Calculation method for distinguishing fractal characteristics of residual water occurrence and migration structure in coal body

Abstract

The invention provides a calculation method for distinguishing fractal characteristics of occurrence and migration structures of residual water in a coal body. The calculation method comprises the following steps: constructing a calculation model for representing coal body structure change by nuclear magnetism and a calculation model for saturation of the residual water in the coal body; establishing an accumulated nuclear magnetic signal T2 map after displacement by means of a calculation model for representing coal body structure change through nuclear magnetism and a calculation model for the saturation degree of residual water in the coal body; dividing a coal body residual water occurrence structure and a migration structure by combining a T2rcut residual cutoff value on the basis of a displaced accumulated nuclear magnetic signal T2 map; a multi-fractal theory is combined to obtain fractal characteristics of the residual water occurrence and migration structure; based on a fractal dimension change rule and in combination with a residual water saturation change trend, analyzing to obtain an influence mechanism of water injection on a residual water occurrence and migration structure; the residual occurrence and migration structure of the coal body is divided from the perspective of nuclear magnetism, the evolution trend of the structure is quantified, and a new method is provided for exploring the structural evolution of the coal body under the water and nitrogen injection condition.

Description

Technical Field

[0001] The present invention relates to the technical field related to mine gas extraction, and in particular to a calculation method for distinguishing fractal characteristics of residual water storage and migration structures in coal bodies. Background Art

[0002] As a major coal producer, my country boasts abundant coalbed methane (CBM) resources, but their exploitation is low, presenting significant development potential. As a mineral associated with coal, efficient utilization of CBM not only helps reduce coal mine gas accidents and ensures safe coal mine production, but also plays a crucial role in optimizing my country's energy structure and reducing greenhouse gas emissions. However, due to the low porosity, low permeability, and strong heterogeneity of my country's coal reservoirs, coupled with the unique characteristic of CBM being primarily stored in an adsorbed state, CBM extraction is relatively difficult.

[0003] Coalbed water injection involves continuously injecting pressurized water into the coal. Under the influence of fluid-solid coupling, the pore and fracture structures of various scales within the coal are continuously expanded and extended as stress is transferred, thereby achieving structural transformation to meet engineering objectives such as stress transfer and permeability enhancement. However, coalbed water injection can increase the water saturation within the coal, that is, the amount of residual water present, which can easily lead to a water lock effect that reduces coalbed methane production. Summary of the Invention

[0004] The present invention aims to provide a calculation method for distinguishing the fractal characteristics of the storage and migration structures of residual water in coal bodies, dividing the storage structure and migration structure of coal bodies, and conducting quantitative analysis through fractal theory, which is conducive to exploring the evolution law of the storage structure and migration structure of residual water in coal bodies under water injection, and can break the water lock effect caused by the increase of residual water storage in coal bodies due to water injection, and provide a theoretical basis for improving the gas extraction effect.

[0005] To this end, the technical solution adopted by the present invention is: a calculation method for distinguishing the fractal characteristics of the storage and migration structure of residual water in coal bodies, which specifically includes the following steps:

[0006] S1. Construct a computational model for NMR characterization of coal structure changes and a computational model for residual water saturation in coal.

[0007] S2. Based on the calculation model of NMR characterization of coal body structure changes and the calculation model of residual water saturation in coal body constructed in S1, the cumulative NMR signal T2 spectrum after saturated water and nitrogen displacement in the coal body is established;

[0008] S3, based on the cumulative nuclear magnetic signal T2 spectrum after saturated water and nitrogen displacement established in S2, combined with T 2rcut Residual cutoff value, dividing the coal body residual water storage structure and migration structure;

[0009] S4. By using the coal body residual water storage structure and migration structure divided in S3 and combining it with the multi-fractal theory, the fractal characteristics of the residual water storage and migration structure are obtained;

[0010] S5. Based on the variation law of fractal dimension and the variation trend of residual water saturation, the influence mechanism of water injection on the storage and migration structure of residual water is analyzed.

[0011] As a preferred embodiment of the above scheme, the specific process of step S1 is: using the NMR technology to utilize the interaction mechanism between the hydrogen-containing fluid and the coal rock, and combining the relaxation signal of the hydrogen-containing fluid in the pores inside the coal rock to characterize the microstructure and fluid migration characteristics of the coal sample;

[0012] Transverse relaxation time, T2, is often used to analyze fluids in coal and rock pores and fissures. It has three different relaxation mechanisms: surface relaxation, diffusion relaxation, and free relaxation.

[0013] It can be expressed as:

[0014]

[0015] Among them, T 2D represents the diffusion relaxation time, ms; T 2B represents the free relaxation time, ms; T 2S represents the surface relaxation time, ms. Since the contributions of free relaxation and diffusion relaxation are much smaller than that of surface relaxation, the formula can be approximately expressed as:

[0016]

[0017] Where ρ2 is the surface relaxation rate, which is a fixed value for the same coal sample, μm / ms; V is the volume of hydrogen-containing fluid in the pores of the coal sample; S is the pore surface area;

[0018] Assuming that the pores inside the coal body are simple structures, the formula can be transformed into:

[0019]

[0020] Among them, F S is the pore geometry factor, and its value is closely related to the pore geometry. For spherical pores, F S The value is 3, columnar pore F S The value is 2; r is the pore radius, nm; Based on the above formula, the relationship between pore characteristic parameters and T2 relaxation value can be established, which is the basis for the quantitative characterization of pore fracture structure using nuclear magnetic resonance technology;

[0021] As the injection pressure changes, the residual water content in different pores and fractures has obvious differences. This difference can be expressed by the following formula: Right now:

[0022]

[0023] in, is the relative residual water saturation; A θ is the accumulated T2 signal of residual water in the coal body after nitrogen displacement; A1 is the accumulated T2 signal when the coal body is relatively saturated.

[0024] Further preferably, the specific process of step S2 is: according to the data obtained from the nuclear magnetic resonance calculation model, the nuclear magnetic resonance data of the coal sample after saturated water and gas drive are plotted into a nuclear magnetic resonance T2 spectrum; its horizontal axis is the relaxation time proportional to the pore size, and the vertical axis is the signal amplitude proportional to the number of pores of corresponding size; and the nuclear magnetic resonance T2 spectrum is converted into a nuclear magnetic cumulative T2 spectrum.

[0025] More preferably, the specific process of step S3 is as follows: based on the nuclear magnetic cumulative T2 spectrum, a line parallel to the X axis is drawn from the maximum value of the T2 spectrum cumulative nuclear magnetic signal curve after gas flooding, and the intersection of the line and the saturated water cumulative nuclear magnetic signal curve is the T value of the coal sample. 2rcut Residual cutoff value.

[0026] More preferably, the specific process of step S4 is as follows: the pore space inside the coal rock shows a certain self-similarity, which indicates that fractal theory can be used to characterize the complexity of different pore structures of the coal body;

[0027] With the help of fractal geometry theory, the fractal geometry formula of coal rock pore space distribution is obtained:

[0028]

[0029] Where r is the pore radius, μm; r max is the maximum pore radius, μm; S is the pore volume ratio within the pore radius range of 0 to r, %; Dr is the pore fractal dimension;

[0030] Based on the NMR relaxation principle, the relationship formula between the coal surface relaxation rate and pore characteristic parameters is combined with the fractal geometry formula to obtain:

[0031]

[0032] Where S v is the percentage of pore volume with surface relaxation rate in the range of 0 to T2 to the total pore volume, %; T2 is the relaxation time, ms; T 2max is the maximum relaxation time, ms; D is the fractal dimension of the pore size determined by the NMR T2 spectrum curve; 2rcut " value, the coal sample T2 spectrum curve is divided into two sections, Dv is given by the formula:

[0033]

[0034] Where, T 2rcut To distinguish the relative relaxation value of the residual water storage part and the migration part of the coal body, ms; T 2min is the minimum transverse relaxation time, ms; D VE With D VR are the fractal dimensions of the residual water migration structure dominated by macropores and cracks and the occurrence structure dominated by micropores, respectively;

[0035] Combining the above two equations and taking the logarithm of both sides, we can get the NMR fractal geometry formula:

[0036]

[0037] Then the fractal dimension is:

[0038] D=3-K

[0039] Where: K is lgS V The slope of the linear fit between 1gT2 and 1gT2.

[0040] More preferably, the specific process of step S5 is: using the above calculation formula and data, obtaining the fractal dimension of the occurrence and migration structure of the coal sample, drawing it into a table, and analyzing its change law to characterize its structural change trend.

[0041] Beneficial effects of the present invention: T2 spectrum obtained by combining saturated water and nitrogen displacement 2rc The residual cutoff value is used to divide the residual water storage structure and migration structure of the coal body; the fractal dimension of the residual storage structure and migration structure of the coal body is obtained by combining the multifractal theory; the evolution trend of its structure is characterized by analyzing the change law of the fractal dimension; the residual storage and migration structure of the coal body is divided from the perspective of nuclear magnetic resonance, and the evolution trend of the structure is quantified, providing a new method for exploring the evolution of coal structure under water and nitrogen injection conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a schematic flow diagram of the present invention.

[0043] Figure 2 It is the nuclear magnetic cumulative T2 spectrum of the present invention.

[0044] Figure 3 The coal sample T of the present invention 2rcut Schematic diagram of residual cutoff values. DETAILED DESCRIPTION

[0045] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0046] like Figure 1-3As shown in FIG, a calculation method for distinguishing the fractal characteristics of the storage and migration structure of residual water in coal bodies specifically includes the following steps:

[0047] S1. Construct a computational model for characterizing coal structure changes using nuclear magnetic resonance and a computational model for residual water saturation in coal.

[0048] The specific process of step S1 is: using the NMR technology to utilize the interaction mechanism between hydrogen-containing fluid and coal rock, and combining the relaxation signal of hydrogen-containing fluid in the pores inside the coal rock to characterize the microstructure and fluid migration characteristics of the coal sample;

[0049] Transverse relaxation time, T2, is often used to analyze fluids in coal and rock pores and fissures. It has three different relaxation mechanisms: surface relaxation, diffusion relaxation, and free relaxation.

[0050] It can be expressed as:

[0051]

[0052] Among them, T 2D represents the diffusion relaxation time, ms; T 2B represents the free relaxation time, ms; T 2S represents the surface relaxation time, ms. Since the contributions of free relaxation and diffusion relaxation are much smaller than that of surface relaxation, the formula can be approximately expressed as:

[0053]

[0054] Where ρ2 is the surface relaxation rate, which is a fixed value for the same coal sample, μm / ms; V is the volume of hydrogen-containing fluid in the pores of the coal sample; S is the pore surface area;

[0055] Assuming that the pores inside the coal body are simple structures, the formula can be transformed into:

[0056]

[0057] Among them, F S is the pore geometry factor, and its value is closely related to the pore geometry. For spherical pores, F S The value is 3, columnar pore F S The value is 2; r is the pore radius, nm; Based on the above formula, the relationship between pore characteristic parameters and T2 relaxation value can be established, which is the basis for the quantitative characterization of pore fracture structure using nuclear magnetic resonance technology;

[0058] As the injection pressure changes, the residual water content in different pores and fractures has obvious differences. This difference can be expressed by the following formula: Right now:

[0059]

[0060] in, is the relative residual water saturation; A θ is the accumulated T2 signal of residual water in the coal body after nitrogen displacement; A1 is the accumulated T2 signal when the coal body is relatively saturated.

[0061] S2. Relying on the calculation model of nuclear magnetic resonance characterization of coal body structure changes and the calculation model of residual water saturation in coal body constructed in S1, the cumulative nuclear magnetic resonance signal T2 spectrum after saturated water and nitrogen displacement in the coal body is established.

[0062] The specific process of step S2 is: according to the data obtained from the nuclear magnetic calculation model, the nuclear magnetic data of the coal sample after saturated water and gas drive are used to draw a nuclear magnetic T2 spectrum; its horizontal axis is the relaxation time proportional to the pore size, and the vertical axis is the signal amplitude proportional to the number of pores of corresponding size; the nuclear magnetic T2 spectrum is converted into a nuclear magnetic cumulative T2 spectrum.

[0063] S3, based on the cumulative nuclear magnetic signal T2 spectrum after saturated water and nitrogen displacement established in S2, combined with T 2rcut The residual cutoff value is used to divide the coal body residual water storage structure and migration structure.

[0064] The specific process of step S3 is as follows: based on the nuclear magnetic cumulative T2 spectrum, draw a line parallel to the X axis from the maximum value of the T2 spectrum cumulative nuclear magnetic signal curve after gas flooding, and the intersection of the line and the saturated water cumulative nuclear magnetic signal curve is the T value of the coal sample. 2rcut Residual cutoff value.

[0065] S4. By combining the coal body residual water storage structure and migration structure divided in S3 and the multi-fractal theory, the fractal characteristics of the residual water storage and migration structure are obtained.

[0066] The specific process of step S4 is as follows: the pore space inside the coal rock shows a certain self-similarity, which indicates that fractal theory can be used to characterize the complexity of different pore structures of the coal body;

[0067] With the help of fractal geometry theory, the fractal geometry formula of coal rock pore space distribution is obtained:

[0068]

[0069] Where r is the pore radius, μm; r max is the maximum pore radius, μm; S is the pore volume ratio within the pore radius range of 0 to r, %; Dr is the pore fractal dimension;

[0070] Based on the NMR relaxation principle, the relationship formula between the coal surface relaxation rate and pore characteristic parameters is combined with the fractal geometry formula to obtain:

[0071]

[0072] Where S v is the percentage of pore volume with surface relaxation rate in the range of 0 to T2 to the total pore volume, %; T2 is the relaxation time, ms; T 2max is the maximum relaxation time, ms; D is the fractal dimension of the pore size determined by the NMR T2 spectrum curve; 2rcut " value, the coal sample T2 spectrum curve is divided into two sections, Dv is given by the formula:

[0073]

[0074] Where, T 2rcut To distinguish the relative relaxation value of the residual water storage part and the migration part of the coal body, ms; T 2min is the minimum transverse relaxation time, ms; D VE With D VR are the fractal dimensions of the residual water migration structure dominated by macropores and cracks and the occurrence structure dominated by micropores, respectively;

[0075] Combining the above two equations and taking the logarithm of both sides, we can get the NMR fractal geometry formula:

[0076]

[0077] Then the fractal dimension is:

[0078] D=3-K

[0079] Where: K is lgS V The slope of the linear fit between 1gT2 and 1gT2.

[0080] S5. Based on the variation law of fractal dimension and the variation trend of residual water saturation, the influence mechanism of water injection on the storage and migration structure of residual water is analyzed.

[0081] The specific process of step S5 is: using the above calculation formula and data, the fractal dimension of the occurrence and migration structure of the coal sample is obtained, drawn into a table, and its change law is analyzed to characterize its structural change trend.

[0082] T2 spectrum obtained by combining saturated water and nitrogen displacement 2rc The residual cutoff value is used to divide the residual water storage structure and migration structure of the coal body; the fractal dimension of the residual storage structure and migration structure of the coal body is obtained by combining the multifractal theory; the evolution trend of its structure is characterized by analyzing the change law of the fractal dimension; the residual storage and migration structure of the coal body is divided from the perspective of nuclear magnetic resonance, and the evolution trend of the structure is quantified, providing a new method for exploring the evolution of coal structure under water and nitrogen injection conditions.

[0083] The following is a study of coal samples from the Wudong Coal Mine, with experimental data provided. The specific experimental plan is as follows:

[0084] The prepared coal samples were divided into two groups, M1 and M2. Before the experiment officially began, the coal samples were placed in a high-temperature and high-pressure holder with a confining pressure of 12 MPa and a temperature of 40°C. The displacement gas was inert nitrogen.

[0085] After the experiment started, the water injection pressure was varied and the nitrogen displacement pressure was constant. The setting parameters are shown in Table 1:

[0086]

[0087] Table 1: Water injection pressure parameters

[0088] The experimental steps are as follows:

[0089] ① Place all coal samples in a drying oven at 105℃ and dry them for more than 24 hours, then vacuum them for 12 hours and weigh them to obtain the mass of the dried coal samples.

[0090] ② Saturate the coal sample in distilled water for 12 hours and weigh it repeatedly until the weight stabilizes to obtain the mass of the saturated coal sample. The initial porosity of each coal sample is determined by weighing. Nuclear magnetic resonance (NMR) measurements of the coal sample in a water-saturated state are performed to obtain a saturated T2 spectrum.

[0091] ③ Dry the saturated coal sample as shown in step ①. After drying, place it in a nuclear magnetic holder and slowly apply confining pressure until it reaches 12 MPa, and then keep it stable.

[0092] ④ The coal samples of groups M1 and M2 were subjected to water injection experiments at pressures of 0MPa, 2MPa, 4MPa, 6MPa, and 8MPa (0MPa is negative pressure saturation). Back pressure was applied during the water injection process and real-time nuclear magnetic resonance monitoring was performed until the T2 spectrum no longer changed. The water injection was terminated and nuclear magnetic resonance imaging was performed.

[0093] After setting the saturation pressure at 0 MPa, a nitrogen displacement experiment was applied to the coal body at a pressure of 6 MPa. Real-time nuclear magnetic resonance monitoring was performed during the displacement process until the T2 spectrum no longer changed. The experiment was terminated and nuclear magnetic resonance imaging was performed. The above steps were then repeated, and experiments were carried out at water injection pressures of 0 MPa, 2 MPa, 4 MPa, 6 MPa, and 8 MPa.

[0094] The measured values ​​of coal samples M1 and M2 under different water injection pressures are calculated according to the calculation method of the fractal characteristics of the residual water storage and migration structure in the coal body, and the experimental data shown in Table 2 below are obtained.

[0095]

[0096] Table 2: Fractal dimensions of coal samples under different water injection pressures

[0097] M1 and M2 in the entire water injection stage D VR With D VE The fractal value changes basically show an upward trend with the increase of water injection pressure. VR The fractal value is less than 2, indicating that the residual occurrence structure of these two coal samples is simple and the number of pores and cracks is small, resulting in weak fractal characteristics. VR The fractal value increases slightly, indicating that the transformation of the occurrence structure dominated by micropores by low water pressure is small. VR With D VE The fractal values ​​all increased significantly, indicating that high-pressure water injection significantly improves the pore development and expansion and fracture extension of the coal mass compared to low-pressure water injection. Overall, coal seam water injection affects both the occurrence structure and the migration structure of the coal mass, with the degree of influence increasing with increasing water pressure. The migration structure is more sensitive to changes in water injection pressure than the occurrence structure.

[0098] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to the embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the claims and their equivalents.

Claims

1. A calculation method for distinguishing the fractal characteristics of the storage and migration structure of residual water in coal bodies, characterized by: The specific steps include: S1. Construct a computational model for NMR characterization of coal structure changes and a computational model for residual water saturation in coal. S2. Based on the calculation model of NMR characterization of coal body structure changes and the calculation model of residual water saturation in coal body constructed in S1, the cumulative NMR signal T2 spectrum after saturated water and nitrogen displacement in the coal body is established; S3, based on the cumulative nuclear magnetic signal T2 spectrum after saturated water and nitrogen displacement established in S2, combined with T 2rcut Residual cutoff value, dividing the coal body residual water storage structure and migration structure; S4. By using the coal body residual water storage structure and migration structure divided in S3 and combining it with the multi-fractal theory, the fractal characteristics of the residual water storage and migration structure are obtained; S5. Based on the variation law of fractal dimension and the variation trend of residual water saturation, the influence mechanism of water injection on the storage and migration structure of residual water is analyzed.

2. The calculation method for distinguishing the fractal characteristics of the residual water storage and migration structure in coal according to claim 1, characterized in that: The specific process of step S1 is: using the NMR technology to utilize the interaction mechanism between the hydrogen-containing fluid and the coal rock, and combining the relaxation signal of the hydrogen-containing fluid in the pores inside the coal rock to characterize the microstructure and fluid migration characteristics of the coal sample; Transverse relaxation time, T2, is often used to analyze fluids in coal and rock pores and fissures. It has three different relaxation mechanisms: surface relaxation, diffusion relaxation, and free relaxation. It can be expressed as: Among them, T 2D represents the diffusion relaxation time, ms; T 2B represents the free relaxation time, ms; T 2S represents the surface relaxation time, ms. Since the contributions of free relaxation and diffusion relaxation are much smaller than that of surface relaxation, the formula can be approximately expressed as: Where ρ2 is the surface relaxation rate, which is a fixed value for the same coal sample, μm / ms; V is the volume of hydrogen-containing fluid in the pores of the coal sample; S is the pore surface area; Assuming that the pores inside the coal body are simple structures, the formula can be transformed into: Among them, F S is the pore geometry factor, and its value is closely related to the pore geometry. For spherical pores, F S The value is 3, columnar pore F S The value is 2; r is the pore radius, nm; Based on the above formula, the relationship between pore characteristic parameters and T2 relaxation value can be established, which is the basis for the quantitative characterization of pore fracture structure using nuclear magnetic resonance technology; As the injection pressure changes, the residual water content in different pores and fractures has obvious differences. This difference can be expressed by the following formula: Right now: in, is the relative residual water saturation; A θ is the accumulated T2 signal of residual water in the coal body after nitrogen displacement; A1 is the accumulated T2 signal when the coal body is relatively saturated.

3. The calculation method for distinguishing the fractal characteristics of the residual water storage and migration structure in coal according to claim 2, characterized in that: The specific process of step S2 is as follows: based on the data obtained from the nuclear magnetic resonance calculation model, the nuclear magnetic resonance data of the coal sample after saturated water and gas flooding are used to draw a nuclear magnetic resonance T2 spectrum; its horizontal axis is the relaxation time proportional to the pore size, and the vertical axis is the signal amplitude proportional to the number of pores of corresponding size; and the nuclear magnetic resonance T2 spectrum is converted into a nuclear magnetic cumulative T2 spectrum.

4. The calculation method for distinguishing the fractal characteristics of the residual water storage and migration structure in coal according to claim 3, characterized in that: The specific process of step S3 is as follows: based on the nuclear magnetic cumulative T2 spectrum, a line parallel to the X axis is drawn from the maximum value of the T2 spectrum cumulative nuclear magnetic signal curve after gas flooding, and the intersection of the line and the saturated water cumulative nuclear magnetic signal curve is the T 2rcut Residual cutoff value.

5. The calculation method for distinguishing the fractal characteristics of the residual water storage and migration structure in coal according to claim 4, characterized in that: The specific process of step S4 is as follows: the pore space inside the coal rock shows a certain self-similarity, which indicates that fractal theory can be used to characterize the complexity of different pore structures of the coal body; With the help of fractal geometry theory, the fractal geometry formula of coal rock pore space distribution is obtained: Where r is the pore radius, μm; r max is the maximum pore radius, μm; S is the pore volume ratio within the pore radius range of 0 to r, %; Dr is the pore fractal dimension; Based on the NMR relaxation principle, the relationship formula between the coal surface relaxation rate and pore characteristic parameters is combined with the fractal geometry formula to obtain: Where S v is the percentage of pore volume with surface relaxation rate in the range of 0 to T2 to the total pore volume, %; T2 is the relaxation time, ms; T 2max is the maximum relaxation time, ms; D is the fractal dimension of the pore size determined by the NMR T2 spectrum curve; 2rcut " value, the coal sample T2 spectrum curve is divided into two sections, Dv is given by the formula: Where, T 2rcut To distinguish the relative relaxation value of the residual water storage part and the migration part of the coal body, ms; T 2min is the minimum transverse relaxation time, ms; D VE With D VR are the fractal dimensions of the residual water migration structure dominated by macropores and cracks and the occurrence structure dominated by micropores, respectively; Combining the above two equations and taking the logarithm of both sides, we can get the NMR fractal geometry formula: Then the fractal dimension is: D=3-K Where: K is lgS V The slope of the linear fit between 1gT2 and 1gT2.

6. The calculation method for distinguishing the fractal characteristics of the residual water storage and migration structure in coal according to claim 5, characterized in that: The specific process of step S5 is: using the above calculation formula and data, the fractal dimension of the occurrence and migration structure of the coal sample is obtained, drawn into a table, and its change law is analyzed to characterize its structural change trend.

Images