Method for representing reservoir capacity of coal rock and roof sandstone reservoir thereof

Abstract

The invention relates to the technical field of petroleum and natural gas exploration, in particular to a method for characterizing the reservoir capacity of coal rocks and roof sandstone reservoirs thereof, which comprises the following steps: collecting coal rock samples in a target area, carrying out physical property analysis, scanning electron microscope observation and low-temperature nitrogen adsorption experiment, and determining the reservoir capacity of the coal rocks and roof sandstone reservoirs. Determining the reservoir capacity of the coal rock reservoir of each layer section in the target area; collecting a roof sandstone reservoir sample corresponding to the coal rock reservoir with good storage capacity, and carrying out physical property analysis, casting body slice observation, high-pressure mercury injection and low-temperature nitrogen adsorption experiments to determine the storage capacity of the roof sandstone reservoir; and determining the influence thickness of the coal rock on the roof sandstone reservoir in combination with the physical properties of the roof sandstone reservoir and the microscopic pore throat structure parameters, and comprehensively representing the storage capacities of the coal rock and the corresponding roof sandstone. According to the method, a favorable sand coal combination can be preferably selected, a favorable roof sandstone reservoir can be found out, the exploration problem of coal bed gas can be effectively solved, and the research of unconventional oil and gas geology is promoted.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas exploration, and is a method for characterizing the reservoir capacity of coal rock and its roof sandstone reservoir. Background Art

[0002] With the continuous breakthroughs in oil and gas exploration and development, the crude oil reserves in conventional high-permeability reservoirs are gradually depleting, and unconventional reservoirs have received extensive attention from oil and gas exploration and development personnel. Coalbed methane is generated during the coalification process of coal. Nearly 90% of the coalbed methane is stored in the nano micropores of the coal reservoir in an adsorbed state, which belongs to a type of unconventional resource with self-generation and self-storage. Research shows that the enrichment of coalbed methane is affected by factors such as geological structure conditions, coal seam burial depth, hydrogeological conditions, sedimentary environment, and physical properties of the coal reservoir. Generally, the gas generation capacity of coal seams is 150 cm 3 / g to 300 cm 3 / g, which is much higher than the adsorption capacity of coal seams of 15 cm 3 / g to 30 cm 3 / g. Although a series of breakthroughs have been made in the research on coalbed methane in recent years, the research on free gas in coal rock still needs to be further deepened. As a source rock and storage carrier, the coalbed methane reservoir has the characteristics of low porosity, low permeability, and strong heterogeneity, which affects the fine quantitative characterization of the coal reservoir. Whether coal rock can be used as an effective reservoir to store natural gas has become a key problem to be solved urgently at present. Therefore, how to characterize the reservoir capacity of coal rock is the top priority for the further exploration and development of coal rock. According to the lithology and spatial combination characteristics of free gas reservoirs, free gas reservoirs can be divided into coal rock roof sandstone type, coal rock roof limestone type, and unconformity surface limestone type. Among them, the coalbed methane reservoir and its roof sandstone have the characteristics of co-generation and accumulation, and there is dynamic conversion and directional migration of free gas, indicating a close relationship between the two. Therefore, the research on the reservoir capacity of the roof sandstone is also of great significance for exploration and development.

[0003] At present, there are few patents on the research of coalbed methane reservoirs and their roof sandstones. The common methods for researching coalbed methane reservoirs include field outcrop observation, high-pressure mercury intrusion method (HMIP), and scanning electron microscopy method (SEM). Among them, the high-pressure mercury intrusion method analyzes the pore volume of the coal reservoir through mercury intrusion data, and it has obvious segmented fractal characteristics. The pore range that this method can measure is 0.005 μm to 360.000 μm, but it cannot accurately and quantitatively analyze micro pores less than 100 nm. Moreover, when the injection pump pressure is too high, it will damage the pore morphology of the sample and affect the experimental results (Li Changfeng, Characterization of pore structure of low-rank coal reservoir based on mercury intrusion method - Taking the southern margin of Junggar Basin as an example, Coal Geology of China, 2018); the information that scanning electron microscopy can reflect is limited and it cannot conduct systematic quantitative evaluation (Zheng Sijian, Research progress and development trend of coalbed methane exploration and development, Geophysical Prospecting for Petroleum, 2022).

[0004] According to the analysis of relevant literature and patent retrieval, the current research methods for characterizing the storage capacity of coalbed methane reservoirs are still insufficient. The existing analysis methods usually focus on the storage capacity of coal rock reservoirs, and the storage capacity analysis of its roof sandstone is missing. As the focus of unconventional oil and gas geological research, providing an effective research method to analyze the storage capacity of coal rock and the influence range of roof sandstone storage capacity is still a key issue to be solved. Summary of the invention

[0005] The present invention provides a method for characterizing the storage capacity of coal rock and its roof sandstone reservoir, which overcomes the shortcomings of the above-mentioned prior art and can effectively solve the problem of the lack of storage capacity analysis of roof sandstone in the prior art, and at the same time provides a method for analyzing the storage capacity of coal rock and the influence range of the storage capacity of roof sandstone.

[0006] The technical solution of the present invention is achieved by the following measures: A method for characterizing the storage capacity of coal rock and its roof sandstone reservoir is carried out according to the following steps: S1, collect coal and rock samples in the target area, conduct physical property analysis, scanning electron microscope observation and low-temperature nitrogen adsorption experiment to determine the storage capacity of coal and rock reservoirs in each layer in the target area; S2, collecting roof sandstone reservoir samples corresponding to the coal rock reservoir with good storage capacity determined in step S1, and conducting physical property analysis, casting thin section observation, high-pressure mercury injection and low-temperature nitrogen adsorption experiments to determine the storage capacity of the roof sandstone reservoir; S3, combining the roof sandstone reservoir physical properties and microscopic pore throat structure parameters in step S2, determining the thickness of the coal rock affecting the roof sandstone reservoir, and comprehensively characterizing the storage capacity of the coal rock and its corresponding roof sandstone.

[0007] The following are further optimizations and / or improvements to the above technical solutions: The specific operations of the above step S1 are: S11, using a core sampler to drill coal rock plug samples for physical property measurement, and selecting coal rock fragments with porosity greater than the lower limit of the porosity of the coal rock reservoir in the target area for scanning electron microscope experiments; S12, observe the microscopic pore and fracture characteristics of coal rock samples through scanning electron microscope experiments, and preliminarily judge the storage capacity of coal rock based on the microscopic pore and fracture characteristics; S13, selecting coal rock samples with relatively developed pores and cracks under a scanning electron microscope, and conducting a low-temperature nitrogen adsorption experiment to obtain a low-temperature nitrogen adsorption-desorption curve of the coal rock and a curve diagram showing the relationship between pore diameter and pore volume; S14, based on the low-temperature nitrogen adsorption-desorption curve of coal rock, the main nanopore morphology of coal rock is determined, and the pore throat distribution characteristics of the nanoscale pores of coal rock samples are obtained based on the relationship curve between pore diameter and pore volume, so as to further determine the storage capacity of coal rock reservoir.

[0008] The lower limit value of the porosity of the coal rock reservoir in the above-mentioned target area is obtained according to the following steps: the lower limit of the porosity of the tight sandstone reservoir in the target area is measured by the oil testing method or the bound water film thickness method, and the lower limit of the porosity of the tight sandstone reservoir in the target area is used as the lower limit value of the porosity of the coal rock reservoir in the target area.

[0009] The specific operations of the above step S2 are: S21, collecting core plug samples of coal rock roof sandstone, measuring the porosity of the core plug samples, and selecting core plug samples with porosity greater than the lower limit of the porosity of the coal rock reservoir in the target area as core plug test samples for the following steps; S22, making the core plug test sample into a casting thin section, and optically observing the casting thin section using a polarizing microscope to observe the development characteristics of pores and cracks in the casting thin section; S23, cutting the core plug test sample, and then performing a high-pressure mercury injection test on the core plug test sample to obtain microscopic pore structure parameters of the roof sandstone sample, wherein the microscopic pore structure parameters of the roof sandstone sample include displacement pressure, maximum pore throat radius, median pressure, median radius, mercury injection curve, capillary pressure and corresponding pore throat radius, mercury injection saturation and high-pressure mercury injection pore size distribution curve; S24, conducting a low-temperature nitrogen adsorption experiment on the core plug test sample to obtain a low-temperature nitrogen adsorption-desorption curve of the coal rock roof sandstone and a curve diagram of the relationship between the pore diameter and the pore volume; S25, converting the mercury injection curve obtained in step S23 and the relationship curve diagram between pore diameter and pore volume obtained in step S24 into a relationship diagram between pore throat radius and porosity component, to obtain the high-pressure mercury injection pore throat radius and porosity component and the low-temperature nitrogen adsorption pore throat radius and porosity component; S26, plotting the high-pressure mercury injection pore throat radius-porosity component curve and the low-temperature nitrogen adsorption pore throat radius-porosity component curve in the same coordinate system, screening the data in the two curves, and obtaining the porosity component curve of the high-pressure mercury injection pore throat radius and the low-temperature nitrogen adsorption pore throat radius to jointly characterize the microscopic pore throat structure of the coal roof sandstone reservoir; S27, converting the pore throat radius in step S26 into a pore throat diameter, combining the porosity ratio, classifying them into micropores, mesopores, macropores, and extra-large pores, and drawing a histogram of the porosity ratio of the roof sandstone at different distances from the sand-coal interface, and determining the storage capacity of the roof sandstone reservoir according to the pore type ratio.

[0010] In the above step S26, the operation of screening the data in the high-pressure mercury injection pore throat radius-porosity component curve and the low-temperature nitrogen adsorption pore throat radius-porosity component curve is as follows: a point that conforms to the overall trend is selected from the overlapping part of the high-pressure mercury injection pore throat radius-porosity component curve and the low-temperature nitrogen adsorption pore throat radius-porosity component curve, and the pore diameter corresponding to the point is recorded as r a , the pore throat radius in the high-pressure mercury injection pore throat radius-porosity component curve is greater than or equal to r a The pore throat radius of the low temperature nitrogen adsorption pore throat radius-porosity component curve is less than r a part is retained.

[0011] In the above step S3, the thickness of the coal rock affecting the roof sandstone reservoir is obtained by the following steps: S31, using gas logging method to obtain the porosity value of the target layer; S32, calculate the average porosity of the target layer; S33, systematically sample the coal rock roof sandstone reservoir from deep to shallow and measure the porosity. When the measured porosity value is less than the average porosity value of step S32, it is considered that the coal rock has basically no impact on the roof sandstone. At this time, the thickness of the coal rock roof sandstone represents the thickness of the impact of the coal rock on the roof sandstone reservoir.

[0012] The method of characterizing the storage capacity of coal rock and its roof sandstone reservoir of the present invention effectively characterizes the storage capacity of coal rock and its roof sandstone from the fundamental point of view. This method determines whether it can be used as a reservoir by characterizing the storage capacity of coal rock, and by determining the influence range of coal rock on the storage capacity of roof sandstone, it helps to select favorable sand-coal combinations and find favorable roof sandstone reservoirs, which will effectively solve the exploration problem of coalbed methane and promote the research of unconventional oil and gas geology. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Attached Figure 1 It is a schematic diagram of the technical process of the present invention.

[0014] Attached Figure 2 This is a scanning electron microscope image of the coal rock in Example 8 of the present invention.

[0015] Attached Figure 3 This is a low-temperature nitrogen adsorption-desorption curve of coal rock in Example 8 of the present invention.

[0016] Attached Figure 4 This is a curve diagram showing the relationship between the pore diameter and pore volume of the coal rock in Example 8 of the present invention.

[0017] Attached Figure 5 This is a polarizing microscope observation image of a roof sandstone casting thin section in Example 8 of the present invention (Well J2).

[0018] Attached Figure 6 This is a polarizing microscope observation image of a roof sandstone casting thin section in Example 8 of the present invention (JS1 Well).

[0019] Attached Figure 7 This is a polarizing microscope observation image of a thin section of the roof sandstone cast in Example 8 of the present invention (Well Y1).

[0020] Attached Figure 8 This is a low-temperature nitrogen adsorption-desorption curve diagram of the roof sandstone in Example 8 of the present invention.

[0021] Attached Fig. 9 This is a curve diagram showing the relationship between the pore diameter and pore volume of the top sandstone in Example 8 of the present invention.

[0022] Attached Fig.10 This is a mercury injection curve diagram obtained from the high-pressure mercury injection test on roof sandstone in Example 8 of the present invention.

[0023] Attached Fig.11 It is the porosity component curve and the porosity cumulative distribution curve of the roof sandstone high-pressure mercury injection and low-temperature nitrogen adsorption in Example 8 of the present invention.

[0024] Attached Fig.12 This is a new porosity component curve diagram obtained in step S2 of Example 8 of the present invention.

[0025] Attached Fig.13 This is a histogram of the proportions of micropores, mesopores, macropores and extra-large pores in the roof sandstone in Example 8 of the present invention. DETAILED DESCRIPTION

[0026] The present invention is not limited by the following embodiments, and specific implementation methods can be determined based on the technical solution of the present invention and actual conditions.

[0027] The present invention will be further described below in conjunction with embodiments: Example 1: The method for characterizing the storage capacity of coal rock and its roof sandstone reservoir is carried out according to the following steps: S1, collect coal and rock samples in the target area, conduct physical property analysis, scanning electron microscope observation and low-temperature nitrogen adsorption experiment to determine the storage capacity of coal and rock reservoirs in each layer in the target area; S2, collecting roof sandstone reservoir samples corresponding to the coal rock reservoir with good storage capacity determined in step S1, and conducting physical property analysis, casting thin section observation, high-pressure mercury injection and low-temperature nitrogen adsorption experiments to determine the storage capacity of the roof sandstone reservoir; S3, combining the roof sandstone reservoir physical properties and microscopic pore throat structure parameters in step S2, determining the thickness of the coal rock affecting the roof sandstone reservoir, and comprehensively characterizing the storage capacity of the coal rock and its corresponding roof sandstone.

[0028] Embodiment 2: As an optimization of the above embodiment, the specific operation of step S1 is: S11, using a core sampler to drill coal rock plug samples for physical property measurement, and selecting coal rock fragments with porosity greater than the lower limit of the porosity of the coal rock reservoir in the target area for scanning electron microscope experiments; S12, observe the microscopic pore and fracture characteristics of coal rock samples through scanning electron microscope experiments, and preliminarily judge the storage capacity of coal rock based on the microscopic pore and fracture characteristics; S13, selecting coal rock samples with relatively developed pores and cracks under a scanning electron microscope, and conducting a low-temperature nitrogen adsorption experiment to obtain a low-temperature nitrogen adsorption-desorption curve of the coal rock and a curve diagram showing the relationship between pore diameter and pore volume; S14, based on the low-temperature nitrogen adsorption-desorption curve of coal rock, the main nanopore morphology of coal rock is determined, and the pore throat distribution characteristics of the nanoscale pores of coal rock samples are obtained based on the relationship curve between pore diameter and pore volume, so as to further determine the storage capacity of coal rock reservoir.

[0029] Example 3: As an optimization of the above example, the lower limit value of the porosity of the coal rock reservoir in the target area is obtained according to the following steps: the lower limit of the porosity of the tight sandstone reservoir in the target area is measured by the oil testing method or the bound water film thickness method, and the lower limit of the porosity of the tight sandstone reservoir in the target area is used as the lower limit value of the porosity of the coal rock reservoir in the target area.

[0030] Embodiment 4: As an optimization of the above embodiment, the specific operation of step S2 is: S21, collecting core plug samples of coal rock roof sandstone, measuring the porosity of the core plug samples, and selecting core plug samples with porosity greater than the lower limit of the porosity of the coal rock reservoir in the target area as core plug test samples for the following steps; S22, making the core plug test sample into a casting thin section, and optically observing the casting thin section using a polarizing microscope to observe the development characteristics of pores and cracks in the casting thin section; S23, cutting the core plug test sample, and then performing a high-pressure mercury injection test on the core plug test sample to obtain microscopic pore structure parameters of the roof sandstone sample, wherein the microscopic pore structure parameters of the roof sandstone sample include displacement pressure, maximum pore throat radius, median pressure, median radius, mercury injection curve, capillary pressure and corresponding pore throat radius, mercury injection saturation and high-pressure mercury injection pore size distribution curve; S24, conducting a low-temperature nitrogen adsorption experiment on the core plug test sample to obtain a low-temperature nitrogen adsorption-desorption curve of the coal rock roof sandstone and a curve diagram of the relationship between the pore diameter and the pore volume; S25, converting the mercury injection curve obtained in step S23 and the relationship curve diagram between pore diameter and pore volume obtained in step S24 into a relationship diagram between pore throat radius and porosity component, to obtain the high-pressure mercury injection pore throat radius and porosity component and the low-temperature nitrogen adsorption pore throat radius and porosity component; S26, draw the high-pressure mercury injection pore throat radius-porosity component curve and the low-temperature nitrogen adsorption pore throat radius-porosity component curve in the same coordinate system, filter the data in the two curves, and obtain the porosity component curve of the high-pressure mercury injection pore throat radius and the low-temperature nitrogen adsorption pore throat radius to jointly characterize the microscopic pore throat structure of the coal roof sandstone reservoir. S27, converting the pore throat radius in step S26 into a pore throat diameter, combining the porosity ratio, classifying them into micropores, mesopores, macropores, and extra-large pores, and drawing a histogram of the porosity ratio of the roof sandstone at different distances from the sand-coal interface, and determining the storage capacity of the roof sandstone reservoir according to the pore type ratio.

[0031] The corresponding porosity proportion remains unchanged, and then the corresponding porosity proportions of different pore throat diameters are accumulated, the micropores are calculated as the cumulative proportion of porosity with pore throat diameters less than 2nm, the mesopores are calculated as the cumulative proportion of porosity with pore throat diameters between 2nm and 50nm, the macropores are calculated as the cumulative proportion of porosity with pore throat diameters between 50nm and 1μm, and the extra-large pores are calculated as the cumulative proportion of porosity with pore throat diameters greater than 1μm; through the above process, the proportions of micropores, mesopores, macropores, and extra-large pores of the roof sandstone at different distances from the sand-coal interface can be obtained, and a histogram is drawn; if the histogram is dominated by micropores, it means that the roof sandstone has basically no storage capacity, if it is dominated by mesopores, it means that the storage capacity is average, and if it is dominated by macropores or extra-large pores, it means that the storage capacity is strong.

[0032] Example 5: As an optimization of the above example, in step S3, the thickness of the coal rock affecting the roof sandstone reservoir is obtained according to the following steps: S31, using gas logging method to obtain the porosity value of the target layer; S32, calculate the average porosity of the target layer; S33, systematically sample the coal rock roof sandstone reservoir from deep to shallow and measure the porosity. When the measured porosity value is less than the average porosity value of step S32, it is considered that the coal rock has basically no impact on the roof sandstone. At this time, the thickness of the coal rock roof sandstone represents the thickness of the impact of the coal rock on the roof sandstone reservoir.

[0033] The influence range of coal rock on the physical properties of roof sandstone can determine the oil and gas reserves, that is, the product of the sand body area and the influence range (thickness) is the volume of the reservoir sand body, and the pore system, that is, the oil and gas reserves, is the product of the volume of the reservoir body and the porosity.

[0034] Embodiment 6: As an optimization of the above embodiment, in step S26, the operation of screening the data in the high-pressure mercury injection pore throat radius-porosity component curve and the low-temperature nitrogen adsorption pore throat radius-porosity component curve is as follows: a point that conforms to the overall trend is selected from the overlapping part of the high-pressure mercury injection pore throat radius-porosity component curve and the low-temperature nitrogen adsorption pore throat radius-porosity component curve, and the pore size corresponding to the point is recorded as r a , the pore throat radius in the high-pressure mercury injection pore throat radius-porosity component curve is greater than or equal to r a The pore throat radius of the low temperature nitrogen adsorption pore throat radius-porosity component curve is less than r a part is retained.

[0035] Example 7: The method for characterizing the storage capacity of coal rock and its roof sandstone reservoir is carried out according to the following steps: S1, collect coal and rock samples in the target area, conduct physical property analysis, scanning electron microscope observation and low-temperature nitrogen adsorption experiment to determine the storage capacity of coal and rock reservoirs in each layer in the target area; (1) Determination of the lower limit of porosity of coal reservoirs in the target area The lower limit value X of the porosity of the coal rock reservoir in the target area is measured by using the oil test method and the lower limit test method of physical properties such as the thickness of the bound water film (hereinafter referred to as X to represent the lower limit value of the porosity of the coal rock reservoir in the target area). Most of the sandstones with porosity <X are dry layers and basically have no ability to store oil and gas. Only the coal rock and roof sandstone with porosity >X have a certain storage capacity. Coal rock, roof sandstone and tight sandstone are all unconventional reservoirs with similar porosities. At present, the research on coal rock reservoirs has just started, and there is no method to calculate the lower limit of coal rock porosity. Therefore, when determining the lower limit value of the porosity of coal rock and roof sandstone in the present invention, the lower limit value of the porosity of the tight sandstone reservoir is determined to be equivalent to the lower limit value of the porosity of coal rock and roof sandstone (i.e., X).

[0036] (2) Physical property determination of coal and rock samples Coal rock plug samples were drilled using a core sampler with a diameter of 2.5 cm and a length of approximately 5 cm. The coal rock plug samples were cut using a wire cutting instrument. Since coal rock samples are fragile, samples with a plug shape and a length of approximately 2 cm to 2.5 cm were screened out as preliminary samples for physical property testing; the remaining fragmented samples were used as preliminary samples for scanning electron microscopy and low-temperature nitrogen adsorption experiments. All experiments were conducted from the same sample to reduce the impact of sample heterogeneity on the experimental results.

[0037] The initial samples for the physical property test are further processed, the sample length is measured, the samples less than 2.5 cm are placed in a container, filled with epoxy resin, and filled until the total length of the plunger sample is 2.5 cm. The physical properties of the filled coal rock samples are measured by the helium test method, and the physical properties of the irregular coal rock samples are calculated by subtracting the filling volume from the total volume; the coal rock fragments with a porosity greater than X are selected for scanning electron microscopy experiments.

[0038] (3) Scanning electron microscope experiment to observe the microscopic pores and cracks of the samples Observe the microscopic pores and cracks of coal rock, observe the type, shape, structure and other characteristics of tiny pores in coal rock that cannot be distinguished by optical microscope, and judge the storage capacity of coal rock.

[0039] The relationship between the microscopic pore and fracture characteristics of coal rock and its storage capacity is as follows: ① Primary pores: The cellular pores remaining in plant tissue cells are more numerous and have poor storage capacity; the inter-debris pores between plant debris are less numerous and contribute less to storage capacity; ② Epigenetic pores: pores formed by the escape of gas generated during coal formation and metamorphism, with a medium number and medium storage capacity; ③ Exogenous pores: pores formed by tectonic action after coal diagenesis, such as crushed pores, breccia pores, friction pores, etc., with a medium number and large storage capacity; ④ Mineral pores: mold pores, dissolution pores, etc. formed by the dissolution of mineral crystals in coal seams are relatively small in number and have average storage capacity. In terms of pore size, nanoscale micropores and small pores are mainly developed. In addition, a large number of cracks can be seen, including tensile cracks, condensation cracks, shear cracks, etc. Among them, cracks not filled with minerals have strong permeability and strong storage capacity; cracks filled with minerals have poor permeability and poor storage capacity.

[0040] (4) Low temperature nitrogen adsorption experiment and spectrum analysis Coal rock samples with relatively developed pores and cracks under a scanning electron microscope were selected, and 3g to 5g of the remaining broken coal samples were weighed with an electronic balance and ground into powder in a grinder. In order to eliminate the residual bound water and capillary moisture in the samples, all samples were pretreated at 300°C for 3 hours before the nitrogen adsorption experiment. Then, high-purity nitrogen with a purity greater than 99.999% was used as the adsorbent, and the nitrogen adsorption amount under different relative pressures was measured at 77K. The low-temperature nitrogen adsorption experiment on coal rock obtained the low-temperature nitrogen adsorption-desorption curve and the relationship curve between pore diameter and pore volume.

[0041] Among them, the low-temperature nitrogen adsorption-desorption curve determines the main nanopore morphology of coal rock. According to the classification standard given by the International Union of Pure and Applied Chemistry (IUPAC) in 1985, type H1 corresponds to cylindrical holes with openings at both ends, type H2 corresponds to ink bottle-shaped holes, type H3 corresponds to parallel plate-like slit holes, and type H4 corresponds to conical slit holes. Specifically: Type H1 generally appears in mesoporous materials with relatively narrow pore size distribution or in aggregates of spherical particles with relatively uniform size; H2 type pores are “narrow-mouthed and wide-body”, with larger pores that store and adsorb more natural gas, but have poor gas migration capabilities due to their narrow throats; The walls of the H3 type parallel plate-shaped slit pores are straight, and the pores have a lower capacity to store and adsorb gas, but the gas is easier to migrate; Type H4 is usually found in activated carbon; Types H1 and H4 are less common in sandstone and coal rocks.

[0042] According to the relationship curve between pore diameter and pore volume, the pore throat distribution characteristics of nano-scale pores in coal rock samples can be obtained. If small pores are dominant, it means that the gas in the coal rock is mainly stored in the form of adsorbed gas. Although some gas can be stored, it is difficult to drain and extract. If large pores account for a large proportion, it means that there is both adsorbed gas and free gas in the coal rock, and the coal rock reservoir has a certain ability to store gas.

[0043] S2, collecting roof sandstone reservoir samples corresponding to the coal rock reservoir with good storage capacity determined in step S1, and conducting physical property analysis, casting thin section observation, high-pressure mercury injection and low-temperature nitrogen adsorption experiments to determine the storage capacity of the roof sandstone reservoir; (1) Physical property determination of roof sandstone samples A plurality of core plug samples of coal rock roof sandstone were collected, with a sample diameter of 2.5 cm and a length greater than 3 cm; the porosity of the collected core plug samples was measured, and before measuring the porosity, the roof sandstone core plug samples were deoiled using ethanol benzene compounds, and the liquid saturated porosity of the core plug samples was measured using a KX-90G dense rock vacuum saturation device and an electronic balance; and core plug samples with a porosity greater than X were screened out as samples for the next step of testing.

[0044] (2) Microscopic observation of the development characteristics of pores and cracks During the optical observation process, a casting slice is made of the sampling sample, and a polarizing microscope is used to perform optical observation on the casting slice. The specific operating steps are: cutting and preparing the casting slice from the screened preliminary sample, and the steps for preparing the casting slice are: cutting the sample slice from the preliminary sample and immersing the sample slice in blue epoxy resin. The entire immersion process is carried out under vacuum to remove the gas in the sample slice. After the blue epoxy resin is cured, the sample slice is polished to a thickness of 30 mm to form a casting slice. A polarizing microscope is used to perform optical observation on the slice to observe the development characteristics of pores and cracks in the casting slice.

[0045] Sandstone mainly develops intergranular pores and intragranular pores. Intergranular pores include intergranular primary pores and intergranular dissolved pores; intragranular pores include feldspar dissolved pores and rock fragment dissolved pores. The more secondary pores formed by intergranular and intragranular dissolution, the stronger the storage capacity of the sandstone.

[0046] Cracks mainly affect the permeability of sandstone reservoirs. The more developed the cracks are, the stronger the permeability of the sandstone reservoir is, the stronger the connectivity between pores is, the stronger the fluid mobility is, and it can further promote dissolution, and the stronger the storage capacity is.

[0047] (3) The top plate sandstone core plug sample remaining in step (2) is cut into a plug sample with a length of 2.5 cm. The core plug sample is subjected to a high-pressure mercury injection test using a pore size meter. Sample three is first deoiled using an ethanol benzene compound, and particles weighing 2 g to 3 g and having a particle size of 2 mm to 3 mm are selected from the deoiled sample three. The particles are dried at a temperature of 110°C, and the dried particles are placed in a dilatometer with a volume of 1 cm3 in a glove box filled with nitrogen. Finally, the dilatometer containing the particles is transferred to a measuring and controlling instrument, and vacuumed under the low-pressure environment of the measuring and controlling instrument for degassing. Liquid mercury is injected into the dilatometer after the degassing treatment. Finally, the dilatometer is placed in a high-pressure mercury injection test instrument for pore detection to measure the microscopic pore structure parameters of the top plate sandstone sample.

[0048] When the measured high-pressure mercury injection displacement pressure or median pressure is less than the displacement pressure or median pressure when the porosity is equal to X, it means that the coal rock roof sandstone has a certain storage capacity, otherwise it can be considered that the roof sandstone has basically no storage capacity.

[0049] Through high-pressure mercury injection experiments, the microscopic pore structure parameters of the roof sandstone samples were obtained, including displacement pressure, maximum pore throat radius, median pressure, median radius, mercury injection curve, capillary pressure and corresponding pore throat radius, mercury injection saturation and high-pressure mercury injection pore size distribution curve.

[0050] (4) The remaining roof sandstone core plug sample after step (3) is subjected to a low-temperature nitrogen adsorption experiment to obtain a low-temperature nitrogen adsorption-desorption curve of the coal rock roof sandstone and a curve diagram showing the relationship between pore diameter and pore volume.

[0051] (5) converting the mercury injection curve obtained in step (3) and the curve diagram of the relationship between the pore diameter and pore volume of the coal rock roof sandstone obtained in step (4) into a relationship diagram between the pore throat radius and the porosity component; Specifically: by multiplying the rock porosity Φ by the mercury saturation increment △S Hg , find the porosity component under different capillary pressure conditions, Porosity component = Φ (porosity) × △S Hg (mercury saturation increment); Among them, the mercury saturation increment △S Hg =S Hg (Small)-S Hg（ Large); mercury saturation increment △S Hg is the difference in mercury saturation between two adjacent points of the measured pore throat radius, S Hg is the mercury saturation, S Hg (Small) refers to the mercury saturation corresponding to the smaller pore throat radius between two adjacent points, S Hg (Large) refers to the mercury saturation corresponding to the smaller pore throat radius between two adjacent points.

[0052] The porosity component of cryogenic nitrogen adsorption can be obtained by multiplying the rock skeleton density by the pore volume; The porosity component corresponding to the pore throat radius of low-temperature nitrogen adsorption = ρ (rock skeleton density) × V (pore volume); Thus, the overall porosity component can be obtained, and then the microscopic pore throat structure of the coal roof sandstone reservoir can be jointly characterized.

[0053] (6) The high-pressure mercury injection pore throat radius and porosity component obtained in step (5) are plotted in a scatter plot. Similarly, the low-temperature nitrogen adsorption pore throat radius and porosity component obtained in step (5) are also placed on the same coordinate system, thereby obtaining the overall porosity component curve; a point that conforms to the overall trend is selected from the overlapping part of the two curves, and the pore diameter corresponding to the point is recorded as r a The pore throat radius in the mercury injection pore throat radius and porosity component curve is greater than or equal to r a The pore throat radius of the low temperature nitrogen adsorption pore throat radius and porosity component curve is less than r a The part of the retained and calculated cumulative porosity, the porosity Φ t , divide the porosity components corresponding to different pore sizes by Φ t , the porosity ratio is obtained, and the cumulative porosity curve corresponding to different pore throat radii is drawn.

[0054] The retained high-pressure mercury injection pore throat radius-porosity component curve and low-temperature nitrogen adsorption pore throat radius-porosity component curve are plotted in the same coordinate system, thereby obtaining a new porosity component curve, and then jointly characterizing the microscopic pore throat structure of the coal roof sandstone reservoir; (7) Multiply the pore throat radius in step (6) by 2 to convert it into the pore throat diameter. The corresponding porosity ratio remains unchanged. Then, the porosity ratios corresponding to different pore throat diameters are accumulated. The cumulative porosity ratios of micropores with pore throat diameters less than 2 nm are calculated, the cumulative porosity ratios of mesopores with pore throat diameters between 2 nm and 50 nm are calculated, the cumulative porosity ratios of macropores with pore throat diameters between 50 nm and 1 μm are calculated, and the cumulative porosity ratios of extra-large pores with pore throat diameters greater than 1 μm are calculated. Through the above process, the ratios of micropores, mesopores, macropores, and extra-large pores of the roof sandstone at different distances from the sand-coal interface can be obtained, and a histogram can be drawn. If the histogram is dominated by micropores, it means that the roof sandstone has basically no storage capacity. If it is dominated by mesopores, it means that the storage capacity is average. If it is dominated by macropores or extra-large pores, it means that the storage capacity is strong.

[0055] In the method of the present invention, steps S1 and S2 can be used to comprehensively evaluate the storage capacity of coal rock and its corresponding roof sandstone; of course, they can also be used separately to evaluate the storage capacity of coal rock or the storage capacity of roof sandstone.

[0056] In addition to steps S1 and S2, when comprehensively characterizing the storage capacity of coal rock and its corresponding roof sandstone, the thickness of coal rock and the thickness of roof sandstone must also be considered, because the thickness of coal rock will affect the thickness of roof sandstone. At the same time, the influence range of coal rock on the physical properties of roof sandstone can determine the oil and gas reserves, that is, the product of sand body area and influence range (thickness) is the volume of reservoir sand body, and the volume of reservoir body multiplied by porosity is the pore system, that is, the oil and gas reserves. Because coal is mainly composed of humic organic matter, and this substance is the main source of type III kerogen, it can produce more organic acids compared to type I kerogen. The acid substances produced in different stages of coal are different. The humic organic matter in the early biochemical stage will be oxidized to produce a large amount of humic acid, while the oxygen-containing groups in type III kerogen will break in the thermodynamic stage to generate a large amount of dibasic organic acids mainly composed of carboxylic acids. At the same time, coal will continuously generate such as CO during the metamorphic diagenesis process. 2 , H 2 , CO, H 2 O and other low molecular weight carbon oxides, among which CO 2 The carbonic acid produced after dissolving in water further makes the formation water acidic. Therefore, the acidic fluid produced by coal provides the material basis for the sandstone dissolution of the coal rock roof.

[0057] These acidic fluids dissolve unstable components of feldspar and rock debris, and transport the dissolution products to other places, thereby improving the physical properties of the coal rock roof. The reason why the dissolution products are transported to other places may be that the coal hydrocarbon pressurization provides power for the acidic fluid to discharge upward, and because the gas also has upward buoyancy, it drives the dissolved materials to move upward, and then the dissolution products are cemented again.

[0058] S3, combining the roof sandstone reservoir physical properties and microscopic pore throat structure parameters in step S2, determining the thickness of the coal rock affecting the roof sandstone reservoir, and comprehensively characterizing the storage capacity of the coal rock and its corresponding roof sandstone.

[0059] (1) Determine the thickness of the roof sandstone: Too thin coal-rock interlayers indicate that the sedimentary water body is unstable. The coal seams may contain mudstone and sandstone, which have little impact on the entire roof sandstone. Therefore, only coal rocks with a thickness of not less than 1 m are meaningful in characterizing the storage capacity. The thickness of the coal rock used as the research object for the thickness of the impact of coal rock on the roof sandstone reservoir in the present invention is not less than 1 m.

[0060] (2) Determine the thickness of the roof sandstone: ① Use the gas measurement method to obtain the porosity value of the target layer; ② Calculate the average porosity of the target layer; ③ Systematically sample the roof sandstone of the coal rock from deep to shallow and measure the porosity. When the measured porosity value is less than the average value, it is considered that the coal rock has basically no effect on the roof sandstone. The thickness at this time represents the thickness of the impact of the coal rock on the roof sandstone reservoir.

[0061] Example 8: The specific application of the method for characterizing the storage capacity of coal rock and its roof sandstone reservoir in the Jurassic coal-bearing strata in the Taibei Sag of the Tuha Basin is as follows: S1, collect coal and rock samples in the target area, conduct physical property analysis, scanning electron microscope observation and low-temperature nitrogen adsorption experiment to determine the storage capacity of coal and rock reservoirs in each layer in the target area; (1) Determination of the lower limit of porosity of coal reservoirs in the target area Using the oil test method and the lower limit test methods of physical properties such as the bound water film thickness, it was measured that the lower limit of the porosity of the tight sandstone reservoir in the target area is about 3%, and the lower limit of the porosity of coal rock pores and roof sandstone is set at 3%. Only coal rock and roof sandstone samples with porosity > 3% have a certain storage capacity.

[0062] (2) Physical property determination of coal and rock samples Coal rock plug samples were drilled using a core sampler with a diameter of 2.5 cm and a length of approximately 5 cm. The coal rock plug samples were cut using a wire cutting instrument. Since coal rock samples are fragile, samples with a plug shape and a length of approximately 2 cm to 2.5 cm were screened out as preliminary samples for physical property testing; the remaining fragmented samples were used as preliminary samples for scanning electron microscopy and low-temperature nitrogen adsorption experiments. All experiments were conducted from the same sample to reduce the impact of sample heterogeneity on the experimental results.

[0063] The initial samples of the physical property test were further processed, and the sample length was measured. The samples less than 2.5 cm were placed in a container and filled with epoxy resin until the total length of the plunger sample was 2.5 cm. The physical properties of the filled coal and rock samples were measured using the helium test method, and the physical properties of the irregular coal and rock samples were calculated by subtracting the filling volume from the total volume. The measurement results are shown in Table 1 (Table of Physical Property Characteristics of Coal and Rock in the Shuixigou Group of Taipei Depression).

[0064] (3) Scanning electron microscope experiment to observe the microscopic pores and cracks of the samples Select the coal rock samples with porosity greater than 3% for scanning electron microscope experiments to observe the microscopic pores and cracks of the coal rock; observe the type, shape, structure and other characteristics of the tiny pores in the coal rock that cannot be distinguished by an optical microscope, and judge the storage capacity of the coal rock. Figure 2 As shown, Figure 2 In the figure, a: HN4 well, 3472.30 m, fractures; b: H8 well, 3983.90 m, residual cellular pores of silky bodies; c: H8 well, 3983.90 m, micro-fractures, pores, and fragmentary pores; d: J2 well, 3223.7 m, pores and inter-fragmentary pores; e: LS1 well, 3940.85 m, dolomite filling the fractures; f: Y1 well, 1412.84 m, fragmentary pores and breccia pores.

[0065] From the perspective of different pore genesis, the pores developed in the coal in the study area include: ① Primary pores: residual pores in plant tissue cells ( Figure 2 b), with a large number and poor storage capacity; inter-clast pores between plant debris ( Figure 2 d) Small in number and less in contribution to storage capacity; ② Post-genetic pores: pores formed by the escape of gas generated during coal metamorphism ( Figure 2 c) medium quantity and medium storage capacity; ③ Exogenous pores: pores formed by tectonic action after coal diagenesis. Such as particle pores, breccia pores ( Figure 2 f), friction holes, etc., with a medium number and large storage capacity; ④ Mineral pores: mold pores, dissolution pores, etc. formed by the dissolution of mineral crystals in coal seams are relatively small in number and have average storage capacity. In terms of pore size, nano-scale micropores and small pores are mainly developed. In addition, a large number of cracks can be seen ( Figure 2 a. Figure 2 e), including the development of tension cracks, condensation cracks, shear cracks, etc., among which the cracks not filled with minerals have strong permeability and strong storage capacity; the cracks filled with minerals have poor permeability and poor storage capacity.

[0066] (4) Low temperature nitrogen adsorption experiment and spectrum analysis Select coal and rock samples with relatively developed pores and cracks under the scanning electron microscope, weigh the remaining broken coal samples of 3g to 5g with an electronic balance, put them into a grinder and grind them into powder. In order to eliminate the bound water and capillary moisture remaining in the samples, all samples are pre-treated at 300℃ for 3h before the nitrogen adsorption experiment. Then, high-purity nitrogen with a purity greater than 99.999% is used as the adsorbent, and the nitrogen adsorption amount under different relative pressures is measured at 77K. The low-temperature nitrogen adsorption experiment instrument uses the Autosorb-iQ type specific surface area and pore size distribution analysis (Quantachrome, USA). The pore size distribution range of this instrument is 1.7nm-300nm, and the minimum specific surface area can be measured to 0.0005m 2 / g; According to the experimental results, the low-temperature nitrogen adsorption-desorption curve of coal rock is Figure 3 ) and the relationship between pore diameter and pore volume ( Figure 4 ); Figure 3 and Figure 4 a: DS1 well, 3873.2m; b: JS1 well, 3780.3m; c: HN4 well, 3472.3m; d: H8 well, 3983.9m.

[0067] The results show that there are many nanopores in the sample coal rock, and the low-temperature nitrogen hysteresis loops are basically H3 type, and the corresponding nanopore microscopic morphology is parallel plate-like slit-type holes.

[0068] The relationship curve between pore diameter and pore volume represents the microscopic pore throat structure of coal rock reservoirs; the results show that the diameter of nanopores in coal rock is mainly distributed around 100nm, which is much larger than the diameter of methane molecules, indicating that coal rock can not only adsorb some natural gas, but also has more nanopores and microcracks to store free natural gas.

[0069] S2, collecting roof sandstone reservoir samples corresponding to the coal rock reservoir with good storage capacity determined in step S1, and conducting physical property analysis, casting thin section observation, high-pressure mercury injection and low-temperature nitrogen adsorption experiments to determine the storage capacity of the roof sandstone reservoir; (1) Physical property determination of roof sandstone samples A plurality of core plug samples of coal rock roof sandstone were collected, with a sample diameter of 2.5 cm and a length of more than 3 cm; the porosity of the collected core plug samples was measured, and before measuring the porosity, the core plug samples were deoiled using ethanol benzene compounds, and the liquid saturated porosity of the core plug samples was measured using a KX-90G dense rock vacuum saturation device and an electronic balance; the test results are shown in Table 2. The core plug samples with a porosity greater than 3% were screened out as the next test samples.

[0070] (2) Microscopic observation of the development characteristics of pores and cracks During the optical observation process, a casting slice is made for the sampling sample, and a polarizing microscope is used to perform optical observation on the casting slice. The specific operation steps are: cutting and preparing the casting slice from the screened preliminary sample, and the steps for preparing the casting slice are: cutting the sample slice from the preliminary sample and immersing the sample slice in blue epoxy resin. The entire immersion process is carried out under vacuum to remove the gas in the sample slice. After the blue epoxy resin is cured, the sample slice is polished to a thickness of 30 mm to form a casting slice. A Zeiss (AX-ioplan 2imaging) polarizing microscope is used to perform optical observation on the slice to observe the development characteristics of pores and cracks in the casting slice.

[0071] according to Figures 5 to 7 From the physical property data of the coal rock and its roof sandstone reservoir, it can be found that the porosity of the sandstone reservoir near the coal rock roof is significantly different from that of the sandstone reservoir 2m to 3m above it, and the physical properties tend to gradually deteriorate from bottom to top; from the microscopic photos, the pores of the sandstone reservoir near the coal rock roof are relatively developed, mainly secondary pores, mainly intergranular dissolved pores, followed by intragranular dissolved pores of feldspar and rock fragments. And the farther from the roof, the lower the dissolved pore content. It is believed that the acidic substances such as organic acids and humic acid produced at the top of the coal rock have the effect of improving the physical properties of the sandstone reservoir and increasing the storage space.

[0072] (3) The remaining top sandstone core plug samples in (2) were cut into plug samples with a length of 2.5 cm. The core plug samples were subjected to high-pressure mercury injection experiments using the AutoPore IV 9505 pore size analyzer (Micromeritics, USA). Sample 3 was first deoiled using ethanol benzene compounds, and particles weighing 2 g to 3 g and with a particle size of 2 mm to 3 mm were selected from the deoiled sample 3. The particles were dried at a temperature of 110°C and placed in a nitrogen-filled glove box with a volume of 1 cm 3The dilatometer with particles is placed in a dilatometer, and finally the dilatometer with particles is transferred to the measuring and controlling instrument. The dilatometer is evacuated under the low pressure environment of the measuring and controlling instrument for degassing. Liquid mercury is injected into the dilatometer after degassing. Finally, the dilatometer is placed in a high-pressure mercury injection test instrument for pore detection. The microscopic pore structure parameters of the roof sandstone sample (including: displacement pressure / MPa, maximum pore throat radius / μm, median pressure / MPa, median radius / μm, mercury injection curve, capillary pressure and its corresponding pore throat radius, mercury injection saturation, high-pressure mercury injection pore size distribution curve, etc.) are measured. Some parameters are shown in Tables 3 and 4. The displacement pressure and median pressure at a porosity of 3% (lower limit of porosity) at a depth of 5149.2m in Well J701 in the study area were measured to be 13.768MPa and 95.858MPa, respectively. When the displacement pressure or median pressure of the roof sandstone is less than the displacement pressure or median pressure when the porosity is equal to 3%, it means that the coal rock roof sandstone has a certain storage capacity. Otherwise, it can be considered that the roof sandstone has basically no storage capacity.

[0073] (4) The remaining roof sandstone core plug sample after step (3) is subjected to a low-temperature nitrogen adsorption experiment (the operation is the same as step (4) in S1), and the low-temperature nitrogen adsorption-desorption curve of the coal rock roof sandstone is obtained ( Figure 8 ) and the relationship between pore diameter and pore volume ( Fig. 9 ); Figure 8 Among them, a is J2 well, 3220.96m; b is J2 well, 3222.65m; c is JS1 well, 3778.54m; d is JS1 well, 3779.22m; e is LS1 well, 3938.27m; f is Y1 well, 1409.8m.

[0074] Fig. 9 Among them, a is J2 well, 3220.96m; b is J2 well, 3222.65m; c is JS1 well, 3778.54m; d is JS1 well, 3779.22m; e is LS1 well, 3938.27m; f is Y1 well, 1409.8m.

[0075] (5) The mercury injection curve obtained in the high-pressure mercury injection experiment ( Fig.10 , where a is J2 well, 3220.96m; b is J2 well, 3222.65m; c is JS1 well, 3778.54m; d is JS1 well, 3779.22m; e is LS1 well, 3938.27m; f is Y1 well, 1409.8m) and the relationship curve between the pore diameter and pore volume of coal rock roof sandstone ( Fig. 9 ) is converted into a relationship diagram between pore throat radius and porosity component.

[0076] Specifically: by multiplying the rock porosity Φ by the mercury saturation increment △S Hg, find the porosity components under different capillary pressure conditions: Porosity component = Φ (porosity) × △S Hg (mercury saturation increment); Among them, the mercury saturation increment △S Hg =S Hg (Small)-S Hg (large); mercury saturation increment △S Hg is the difference in mercury saturation between two adjacent points of the measured pore throat radius, S Hg is the mercury saturation, S Hg (Small) refers to the mercury saturation corresponding to the smaller pore throat radius between two adjacent points, S Hg (Large) refers to the mercury saturation corresponding to the smaller pore throat radius between two adjacent points. The pore throat radius, pore volume and porosity component data of Well J2 are shown in Table 5.

[0077] The porosity component of cryogenic nitrogen adsorption can be obtained by multiplying the rock skeleton density by the pore volume; The porosity component corresponding to the pore throat radius of low-temperature nitrogen adsorption = ρ (rock skeleton density) × V (pore volume).

[0078] (6) The high-pressure mercury injection pore throat radius and porosity component obtained in step (5) are made into a scatter plot. Similarly, the low-temperature nitrogen adsorption pore throat radius and porosity component obtained in step (5) are also placed on the same coordinate system. Thus, the overall porosity component can be obtained, such as Fig.11 When the pore throat radius is 0.01 μm to 0.1 μm, the two curves overlap. A point that conforms to the overall trend is selected from the overlapping part of the curves, and the pore diameter corresponding to the point is recorded as r a The pore throat radius in the mercury injection pore throat radius and porosity component curve is greater than or equal to r a The pore throat radius of the low temperature nitrogen adsorption pore throat radius and porosity component curve is less than r a The part of the retained and calculated cumulative porosity, the porosity Φ t , divide the porosity components corresponding to different pore sizes by Φ t , the porosity ratio is obtained, and the cumulative porosity curve corresponding to different pore throat radii is drawn. The results are as follows Fig.11 As shown in (b), Fig.11 b is an example of 3220.96 m in J2 well, where (a) is the porosity component of mercury injection and low-temperature nitrogen adsorption, and (b) is the cumulative distribution of porosity of mercury injection and low-temperature nitrogen adsorption.

[0079] The retained high-pressure mercury injection pore throat radius-porosity component curve and low-temperature nitrogen adsorption pore throat radius-porosity component curve are plotted in the same coordinate system, thereby obtaining a new porosity component curve diagram. The results are shown in Fig.12 ( Fig.12 In the figure, a is 3220.96m for J2 well, b is 3222.65m for J2 well, c is 1409.8m for Y1 well, and d is 1412.08m for Y1 well), and then the microscopic pore throat structure of the coal rock roof sandstone reservoir is jointly characterized. The pore throat radius of the roof sandstone is mainly between 0.01μm and 0.1μm.

[0080] (7) Multiply the pore throat radius in step (6) by 2 to convert it into the pore throat diameter. The corresponding porosity ratio remains unchanged. Then, the porosity ratios corresponding to different pore throat diameters are accumulated. The cumulative porosity ratios of micropores with pore throat diameters less than 2 nm are calculated. The cumulative porosity ratios of mesopores with pore throat diameters between 2 nm and 50 nm are calculated. The cumulative porosity ratios of macropores with pore throat diameters between 50 nm and 1 μm are calculated. The cumulative porosity ratios of extra-large pores with pore throat diameters greater than 1 μm are calculated. Through the above process, the ratios of micropores, mesopores, macropores, and extra-large pores in the roof sandstone at different distances from the sand-coal interface can be obtained, and a histogram can be drawn. The results are shown in Figure 2. Fig.13 ( Fig.13 In the figure, a is 3220.96m from J2 well, 2.69m from the sand-coal interface; b is 3222.65m from J2 well, 1m from the sand-coal interface; c is 1409.8m from Y1 well, 2.2m from the sand-coal interface; d is 1412.08m from Y1 well, 0.12m from the sand-coal interface). Fig.13 It can be seen that at 2.69m from the sand-coal interface in Well J2, the microscopic pores are mainly mesopores and macropores, accounting for about 36.74% and 53.94% respectively, and the macropores account for only 8.98%; when it is only 1m away from the sand-coal interface, the microscopic pores are mainly macropores and extra-large pores, accounting for about 48.01% and 39.73% respectively. Similarly, at 2.2m from the sand-coal interface in Well Y1, the microscopic pores are mainly mesopores and macropores, accounting for about 80.79% and 15.67% respectively, and when it is only 0.12m away from the sand-coal interface, the microscopic pores are mainly macropores and extra-large pores, accounting for about 53.94% and 34.28% respectively, indicating that the closer to the sand-coal interface, the more pores with large pore sizes account for, and the better the storage capacity.

[0081] S3, combining the roof sandstone reservoir physical properties and microscopic pore throat structure parameters in step S2, determining the thickness of the coal rock affecting the roof sandstone reservoir, and comprehensively characterizing the storage capacity of the coal rock and its corresponding roof sandstone.

[0082] (1) Determine the thickness of the roof sandstone: According to statistics, the thickness of the coal seam in Well J2 is 0.88m to 4.79m, the thickness of the coal seam in Well JS1 is 1m to 6.02m, the thickness of the coal seam in Well LS1 is 1m to 3m, and the thickness of the coal seam in Well Y1 is 1m to 4m.

[0083] (2) Determine the thickness of the roof sandstone: ① Use the gas measurement method to obtain the porosity value of the target layer; ② Calculate the average porosity of the target layer; ③ Systematically sample the roof sandstone of the coal rock from deep to shallow and measure the porosity. When the measured porosity value is less than the average value, it is considered that the coal rock has basically no impact on the roof sandstone. The final determined impact range is shown in Table 6.

[0084] The thickness at this time represents the thickness of the influence of coal rock on the roof sandstone reservoir. The influence range of coal rock on the physical properties of roof sandstone can determine the oil and gas reserves, that is, the product of the sand body area and the influence range (thickness) is the volume of the reservoir sand body. Multiplying the volume of the reservoir body by the porosity is the pore system, which is the oil and gas reserves.

[0085] After steps one to three, it is generally believed that the coal rock and its roof sandstone in the Y1 well in the study area have the best storage capacity, followed by J2, JS1, and LS1 wells.

[0086] In summary, the present invention proposes a comprehensive analysis method by measuring the physical properties of coal rock reservoirs and roof sandstones and combining them with experimental analysis, so as to predict the storage capacity of coal rock and its roof sandstone, which is helpful to optimize favorable sand-coal combinations and find favorable roof sandstone reservoirs, which will effectively solve the exploration problem of coalbed methane and promote the research of unconventional oil and gas geology.

[0087] The above technical features constitute the embodiments of the present invention, which have strong adaptability and implementation effect. Non-essential technical features can be added or reduced according to actual needs to meet the requirements of different situations.

Claims

1. A method for characterizing the storage capacity of coal rock and its roof sandstone reservoir, Features Follow these steps: S1, collect coal and rock samples in the target area, conduct physical property analysis, scanning electron microscope observation and low-temperature nitrogen adsorption experiment to determine the storage capacity of coal and rock reservoirs in each layer in the target area; S2, collecting roof sandstone reservoir samples corresponding to the coal rock reservoir with good storage capacity determined in step S1, and conducting physical property analysis, casting thin section observation, high-pressure mercury injection and low-temperature nitrogen adsorption experiments to determine the storage capacity of the roof sandstone reservoir; S3, combining the roof sandstone reservoir physical properties and microscopic pore throat structure parameters in step S2, determining the thickness of the coal rock affecting the roof sandstone reservoir, and comprehensively characterizing the storage capacity of the coal rock and its corresponding roof sandstone.

2. The method for characterizing the storage capacity of coal rock and its roof sandstone reservoir according to claim 1, Features The specific operations of step S1 are: S11, using a core sampler to drill coal rock plug samples for physical property measurement, and selecting coal rock fragments with porosity greater than the lower limit of the porosity of the coal rock reservoir in the target area for scanning electron microscope experiments; S12, observe the microscopic pore and fracture characteristics of coal rock samples through scanning electron microscope experiments, and preliminarily judge the storage capacity of coal rock based on the microscopic pore and fracture characteristics; S13, selecting coal rock samples with relatively developed pores and cracks under a scanning electron microscope, and conducting a low-temperature nitrogen adsorption experiment to obtain a low-temperature nitrogen adsorption-desorption curve of the coal rock and a curve diagram showing the relationship between pore diameter and pore volume; S14, based on the low-temperature nitrogen adsorption-desorption curve, the main nanopore morphology of coal rock is determined, and the pore throat distribution characteristics of the nanoscale pores of the coal rock sample are obtained based on the relationship curve between the pore diameter and the pore volume, so as to further determine the storage capacity of the coal rock reservoir.

3. The method for characterizing the storage capacity of coal rock and its roof sandstone reservoir according to claim 2, Features The lower limit value of the porosity of the coal rock reservoir in the target area is obtained according to the following steps: the lower limit of the porosity of the tight sandstone reservoir in the target area is measured by the oil testing method or the bound water film thickness method, and the lower limit of the porosity of the tight sandstone reservoir in the target area is used as the lower limit value of the porosity of the coal rock reservoir in the target area.

4. The method for characterizing the storage capacity of coal rock and its roof sandstone reservoir according to any one of claims 1 to 3, Features The specific operations of step S2 are: S21, collecting core plug samples of coal rock roof sandstone, measuring the porosity of the core plug samples, and selecting core plug samples with porosity greater than the lower limit of the porosity of the coal rock reservoir in the target area as core plug test samples for the following steps; S22, making the core plug test sample into a casting thin section, and optically observing the casting thin section using a polarizing microscope to observe the development characteristics of pores and cracks in the casting thin section; S23, cutting the core plug test sample, and then performing a high-pressure mercury injection test on the core plug test sample to obtain microscopic pore structure parameters of the roof sandstone sample, wherein the microscopic pore structure parameters of the roof sandstone sample include displacement pressure, maximum pore throat radius, median pressure, median radius, mercury injection curve, capillary pressure and corresponding pore throat radius, mercury injection saturation and high-pressure mercury injection pore size distribution curve; S24, conducting a low-temperature nitrogen adsorption experiment on the core plug test sample to obtain a low-temperature nitrogen adsorption-desorption curve of the coal rock roof sandstone and a curve diagram of the relationship between the pore diameter and the pore volume; S25, converting the mercury injection curve obtained in step S23 and the relationship curve diagram between pore diameter and pore volume obtained in step S24 into a relationship diagram between pore throat radius and porosity component, to obtain the high-pressure mercury injection pore throat radius and porosity component and the low-temperature nitrogen adsorption pore throat radius and porosity component; S26, plotting the high-pressure mercury injection pore throat radius-porosity component curve and the low-temperature nitrogen adsorption pore throat radius-porosity component curve in the same coordinate system, screening the data in the two curves, and obtaining the porosity component curve of the high-pressure mercury injection pore throat radius and the low-temperature nitrogen adsorption pore throat radius to jointly characterize the microscopic pore throat structure of the coal roof sandstone reservoir; S27, converting the pore throat radius in step S26 into a pore throat diameter, combining the porosity ratio, classifying them into micropores, mesopores, macropores, and extra-large pores, and drawing a histogram of the porosity ratio of the roof sandstone at different distances from the sand-coal interface, and determining the storage capacity of the roof sandstone reservoir according to the pore type ratio.

5. The method for characterizing the storage capacity of coal rock and its roof sandstone reservoir according to claim 4, Features In step S26, the operation of screening the data in the high-pressure mercury injection pore throat radius-porosity component curve and the low-temperature nitrogen adsorption pore throat radius-porosity component curve is as follows: a point that conforms to the overall trend is selected from the overlapping part of the high-pressure mercury injection pore throat radius-porosity component curve and the low-temperature nitrogen adsorption pore throat radius-porosity component curve, and the pore diameter corresponding to the point is recorded as r a , the pore throat radius in the high-pressure mercury injection pore throat radius-porosity component curve is greater than or equal to r a The pore throat radius of the low temperature nitrogen adsorption pore throat radius-porosity component curve is less than r a part is retained.

6. The method for characterizing the storage capacity of coal rock and its roof sandstone reservoir according to any one of claims 1 to 5, Features In step S3, the thickness of the coal rock affecting the roof sandstone reservoir is obtained by the following steps: S31, using gas logging method to obtain the porosity value of the target layer; S32, calculate the average porosity of the target layer; S33, systematically sample the coal rock roof sandstone reservoir from deep to shallow and measure the porosity. When the measured porosity value is less than the average porosity value of step S32, it is considered that the coal rock has basically no impact on the roof sandstone. At this time, the thickness of the coal rock roof sandstone represents the thickness of the impact of the coal rock on the roof sandstone reservoir.

Images