A method for distinguishing between the fractal characteristics of residual water occurrence and migration structure in coal

Abstract

The application provides a calculation method for distinguishing fractal characteristics of residual water occurrence and migration structure in coal bodies, and comprises the following steps: constructing a calculation model for representing coal body structure change by nuclear magnetic resonance, and a calculation model for residual water saturation in coal bodies; establishing a cumulative nuclear magnetic signal T2 spectrum after displacement based on the calculation model for representing coal body structure change by nuclear magnetic resonance and the calculation model for residual water saturation in coal bodies; dividing residual water occurrence structure and migration structure of coal bodies based on the cumulative nuclear magnetic signal T2 spectrum after displacement, combining T 2rcut residual cutoff value; obtaining fractal characteristics of residual water occurrence and migration structure by combining multiple fractal theories; analyzing an influence mechanism of water injection on residual water occurrence and migration structure based on variation rules of fractal dimensions and variation trends of residual water saturation; dividing residual occurrence and migration structure of coal bodies from the nuclear magnetic angle, and quantifying evolution trends of the structure, so that a new method is provided for exploring coal body structure evolution under water injection and nitrogen injection conditions.

Description

Technical Field

[0001] This invention relates to the technical field of mine gas extraction, and in particular to a calculation method for distinguishing the fractal characteristics of residual water occurrence and migration structure within coal seams. Background Technology

[0002] As a major coal-producing country, my country possesses abundant coalbed methane (CBM) resources, but their exploitation level is low, indicating extremely high development potential. As a associated mineral resource of coal, the efficient utilization of CBM is not only beneficial for reducing coal mine gas accidents and ensuring safe coal mine production, but also plays a significant role in optimizing my country's energy structure and reducing greenhouse gas emissions. However, due to the generally low porosity, low permeability, and strong heterogeneity of my country's coal reservoirs, coupled with the unique characteristic that CBM is mostly found in adsorbed states within these reservoirs, CBM extraction is quite challenging.

[0003] Coal seam water injection involves continuously injecting pressurized water into the coal seam. Under the action of fluid-structure interaction, the pore and fracture structures of various scales within the coal seam are continuously expanded and extended as stress is transferred, thereby achieving structural modification to meet engineering objectives such as stress transfer and increased permeability. However, coal seam water injection will increase the water saturation within the coal seam, that is, increase the content of residual water, which can easily cause a water-locking effect and reduce coalbed methane production. Summary of the Invention

[0004] This invention aims to provide a calculation method for distinguishing the fractal characteristics of residual water occurrence and migration structures in coal bodies. It divides the residual water occurrence and migration structures in coal bodies and performs quantitative analysis through fractal theory. This method is beneficial for exploring the evolution law of residual water occurrence and migration structures in coal bodies under water injection, and can break the water-locking effect caused by the increase in residual water occurrence in coal bodies due to water injection, thus providing a theoretical basis for improving gas extraction efficiency.

[0005] Therefore, the technical solution adopted by this invention is: a calculation method for distinguishing the fractal characteristics of residual water occurrence and migration structure within coal, specifically including the following steps:

[0006] S1. Construct a calculation model for NMR characterization of coal body structural changes, and a calculation model for residual water saturation in coal body;

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

[0008] S3. Based on the cumulative NMR signal T2 spectrum established in S2 after displacement by saturated water and nitrogen, combined with T... 2rcut Residual cutoff value is used to classify the residual water occurrence and migration structure of the coal body;

[0009] S4. Based on the residual water occurrence and migration structure of the coal body as defined in S3, and combined with multifractal theory, the fractal characteristics of the residual water occurrence and migration structure are obtained.

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

[0011] As a preferred embodiment of the above scheme, the specific process of step S1 is as follows: using 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 transport characteristics of the coal sample.

[0012] The analysis of fluids in coal and rock pores and fractures often uses the transverse relaxation time, T2; it has three different relaxation mechanisms: surface relaxation, diffusion relaxation, and free relaxation.

[0013] It can be represented as:

[0014]

[0015] Among them, T 2D Represents the diffusion relaxation time, in milliseconds (ms); T 2B T represents the free relaxation time, in milliseconds (ms); 2S denoted by , representing the surface relaxation time in milliseconds; since the contributions of free relaxation and diffusion relaxation are much smaller than those of surface relaxation, the formula can be approximated as:

[0016]

[0017] In the formula, ρ2 is the surface relaxation rate, and ρ2 is a fixed value for the same coal sample, in 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 the internal pores of the coal body have a simple structure, the formula can be transformed into:

[0019]

[0020] Among them, F S F is the pore geometry factor, whose value is closely related to the pore geometry. For spherical pores... S The value is 3, and the columnar pore size 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 quantitative characterization of pore structure using nuclear magnetic resonance technology;

[0021] With changes in injection pressure, the residual water content in different pores and fissures varies significantly. This difference can be expressed by the following formula, defining the variable S. ∂ ,Right now:

[0022]

[0023] Among them, S ∂ Relative residual water saturation; A θ A1 represents the cumulative T2 signal of residual water in the coal body after nitrogen displacement; A2 represents the cumulative T2 signal when the coal body is relatively saturated.

[0024] Further preferably, the specific process of step S2 is as follows: based on the data obtained from the NMR calculation model, the NMR data of the coal sample after saturated water and gas driving are plotted into an NMR T2 spectrum; the 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 the corresponding size; the NMR T2 spectrum is converted into an NMR cumulative T2 spectrum.

[0025] More preferably, the specific process of step S3 is as follows: Based on the cumulative NMR T2 spectrum, draw an X-axis parallel line from the maximum value of the cumulative NMR signal curve of the T2 spectrum after gas driving; the intersection of this line with the cumulative NMR signal curve of saturated water is the T2 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 and rock exhibits a certain degree of self-similarity, which indicates that fractal theory can be used to characterize the complexity of different pore structures in coal.

[0027] Using fractal geometry theory, the fractal geometric formula for the spatial distribution of coal and rock pores is obtained:

[0028]

[0029] In the formula, r is the pore radius, µm; r max denoted as the maximum pore radius, µm; S is the pore volume ratio within the pore radius range of 0 to r, %; Dr is the fractal dimension of the pore size.

[0030] Based on the NMR relaxation principle, the relationship between the relaxation rate of the coal surface and the pore characteristic parameters is obtained by combining the formula with the fractal geometry formula:

[0031]

[0032] In the formula, S v T is the percentage of pore volume in the total pore volume with a surface relaxation rate in the range of 0 to T2, expressed as %; T2 is the relaxation time, expressed as ms. 2max The maximum relaxation time is given in milliseconds (ms); D is the fractal dimension of the pore size determined by the NMR T2 spectrum; the determined "T" is used to define the maximum relaxation time. 2rcut The value divides the T2 spectrum curve of the coal sample into two segments, and Dv is given by the formula:

[0033]

[0034] In the formula, T 2rcut To distinguish the relative relaxation values ​​of the residual water content and the transported portion of the coal seam, ms; T 2min The minimum relaxation time is in milliseconds (ms); D VE With D VR These are the fractal dimensions of residual water transport structures dominated by macropores and the occurrence structures dominated by micropores, respectively.

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

[0036]

[0037] The fractal dimension is then:

[0038]

[0039] In the formula: K is lgS V The slope of the linear fit between lgT2 and lgT2.

[0040] More preferably, the specific process of step S5 is as follows: using the above calculation formula and data, the fractal dimension of the coal sample occurrence and migration structure is obtained, plotted in a table, and its variation law is analyzed to characterize its structural change trend.

[0041] The beneficial effects of this invention: T obtained by combining the T2 spectra of saturated water and nitrogen displacement 2rc The residual cutoff value was used to classify the residual water occurrence and transport structure of the coal body. The fractal dimension of the residual occurrence and transport structure of the coal body was obtained by combining multifractal theory. The evolution trend of the structure was characterized by analyzing the variation law of the fractal dimension. The residual occurrence and transport structure of the coal body was classified from the perspective of nuclear magnetic resonance and the evolution trend of the structure was quantified, providing a new method for exploring the evolution of coal body structure under water and nitrogen injection conditions. Attached Figure Description

[0042] Figure 1 This is a flowchart illustrating the present invention.

[0043] Figure 2 This is the cumulative T2 NMR spectrum of the present invention.

[0044] Figure 3 The coal sample T of this invention 2rcut Schematic diagram of residual cutoff value. Detailed Implementation

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

[0046] like Figure 1-3 As shown, a calculation method for distinguishing the fractal characteristics of residual water occurrence and migration within coal seams includes the following steps:

[0047] S1. Construct a calculation model for NMR characterization of coal body structural changes, and a calculation model for residual water saturation in coal body.

[0048] The specific process of step S1 is as follows: using 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 transport characteristics of the coal sample.

[0049] The analysis of fluids in coal and rock pores and fractures often uses the transverse relaxation time, T2; it has three different relaxation mechanisms: surface relaxation, diffusion relaxation, and free relaxation.

[0050] It can be represented as:

[0051]

[0052] Among them, T 2D Represents the diffusion relaxation time, in milliseconds (ms); T 2B T represents the free relaxation time, in milliseconds (ms); 2S denoted by , representing the surface relaxation time in milliseconds; since the contributions of free relaxation and diffusion relaxation are much smaller than those of surface relaxation, the formula can be approximated as:

[0053]

[0054] In the formula, ρ2 is the surface relaxation rate, and ρ2 is a fixed value for the same coal sample, in 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 the internal pores of the coal body have a simple structure, the formula can be transformed into:

[0056]

[0057] Among them, F S F is the pore geometry factor, whose value is closely related to the pore geometry. For spherical pores... S The value is 3, and the columnar pore size 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 quantitative characterization of pore structure using nuclear magnetic resonance technology;

[0058] With changes in injection pressure, the residual water content in different pores and fissures varies significantly. This difference can be expressed by the following formula, defining the variable S. ∂ ,Right now:

[0059]

[0060] Among them, S ∂ Relative residual water saturation; A θ A1 represents the cumulative T2 signal of residual water in the coal body after nitrogen displacement; A2 represents the cumulative T2 signal when the coal body is relatively saturated.

[0061] S2. Based on the calculation model for NMR characterization of coal body structural changes and the calculation model for residual water saturation in coal body constructed in S1, the cumulative NMR signal T2 spectrum after displacement by saturated water and nitrogen in coal body is established.

[0062] The specific process of step S2 is as follows: Based on the data obtained from the NMR calculation model, the NMR data of the coal sample after saturation water and gas driving are plotted into an NMR T2 spectrum; the horizontal axis is the relaxation time, which is proportional to the pore size, and the vertical axis is the signal amplitude, which is proportional to the number of pores of the corresponding size; the NMR T2 spectrum is converted into an NMR cumulative T2 spectrum.

[0063] S3. Based on the cumulative NMR signal T2 spectrum established in S2 after displacement by saturated water and nitrogen, combined with T... 2rcut The residual cutoff value is used to classify the residual water occurrence structure and migration structure of the coal body.

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

[0065] S4. Based on the residual water occurrence and migration structure of the coal body as defined in S3, and combined with multifractal theory, the fractal characteristics of the residual water occurrence and migration structure are obtained.

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

[0067] Using fractal geometry theory, the fractal geometric formula for the spatial distribution of coal and rock pores is obtained:

[0068]

[0069] In the formula, r is the pore radius, µm; r max denoted as the maximum pore radius, µm; S is the pore volume ratio within the pore radius range of 0 to r, %; Dr is the fractal dimension of the pore size.

[0070] Based on the NMR relaxation principle, the relationship between the relaxation rate of the coal surface and the pore characteristic parameters is obtained by combining the formula with the fractal geometry formula:

[0071]

[0072] In the formula, S v T is the percentage of pore volume in the total pore volume with a surface relaxation rate in the range of 0 to T2, expressed as %; T2 is the relaxation time, expressed as ms. 2max The maximum relaxation time is given in milliseconds (ms); D is the fractal dimension of the pore size determined by the NMR T2 spectrum; the determined "T" is used to define the maximum relaxation time. 2rcut The value divides the T2 spectrum curve of the coal sample into two segments, and Dv is given by the formula:

[0073]

[0074] In the formula, T 2rcut To distinguish the relative relaxation values ​​of the residual water content and the transported portion of the coal seam, ms; T 2min The minimum relaxation time is in milliseconds (ms); D VE With D VR These are the fractal dimensions of residual water transport structures dominated by macropores and the occurrence structures dominated by micropores, respectively.

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

[0076]

[0077] The fractal dimension is then:

[0078]

[0079] In the formula: K is lgS V The slope of the linear fit between lgT2 and lgT2.

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

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

[0082] T obtained by combining water-saturated and nitrogen-displaced T2 spectra 2rc The residual cutoff value was used to classify the residual water occurrence and transport structure of the coal body. The fractal dimension of the residual occurrence and transport structure of the coal body was obtained by combining multifractal theory. The evolution trend of the structure was characterized by analyzing the variation law of the fractal dimension. The residual occurrence and transport structure of the coal body was classified from the perspective of nuclear magnetic resonance and the evolution trend of the structure was quantified, providing a new method for exploring the evolution of coal body structure under water and nitrogen injection conditions.

[0083] The following experimental data are presented using coal samples from the Wudong Coal Mine as the research object. The specific experimental scheme 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 displacing gas was inert nitrogen.

[0085] After the experiment began, varying water injection pressure and a constant nitrogen displacement pressure were applied. The parameters were set as shown in Table 1.

[0086] Table 1: Water Injection Pressure Parameter Table

[0087] The experimental steps are as follows:

[0088] ① Place all coal samples in a drying oven at 105℃ for more than 24 hours, then vacuum for 12 hours, and weigh to obtain the mass of the coal samples after drying.

[0089] ② The coal sample was saturated in distilled water for 12 hours and weighed repeatedly until the weight stabilized to obtain the mass of the saturated coal sample. The initial porosity of each coal sample was obtained by weighing. Nuclear magnetic resonance (NMR) measurements were performed on the coal sample under saturated water conditions to obtain the saturated T2 spectrum.

[0090] ③ Dry the saturated coal sample as shown in step ①. After drying, place it in an NMR holder and slowly apply confining pressure until it reaches 12 MPa, then keep it stable.

[0091] ④ The coal samples from groups M1 and M2 were subjected to water injection experiments at pressures of 0 MPa, 2 MPa, 4 MPa, 6 MPa and 8 MPa respectively (0 MPa is negative pressure saturation). During the water injection process, back pressure was applied and real-time nuclear magnetic resonance monitoring was performed until the T2 spectrum no longer changed. The water injection was then stopped and nuclear magnetic resonance imaging was performed.

[0092] After setting the pressure to 0 MPa for water saturation, a nitrogen displacement experiment was conducted on the coal body at a pressure of 6 MPa. Real-time NMR monitoring was performed during the displacement process until the T2 spectrum no longer changed. The experiment was then terminated and NMR imaging was performed. The above steps were then repeated, with experiments conducted sequentially at water injection pressures of 0 MPa, 2 MPa, 4 MPa, 6 MPa, and 8 MPa.

[0093] The measured values ​​of coal samples M1 and M2 under different water injection pressures were calculated according to the calculation method of the fractal characteristics of residual water occurrence and transport structure in the coal body, and the experimental data are shown in Table 2 below.

[0094]

[0095] Table 2: Fractal Dimension of Coal Samples under Different Injection Pressures

[0096] M1 and M2 during the entire water injection phase D VR With D VE The fractal value generally increases with increasing injection pressure. M1 and M2 are in the 0MPa negative pressure saturation stage. VR A fractal value less than 2 indicates that the residual structure of these two coal samples is simple and the number of pores and fractures is small, resulting in weak fractal characteristics. During low-pressure water injection, D VR The fractal value increases slightly, indicating that the lower water pressure has a minimal impact on the micropore-dominated structure. As the injection pressure increases, D... VR With D VE The fractal values ​​all showed a significant increase, indicating that high-pressure water injection has a "fault-like" improvement on the generation, expansion, and fracture extension of coal pores compared to low-pressure water injection. Overall, coal seam water injection affects both the coal's occurrence and migration structures, with the degree of influence increasing with water pressure. Furthermore, the migration structure is more sensitive to changes in water injection pressure than the occurrence structure.

[0097] Although embodiments of the invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. A calculation method for distinguishing the fractal characteristics of residual water occurrence and transport within coal, characterized in that: Specifically, the following steps are included: S1. Construct a calculation model for NMR characterization of coal body structural changes, and a calculation model for residual water saturation in coal body; S2. Based on the calculation model for NMR characterization of coal body structure changes and the calculation model for residual water saturation in coal body constructed in S1, establish the cumulative NMR signal T2 spectrum after saturated water and nitrogen displacement in coal body. The specific process of step S2 is as follows: Based on the data obtained from the NMR calculation model, the NMR data of the coal sample after saturated water and gas driving are plotted as NMR 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 the corresponding size; the NMR T2 spectrum is converted into NMR cumulative T2 spectrum; S3. Based on the cumulative NMR signal T2 spectrum established in S2 after displacement by saturated water and nitrogen, combined with T... 2rcut Residual cutoff value is used to classify the residual water occurrence and migration structure of the coal body; The specific process of step S3 is as follows: Based on the cumulative NMR T2 spectrum, draw a line parallel to the X-axis from the maximum value of the cumulative NMR signal curve of the T2 spectrum after gas driving. The intersection of this line with the cumulative NMR signal curve of saturated water is the T2 value of the coal sample. 2rcut Residual cutoff value; S4. Based on the residual water occurrence and migration structure of the coal body as defined in S3, and combined with multifractal theory, the fractal characteristics of the residual water occurrence and migration structure are obtained. The specific process of step S4 is as follows: the pore space inside the coal and rock exhibits a certain degree of self-similarity, which indicates that fractal theory can be used to characterize the complexity of different pore structures in coal. Using fractal geometry theory, the fractal geometric formula for the spatial distribution of coal and rock pores is obtained: In the formula, r is the pore radius, µm; r max denoted as the maximum pore radius, µm; S is the pore volume ratio within the pore radius range of 0 to r, %; Dr is the fractal dimension of the pore size. Based on the NMR relaxation principle, the relationship between the relaxation rate of the coal surface and the pore characteristic parameters is obtained by combining the formula with the fractal geometry formula: In the formula, S v The percentage of pore volume in the total pore volume with a surface relaxation rate in the range of 0 to T2, expressed as % . T2 is the relaxation time, in milliseconds; T 2max The maximum relaxation time is given in milliseconds (ms); D is the fractal dimension of the pore size determined by the NMR T2 spectrum; the determined "T" is used to define the maximum relaxation time. 2rcut The value divides the T2 spectrum curve of the coal sample into two segments, and Dv is given by the formula: In the formula, T 2rcut To distinguish the relative relaxation values ​​of the residual water content and the transported portion of the coal seam, ms; T 2min The minimum relaxation time is in milliseconds (ms); D VE With D VR These are the fractal dimensions of residual water transport structures dominated by macropores and the occurrence structures dominated by micropores, respectively. Combining the above two equations and taking the logarithm of both sides, we obtain the NMR fractal geometry formula: The fractal dimension is then: In the formula: K is lgS V The slope of the linear fit between lgT2; S5. Based on the fractal dimension variation law and the residual water saturation variation trend, the influence mechanism of water injection on the residual water occurrence and migration structure is analyzed.

2. The calculation method for distinguishing the fractal characteristics of residual water occurrence and migration structure within coal as described in claim 1, characterized in that: The specific process of step S1 is as follows: using 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 transport characteristics of the coal sample. The analysis of fluids in coal and rock pores and fractures often uses the transverse relaxation time, T2; it has three different relaxation mechanisms: surface relaxation, diffusion relaxation, and free relaxation. It can be represented as: Among them, T 2D Represents the diffusion relaxation time, in milliseconds (ms); T 2B T represents the free relaxation time, in milliseconds (ms); 2S denoted by , representing the surface relaxation time in milliseconds; since the contributions of free relaxation and diffusion relaxation are much smaller than those of surface relaxation, the formula can be approximated as: In the formula, ρ2 is the surface relaxation rate, and ρ2 is a fixed value for the same coal sample, in μm / ms; V is the volume of hydrogen-containing fluid in the pores of the coal sample; S is the pore surface area. Assuming the internal pores of the coal body have a simple structure, the formula can be transformed into: Among them, F S F is the pore geometry factor, whose value is closely related to the pore geometry. For spherical pores... S The value is 3, and the columnar pore size 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 quantitative characterization of pore structure using nuclear magnetic resonance technology; With changes in injection pressure, the residual water content in different pores and fissures varies significantly. This difference can be expressed by the following formula, defining the variable S. ∂ ,Right now: Among them, S ∂ Relative residual water saturation; A θ A1 represents the cumulative T2 signal of residual water in the coal body after nitrogen displacement; A2 represents the cumulative T2 signal when the coal body is relatively saturated.

3. The calculation method for distinguishing the fractal characteristics of residual water occurrence and migration structure within coal as described in claim 1, characterized in that: The specific process of step S5 is as follows: using the above calculation formula and data, the fractal dimension of the coal sample occurrence and migration structure is obtained, plotted in a table, and its variation law is analyzed to characterize its structural change trend.

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