithology, lithofacies, and the structure and texture of throat-bearing rocks.

3) Characteristics of throat shape and size

In terms of their complex origins and variable radii and length, pore-contraction throats can be divided into four types: large long, large short, small long, and small short (Figure 11.16a and b). Large-long throats are primarily developed between intergranular pores with a large radius and are in the shape of sandglass-like tubes (Figure 11.16a). Large-short throats are most common between vesicles or intergranular pores with a large radius and are in the shape of dumbbells (Figure 11.16b). Small-long throats are found commonly between intergranular pores with a small radius and are in the shape of slender sandglasses. Small-short throats are developed between vesicles and matrix micropores with a small radius and are in the shape of minute dumbbells.

2 Identification of dissolution throats

Compared with pore-contraction throats, dissolution throats are of complex origins and variable morphology, developed in a wide range of lithologies and lithofacies. Therefore, dissolution throats can be effectively identified based on throat shape and size as well as dissolution trace markers in combination with the lithology, lithofacies, structure, and texture of the rocks in which the throats are developed.

3 Identification of fractured throats

Fractured throats are formed from rock contraction, cryptoexplosion, and tectonic activities, and they differ from matrix throats in that they have their own distinct origins and shapes. Therefore, fractured throats can be identified systematically based on their origins, shapes, and sizes, in combination with the characteristics of lithology, lithofacies, structure, and texture of the rocks in which fractures are developed.

1 Characterization of throat shape

Because throat has a small radius, cast thin section examination is mainly used for SEM observation of the shape and distribution of various types of throats in volcanic reservoirs. This provides a basis for the characterization of microstructures in volcanic reservoirs.

1) Pore-contraction throats

These throats are the constricted parts between vesicles or intergranular pores and normally have a tubular shape. The vesicle contraction throats usually have the large-short and small-short tubular features, with a sandglass-like outline (Table 11.14b and d). Crystal growth throats are characterized by a rough inner wall and an irregular shape, usually of the small-long tubular category, with a sandglass outline. Most contracted throats in intergranular pores are of the large-long and small-long types, with a dumbbell outline (Table 11.15a and c). In the XX gas field, where volcanic lava is the major lithology in the volcanic reservoir, vesicles with a small radius are predominant in pores. Small-short throats are usually the well-developed type of pore-contraction throat, accounting for about 55%; the large-short type accounts for about 20%; the small-long type accounts for about 15%; and the large-long type accounts for about 10%.

Table 11.14

Shape Classification of Pore-Contraction Throats in Volcanic Reservoirs

Throat Type	Origin	Shape Type
Pore- contraction throat	Formed by contraction of intergranular pores or vesicles
		(a) Large-long tubular, sandglass like (volcanic breccia).	(b) Large-short tubular and dumbbell like (rhyolite).
		(c) Small-long tubular, sandglass like (volcanic breccia).	(d) Small-short tubular and dumbbell like (rhyolite).

Table 11.15

Shape Classification of Dissolution Throats in Volcanic Reservoirs

Throat Type	Origin	Shape Type (Irregular Tubular)
Dissolution throat	Through dissolution, pore-contraction throats are reformed, or the soluble components are dissolved in matrix
		(a) Large-long type, sandglass-like in outline.	(b) Large-short type, dumbbell-like in outline.	(c) Large-short type, thorny in outline.

3) Fractured throats

These are largely sheetlike throats. In the large-long type formed by large fractures, throats originating from structural fractures have a large size and a linear pattern in plan view and may be developed in various types of pores (Table 11.16a). Those associated with dissolution fractures have larger sizes than those of precursory fractures, as their shape is generally controlled by the precursory fractures, but the inner wall tends to become rough with an irregular the outline, and they may occur in various types of pores. The throats originating from columnar joint fractures are large in size, manifesting as crevasse cracks in the cross section and a linear pattern in the longitudinal section, and they develop mainly between macrovesicles or small vesicles in volcanic lava of the volcanic conduit facies. In the small-long throats originating from small fractures, those originating from contraction fractures are characterized by concentric rings and crevasse cracks, and they develop between various types of vesicles in volcanic lava of the effusive facies (Table 11.16b); those originating from blast fractures have the smallest size, usually showing a flat to slightly curved linear pattern, and they are found commonly between various types of intergranular pores in volcanic clastic rocks of the explosive facies and autoclastic breccia lava of the volcanic conduit facies (Table 11.16c).

Table 11.16

Shape Classification of Fractured Throats in Volcanic Reservoirs

Throat Type	Origin	Shape Type (Sheetlike Space)
Fractured throat	Fracture plays a role in pore communication and results in fractured throats
		(a) Large-long type (structural fracture), linear pattern.	(b) Small-long type (contraction fracture), linear pattern.	(c) Small-long type, as crevasse cracks.

2 Characterization of throat size in reservoirs

Throat size determines the fluid flow capacity of a reservoir. In terms of throat features in volcanic reservoirs, the size characteristics of various types of throats can be clarified by integrating core observation, cast thin section examination, capillary pressure curve analysis, constant-rate mercury injection analysis, and log interpretation. This provides a basis for characterizing the accumulation-permeation capacity of throats.

2) Mercury injection

Mercury injection experiments measure the mercury volumes injected into a core plug under different pressures; the capillary pressure curves are derived and then the throat size and pore volume controlled by the throats are calculated using the J function [13]. Through processing and statistical analysis of a large number of capillary pressure curves, the size and distribution characteristics of various types of throats in a volcanic reservoir matrix can be assessed.

In the XX gas field, capillary pressure curves were analyzed with conventional mercury injection information derived from 206 pieces of volcanic rock core samples, and the radius distributions of various types of matrix throat were obtained (Table 11.17). The largest matrix throat radius is 0.64 μm and the radii range from 0.005 to 0.4 μm with an average of 0.07 μm (Figure 11.17a). In terms of size, microthroats are dominant (accounting for about 76.6%), small throats rank second (accounting for about 18%), and medium throats and large throats are poorly developed (accounting for about 4.9% and 0.5%, respectively) (Figure 11.17b). In matrix throats, the pore-contraction throats have the largest radius at 0.38 μm, with the radius values ranging from 0.008 to 0.2 μm and averaging 0.06 μm; in terms of size, microthroats are dominant (accounting for about 83.9%), small throats account for about 13.4%, medium throats account for only about 2.7%, and large throats are not developed (Figure 11.17c). The dissolution throats have a maximum radius of 0.64 μm, with the radii ranging from 0.01 to 0.4 μm and averaging 0.11 μm; in terms of size, microthroats are dominant (accounting for about 57.9%), small throats account for about 29.8%, medium throats account for only about 10.5%, and large throats are poorly developed (accounting for about 1.8%) (Figure 11.17d).

Table 11.17

Statistics on Throat Sizes in Volcanic Gas Reservoirs (XX Gas Field)

Characterization Method	Matrix Throat Radius, μm			Pore-Contraction Throat Radius, μm			Dissolution Throat Radius, μm			Fractured Throat Radius, μm
	Max	Range	Min	Max	Range	Min	Max	Range	Min	Max	Range	Min
Core observation										3000	30-300	150
Mercury injection	0.64	0.008-0.4	0.07	0.38	0.008-0.2	0.06	0.64	0.01-0.4	0.11
Constant-rate mercury injection	5.4	0.16-1.2	0.6
Log interpretation										750	2.5-25	14

3) Constant-rate mercury injection

In this method, mercury is injected into rock pores in a process of quasi-static state. Pore and throat are identified by the pressure drop that occurs when mercury breaks through the throat, and the characteristic parameters of microscopic porous structure in rocks, such as distributions of pore radius and throat radius controlled by large throats (≥ 0.1 μm), can be derived. In the XX gas field, 24 pieces of volcanic rock sample were analyzed in the constant-rate mercury injection experiments. The results (Figure 11.18 and Table 11.17) indicate that the maximum radius of a matrix throat in the volcanic reservoir is 5.4 μm, with the radius values distributed from 0.16 to 1.2 μm and averaging 0.6 μm; medium throats are dominant, accounting for about 41.7%; small throats rank second, accounting for about 33.3%; and very few large throats are developed (about 25%). Because mercury injection is a quasi-static process, micropores cannot be detected.

3 Characterization of reservoir permeation capacity

Permeation capacity is an important indicator for evaluating the sizes of various types of throats and their fluid flow properties. Permeability and displacement pressure are two parameters normally used to characterize the permeation capacity of throats in volcanic reservoirs.

1) Permeation capacity of matrix throats

By integrated use of core porosity and permeability tests, mercury injection experiments, and log interpretations, the overall permeation capacity of matrix throats and various individual types of matrix throats in volcanic reservoirs are characterized.

Overall permeation capacity of reservoir matrix

In volcanic reservoirs, matrix throats have complex origins and highly variable shapes and radius sizes, resulting in significant variations in their permeation capacities. In the XX gas field (Figure 11.19 and Table 11.18), the volcanic rock cores have a maximum permeability of 17.3 mD, ranging from 0.02 to 0.2 mD with an average of 0.14 mD. The matrix permeability derived from logging interpretation has a maximum of 3.2 mD, with a range of 0.01 to 0.15 mD and an average 0.07 mD. The displacement pressure derived from mercury injection experiments has a wide distribution, with an average of 9.77 MPa. It is thus confirmed that the matrix permeation capacity in the volcanic reservoir is relatively weak and varies significantly.

Table 11.18

Statistics on the Permeation Capacity of Various Types of Throats in Volcanic Gas Reservoirs (XX Gas Field)

Throat Type	Typical Rock	Matrix Permeability Derived from Logging, mD	Experimental Matrix Permeability, mD	Expulsion Pressure, MPa
Pore-contraction throat	Rhyolitic volcanic breccia	0.05	0.08	10.57
Dissolution throat	Rhyolitic breccia lava	0.16	0.28	8.06
Matrix throat	All volcanic rocks	0.07	0.15	9.77

Permeation capacity of various types of throats by origin

The two types of matrix throat—that is, the pore-contraction throats and dissolution throats—have different origins, shapes, and sizes, leading to different permeation capacities (see Table 11.18). For pore-contraction throats, the matrix permeabilities derived from logs and experiments are 0.05 mD and 0.08 mD, respectively, and the displacement pressure is 10.57 MPa; whereas these values for dissolution throats are 0.08 mD, 0.28 mD, and 8.06 MPa, respectively. This suggests a relatively good permeation capacity for dissolution throats.

Permeation capacity of various types of throats by size

Throats of different sizes and lengths have different permeation capacities. In general, a larger throat size corresponds to a stronger permeation capacity. In the pore-contraction throat example (Table 11.19) from the XX gas field, the matrix permeability of large-short throats is the highest (average 0.124 mD) and their displacement pressure is the lowest (average 9.16 MPa), indicating the strongest permeation capacity. The matrix permeability of large-long throats is fairly high (average 0.108 mD) and their displacement pressure is fairly low (average 9.86 MPa), showing a relatively good permeation capacity. The matrix permeability of small-long throats is fairly low (average 0.048 mD) and their displacement pressure is high (average 12.31 MPa), showing a weak permeation capacity. Finally, the matrix permeability of small-short throats is the lowest (average 0.032 mD) and their displacement pressure the highest (average 16.17 MPa), representing the weakest permeation capacity.

Table 11.19

Statistics on the Permeation Ability of Various Types of Throats in the Volcanic Reservoirs of XX Gas Field (in Terms of Subfacies)

Throat Type		Typical Subfacies	Matrix Permeability, mD	Displacement Pressure, MPa
Pore-contraction throat	Large-short	Upper subfacies of effusive facies	0.124	9.16
	Large-long	Fallout subfacies of explosive facies	0.108	9.86
	Small-long	Fallout subfacies of explosive facies	0.048	12.31
	Small-short	Lower subfacies of effusive facies Hot base surge subfacies of explosive facies	0.032	16.17

1 Single accumulation-permeation (A-P) pattern

A single A-P pattern denotes that the reservoir spaces are simple and that throats and reservoir spaces are related in origin. In light of the genetic classification and assemblage relationship of reservoir spaces and throats, the single pattern in volcanic gas reservoirs includes four types: vesicular, intergranular porous, microporous, and fractured. Because of the differences in pore types, throat types, and characteristics of lithology and lithofacies, different types of A-P patterns have different accumulation-permeation capacities, resulting in distinct dynamic characteristics of deliverability and stable production capability in volcanic reservoirs (see Table 11.20):

1. Vesicular A-P pattern. This pattern is mainly developed in vesicular lava of top and upper subfacies of the effusive facies; vesicles with a large radius are the predominant reservoir space, characterized by a bead-string-like distribution; most throats are of the large-short type formed from vesicle contraction, and the small-short type ranks second. Fractures are poorly developed. Hence, this pattern provides a high accumulation capacity and moderate permeation capacity.

2. Intergranular porous A-P pattern. This pattern is mainly formed in volcanic breccia of fallout subfacies of explosive facies and welded breccia of hot clastic flow subfacies of explosive facies. Intergranular pores with a large radius are predominant in reservoir spaces and most throats originate from intergranular pore contraction, and large and long, large and short, small and long, and small and short throats are all developed; fractures are poorly developed. Therefore, this pattern has features of moderate accumulation ability and moderate or poor permeation ability.

3. Microporous A-P pattern. This pattern occurs primarily in welded tuff of hot clastic flow subfacies of the explosive facies and crystal tuff of hot base surge subfacies of the explosive facies. The reservoir space is dominated by matrix micropores with a small radius. The small-short types of throats that originated from matrix micropore contraction are predominant; fractures are poorly developed. Thus, this pattern usually has relatively poor accumulation and permeation capacities.

4. Fractured A-P pattern. This pattern is found mainly in tight volcanic rocks of lower and middle subfacies of the effusive facies as well as hot base surge subfacies of the explosive facies. Structural and contraction fractures are predominant, making up the major reservoir spaces and seepage pathways. Most of the matrix pores are scattered small vesicles, microvesicles, and matrix micropores. Fractured throats are predominant, including the large-long throat type formed from large fractures as well as the small-long throat type that originates from small fractures. Therefore, this pattern is characterized by poor accumulation capacity but good permeation capacity.

2 Combined A-P pattern

In a combined A-P pattern, various pores act as reservoir spaces and matrix throats, and fractured throats act as seepage pathways. Because the permeation capacity of fractured throats is much higher than that of matrix throats, the matrix throats in combined pattern have a smaller contribution to the permeation capacity of reservoirs. In light of the genetic types of reservoir spaces, the combined A-P patterns fall into four types: fractured-vesicular, fractured–intergranular porous, fractured-microporous, and fractured–dissolution-porous. In different types of combined A-P patterns, there are differences in pore types, fracture types, lithology, and lithofacies, and consequently there are differences in accumulation capacity, permeation capacity, and reservoir deliverability (see Table 11.20):

1. Fractured-vesicular A-P pattern. This A-P pattern is developed mainly in vesicular lava of the top and upper subfacies of the effusive facies. The reservoir space is dominated by vesicles with a large radius, featuring a bead-string-like distribution. Most of the throats are of the vesicle contraction and fractured types. Vesicle contraction throats include the large-short and small-short categories, whereas fractured throats include the large-long and small-long categories. Most fractures are well developed and are of the structural and contraction types. Hence, the fractured-vesicular A-P pattern has good accumulation and permeation capacities.

2. Fractured–intergranular porous A-P pattern. This A-P pattern is mainly formed in volcanic breccia of fallout subfacies of the explosive facies and welded breccia of hot clastic flow subfacies of the explosive facies. Most of the reservoir spaces are intergranular pores with a large radius. Intergranular pore-contraction throats and fractured throats are dominant, with large-long, large-short, small-long, and small-short types of throats developed. Structural fractures are predominant and well developed. The fractured–intergranular porous A-P pattern, therefore, has a moderate accumulation capacity and a fairly good permeation capacity.

3. Fractured-microporous A-P pattern. This A-P pattern is found primarily in welded tuff of hot clastic flow subfacies of the explosive facies and crystalline tuff of hot base surge subfacies of the explosive facies. Most of the reservoir spaces are matrix micropores with a small radius. Pore-contraction throats and fractured throats are predominant, usually including the large-long, small-long, and small-short types. Structural and blast fractures are common and well developed. As a result, this A-P pattern usually has a moderate-low accumulation capacity and a moderate permeation capacity.

4. Fractured–dissolution-porous A-P pattern. This A-P pattern occurs mainly in volcanic rocks of various lithofacies. The reservoir spaces are primarily composed of dissolution pores of variable radii, including dissolution cavities, dissolution pores, and matrix dissolution pores. Dissolution and fractured throats are predominant. Structural contraction and blast fractures are well developed. Therefore, this pattern has a moderate-strong accumulation capacity and a good permeation capacity.

1 Vesicular A-P pattern

A vesicular A-P pattern is developed mainly in volcanic lava of the effusive facies. Its accumulation capacity is primarily affected by vesicle size and the degrees of vesicle development, as well as reservoir extension, whereas the permeation capacity is influenced by the radius of vesicle contraction throats and pore-throat configuration. From the middle-lower subfacies to the upper-top subfacies of the effusive facies, the radii of vesicles and throats increase because of the increased degassing of volatile constituents, with better developed and better communicated vesicles, resulting in improved accumulation-permeation capacity, deliverability, and stable production capability for the reservoirs.

Horizon No. 93 in Well XSS2-1 has a typical vesicular A-P pattern, characterized by vesicular rhyolite of the upper subfacies of the effusive facies. Its reservoir space is dominated by macrovesicles with a bead-string-like distribution. Most of the throats are of the large-short type that originated from vesicle contraction, exhibiting good communication between pores. Fracture is not observed in cores (Figure 11.20a). In this horizon, the effective porosity is 13.2%, the average permeability is 0.74 mD, and the probe radius for the inner zone is 65 m from well test interpretation with an infinite outer zone, indicating a good accumulation capacity but a moderate permeation capacity. During gas testing, the natural deliverability was 15.1 × 10⁴ m³/d, with a pilot production yield of 12.15 × 10⁴ m³/d and open flow potential of 40.93 × 10⁴ m³/d. After stream injection, a stable production of 9 to 15.7 × 10⁴ m³/d is maintained, with a slow pressure decline and a gas production per pressure drop at 7839 × 10⁴ m³/MPa. So far, the cumulative gas production has been 0.6 × 10⁸ m³, and the stable production duration is estimated to be 10 years (Figure 11.20b and Table 11.21). Therefore, these rocks of the upper and top subfacies with a vesicular A-P pattern have good accumulation-permeation capacities, high production levels, and a strong potential for stable production.

2 Intergranular porous A-P pattern

This A-P pattern occurs primarily in large-grained volcanic clastic rocks of the explosive facies. Its accumulation-permeation ability is affected by many factors, such as the clastic grain size, sorting, accumulation, and compaction. Large and well-sorted grains, strong grain support, and weaker compaction usually correspond to large, well-developed, and well-connected intergranular pores and, consequently, a strong accumulation-permeation capacity. Due to weak resistance against compaction, matrix filling usually occurs within the intergranular pores. Under deep burial conditions, therefore, the intergranular porous A-P pattern tends to have poor accumulation-permeation capacities, deliverability, and stable production potential.

In Well XX3, horizons No. 165, 166, and 170-I have a typical intergranular porous A-P pattern, hosted in welded tuff of the hot clastic flow subfacies. The reservoir space is predominantly composed of intergranular pores with a small radius. Most of the throats belong to the small-long category and originated from contraction of intergranular pores. Fractures are not developed, resulting in poor communication between pores (Figure 11.21a). In this reservoir interval, the average effective porosity is 5.51%, average permeability is 0.12 mD, and the probe radius is 67.6 m, indicating a poor accumulation and permeation capacity. Gas testing in the interval produced the following results: natural deliverability 0.08 × 10⁴ m³/d, deliverability up to 5.27 × 10⁴ m³/d after fracturing, and open flow potential 7.49 × 10⁴ m³/d (see Table 11.21). The discharge curve in production tests (Figure 11.21b) shows that this reservoir interval has a certain level of capacity for stable production, at a low yield of 2 to 3 × 10⁴ m³/d.

3 Microporous A-P pattern

A microporous A-P pattern occurs mainly in welded tuff of the hot clastic flow subfacies and crystalline tuff of the hot base surge subfacies. The reservoir space is dominated by matrix micropores and their associated small-short throats, resulting in poor accumulation and permeation capacities. Generally speaking, the volcanic reservoir with this A-P pattern has no natural deliverability, although a medium-low yield and relatively weak stable production can be achieved by fracturing.

Horizon No. 129 in Well XX1-4 has this type of A-P pattern, found in rhyolitic breccia and crystalline tuff of the hot base surge subfacies. The reservoir space is dominated by matrix micropores, connected by small-short throats that originated from the contraction of matrix micropores, without fractures (Figure11.22a). In this horizon, the average effective porosity is 4.87%, the average permeability is 0.08 mD, and the probe radius is 213.5 m, showing a moderate accumulation capacity but poor permeation capacity. After gas testing, natural production reached 0.28 × 10⁴ m³/d, and postfracturing production rose to 12.64 × 10⁴ m³/d; the yield during production testing was 6.4 × 10⁴ m³/d, and open flow potential was 19.49 × 10⁴ m³/d. After stream injection, it produced intermittently at a yield of 3.4 to 6.43 × 1 0⁴ m³/d, showing a rapid pressure drop when the well was open and a slow pressure buildup after shut-in. So far, the cumulative gas production has been up to 0.12 × 10⁸ m³ (Figure 11.22b and Table 11.21). This suggests that the microporous A-P pattern has poor accumulation-permeation capacity, low deliverability, and weak stable production potential.

4 Fractured A-P pattern

This pattern may be developed in various types of lithologies and lithofacies. Fractures are predominant in reservoir spaces and seepage pathways, characterized by poor accumulation capacity but good permeation capacity. The characteristics of the production performance of volcanic reservoirs with a fractured A-P pattern show that a certain level of production can be obtained at the initial stage when the reservoir is opened, but it declines rapidly and fails to maintain at a stable level. Therefore, a volcanic reservoir with this A-P pattern is rarely opened directly; instead, the reservoir is put into development indirectly by opening overlying and underlying pay zones.

Horizon No. 224 (3737 to 3822 m) in Well XX21 is a typical example of a fractured reservoir that occurs in tight rhyolite of the middle subfacies of the effusive facies. Vesicles are absent but structural, blast, and contraction fractures are developed (Figure 11.23a). In this horizon, the average effective porosity is 1.44 %, and the average permeability is 8.09 mD, making it a low-porosity and high-permeability reservoir. No gas tests or production tests have been conducted in this horizon. However, in its overlying formation, No. 223 (3656 to 3737 m, with an effective porosity of 4% to 8%, and permeability of 0.1 to 8 mD), a natural deliverability of 3 × 10⁴ m³/d was obtained during gas testing, postfracturing production rose to 41.96 × 10⁴ m³/d, and the open flow capacity was 55.18 × 10⁴ m³/d (see Table 11.21). In horizon No. 223, the yield during production testing was stabilized at 12 × 10⁴ m³/d; the pressure dropped rapidly but built up quickly after shut-in, and then increased gradually to approach a stable level (Figure 11.23b). This implies that some of the initial deliverability of No. 223 horizon is the contribution from the fractures in horizons No. 223 and No. 224, and the rapid production decline is consistent with the deliverability characteristics of a fractured A-P pattern.

5 Fractured-vesicular A-P pattern

This A-P pattern is primarily developed in vesicular rhyolite of the top and upper subfacies of the effusive facies as well as breccia lava of the splash subfacies of the explosive facies. Its accumulation capacity is mainly controlled by the vesicular development and reservoir extension. The permeation capacity is affected by many factors such as fracture types and their degree of development, the radius of vesicle contraction throat, and the assemblage relationships of fractures and vesicles. This A-P pattern generally has a moderate accumulation-permeation capacity, and the volcanic reservoir of this pattern has a moderate deliverability and some stable production potential.

Horizons No. 59 II (3592 to 3600 m) and No. 60 IV (3665 to 3675 m) of Well XX9 consist of the upper subfacies vesicular rhyolite of the effusive facies. Reservoir spaces are predominantly the uniformly distributed small vesicles with well-developed structural and contraction fractures well connected by fractured and vesicle contraction throats, representing a typical fractured-vesicular A-P pattern (Figure 11.24a). In these formations, the average effective porosity is 6.3%, the average permeability is 1.19 mD, and the probe radius is 19.8 to 27 m, indicating a moderate accumulation and permeation capacity. Gas tests produced the following results: natural deliverability 0.05 × 10⁴ m³/d, postfracturing production 20.9 × 10⁴ m³/d, and open flow potential 23.16 × 10⁴ m³/d; after production commenced, the yield was maintained at 6 × 10⁴ m³/d, pressure dropped quickly and built up rapidly after well shut-in, and the gas production per pressure drop was 86.3 × 10⁴ m³/d/MPa. So far, the cumulative gas production has been 0.3 × 10⁸ m³, with the stable production period estimated at four years. This demonstrates that these reservoirs have a moderate accumulation-permeation capacity but a relatively low to poor potential for stable production (Figure 11.24b and Table 11.21).

6 Fractured–intergranular porous A-P pattern

This A-P pattern occurs mainly in volcanic breccia of the fallout subfacies and welded breccia of the hot clastic flow subfacies, both of the explosive facies. Its accumulation capacity is affected by the degree of intergranular pore development and reservoir extension, whereas the permeation capacity is affected by multiple factors such as fracture types and their development, throat radius, and the assemblage relationships of fractures and intergranular pores. When intergranular pores and fractures are well developed, the reservoir permeation capacity is enhanced by fractures. Volcanic reservoirs with this A-P pattern tend to have high deliverability, a slow production decline, and strong potential for stable production.

Horizon No. 150 in Well XX1 is a volcanic breccia of the fallout subfacies, with large grain sizes. The reservoir space is dominated by intergranular pores of medium radius, with structural and blast fractures and intergranular pore-contraction throats and fractured throats. These features point to a typical fractured–intergranular porous A-P pattern (Figure 11.25a). In this reservoir, the average effective porosity is 8.6%, average permeability is 3.71 mD, and the probe radius is 205 m, suggesting good accumulation-permeation capacity. In this horizon, a deliverability of 53 × 10⁴ m³/d was obtained during gas testing after fracturing, the yield of production testing was 14.07 × 10⁴ m³/d, and the open flow potential was 118 × 10⁴ m³/d. After stream injection, it reached a production level of 12 to 24 × 10⁴ m³/d, with a slow pressure drop; the gas production per pressure drop was up to 2630.4 × 10⁴ m³/MPa. So far, the cumulative gas production has reached 2.5 × 10⁸ m³, with the stable production period estimated at seven years (see Table 11.21).

7 Fractured-microporous A-P pattern

This A-P pattern is found primarily in crystalline tuff of the hot base surge subfacies and welded tuff of the hot crystal flow subfacies (both of the explosive facies). Its accumulation capacity depends on the development of matrix micropores and the reservoir extension, whereas the ability of permeation is affected by the type, development, and distribution of fractures. Because fractures can improve the reservoir permeation capacity, a volcanic reservoir with this A-P pattern tends to have a high initial production, but rapid production decline, and poor potential for stable production.

Horizon No. 141-I in Well XX6-107 is characterized by the fractured-microporous A-P pattern, comprising rhyolitic breccia and crystalline tuff of the hot base surge subfacies. Matrix micropores are predominant in the reservoir spaces, with well-developed structural and blast fractures (Figure 11.26a). In this horizon, the average effective porosity is 6.31%, average permeability is 0.42 mD, and the probing radius is 53 m, indicating a fairly poor accumulation and permeation capacity. During gas testing, a natural deliverability of 0.17 × 10⁴ m³/d was obtained, postfracturing gas testing production rose to 9.7 × 10⁴ m³/d, the pilot gas production was 7.6 × 10⁴ m³/d, and open flow potential was 11.24 × 10⁴ m³/d. After production commenced intermittently, the pressure declined rapidly and built up slowly after well shut-in, indicating insufficient gas deliverability and a very low potential for stable production (Figure 11.26b and Table 11.21).

8 Fractured–dissolution-porous A-P pattern

This A-P pattern may occur in various volcanic rocks, especially common in the splash subfacies of the explosive facies, volcanic neck subfacies of the volcanic conduit facies, and top and upper subfacies of the effusive facies. The accumulation capacity of this pattern is controlled mainly by the development of dissolution pores and some primary pores, whereas the permeation capacity is controlled by the type and development of fractures, the radius of dissolution throats, and the assemblage relationships of fractures and dissolution pores. Dissolution increases the pore and throat sizes, whereas fractures further improve the reservoir permeation capacity. As a result, the fractured–dissolution-porous A-P pattern is generally characterized by good petrophysical properties, good communication between pores, and strong accumulation-permeation capacity. The volcanic reservoir with this A-P pattern generally has a high natural deliverability.

In Well XX8, horizon No. 74 XI consists of brecciated lava of the volcanic neck subfacies. Dissolution pores among large-sized grains are predominant in reservoir spaces, with additional vesicles and minor intergranular pores. Structural fractures, contraction fractures, and dissolution fractures can be seen in cores and are well developed. Due to its favorable pore-fracture association, the fractures play a large role in pore communication. These features denote a typical fractured–dissolution-porous A-P pattern (Figure11.27a). In this horizon, the average effective porosity is 16.7%, average permeability is 25.2 mD, and the probe radius is 36.6 m, indicating a fairly good accumulation-permeation capacity. During gas testing in this horizon, the natural deliverability reached 22.6 × 10⁴ m³/d and the open flow potential was 43.81 × 10⁴ m³/d. Due to its high CO₂ content (up to 24.52%), this well has not been put on stream so far. Production testing for more than three months (Figure 11.27b) produced a yield value of 10 to 15 × 10⁴ m³/d, with a stable pressure and a high ratio of gas yield per pressure drop at 1792.8 × 10⁴ m³/MPa. These processes encountered a problem of upward breakthrough of bottom water along fractures. So far, the cumulative gas production has reached 0.12 × 10⁸ m³, and the stable production period is estimated to be six years (see Table 11.21).

In summary, among the eight A-P patterns discussed, the fractured–dissolution-porous pattern has the highest accumulation-permeation capacity, the highest natural deliverability, and the best stable production potential; the vesicular, fractured-vesicular, and fractured–intergranular porous A-P patterns rank next; and the microporous pattern has the poorest properties.

1 Irreducible water film thickness analysis

A layer of stable irreducible water film is absorbed on the inner wall of pores and throats, which is difficult to remove under the pressure differential of the gas layer [17]. According to the adsorption theory, the sum of irreducible water film thickness on the inner wall and the motion diameter of the natural gas molecule (10 ⁻⁴ to 10 ⁻³ μm) is the throat diameter cutoff threshold for gas production.

In the laboratory, the thickness of irreducible water film is determined by the following procedures [18]. A core is taken from the net gas pay zones with oil-based drilling fluid, and then the microwave method or extraction method is used to measure the irreducible water volume in these cores. For the same core samples, high-pressure (over 400 MPa) mercury injection and adsorption methods are used to measure the specific surface of pores and throats. The irreducible water volume divided by the specific surface of pores and throats is the thickness of the irreducible water film. At present, the experiment to measure the thickness of irreducible water film is not yet performed for volcanic reservoirs. Therefore, the measurement results in carbonate reservoirs are used for reference. In the Sinian dolomite reservoirs of the Weiyuan gas field, the thickness of irreducible water film under different porosity conditions is measured. The cutoff value of the throat radius in effective reservoirs is calculated on the basis of the molecular diameter of natural gas at 0.001 μm (Table 11.22). As Table 11.22 shows, when porosity is in the range of 3% to 4%, the average thickness of irreducible water film is 0.0113 μm. Twice the water film thickness plus the molecular diameter of natural gas is 0.0235 μm—that is, when the throat diameter is less than 0.0235 μm, it will be difficult for natural gas molecule to flow through the throat. Therefore, the cutoff of the throat radius is half of the diameter—that is, about 0.0118 μm.

The porosity cutoff for volcanic gas reservoirs in the XX gas field determined by dynamic analysis and empirical statistics is about 3% to 4%. With reference to the thickness of the irreducible water film in the Sinian dolomite reservoir, the throat radius cutoff for volcanic gas reservoirs in this gas field is determined to be 0.01 μm.

2 Empirical formula method

Xiang and colleagues [19] studied the water film thickness in tight sandstone gas reservoirs. Based on the theoretical equation of water film thickness in a soil grain surface, the equation to calculate the core water film thickness is derived as follows:

${\mathit{d}}_{\mathrm{i}}=7.142{\varphi }_{\mathrm{e}}{S}_{\mathrm{wi}}/\left(A_{\mathrm{specific}}\right)\rho$

  (11.5.1)

Where,

d_i, water film thickness in core, μm;

ϕ_e, core porosity;

S_wi, irreducible water saturation in core;

A_specific, specific surface area of rock, m²/g;

ρ, rock density, g/cm³.

With reference to this empirical formula and the characteristics of volcanic gas reservoirs, the irreducible water film thickness in volcanic gas reservoirs is calculated to determine the throat cutoff in reservoirs.

The experimental data on relative permeability in 46 core samples from the XX volcanic gas reservoir revealed that the reservoir porosity is 2.5% to 16.9 %, irreducible water saturation is 32.61% to 53.02%, rock density is 2.13 to 2.65 g/cm³, and the specific surface area of rock is 3.25 m²/g [20]. Using Equation 11.5.1, the irreducible water film thickness is calculated to be 0.0106 to 0.061 μm. If the motion diameter of natural gas molecules is 0.001 μm, the throat radius cutoff will fall in the range of 0.0113 to 0.0615 μm. According to the calculation, the relationship between core porosity and water film thickness is plotted in Figure 11.28, and the cutoff distribution of the throat radius under different porosities is determined (Table 11.23). Based on these results as well as the porosity cutoff for volcanic gas reservoirs in the XX gas field determined by other methods, the thickness of irreducible water film is determined to be 0.0141 to 0.0161 μm, and the cutoff of the throat radius is 0.0146 to 0.0166 μm, with the average taken at 0.015 μm.

3 Centrifuge experiment method

For different sized throats, the centrifugal force required for fluid separation in a centrifuge experiment is different. According to the relationship between centrifugal force and throat radius, the centrifuge experiment can be used to determine the throat radius cutoff threshold.

Based on the capillary force formula, the relationship between centrifuge force and throat radius for a volcanic rock core sample is expressed by the following equation:

$p=\frac{2\sigma cos\theta }{r}r=\frac{2\sigma cos\theta }{p}$

  (11.5.2)

Where:

p, centrifugal force, dsi;

r, throat radius, μm;

σ, water - gas interfacial tension, mN/m, taking 72 mN/m;

θ, wetting angle (= 0°)

Yang and coworkers [21] selected 12 volcanic core samples from the XX gas field and used six of them to perform a centrifuge experiment and to measure the residual water saturation after the centrifuge process was measured (Table 11.24). The results are used to plot the relationship between centrifugal force and residual water saturation (Figure 11.29). The plot shows that the residual water saturation decreases with the increase of centrifugal force; when the centrifugal force is more than 2.07 MPa, the range of variation in residual water saturation narrows down. In these experiments, the characteristics of mobile (movable) water saturation controlled by different throats were obtained (Figure 11.30). The mobile (movable) water saturation decreases with the decrease in throat radius. At a throat radius of 0.04 μm, the saturation of mobile water is only 1.14% on average. However, the residual water saturation (55.24% on average) is slightly more than the irreducible water saturation (52.46% on average). This indicates that the throat with a radius smaller than 0.04 μm still controls 2.78% of the mobile water. In these experiments, no data are available to describe what occurs when the centrifugal force is higher than 3.45 MPa, so the trend of change is unclear for that range. It is believed that the cutoff of the throat radius in volcanic gas reservoirs shall be less than 0.04 μm.

4 Throat cutoff threshold in volcanic reservoirs

Among the three methods used to determine throat cutoff values (Table 11.25), water film thickness analysis and the empirical formula method are based on the adsorption theory, which involves integrating the method of experiment and empirical formula, respectively, to determine the irreducible water film thickness under different porosity conditions and then combining the result with the natural gas molecular diameter to determine the minimum throat radius that will allow natural gas flow. The throat cutoff values determined by these two methods are relatively close (0.01 to 0.015 μm). According to the centrifuge experiment and the capillary force equation, the residual water saturations under different centrifugal forces are measured to characterize the mobile water saturation controlled by throats of different sizes. The throat cutoff threshold is determined by comparison with the irreducible water saturation determined by Nuclear-Magnetic Resonance (NMR) experiment. The comprehensive analysis suggests that when the throat radius is more than 0.04 μm and the centrifugal force is lower than 3.45 MPa, most of the mobile fluid may be produced; when the throat radius is between 0.01 to 0.04 μm, increased centrifugal force will produce part of the mobile fluid; when the throat radius is less than 0.01 μm, no mobile fluid exists in the reservoir. Therefore, the throat radius cutoff threshold in volcanic gas reservoirs is determined to be 0.01 μm.

1 Effective pore volume controlled by different throats and permeability

In general, a larger pore volume associated with large throats corresponds to a higher reservoir permeability and better reservoir fluid-producing capacity. The following information on pore volumes controlled by different throats has been obtained using mercury injection test data of volcanic gas reservoirs in the CC gas field (Table 11.27):

1. The effective throats (radius > 0.01 μm) in volcanic rocks control 66.40% of pore volume. Among these, the throats with a radius of 0.25 to 0.05 μm control the most pore volume (accounting for 38.54%), the throats with a radius larger than 0.25 μm control 20.49%, and the throats with a radius of 0.05 to 0.01 μm control the least amount of pore volume (7.37%). This implies that most of the volcanic gas reservoir pores in this gas field are controlled by larger throats.

2. A better reservoir type has a larger pore volume controlled by effective throats. The effective throats in types I, II, and III reservoirs control 82.53%, 64.3%, and 52.36% of pore volumes, respectively. With the improvement of reservoir type, the pore volumes controlled by effective throats increase notably.

3. A better reservoir type has a larger effective pore volume controlled by large throats. For a type I reservoir, the effective pore volume controlled by throats with a radius larger than 0.25 μm is the largest (accounting for 41.97%), followed by throats with a radius of 0.25 to 0.05 μm (36.73%). For type II reservoirs, the effective pore volume controlled by throats with a radius of 0.25 to 0.05 μm is the largest (accounting for 39.62%), and throats with a radius of more than 0.25 μm account for 17.52%. For type III reservoirs, the effective pore volume controlled by throats with a radius of 0.25 to 0.05 μm is the largest (39.26%), followed by throats with a radius of 0.05 to 0.01 μm (11.12%).

Matrix permeability is correlated to throat radius (see Figure 11.30). The pore volume controlled by different grades of permeability has the same characteristics as the pore volume controlled by different throats (see Table 11.27): (1) the effective volcanic reservoirs control 66.40% of the pore volume, which is higher than that controlled by matrix permeability over 0.015 Md; (2) a better reservoir type will have a larger effective pore volume controlled by high matrix permeability.

2 Effective pore volume controlled by different displacement pressures

A mercury injection test is performed to characterize the pore volume controlled by different displacement pressures, which can provide a basis for determining the appropriate production pressure differential for gas reservoir development. The displacement pressure of volcanic reservoirs has a good correlation with the throat radius (Figure 11.31). The effective pore volume controlled by different displacement pressures has the same characteristics of effective pore volume controlled by different grades of throats (Figure 11.32, Table 11.28):

1. When displacement pressure is greater than 40 MPa, the effective pore volume in volcanic reservoirs accounts for 65.48% of total pore volume; when displacement pressure is less than 1, 5, 10, 15, and 25 MPa, the effective pore volumes controlled by displacement pressure are 10.91%, 27.98%, 40.67%, 49.26%, and 58.26%, respectively. This suggests that a lower displacement pressure controls the effective pore volume in volcanic reservoirs.

2. A better reservoir will have a larger effective pore volume controlled by a lower displacement pressure. For a type I reservoir, more than half of the effective pore volume (53.71%) is associated with a displacement pressure < 5 MPa; for a type II reservoir, more than half of the effective pore volume (33.40%) is controlled by a displacement pressure < 10 MPa, with the main controlling pressure ranging from 1 to 25 MPa; for a type III reservoir, most of the effective pore volume (29.88%) is controlled by a displacement pressure < 15 MPa, with the main controlling pressure ranging from 5 to 25 MPa.

1 Mobile fluid volume controlled by different throats and permeabilities

Throats control the fluid flow capacity of reservoirs. The saturation of mobile fluid in reservoirs is positively correlated to the throat radius (Figure 11.33). In the XX gas field, NMR tests and mercury injection tests are combined to quantitatively characterize the mobile fluid volume controlled by different throats (Figure 11.34, Table 11.29). The tests show the following results:

1. The effective throats (> 0.01 μm) control 45.67% of mobile fluid volume, among which throats with a radius of 0.1 μm control 40.51% of mobile fluid volume. This indicates that the mobile fluid volume in volcanic reservoirs is small, controlled by larger throats.

2. A better reservoir corresponds to a larger mobile fluid volume controlled by effective throats. The effective throats in type I, type II, and type III reservoirs control 65.19%, 47.40%, and 24.43%, respectively, of mobile fluid volume. The mobile fluid volume controlled by effective throats increases notably with better reservoir properties.

3. A better reservoir also corresponds to a larger movable fluid volume controlled by large throats. In type I reservoirs, the mobile fluid volume controlled by throats with a radius that is > 0.75 μm and in the 0.5 to 0.25 μm range is the largest (21.38%, 22.47%, respectively), and that controlled by throats with a radius of 0.25 to 0.1 μm ranks second (12.96%). In type II reservoirs, the mobile fluid volume controlled by throats with a radius of 0.25 to 0.1 μm is the largest (26.55%), and that controlled by throats with a radius of 0.5 to 0.25 μm ranks next (15.18%). In type III reservoirs, the movable fluid volume controlled by throats with a radius of 0.1 to 0.05 μm is the largest (8.05%), and that controlled by throats with a radius of 0.25 to 0.1 μm ranks second (7.46%).

Similarly, the mobile fluid volume controlled by different permeabilities is analogous to that controlled by different throats (see Table 11.29): (1) the effective reservoirs in volcanic rocks control 45.67% of the movable fluid volume, among which the movable fluid volume controlled by a matrix permeability higher than 0.1 mD is the largest (40.51%); and (2) the better the reservoir, the larger the mobile fluid volume controlled by a high matrix permeability.

2 Mobile fluid volume controlled by different displacement pressures

Mobile fluid saturation is well correlated to the displacement pressure (Figure 11.35). Lower displacement pressure in a volcanic reservoir corresponds to a higher movable fluid saturation. The mobile fluid volume controlled by different displacement pressures is related to the reservoir types (see Figure 11.36, Table 11.30):

1. In the XX gas field, the mobile fluid volume in volcanic reservoirs is about 45.67%, and the displacement pressure is less than 25 MPa. When the displacement pressure is less than 1, 3, 5, 10, and 15 MPa, the mobile fluid volume controlled by the displacement pressure is 10.54%, 27.3%, 36.57%, 43.69%, and 45.27%, respectively. Most movable fluid is distributed in reservoirs with displacement pressures less than 10 MPa.

2. A better reservoir has a larger mobile fluid volume controlled by low displacement pressure and better mobility of the reservoir fluid. For type I reservoirs, displacement pressures at ranges less than 2 MPa and 3 MPa correspond to 42.46% and 55.25% mobile fluid, respectively; for type II reservoirs, most mobile fluid (38.08%) corresponds to displacement pressures of 2 to 10 MPa. In type III reservoirs, most of the mobile fluid (15.83%) is related to displacement pressures of 3 to 15 MPa. Therefore, the better the reservoir, the more mobile the reservoir fluid, and the lower the production pressure differential required.

Considering the complexity of microtexture in volcanic gas reservoirs, this chapter has focused on the characterization of reservoir spaces and the shapes, sizes, and accumulation-permeation capacity of throats, using the methods of core observation, thin section examination, laboratory testing, log interpretation, and dynamic analysis. On the basis of these studies, the characteristics of effective pore volume and mobile fluid volume controlled by different throats, permeabilities, and displacement pressures are analyzed. Ultimately, the volcanic gas reservoir microtexture characterization is developed, which provides technical support for evaluating the producing capacity of different types of effective reservoirs. For the volcanic gas reservoir in the XX gas field (Table 11.31), NMR tests and mercury tests show the following features:

1. In a type I reservoir in this gas field, the effective pore volume accounts for 93.78% of the total pore volume, and the mobile fluid volume accounts for 65.19% of the total pore volume. The reservoir fluid has high seepage and good producing capacities. Its natural gas reserves (about 10% of the proven in situ gas) can be recovered effectively under present economic and technological conditions.

2. In a type II reservoir, the effective pore volume accounts for 82.69% of the total pore volume, and the mobile fluid volume accounts for 47.40% of the total pore volume. The reservoir fluid has medium seepage and relatively good producing capacities. Its natural gas reserves (about 30%) can be recovered effectively by applying new technologies such as staged fracturing in horizontal wells.

3. In a type III reservoir, the effective pore volume accounts for 61.75% of the total pore volume, and the mobile fluid volume accounts for 24.43% of the total pore volume. The reservoir fluid has poor seepage and producing capacities. For its natural gas reserves (about 60% of the proven in situ gas), about 35% can be recovered effectively by applying new research and development technologies, but the remaining 25% will be difficult to extract at present.