Coal rock gas diffusivity and preservation evaluation method based on multi-constraint coupling

Abstract

The application discloses a coal rock gas diffusion quantity and preservation quantity evaluation method based on multi-constraint coupling, relates to the technical field of oil and gas engineering and unconventional natural gas resource evaluation, and comprises the following steps: a thermal evolution model is established to obtain accumulated gas generation quantity; a reservoir energy constraint model of a sedimentary burial stage is established to obtain gas storage energy; an energy adjustment model of a tectonic uplift stage is established to obtain gas storage energy after uplift; the gas storage energy is converted into equivalent methane preservation capacity; a tectonic diffusion coefficient composed of various indexes weighted is established; and the accumulated gas generation quantity, the equivalent methane preservation capacity and the tectonic diffusion coefficient are comprehensively considered to calculate energy equivalent theoretical preservation capacity at a maximum burial depth stage, energy equivalent theoretical diffused gas quantity at the tectonic uplift stage and present energy equivalent theoretical preservation capacity. The application can realize quantitative evaluation of the whole process of unconventional natural gas generation, preservation and diffusion, the evaluation result is reliable, the application range is wide, and the application can provide technical support for shale gas and coal rock gas resource preservation evaluation.

Description

Technical Field

[0001] This invention relates to the field of oil and gas engineering and unconventional natural gas resource evaluation technology, and in particular to a method for evaluating the escape and retention of coal and rock gas based on multi-constraint coupling. Background Technology

[0002] Coal-rock gas is a typical unconventional natural gas found in coal seams. Its formation is characterized by self-generation and self-storage, near-source enrichment, and multi-form occurrence. The resource scale is jointly controlled by the degree of coalification, reservoir porosity and cleavage structure, temperature and pressure conditions, tectonic evolution, and preservation conditions. Accurately and quantitatively evaluating the gas production, preservation, and dissipation of coal-rock gas during sedimentary burial and tectonic uplift evolution is a core technical prerequisite for resource potential accounting, exploration target selection, and development deployment.

[0003] Currently, coal-rock gas resource assessment commonly employs basin simulation software such as PetroMod to reconstruct burial history, thermal history, and hydrocarbon generation history, and to quantitatively calculate cumulative gas production. However, cumulative gas production only characterizes the gas supply capacity of the source rock and cannot be equated with the actual gas volume preserved in the reservoir. The actual gas volume preserved in the reservoir is simultaneously constrained by both the reservoir's storage capacity and tectonic dissipation conditions. The assessment method that directly substitutes gas production for resource volume deviates significantly from the actual hydrocarbon accumulation patterns.

[0004] During sedimentation and burial, overlying strata loading and increased formation temperature can induce reservoir pore compression, matrix elastic deformation, changes in pore pressure, and adjustments in methane occurrence. During tectonic uplift, decreased confining pressure, decreased temperature, pore rebound, microfracture opening, and fracture activity all directly affect the preservation and release of natural gas. Existing evaluation methods mostly focus on analyzing single factors such as gas production, gas content, pressure coefficient, or capping conditions, making it difficult to simultaneously and quantitatively characterize the intrinsic relationship between the gas volume preserved at the maximum burial depth, the gas volume released during tectonic uplift, and the current gas volume.

[0005] Therefore, there is an urgent need to establish a multi-constraint coupled evaluation method for coal and rock gas that comprehensively considers gas supply, reservoir energy storage capacity, and tectonic dissipation conditions, so as to more rationally evaluate the generation, preservation, dissipation laws, and current enrichment status of coal and rock gas in the geological history evolution process. Summary of the Invention

[0006] To address the aforementioned technical issues, this invention provides a method for evaluating the escape and retention of coal and rock gas based on multi-constraint coupling.

[0007] The technical solution adopted by this invention to solve its technical problem is: a method for evaluating the escape and retention of coal and rock gas based on multi-constraint coupling, characterized by including the following steps:

[0008] S1. Collect basic parameters of the target coal and rock reservoir;

[0009] S2. Use PetroMod software to establish a thermal evolution model of the target strata and obtain the cumulative gas production of the target reservoir during its geological history.

[0010] S3. Based on the burial depth, temperature, pore pressure and effective stress parameters of the target reservoir during the sedimentation and burial process, establish a reservoir energy constraint model for the sedimentation and burial stage and solve for the gas storage energy during the sedimentation and burial stage.

[0011] S4. Based on the process of confining pressure reduction, pore rebound and matrix elastic energy release during tectonic uplift, establish a reservoir energy adjustment model for the tectonic uplift stage and solve for the gas storage energy during the tectonic uplift stage.

[0012] S5. Based on the gas storage energy, reservoir temperature, pore pressure and methane compressibility factor during the sedimentation and burial stages and the tectonic uplift stages, establish an equivalent methane storage capacity conversion model to convert the gas storage energy into the equivalent methane storage capacity under standard conditions.

[0013] S6. Based on the degree of fracture development, caprock sealing, caprock breakthrough pressure, and methane diffusion capacity, a quantitative model of tectonic escape coefficient is constructed.

[0014] S7. Calculate the theoretical energy equivalent storage capacity at the maximum burial depth stage, the theoretical energy equivalent gas volume during the tectonic uplift stage, and the current theoretical energy equivalent storage capacity by combining the cumulative gas volume, equivalent methane storage capacity, and tectonic escape coefficient.

[0015] Furthermore, in step S2, the cumulative gas volume is obtained through the kerogen conversion rate model, and its calculation formula is as follows: ;

[0016] In the formula, M HC The total amount of oil and gas generated from the conversion of kerogen; TOC0 is the original organic carbon content; HI0 is the original hydrogen index; TR is the conversion rate, indicating the proportion of kerogen that has been converted; M rock To determine the source rock mass for hydrocarbon generation, the volumetric method was used for calculation: ;

[0017] In the formula, A is the coal and rock area; h is the effective coal and rock thickness; and ρ is the coal and rock density.

[0018] Furthermore, in step S3, the reservoir energy constraint model for the deposition and burial stage is as follows:

[0019]

[0020] In the formula, I H1 This refers to the mechanical input energy or pore compression work caused by the loading of overlying strata during the sedimentation and burial stage, i.e., the gravity input energy; I T1The thermodynamic constraint term corresponding to the burial temperature rise, i.e., the thermodynamic input energy; T M1 This refers to the elastic energy stored in the rock matrix, i.e., the matrix elastic energy; T G1 D1 represents the equivalent gas storage energy of methane formed under given conditions in a pore-fracture system; D1 is the energy dissipation term.

[0021] Furthermore, the gravitational input energy I H1 The calculation formula is:

[0022] ;

[0023] In the formula, φ0 is the initial porosity; V rock ρ is the target coal and rock volume; c is the pore compressibility coefficient; ρ is the average density of the overlying strata; g is the gravitational acceleration; h max The maximum burial depth;

[0024] Thermal input energy I T1 The calculation formula is:

[0025] ;

[0026] In the formula, C p ρ represents the specific heat capacity of the target coal seam; rock T represents the coal and rock density of the target seam. i V represents the temperature of the target layer at a certain burial depth at a given time. rock T0 represents the rock volume of the target layer; T0 represents the temperature of the target layer under the initial burial depth conditions.

[0027] Matrix elasticity T M1 The calculation formula is:

[0028] ;

[0029] In the formula, ν is Poisson's ratio; E is the elastic modulus; h max ρ is the maximum burial depth; ρ is the average density of the overlying rock strata; g is the acceleration due to gravity; V rock The target seam coal and rock volume.

[0030] Furthermore, the formula for calculating the gas storage energy during the sedimentation and burial stage is as follows:

[0031] ;

[0032] In the formula, ξ H ξ T and ξ M These are the effective contribution coefficients of gravity input energy, thermal input energy, and matrix elasticity to the equivalent gas storage capacity, respectively, with values ​​ranging from 0 to 1; D1 is the energy dissipation term.

[0033] Under ideal closed conditions and with negligible energy dissipation, let ξ H =ξ T =ξ M =1, D1=0;

[0034] Then T G1 ≈I H1 +I T1 -T M1 .

[0035] Furthermore, in step S4, the formula for calculating the gas energy storage during the lifting stage is as follows:

[0036] ;

[0037] In the formula, To reduce the amount of gravity input energy during the lifting process, ;

[0038] To reduce the amount of heat input during the lifting process, ;

[0039] This represents the amount of elastic energy released by the matrix during the lifting process. ;

[0040] This refers to the work done on the expansion during pore rebound or pore expansion.

[0041] ;

[0042] In the formula, P p V represents the reservoir pore pressure. P1 V represents the pore volume at the maximum burial depth stage. P2 This represents the pore volume after lifting.

[0043] Furthermore, in step S5, the equivalent methane storage capacity conversion model is as follows:

[0044] ;

[0045] In the formula, T is the equivalent volume of methane under standard conditions, i.e., the equivalent methane storage capacity; G For gas energy storage; T std Standard temperature; Z is the methane compressibility factor; T is the reservoir temperature; P std Standard pressure; P p P0 is the reservoir pore pressure; P0 is the reference pressure.

[0046] Furthermore, in step S6, the constructed dissipation coefficient A weighted summation model for four indicators:

[0047] .

[0048] In the formula, I F I is the fracture dispersion index. C I is the cap layer unsealing index. B To break through the risk index, I D ω is the diffusion exponent; F ,ω C ,ω B and ω D For the corresponding weights of each indicator, satisfy ω F +ω C +ω B +ω D =1.

[0049] Furthermore, in step S7, the energy equivalent theoretical storage capacity N at the maximum burial depth stage... G1 Calculate using the following formula:

[0050] ;

[0051] In the formula, η c For effective fill-and-hold coefficient; The cumulative gas volume at maximum burial depth; N E1 The equivalent gas storage capacity obtained from gas energy storage at the maximum burial depth stage is calculated using the following formula:

[0052] ;

[0053] In the formula, T G1 For gas energy storage at the maximum burial depth stage; Z max R is the methane compressibility factor under the maximum burial depth temperature and pressure conditions; T is the gas constant; max The reservoir temperature at maximum burial depth; P p,max P0 represents the pore pressure at the maximum burial depth stage; P0 is the reference pressure.

[0054] Furthermore, the theoretical energy equivalent of the escaped gas volume N during the lifting phase is constructed. loss The formula is:

[0055] ;

[0056] Where, λ loss To construct the dissipation coefficient, the value range is 0-1; N G1 The theoretical energy equivalent storage capacity at the maximum burial depth stage; N G2 To preserve capacity for current energy equivalence theory.

[0057] The beneficial effects of this invention are:

[0058] This invention overcomes the limitations of traditional evaluation methods that focus solely on single factors such as gas production, gas content, or capping conditions. It organically couples gas supply, reservoir energy preservation capacity, and tectonic dispersal conditions, enabling a more comprehensive and realistic reflection of the enrichment and preservation process of unconventional natural gas throughout geological history. By establishing energy evolution models for the sedimentary burial and tectonic uplift stages, it achieves quantitative calculations of cumulative gas production, the theoretical energy equivalent preservation capacity at the maximum burial depth stage, the theoretical energy equivalent dispersal gas volume during the tectonic uplift stage, and the current theoretical energy equivalent preservation capacity. This comprehensively describes the entire process of natural gas generation, preservation, and dispersal, avoiding the empirical assumption of directly equating cumulative gas production with actual preserved gas volume. The evaluation results have clear physical meaning and geological basis. Furthermore, by introducing a tectonic dispersal coefficient, a weighted combination of the fault dispersal index, capping failure index, breakthrough risk index, and diffusion dispersal index, this application can quantitatively characterize the intensity of natural gas dispersal during tectonic uplift, making the method applicable to areas with complex structures and variable preservation conditions, and the evaluation results more consistent with geological realities. Furthermore, this method is not only applicable to coalbed methane, but can also be extended to various unconventional natural gas reservoirs such as shale gas and tight sandstone gas, demonstrating good versatility and portability. It can provide scientific and quantitative technical support for the evaluation of unconventional natural gas resource potential, analysis of preservation conditions, and selection of favorable exploration areas in complex structural regions. Attached Figure Description

[0059] Figure 1 This is a schematic diagram of the overall process of an evaluation method for coal and rock gas emission and retention based on multi-constraint coupling according to the present invention. Detailed Implementation

[0060] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application will be clearly and completely described below. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be arbitrarily combined with each other.

[0061] This invention discloses a method for evaluating the amount of coal and rock gas escape and retention based on multi-constraint coupling.

[0062] A method for evaluating the escape and retention of coal and rock gas based on multi-constraint coupling, specifically including the following steps:

[0063] S1. Obtain the basic parameters of the target reservoir, including but not limited to the burial depth of the target layer, formation temperature, reservoir pore pressure, initial and current porosity, rock density, elastic modulus, Poisson's ratio, original organic carbon content, original hydrogen index, caprock breakthrough pressure, methane diffusion coefficient, pore compressibility coefficient, pore resilience coefficient, Biot effective stress coefficient, etc. All parameters can be determined by combining core drilling tests, well logging interpretation and regional geological data. These parameters are used for subsequent model building and calculation.

[0064] S2. Use PetroMod software to establish a thermal evolution model of the target stratum, and obtain the cumulative gas production of the target reservoir during the geological history process based on the burial history, thermal history and hydrocarbon generation kinetic parameters.

[0065] PetroMod software calculates cumulative gas production using a kerogen conversion rate model, and the calculation formula is as follows:

[0066] ;

[0067] In the formula, M HC Total hydrocarbons generated from kerogen conversion, in kg; TOC0 is the original organic carbon content, in wt%; TOC0 / 100 indicates the proportion of organic carbon in the rock; HI0 is the original hydrogen index, in mgHC / gTOC; HI0 / 1000 indicates the hydrocarbon generation potential of organic carbon, converted to a mass ratio; M rock The mass of the source rock involved in hydrocarbon generation, expressed in kg, is calculated using the volumetric method:

[0068] ;

[0069] In the formula, The source rock area is expressed in meters. 2 ; Effective source rock thickness, in meters (m). Density of rock, unit: kg / m³ 3 .

[0070] TR stands for kerogen conversion rate, which is dimensionless and represents the proportion of kerogen that has been converted. It generally varies dynamically with burial depth and geological time and is automatically calculated by PetroMod software based on the embedded hydrocarbon generation kinetic model.

[0071] S3. Based on the burial depth, temperature, pore pressure and effective stress parameters of the target reservoir during the sedimentation and burial process, establish a reservoir energy constraint model for the sedimentation and burial stage, and solve for the gas storage energy during the sedimentation and burial stage.

[0072] The reservoir energy constraint model during the sedimentation and burial stage is as follows: ;

[0073] In the formula, IH1 The mechanical input energy or pore compression work caused by the loading of overlying strata during the sedimentation and burial stage, i.e., gravity input energy, is measured in J; T1 The thermodynamic constraint term corresponding to the temperature rise during burial is the thermal input energy, expressed in J / T. M1 The elastic energy stored in the rock matrix, also known as matrix elastic energy, is measured in J or T. G1 D1 represents the equivalent gas storage energy of methane gas in a pore-fracture system under given conditions, mainly reflecting the capacity for free gas compression storage and adsorbed gas accumulation, in units of J; D2 represents the energy dissipation caused by processes such as deposition compaction, plastic deformation, irreversible adjustment of pore structure, heat loss, and fluid migration, in units of J.

[0074] Under ideally closed conditions and with negligible energy dissipation, D1 in the equation can be approximated as 0. In this case, the input energy is mainly converted into the elastic energy of the rock matrix and the energy stored in the pore gas. It should be noted that T in the equation... G1 This is for the purpose of preserving the equivalent energy term in terms of capacity, and does not represent the chemical energy released or absorbed by the actual formation of methane from organic matter.

[0075] The process of determining the gas storage energy during the sedimentation and burial stage includes the following steps:

[0076] S3.1 Calculate the gravity input energy based on the static rock pressure and pore volume compression of the target reservoir during the sedimentation and burial stage. .

[0077] The formula for calculating static rock pressure is as follows: ;

[0078] In the formula, The average density of the overlying rock strata is expressed in kg / m³. 3 ; The acceleration due to gravity is taken as 9.81 m / s². 2 ; The thickness of the overlying strata is expressed in meters (m).

[0079] The product of pressure and volume change represents pressure-volume work. Therefore, the pressure-volume work corresponding to the pore volume compression of the target layer under static rock pressure is taken as the energy constraint term of gravity loading on the reservoir pore system during the sedimentation and burial stage. It should be noted that this term reflects the mechanical work required for pore compression and pore pressure adjustment and should not be measured repeatedly with the matrix elastic energy.

[0080] During the deposition and burial process, the target layer undergoes compaction, and the porosity decreases exponentially with increasing effective stress. The porosity compression equation is as follows: ;

[0081] In the formula, Represents porosity under different burial depths; dimensionless. The initial porosity is dimensionless. The pore compressibility coefficient is expressed in Pa. -1 ; Effective stress, unit: Pa.

[0082] When pore pressure changes are ignored during sedimentation and burial, or when the influence of pore pressure is incorporated into the equivalent compressibility coefficient, the effective stress can be approximated by the static rock pressure. Therefore, the pore volume compression of the target layer... Represented as: ;

[0083] In the formula, This is the pore volume compression, in meters. 3 , The target seam coal and rock volume, in meters. 3 .

[0084] Gravity input energy By integrating the pressure-volume work, we obtain: ;

[0085] Through integral derivation, we can obtain: .

[0086] In the formula, φ0 is the initial porosity, which is dimensionless; V rock The target seam coal and rock volume, in meters. 3 c is the porosity compressibility coefficient, Pa -1 ρ is the average density of the overlying rock strata, in kg / m³. 3 g is the acceleration due to gravity, with a value of 9.81 m / s². 2 h max Maximum burial depth, in meters; P h,max Maximum static rock pressure, unit: Pa, P h,max =ρgh max .

[0087] S3.2 Calculate the thermal input energy based on the temperature change process of the target layer. .

[0088] During the burial process, the target layer's temperature rises from an initial temperature T0 to the temperature T at a certain burial depth. i The absorbed heat is partially converted into gas energy storage in the pore-fracture system. The formula for calculating the thermal input energy is:

[0089] ;

[0090] In the formula, C p ρ represents the specific heat capacity of the rock in the target stratum, in J / (kg·°C); rock Target reservoir rock density, unit kg / m³3 V rock The target reservoir coal and rock volume, in meters. 3 ;T i Both T0 and T0 are in °C. This formula assumes that the specific heat capacity of the rock remains approximately constant within the temperature range under study.

[0091] S3.3 Calculate the matrix elastic properties based on the target reservoir's elastic modulus, Poisson's ratio, and triaxial principal stress parameters. .

[0092] The matrix elastic properties are calculated based on the rock elasticity theory under triaxial stress, and the relationship between strain and principal stress is as follows:

[0093] ;

[0094] In the formula, E is the elastic modulus of the target reservoir rock, in Pa; ν is Poisson's ratio, dimensionless; , , These are the three principal stresses, in Pa; , , These are the strains in the three principal stress directions, and are dimensionless.

[0095] Elastic energy density of rock matrix under single burial depth conditions Represented as:

[0096] ;

[0097] Substituting the principal strain expression into the above equation, we get:

[0098] ;

[0099] Therefore, the total matrix elasticity of the target layer can be expressed as: ;

[0100] Right now: .

[0101] If measured horizontal stress data are unavailable, the stress coefficient can be used as an approximation:

[0102] ; ; ;

[0103] In the formula, and These are the maximum and minimum horizontal stress coefficients, respectively. If we further approximate this as an ideal isotropic static rock pressure, then: ;

[0104] The matrix elastic energy density can then be simplified as:

[0105] ;

[0106] The corresponding total matrix elastic properties are:

[0107] ;

[0108] In the formula, ν is Poisson's ratio, dimensionless; E is the elastic modulus, in Pa; h max The maximum burial depth is expressed in meters (m); ρ is the average density of the overlying strata, expressed in kg / m³. 3 g is the acceleration due to gravity; V rock The target seam coal and rock volume, in meters. 3 .

[0109] S3.4. Based on the gravity input energy, thermal input energy and matrix elastic energy during the sedimentation and burial stage, calculate the reservoir gas storage energy during the sedimentation and burial stage.

[0110] Gas storage energy T during the sedimentation and burial stage G1 The energy input is determined by gravity, thermal input, and matrix elasticity, while also considering energy dissipation and contribution coefficients. The formula is as follows:

[0111] ;

[0112] In the formula, D1 represents the energy dissipation caused by compaction, plastic deformation, irreversible adjustment of pore structure, heat loss, and fluid migration during the deposition and burial process, in J; ξ H ξ T and ξ M These are the effective contribution coefficients of gravitational input energy, thermal input energy, and matrix elasticity to the equivalent gas storage capacity, respectively, with values ​​ranging from 0 to 1. The purpose of introducing these effective contribution coefficients is to avoid redundant measurement of gravitational input energy, thermal input energy, and matrix elasticity. Furthermore, , , ,and .

[0113] Under conditions of good closure and weak dissipation, it can be approximated as:

[0114] ;

[0115] Substituting the gravity input energy, thermal input energy, and matrix elastic energy, the reservoir gas storage energy during the sedimentation and burial stage can be obtained. The semi-quantitative expression:

[0116] ;

[0117] in, , and It should be corrected based on the measured gas content, pressure coefficient, pore structure and storage conditions.

[0118] To simplify the calculations, this embodiment sets the parameters based on ideal geological conditions of complete energy conservation. , and If all values ​​are 1, then: ;

[0119] Right now: .

[0120] S4. Based on the process of confining pressure reduction, pore pressure change, pore rebound and matrix elastic energy release during tectonic uplift, establish a reservoir energy adjustment model for the tectonic uplift stage and solve for gas storage energy during the tectonic uplift stage.

[0121] The reservoir energy adjustment model during the structural uplift stage is as follows:

[0122] ;

[0123] In the formula, The energy stored in methane gas in the enhanced pore-fracture system is expressed in J. The energy stored in methane gas in the pore-fracture system at the maximum burial depth stage is expressed in J. The reduction in mechanical loading energy due to the decrease in static rock pressure during the lifting process, expressed in J; The reduction in thermal constraint terms during the lifting and cooling process, expressed in J; The unit is the amount of elastic energy released from the rock matrix during the lifting and unloading process, expressed in J. This is the effective coefficient for compensating the matrix elasticity for gas storage capacity, with a value of 0–1, and is dimensionless. The work done by expansion during pore rebound or microcrack opening, measured in J; This represents energy dissipation during the uplift process caused by tectonic disturbances, fluid escape, frictional damage, and heat loss, measured in J. All of the above parameters should be understood as storage capacity adjustment terms, not methane formation terms.

[0124] When the dissipation term is small and the parameters are corrected by actual measurement, it can be ignored. Furthermore, to simplify calculations, this is based on the ideal law of complete energy conservation. , , ,and If both are 1, then: .

[0125] , , , It can be calculated using the following formula:

[0126] ;in, The residual gravitational input energy after lifting, in J; The total gravitational input energy during the deposition and burial process is expressed in J.

[0127] ;in, The total thermal input energy during the deposition and burial process is expressed in J. The residual thermal input energy after lifting is expressed in J.

[0128] ;in, Elastic energy accumulated in the target reservoir under maximum burial depth conditions, in J; This refers to the elastic energy stored in the rock matrix after uplift, i.e., the elastic energy of the preserved matrix, expressed in J.

[0129] ;

[0130] in, The work done by expansion during pore rebound or pore expansion is expressed in J. The reservoir pore pressure at different burial depths under uplift conditions is expressed in Pa. The pore volume at the maximum burial depth stage, in meters (m). 3 ; The volume of pores after lifting is expressed in meters. 3 .

[0131] During the tectonic uplift stage, as the overlying strata are eroded and thinned, the total vertical stress of the target layer decreases, and the reservoir transitions from a compacted state at maximum burial depth to a rebound state. The residual static rock pressure P after uplift... l Only with the remaining burial depth h l The relevant formula is... ;

[0132] In the formula, The residual static rock pressure after uplift, in Pa; The average density of the overlying rock strata is expressed in kg / m³. 3 ; The acceleration due to gravity is m / s². 2 ; The thickness of the remaining overlying strata after uplift is expressed in meters (m).

[0133] The actual residual pore compression after lifting can be approximated as the difference between the pore compression under residual gravity and the unloading rebound driven by pore pressure. Without considering pore rebound, the residual burial depth... The volumetric compression of the lower pores is expressed as: ;

[0134] In the formula, ρ is the initial porosity, dimensionless; c is the porosity compressibility coefficient, in Pa. -1 ; The average density of the overlying rock is expressed in kg / m³; g is the acceleration due to gravity, expressed in h. l The remaining burial depth after structural uplift, in meters; The target seam coal and rock volume, in meters. 3 .

[0135] During the structural uplift process, the pores will rebound to a certain extent due to the action of pore fluid pressure and the reduction of effective stress.

[0136] Let the pore rebound be: ;

[0137] In the formula, Porosity rebound is a dimensionless quantity.

[0138] The actual residual porosity compression after lifting should be: ;

[0139] Right now: .

[0140] Simultaneously, the pore rebound is less than the pore compression, and the actual pore compression under residual gravity conditions is reduced. The following conditions should be met: ;

[0141] During the structural uplift process, due to the decrease in effective stress, the pores undergo unloading and springback. The pore springback equation is: ;

[0142] In the formula, To construct the target reservoir porosity at different burial depths under uplift conditions, dimensionless; It is the minimum porosity of the target reservoir under the maximum burial depth condition, and is dimensionless. is the porosity resilience coefficient, which is dimensionless; Total stress in the overlying strata under maximum burial depth conditions, in Pa; Pore ​​fluid pressure at maximum burial depth, in Pa; It is the total stress in the overlying strata when the structure is uplifted to a certain depth, expressed in Pa. The pore fluid pressure at a certain depth is measured in Pa. is the Biot effective stress coefficient, used to correct the efficiency of fluid pressure on the rock skeleton. For dense shale, it is usually between 0.6 and 1.0, and is often taken as 1.

[0143] Based on this, the pore rebound is: ;

[0144] Right now: .

[0145] In the formula, Pore ​​pressure at maximum burial depth, in Pa; The pore pressure after elevation is expressed in Pa. This represents the effective stress coefficient of Biot.

[0146] At the same time, the rebound amount should be constrained by the residual compression:

[0147] ;

[0148] Based on this, strictly from the perspective of pressure-volume work, the residual gravitational input energy for:

[0149] ;

[0150] in: ;

[0151] but: ;

[0152] It can be approximated as: ;

[0153] in: ;

[0154] This represents the gravity input energy at the residual burial depth without considering pore rebound.

[0155] The correction term caused by pore springback is: ;

[0156] Therefore, the gravity input energy at the remaining burial depth is:

[0157]

[0158] Based on the above formula, gravitational loss energy for:

[0159] .

[0160] The residual thermal input energy during the tectonic uplift stage can also be estimated through the temperature change of the target layer. The temperature evolution curve of the target layer during the uplift process can be obtained from PetroMod simulation results. Assuming that the specific heat capacity of the target layer rock remains approximately constant within the studied temperature range, the thermal input energy retained by the target layer relative to its initial state when uplifted to a certain residual burial depth is: ;

[0161] and then: ;

[0162] In the formula, m is the mass of the target layer rock, in kg; C p Specific heat capacity of the target layer, in J / (kg·°C); T x T0 represents the temperature of the target stratum at a certain depth during the uplift phase, in °C; T0 represents the temperature of the target stratum at the initial depth .... max Temperature of the target layer at maximum burial depth, in °C.

[0163] Preservation of matrix elastic properties during the lifting phase Under deep reservoir conditions, i.e., burial depth greater than 1000 meters, :

[0164] but: ;

[0165] and then ;

[0166] In the formula, Elastic modulus, in Pa; Poisson's ratio, dimensionless. The average density of the overlying rock strata is expressed in kg / m³. 3 ; The acceleration due to gravity is taken as 9.81 m / s². 2 ; The thickness of the remaining overlying strata after uplift is expressed in meters (m).

[0167] Pore ​​rebound energy is obtained by multiplying the pore pressure and the pore rebound amount, as shown in the following formula:

[0168] ;

[0169] in, ;

[0170] but:

[0171] In the formula, The work done by expansion during pore rebound or pore expansion is expressed in J. The reservoir pore pressure is expressed in Pa. This refers to the reservoir pore volume; The ultimate pore volume under the condition of maximum burial depth, m 3 ; The pore volume at a certain residual burial depth, expressed in meters. 3 ; Porosity is a dimensionless quantity.

[0172] The amount of porosity rebound during structural uplift can be characterized by the porosity recovery caused by the reduction in effective stress:

[0173] ;

[0174] Since the path of pore pressure variation can be approximated as a linear equation that changes with depth, this embodiment uses average pore pressure as an approximation: ;

[0175] in, ;

[0176] but: ;

[0177] In the formula, W Pore The work done by expansion during pore rebound or pore expansion is expressed in J or P. p,max P represents the pore pressure at maximum burial depth, in Pa. p,x The pore pressure at a certain depth, expressed in Pa; V rock ρ is the target coal and rock volume in m³; Cr is the porosity resilience coefficient, dimensionless; ρ is the average density of the overlying strata in kg / m³; g is the gravitational acceleration, taken as 9.81 m / s². 2 h max Maximum burial depth, in meters (m); h x The depth at which the rock is raised to a certain depth is measured in meters (m); α is the Biot effective stress coefficient, which is dimensionless.

[0178] In summary, the energy stored after structural uplift... The calculation formula is:

[0179] ;

[0180] S5. Convert the gas storage energy into equivalent methane storage capacity: Calculate the equivalent methane volume under standard conditions based on the reservoir gas storage energy, reservoir temperature, pore pressure and methane compressibility factor.

[0181] If methane gas is approximated as a real gas and corrected using the methane compressibility factor Z, then the value of a unit mole of methane is determined by the reference pressure. Compressed to reservoir pore pressure The corresponding compressed energy storage can be expressed as:

[0182] ;

[0183] In the formula, Z is the methane compressibility factor, which is dimensionless; R is the gas constant; and T is the reservoir temperature, in K.

[0184] Therefore, gas storage energy The corresponding equivalent amount of methane is: ;

[0185] The corresponding equivalent methane mass is: ;

[0186] In the formula, The equivalent amount of methane is expressed in moles. Equivalent methane mass, unit: kg; Z is the gas storage energy, in J; Z is the methane compressibility factor, dimensionless, calculated using the Peng-Robinson equation of state; R is the gas constant, taken as 8.314 J / (mol*K); T is the reservoir temperature, in K. The reservoir pore pressure is expressed in Pa. Reference pressure, unit: Pa; The value is the molar mass of methane, taken as 0.01604 kg / mol.

[0187] The corresponding volume of methane under standard conditions is: ;

[0188] Right now: ;

[0189] In the formula, =273.15K, =101325 Pa, T G T can be taken separately G1 T G2 The difference between the two represents the equivalent preservation capacity at the maximum burial depth stage, the equivalent preservation capacity after uplift, and the change in equivalent capacity during the tectonic uplift process, respectively.

[0190] Methane in unconventional natural gas reservoirs typically includes both free and adsorbed gas; therefore, the total reservoir capacity can be further expressed as: ;

[0191] The free gas storage capacity is expressed as: ;

[0192] The adsorbed gas storage capacity is approximated using the Langmuir model as follows: ;

[0193] In the formula, The reservoir pore volume is expressed in cubic meters (m³). 3 Z is the methane compressibility factor; R is the gas constant; T is the reservoir temperature in K. The mass of the reservoir rock is expressed in kg. Langmuir volume, in meters (m). 3 / kg; Langmuir pressure, in Pa.

[0194] The energy evolution model mainly affects the storage capacity of free gas and adsorbed gas by controlling pore volume, pore pressure, effective stress and pore resilience.

[0195] S6. Establish the tectonic escape coefficient based on the degree of fracture development, caprock sealing, breakthrough pressure, and diffusion capacity.

[0196] The tectonic escape coefficient is determined by the degree of fracture development, caprock sealing, breakthrough pressure, and diffusion capacity. To quantitatively determine... It is expressed as the fracture dissipation index. Cap layer unsealing index Breakthrough Risk Index and diffusion extinction index Weighting function: ;

[0197] In the formula, The fracture dispersion index is mainly determined by fracture density, fracture scale, fracture connectivity, and whether the fracture cuts through the cover layer. The caprock unsealing index is mainly determined by caprock thickness, lithological assemblage, continuity, and permeability. The risk index is mainly determined by the ratio of the gas escape driving force to the caprock breakthrough pressure. The diffusion exponent is mainly determined by the effective diffusion coefficient of methane, the duration of the rise, and the length of the effective diffusion path. , , , These are the weights of each indicator, and they satisfy... .

[0198] In the absence of a large number of measured samples, semi-quantitative values ​​can be assigned based on structural geological interpretation, caprock breakthrough pressure experiments, methane diffusion experiments, and measured gas content. When multi-well measured gas content data is available, values ​​can be assigned based on currently preserved measured gas content. Perform inversion and correction.

[0199] S7. Based on the cumulative gas production, equivalent methane storage capacity, and tectonic escape coefficient, calculate the theoretical energy equivalent storage capacity at the maximum burial depth stage, the theoretical energy equivalent escape gas volume during the tectonic uplift stage, and the current theoretical energy equivalent storage capacity. The specific calculations are as follows:

[0200] At the maximum burial depth stage, the cumulative gas volume obtained from PetroMod simulation is:

[0201] ;

[0202] In the formula, The cumulative gas volume at the maximum burial depth stage, in meters (m). 3 ;t max The geological time required to reach the maximum burial depth.

[0203] The equivalent gas storage capacity calculated from the gas storage energy at the maximum burial depth stage is:

[0204] ;

[0205] In the formula, T G1 The gas storage energy at the maximum burial depth stage is expressed in J; Z. max The methane compressibility factor under the maximum burial depth temperature and pressure conditions is dimensionless; R is the gas constant; T max Reservoir temperature at maximum burial depth, in Kelvin (K); P p,max P0 represents the pore pressure at the maximum burial depth stage, in Pa; P0 is the reference pressure, in Pa.

[0206] Theoretical Preservation of Actual Energy Equivalent at Maximum Burial Depth Stage Simultaneously controlled by both the gas supply and reservoir storage capacity. When the cumulative gas production is less than the reservoir storage capacity, the stored gas volume is primarily controlled by the gas supply; when the cumulative gas production is greater than the reservoir storage capacity, the stored gas volume is primarily controlled by the reservoir storage capacity. Therefore, it can be expressed as:

[0207] ;

[0208] In the formula, The effective charge-storage coefficient, dimensionless, represents the proportion of PetroMod gas that can enter the target reservoir and participate in its preservation. For self-generated and self-storage shale gas systems, it can be approximated as... If the study area has well-developed fractures or poor preservation conditions, then the measured gas content, pressure coefficient, breakthrough pressure, and diffusion coefficient should be considered in conjunction with the analysis. Perform corrections.

[0209] This formula can also be written in a form that preserves coefficients: ;

[0210] in, ;

[0211] In the formula, This is the comprehensive preservation coefficient at the maximum burial depth stage, dimensionless. When the gas production in the formation is less than the energy-constrained gas storage capacity... near This indicates that the storage capacity is mainly controlled by the gas supply; when the gas supply from the formation exceeds the energy-constrained storage capacity, This indicates that the amount of reservoir energy is mainly controlled by the reservoir energy-pore system's preservation capacity.

[0212] Similarly, the theoretical energy equivalent storage capacity N under current burial depth conditions... G2 The same calculation method can be used, simply replacing the parameters with the temperature, pressure, and gas storage energy T corresponding to the current burial depth. G2 .

[0213] The energy equivalent theoretical escape volume N during the structural uplift phase loss The calculation formula is as follows:

[0214] ;

[0215] In the formula, λ loss The escape coefficient is used to characterize the proportion of gas that actually escapes during tectonic uplift beyond the reservoir's current storage capacity; its value ranges from 0 to 1. When the value is close to 0, it indicates that although tectonic uplift may lead to a decrease in storage capacity, the lack of fracture development, strong caprock sealing, high breakthrough pressure, or weak diffusion capacity mean that the gas exceeding the storage capacity has not significantly escaped; when When the value is close to 1, it indicates that the fracture is well-developed, the caprock is poorly sealed, the breakthrough pressure is low, or the diffusion capacity is strong, and most of the gas exceeding the storage capacity has escaped.

[0216] Without considering the gas escape coefficient during the tectonic uplift stage, it is expressed as: .

[0217] It should be noted that the above calculations are... , , This reflects the theoretical upper limit of reservoir preservation under given energy, pressure, and temperature conditions, i.e., the energy-equivalent theoretical preservation capacity and the amount of escaping gas, and is not equivalent to the actual geological reserves. The actual geological reserves are also controlled by multiple factors such as the effective reservoir volume fraction, gas saturation, pore connectivity, structural preservation conditions, fracture escaping, and hydrocarbon expulsion retention efficiency, and should be corrected according to the actual geological development characteristics of different structural units.

[0218] The correction factor can be expressed as: ;

[0219] In the formula, η ret η is the retention coefficient, representing the proportion of generated gas ultimately retained within the shale system; it is dimensionless. eff The effective reservoir coefficient represents the proportion of effective shale formations, high-quality reservoirs, and organic-rich formations; it is dimensionless. η sat η is a gas saturation correction factor, reflecting the spatial competition between free gas, adsorbed gas, and bound water in the pores; it is dimensionless. seal To preserve the condition coefficient, reflecting the sealing properties of the caprock, breakthrough pressure, fracture activity, and tectonic preservation conditions, it is dimensionless; η fracThe effective pore-fracture connectivity coefficient reflects pore connectivity, fracture opening degree, and the proportion of effective storage space; it is dimensionless.

[0220] It should be noted that the method proposed in this application is applicable to various unconventional natural gas reservoirs, such as shale gas reservoirs, coal gas reservoirs, and tight sandstone gas reservoirs.

[0221] The method of this application will be verified through a specific implementation case below.

[0222] The Longmaxi Formation shale reservoir in a certain area was selected as the research object. The current burial depth of this target stratum is 2634m, and the maximum burial depth is 4604m. The current formation temperature is 50℃, and the formation temperature at the maximum burial depth stage is 196℃. The average TOC of the target stratum is 4.12%, the average porosity is 4.37%, and the average density of the overlying strata rocks is 2.356g / cm³. 3 The average density of shale rock is 2.46 g / cm³. 3 The elastic modulus is 17.20 GPa; Poisson's ratio is 0.21; and the methane diffusion coefficient at the target layer is 2.003 × 10⁻⁶. - 9 cm 2 / s; Breakthrough pressure is 4.93 MPa; Pore compressibility coefficient c = 0.023; Pore resilience coefficient is selected based on regional experimental data.

[0223] The cumulative gas volume at the maximum burial depth stage was obtained using PetroMod simulation: =2274×10 8 m 3 / km 3 The calculated gravitational input energy I H1 =5.40×10 15 J / km 3 Thermal input energy I T1 =3.94×10 17 J / km 3 Matrix elastic properties T M1 =5.73×10 14 J / km 3 Taking a contribution coefficient of 1 and neglecting dissipation, the gas storage energy T is obtained. G1 =3.99×10 17 J / km 3 .

[0224] The structure was uplifted to its current burial depth, and the calculated reduction in gravity input energy was obtained. =2.47×10 15 J / km 3 The amount of heat input can be reduced. =3.31×10 17 J / km3 ; Matrix elasticity release =3.86×10 14 J / km 3 ; Pore rebound amount 0.5%, pore rebound energy W Pore =2.08×10 14 J / km 3 Take β M =0.72, so the gas storage energy T after lifting is obtained. G2 =6.59×10 16 J / km 3 .

[0225] Equivalent methane storage capacity conversion:

[0226] Maximum burial depth stage: T max =467.15K, P p,max =106.41MPa, Z max =1.233, N is calculated. E1 =2680×10 8 m 3 / km 3 ;

[0227] Current stage: T now =323.15K, P p,now =60.88MPa, Z now =0.859, N is calculated. E2 =1000×10 8 m 3 / km 3 .

[0228] Take η c =1, then the theoretical energy equivalent storage capacity N at the maximum burial depth stage G1 =2274×10 8 m 3 / km 3 This region is a normal-pressure reservoir; the tectonic escape coefficient λ is taken. loss =1, thus obtaining the theoretically equivalent escaping gas volume N during the structural lifting stage. loss =1274×10m 3 / km 3 .

[0229] This specific implementation case demonstrates that the method proposed in this application can simultaneously achieve quantitative evaluation of the cumulative gas production of unconventional natural gas, the energy equivalent theoretical gas storage volume at the maximum burial depth stage, the energy equivalent theoretical gas escape volume during the tectonic uplift stage, and the current energy equivalent theoretical gas storage volume. It can also better reflect the impact of reservoir energy evolution on natural gas storage capacity during the sedimentation-burial-tectonic uplift process.

[0230] Although the embodiments disclosed in this application are as described above, the content is merely for the purpose of understanding this application and is not intended to limit this application. Any person skilled in the art to which this application pertains may make any modifications and changes in the form and details of the implementation without departing from the spirit and scope disclosed in this application; however, the scope of patent protection of this application shall still be determined by the scope defined in the appended claims.

Claims

1. A method for evaluating the escape and retention of coal and rock gas based on multi-constraint coupling, characterized in that: Includes the following steps: S1. Collect basic parameters of the target coal and rock reservoir; S2. Use PetroMod software to establish a thermal evolution model of the target strata and obtain the cumulative gas production of the target reservoir during its geological history. S3. Based on the burial depth, temperature, pore pressure and effective stress parameters of the target reservoir during the sedimentation and burial process, establish a reservoir energy constraint model for the sedimentation and burial stage and solve for the gas storage energy during the sedimentation and burial stage. S4. Based on the process of confining pressure reduction, pore rebound and matrix elastic energy release during tectonic uplift, establish a reservoir energy adjustment model for the tectonic uplift stage and solve for the gas storage energy during the tectonic uplift stage. S5. Based on the gas storage energy, reservoir temperature, pore pressure and methane compressibility factor during the sedimentation and burial stages and the tectonic uplift stages, establish an equivalent methane storage capacity conversion model to convert the gas storage energy into the equivalent methane storage capacity under standard conditions. S6. Based on the degree of fracture development, caprock sealing, caprock breakthrough pressure, and methane diffusion capacity, a quantitative model of tectonic escape coefficient is constructed. S7. Calculate the theoretical energy equivalent storage capacity at the maximum burial depth stage, the theoretical energy equivalent gas volume during the tectonic uplift stage, and the current theoretical energy equivalent storage capacity by combining the cumulative gas volume, equivalent methane storage capacity, and tectonic escape coefficient.

2. The method for evaluating the emission and retention of coal and rock gas based on multi-constraint coupling according to claim 1, characterized in that: In step S2, the cumulative gas production is obtained using the kerogen conversion rate model, and its calculation formula is as follows: ； In the formula, M HC The total amount of oil and gas generated from the conversion of kerogen; TOC0 is the original organic carbon content; HI0 is the original hydrogen index; TR is the conversion rate, indicating the proportion of kerogen that has been converted; M rock To determine the source rock mass involved in hydrocarbon generation, the volumetric method was used for calculation: ; In the formula, A is the coal and rock area; h is the effective coal and rock thickness; and ρ is the coal and rock density.

3. The method for evaluating the emission and retention of coal and rock gas based on multi-constraint coupling according to claim 2, characterized in that: In step S3, the reservoir energy constraint model for the sedimentation and burial stage is as follows: ； In the formula, I H1 This refers to the mechanical input energy or pore compression work caused by the loading of the overlying rock layers during the sedimentation and burial stage, i.e., the gravity input energy. I T1 The thermodynamic constraint term corresponding to the burial temperature rise, i.e., the thermodynamic input energy; T M1 This refers to the elastic energy stored in the rock matrix, i.e., the matrix elastic energy; T G1 D1 represents the equivalent gas storage energy of methane formed under given conditions in a pore-fracture system; D1 is the energy dissipation term.

4. The method for evaluating the escape and retention of coal and rock gas based on multi-constraint coupling according to claim 3, characterized in that: Gravity input energy I H1 The calculation formula is: ； In the formula, φ0 is the initial porosity; V rock ρ is the target coal and rock volume; c is the pore compressibility coefficient; ρ is the average density of the overlying strata; g is the gravitational acceleration; h max The maximum burial depth; Thermal input energy I T1 The calculation formula is: ； In the formula, C p ρ represents the specific heat capacity of the target coal seam; rock T represents the coal and rock density of the target seam. i V represents the temperature of the target layer at a certain burial depth at a given time. rock T0 represents the target layer's coal and rock volume; T0 represents the target layer's temperature under the initial burial depth conditions. Matrix elasticity T M1 The calculation formula is: ； In the formula, ν is Poisson's ratio; E is the elastic modulus; h max ρ is the maximum burial depth; ρ is the average density of the overlying rock strata; g is the acceleration due to gravity; V rock The target seam coal and rock volume.

5. The method for evaluating the escape and retention of coal and rock gas based on multi-constraint coupling according to claim 4, characterized in that: The formula for calculating the gas storage energy during the sedimentation and burial stage is as follows: ； In the formula, ξ H ξ T and ξ M These are the effective contribution coefficients of gravity input energy, thermal input energy, and matrix elasticity to the equivalent gas storage capacity, respectively, with values ​​ranging from 0 to 1; D1 is the energy dissipation term. Under ideal closed conditions and with negligible energy dissipation, let ξ H =ξ T =ξ M =1, D1=0; Then T G1 ≈I H1 +I T1 -T M1 .

6. The method for evaluating the escape and retention of coal and rock gas based on multi-constraint coupling according to claim 1, characterized in that: In step S4, the formula for calculating the gas energy storage during the lifting stage is as follows: ； In the formula, To reduce the amount of gravity input energy during the lifting process, ; To reduce the amount of heat input during the lifting process, ; This represents the amount of elastic energy released by the matrix during the lifting process. ; This refers to the work done on the expansion during pore rebound or pore expansion. ； In the formula, P p V represents the reservoir pore pressure. P1 V represents the pore volume at the maximum burial depth stage. P2 This represents the pore volume after lifting.

7. The method for evaluating the escape and retention of coal and rock gas based on multi-constraint coupling according to claim 1, characterized in that: In step S5, the equivalent methane storage capacity conversion model is as follows: ； In the formula, T is the equivalent volume of methane under standard conditions, i.e., the equivalent methane storage capacity; G For gas energy storage; T std The standard temperature is Z; Z is the methane compressibility factor. T is the reservoir temperature; P std Standard pressure; P p P0 is the reservoir pore pressure; P0 is the reference pressure.

8. The method for evaluating the escape and retention of coal and rock gas based on multi-constraint coupling according to claim 1, characterized in that: In step S6, the constructed dissipation coefficient A weighted summation model for four indicators: ； In the formula, I F I is the fracture dispersion index. C I is the cap layer unsealing index. B To break through the risk index, I D ω is the diffusion exponent; F ,ω C ,ω B and ω D For the corresponding weights of each indicator, satisfy ω F +ω C +ω B +ω D =1.

9. The method for evaluating the escape and retention of coal and rock gas based on multi-constraint coupling according to claim 1, characterized in that: In step S7, the energy equivalent theoretical storage capacity N at the maximum burial depth stage is... G1 Calculate using the following formula: ； In the formula, η c For effective fill-and-hold coefficient; The cumulative gas volume at maximum burial depth; N E1 The equivalent gas storage capacity obtained from gas energy storage at the maximum burial depth stage is calculated using the following formula: ； In the formula, T G1 For gas energy storage at the maximum burial depth stage; Z max R is the methane compressibility factor under the maximum burial depth temperature and pressure conditions; T is the gas constant; max The reservoir temperature at maximum burial depth; P p,max P0 represents the pore pressure at the maximum burial depth stage; P0 is the reference pressure.

10. The method for evaluating the escape and retention of coal and rock gas based on multi-constraint coupling according to claim 9, characterized in that: The energy equivalent theoretical escape volume N during the structural uplift phase loss The formula is: ； Where, λ loss To construct the dissipation coefficient, the value range is 0-1; N G1 The theoretical energy equivalent storage capacity at the maximum burial depth stage; N G2 To preserve capacity for current energy equivalence theory.

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