A method for evaluating carbon dioxide adsorption and sequestration capacity of coal seams containing methane

Abstract

This invention discloses a method for assessing the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams, belonging to the field of carbon dioxide geological sequestration technology. The method first obtains the basic physical properties of the coal seam, the competitive adsorption parameters of CO2 and CH4, the geological parameters of the coal seam, and the rock mechanics parameters, breakthrough pressure parameters, and in-situ stress parameters of the roof and floor. Based on the rock mechanics, in-situ stress, and breakthrough pressure parameters of the roof and floor, the sealing pressure of the roof and floor is determined, defining the upper limit pore pressure of the coal seam. Under the constraint of the upper limit pore pressure, combined with the competitive adsorption parameters of CO2 and CH4 and the basic physical properties of the coal seam, the partial pressures of CO2 and CH4 under equilibrium conditions are determined. Based on the partial pressures of CO2 and CH4, the CO2 adsorption capacity per unit coal rock is determined, and the carbon dioxide adsorption and sequestration capacity of the coal seam is obtained by coupling the basic physical properties and geological parameters. This invention provides accurate, safe, and logically complete assessments, suitable for rapid evaluation of coal seam sequestration potential, and provides reliable support for engineering implementation.

Description

Technical Field

[0001] This invention belongs to the field of carbon dioxide geological storage technology, and specifically relates to a method for evaluating the carbon dioxide adsorption and storage capacity of methane-containing coal seams. Background Technology

[0002] Coal seams, due to their well-developed micropores and large specific surface area, possess a strong adsorption capacity for carbon dioxide and are considered one of the most promising geological carbon dioxide sequestration media. After carbon dioxide is injected into a coal seam, it mainly exists in an adsorbed state; therefore, the amount of adsorbed carbon dioxide stored is a core indicator for assessing the carbon dioxide sequestration potential of a coal seam.

[0003] Most existing methods for assessing the carbon dioxide adsorption and sequestration capacity of coal seams rely on pure carbon dioxide isothermal adsorption experimental data, directly determining it using the Langmuir equation. These methods have significant drawbacks:

[0004] 1. The competitive adsorption effect between methane and carbon dioxide in the coal seam was ignored, resulting in an overestimation of the sequestration volume; 2. Failure to incorporate sealing constraints such as the rock mechanical strength and breakthrough pressure of the top and bottom plates into the evaluation system may lead to overpressure leakage risks. 3. The determination of adsorption capacity is disconnected from the safety pressure boundary, making it impossible to obtain an assessment result that balances accuracy and engineering safety.

[0005] Therefore, there is an urgent need for a method to assess the carbon dioxide adsorption and sequestration capacity of coal seams that can take into account competitive adsorption, sealing constraints, and achieve a unified logic between parameters and determination process. Summary of the Invention

[0006] To address the aforementioned problems, the purpose of this invention is to provide a method for evaluating the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams, thereby achieving an accurate, safe, and logically sound determination of the carbon dioxide adsorption and sequestration capacity of coal seams.

[0007] The technical solution of this invention is: a method for evaluating the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams, comprising: Obtain basic physical properties of the coal seam, competitive adsorption parameters of CO2 and CH4, geological parameters of the coal seam, as well as rock mechanical parameters, breakthrough pressure parameters, and geostress parameters of the roof and floor.

[0008] The sealing pressures of the roof and floor are determined by using the rock mechanics parameters, breakthrough pressure parameters, and geostress parameters of the roof and floor. The sealing pressures of the roof and floor include the sealing pressure of the roof and the sealing pressure of the floor. The upper limit pore pressure of the coal seam is then determined by using the sealing pressures of the roof and floor.

[0009] Under the constraint of the upper limit pore pressure of the coal seam, and by combining the competitive adsorption parameters of CO2 and CH4 with the basic physical properties of the coal seam, the partial pressures of CO2 and CH4 under equilibrium conditions are determined.

[0010] The CO2 adsorption capacity per unit coal rock is determined based on the CO2 partial pressure and the CH4 partial pressure. The total mass of the coal seam is determined based on the basic physical properties and geological parameters of the coal seam. The carbon dioxide adsorption and storage capacity of the coal seam is determined based on the CO2 adsorption capacity per unit coal rock and the total mass of the coal seam.

[0011] Furthermore, the rock mechanical parameters of the top plate and the bottom plate include the rock mechanical parameters of the top plate and the rock mechanical parameters of the bottom plate. The rock mechanical parameters of the top plate include the cohesion, internal friction angle and tensile strength of the top plate, and the rock mechanical parameters of the bottom plate include the cohesion, internal friction angle and tensile strength of the bottom plate.

[0012] The breakthrough pressure parameters of the top plate and the bottom plate include the breakthrough pressure parameters of the top plate and the breakthrough pressure parameters of the bottom plate. The breakthrough pressure parameter of the top plate is the breakthrough pressure of the top plate; the breakthrough pressure parameter of the bottom plate is the breakthrough pressure of the bottom plate.

[0013] The geostress parameters of the top plate and the bottom plate include the geostress parameters of the top plate and the geostress parameters of the bottom plate; the geostress parameters of the top plate include the vertical geostress of the top plate, the maximum horizontal principal stress of the top plate, and the minimum horizontal principal stress of the top plate; the geostress parameters of the bottom plate include the vertical geostress of the bottom plate, the maximum horizontal principal stress of the bottom plate, and the minimum horizontal principal stress of the bottom plate.

[0014] The specific steps for determining the sealing pressure of the top plate and the sealing pressure of the bottom plate are as follows: The maximum and minimum principal stresses of the top and bottom plates are determined based on the values ​​of the vertical ground stress, the maximum horizontal principal stress, and the minimum horizontal principal stress, respectively.

[0015] The tensile rupture pressures of the top and bottom plates are determined based on the sum of the minimum principal stresses and tensile strengths of the top and bottom plates, respectively.

[0016] According to formula P respectively f =(P max -K×P min -2C×K 1 / 2 The critical pore pressure for shear failure of the top and bottom plates is determined by P / (1-K); where P f P represents the critical pore pressure at which shear fracture occurs. max P represents the maximum principal stress, K represents the stress coefficient, and P represents the maximum principal stress. min The minimum principal stress, C represents cohesion; the stress coefficient K is determined by the internal friction angle.

[0017] The minimum absolute value among the tensile rupture pressure, shear rupture critical pore pressure, and breakthrough pressure of the top and bottom plates is determined as the sealing pressure of the top and bottom plates, respectively.

[0018] Furthermore, the stress coefficient is determined according to the formula K=(1+sinφ) / (1-sinφ), where K represents the stress coefficient and φ represents the internal friction angle.

[0019] Furthermore, the upper limit pore pressure of the coal seam is determined based on the numerical values ​​of the sealing pressure of the roof and the sealing pressure of the floor. The smaller of the two pressure values ​​is selected as the candidate pressure. If the candidate pressure is greater than the original formation pressure, then the candidate pressure is determined as the upper limit pore pressure of the coal seam.

[0020] Further, the CO2 and CH4 competing adsorption parameters include CO2 Langmuir volume, CO2 Langmuir pressure, CH4 Langmuir volume, and CH4 Langmuir pressure; determining the partial pressure of CO2 and the partial pressure of CH4 specifically includes: Based on the extended Langmuir model, the adsorption phase composition equations for CO2 and CH4 are established by combining the Langmuir volume of CO2, the Langmuir pressure of CO2, the Langmuir volume of CH4, and the Langmuir pressure of CH4.

[0021] Establish the total pressure constraint equation, establish the methane conservation equation, and combine the adsorption phase composition equation with the total pressure constraint equation to determine the partial pressures of CO2 and CH4 under equilibrium conditions.

[0022] Furthermore, the composition equation of the adsorbed phase is as follows: V CO2 =VL CO2 ×P CO2 / PL CO2 / (1+P CO2 / PL CO2 +P CH4 / PL CH4 ); V CH4 =VL CH4 ×P CH4 / PL CH4 / (1+P CO2 / PL CO2 +P CH4 / PL CH4 ); Among them, V CO2 VL represents the amount of CO2 adsorbed per unit mass of coal. CO2 P represents the Langmuir volume of CO2. CO2 PL represents the partial pressure of CO2 in equilibrium. CO2 P represents the Langmuir pressure of CO2.CH4 PL represents the partial voltage of CH4 in equilibrium. CH4 Indicates CH4 Langmuir pressure; V CH4 VL represents the amount of CH4 adsorbed per unit mass of coal. CH4 This represents the Langmuir volume of CH4.

[0023] Furthermore, the total pressure constraint equation is: P CO2 +P CH4 =P max Among them, P CO2 P represents the partial pressure of CO2 in equilibrium. CH4 P represents the partial pressure of CH4 in equilibrium. max This indicates the upper limit of the pore pressure in the coal seam.

[0024] Furthermore, the basic physical properties of the coal seam include coal seam porosity and coal-rock density; the geological parameters of the coal seam include coal seam thickness, gas-bearing area of ​​the coal seam, and original coal seam pore pressure. The methane conservation equation is:

[0025] V 0CH4 ×ρ c ×A×h+(P0 / RT)×φ1×A×h=M 0CH4 Among them, M 0CH4 V represents the original total amount of CH4; 0CH4 This represents the initial CH4 adsorption capacity, determined by the initial coal seam pore pressure; ρ c φ represents the coal and rock density; A represents the gas-bearing area of ​​the coal seam; h represents the coal seam thickness; P0 represents the original coal seam pore pressure; R represents the universal gas constant; T represents the formation thermodynamic temperature; φ1 represents the coal seam porosity.

[0026] Furthermore, determining the amount of carbon dioxide adsorbed and stored in the coal seam specifically includes: Based on the partial pressures of CO2 and CH4, the CO2 adsorption capacity per unit mass of coal rock is determined according to the extended Langmuir adsorption equation.

[0027] The total mass of a coal seam is determined based on its density, gas-bearing area, and thickness.

[0028] The amount of CO2 adsorbed per unit mass of coal rock is multiplied by the total mass of the coal seam to obtain the amount of carbon dioxide adsorbed and stored in the coal seam.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention achieves accurate assessment of the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams through a complete technical route of "parameter acquisition - sealing constraint determination - equilibrium partial pressure solution - sequestration capacity determination", which has significant advantages over existing technologies.

[0030] This invention incorporates the sealing capacity of the roof and floor into the evaluation system. It determines the sealing pressure of the roof and floor by comprehensively considering rock mechanics parameters and breakthrough pressure, thereby limiting the upper limit of pore pressure in the coal seam. This avoids the risk of overpressure rupture and gas leakage from the source, ensuring that the evaluation results are both engineering-safe and practically operable. Simultaneously, the method fully considers the competitive adsorption effect of methane and carbon dioxide in situ within the coal seam. Based on the extended Langmuir model, the methane conservation equation, and the total pressure constraint equation, it simultaneously solves for the equilibrium partial pressure, abandoning the traditional single-component CO2 gas adsorption calculation model. This significantly improves the accuracy of the CO2 adsorption and storage results, effectively solving the problems of overestimation of CO2 adsorption and storage and large deviations from actual field conditions.

[0031] The overall technical solution features a closed-loop logic, with parameter acquisition, pressure determination, partial pressure solution, and storage capacity calculation linked step-by-step. It is free of redundancy and repeatable, making it suitable for rapid evaluation of coal seam exploration and storage potential. The method considers both geological sealing constraints and the competitive adsorption mechanism of reducing gases, providing reliable and quantitative theoretical basis and data support for coal seam carbon dioxide storage projects. It is highly practical, widely applicable, and has significant engineering application and promotion value. Attached Figure Description

[0032] Figure 1 This is a flowchart of the present invention. Detailed Implementation

[0033] The following is combined with Figure 1 The specific embodiments of the present invention will be described in detail below.

[0034] Example like Figure 1 The method shown here is for assessing the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams, comprising: Obtain basic physical properties of the coal seam, competitive adsorption parameters of CO2 and CH4, geological parameters of the coal seam, as well as rock mechanical parameters, breakthrough pressure parameters, and geostress parameters of the roof and floor.

[0035] The sealing pressures of the roof and floor are determined by using the rock mechanics parameters, breakthrough pressure parameters, and geostress parameters of the roof and floor. The sealing pressures of the roof and floor include the sealing pressures of the roof and floor. The upper limit pore pressure of the coal seam is then determined by using the sealing pressures of the roof and floor.

[0036] Under the constraint of the upper limit pore pressure of the coal seam, the partial pressures of CO2 and CH4 under equilibrium conditions are determined by combining the competitive adsorption parameters of CO2 and CH4 with the basic physical properties of the coal seam.

[0037] The CO2 adsorption capacity per unit coal rock is determined based on the partial pressure of CO2 and CH4. The total mass of the coal seam is determined based on the basic physical properties and geological parameters of the coal seam. The carbon dioxide adsorption and storage capacity of the coal seam is determined based on the CO2 adsorption capacity per unit coal rock and the total mass of the coal seam.

[0038] Preferably, the rock mechanical parameters of the top plate and the bottom plate include the rock mechanical parameters of the top plate and the rock mechanical parameters of the bottom plate. The rock mechanical parameters of the top plate include the cohesion, internal friction angle and tensile strength of the top plate, and the rock mechanical parameters of the bottom plate include the cohesion, internal friction angle and tensile strength of the bottom plate.

[0039] The breakthrough pressure parameters for the top and bottom plates include the breakthrough pressure parameters for the top plate and the breakthrough pressure parameters for the bottom plate. The breakthrough pressure parameter for the top plate is the breakthrough pressure of the top plate; the breakthrough pressure parameter for the bottom plate is the breakthrough pressure of the bottom plate.

[0040] The geostress parameters of the top and bottom plates include the geostress parameters of the top plate and the geostress parameters of the bottom plate; the geostress parameters of the top plate include the vertical geostress, the maximum horizontal principal stress, and the minimum horizontal principal stress of the top plate; the geostress parameters of the bottom plate include the vertical geostress, the maximum horizontal principal stress, and the minimum horizontal principal stress of the bottom plate.

[0041] The specific steps for determining the sealing pressure of the top plate and the sealing pressure of the bottom plate are as follows: The maximum and minimum principal stresses of the top and bottom plates are determined based on the values ​​of the vertical ground stress, the maximum horizontal principal stress, and the minimum horizontal principal stress, respectively.

[0042] The tensile rupture pressures of the top and bottom plates are determined based on the sum of the minimum principal stresses and tensile strengths of the top and bottom plates, respectively.

[0043] According to formula P respectively f =(P max -K×P min -2C×K 1 / 2 The critical pore pressure for shear failure of the top and bottom plates is determined by P / (1-K); where P f P represents the critical pore pressure at which shear fracture occurs. max P represents the maximum principal stress, K represents the stress coefficient, and P represents the maximum principal stress. min The minimum principal stress, C represents cohesion; the stress coefficient K is determined by the internal friction angle.

[0044] The minimum absolute value among the tensile rupture pressure, shear rupture critical pore pressure, and breakthrough pressure of the top and bottom plates is determined as the sealing pressure of the top and bottom plates, respectively.

[0045] Preferably, the stress coefficient is determined according to the formula K=(1+sinφ) / (1-sinφ), where K represents the stress coefficient and φ represents the internal friction angle.

[0046] Preferably, the upper limit pore pressure of the coal seam is determined based on the magnitude of the sealing pressure of the roof and the sealing pressure of the floor. The smaller of the two pressure values ​​is selected as the candidate pressure. If the candidate pressure is greater than the original formation pressure, the candidate pressure is determined as the upper limit pore pressure of the coal seam.

[0047] Preferably, the adsorption parameters for the competition between CO2 and CH4 include the Langmuir volume of CO2, the Langmuir pressure of CO2, the Langmuir volume of CH4, and the Langmuir pressure of CH4; determining the partial pressures of CO2 and CH4 specifically includes: Based on the extended Langmuir model, the adsorption phase composition equations for CO2 and CH4 are established by combining the Langmuir volume of CO2, the Langmuir pressure of CO2, the Langmuir volume of CH4, and the Langmuir pressure of CH4.

[0048] Establish the total pressure constraint equation, establish the methane conservation equation, and combine the adsorption phase composition equation with the total pressure constraint equation to determine the partial pressures of CO2 and CH4 under equilibrium conditions.

[0049] Preferably, the composition equation of the adsorbed phase is: V CO2 =VL CO2 ×P CO2 / PL CO2 / (1+P CO2 / PL CO2 +P CH4 / PL CH4 ); V CH4 =VL CH4 ×P CH4 / PL CH4 / (1+P CO2 / PL CO2 +P CH4 / PL CH4 ).

[0050] Among them, V CO2 VL represents the amount of CO2 adsorbed per unit mass of coal. CO2 P represents the Langmuir volume of CO2. CO2 PL represents the partial pressure of CO2 in equilibrium. CO2 P represents the Langmuir pressure of CO2. CH4 PL represents the partial voltage of CH4 in equilibrium. CH4 Indicates CH4 Langmuir pressure; V CH4 VL represents the amount of CH4 adsorbed per unit mass of coal. CH4This represents the Langmuir volume of CH4.

[0051] Preferably, the total pressure constraint equation is: P CO2 +P CH4 =P max Among them, P CO2 P represents the partial pressure of CO2 in equilibrium. CH4 P represents the partial pressure of CH4 in equilibrium. max This indicates the upper limit of the pore pressure in the coal seam.

[0052] Preferably, the basic physical properties of the coal seam include coal seam porosity and coal-rock density; the geological parameters of the coal seam include coal seam thickness, gas-bearing area of ​​the coal seam, and original coal seam pore pressure. The methane conservation equation is:

[0053] V 0CH4 ×ρ c ×A×h+(P0 / RT)×φ1×A×h=M 0CH4 Among them, M 0CH4 V represents the original total amount of CH4; 0CH4 This represents the initial CH4 adsorption capacity, determined by the initial coal seam pore pressure; ρ c φ represents the coal and rock density; A represents the gas-bearing area of ​​the coal seam; h represents the coal seam thickness; P0 represents the original coal seam pore pressure; R represents the universal gas constant; T represents the formation thermodynamic temperature; φ1 represents the coal seam porosity.

[0054] Preferably, determining the carbon dioxide adsorption and sequestration capacity of the coal seam specifically includes: Based on the partial pressures of CO2 and CH4, the CO2 adsorption capacity per unit mass of coal rock is determined according to the extended Langmuir adsorption equation.

[0055] The total mass of a coal seam is determined based on its density, gas-bearing area, and thickness.

[0056] The amount of CO2 adsorbed per unit mass of coal rock is multiplied by the total mass of the coal seam to obtain the amount of carbon dioxide adsorbed and stored in the coal seam.

[0057] This embodiment obtains Langmuir parameters through CO2 and CH4 gas adsorption experiments, establishes a mass balance equation based on the extended Langmuir adsorption model, and solves for the actual CO2 partial pressure after CO2 injection into the coal seam using the total pressure constraint equation. This realistically simulates the negative feedback mechanism where CH4 is replaced after CO2 injection, leading to an increase in CH4 partial pressure and thus inhibiting CO2 adsorption. The adsorption capacity is determined based on the equilibrium partial pressure, avoiding the overestimation of CO2 adsorption caused by neglecting the CH4 partial pressure in traditional pure CO2 adsorption models.

[0058] Adsorption determination and safety boundary are unified to form a complete evaluation framework: safety constraints and adsorption thermodynamics are coupled in the same evaluation process, which solves the problem of separation between adsorption amount determination and safety boundary in the existing technology, so that the evaluation results reflect the adsorption capacity of coal seams and meet engineering safety requirements.

[0059] Fully indoor experimental drive, low cost and short cycle: all parameters can be obtained through indoor core experiments, eliminating the need for expensive and time-consuming field injection tests, making it suitable for rapid screening and multi-block comparison in the early stages of exploration.

[0060] The theoretical adsorption and storage capacity is clearly defined: This method outputs the theoretical maximum CO2 adsorption and storage capacity under the upper limit of safe pressure and after considering competitive adsorption equilibrium. It reflects the limit of the coal seam's ability to safely adsorb CO2 under ideal conditions, and provides a scientific basis for subsequent engineering design and further evaluation.

[0061] Application examples Taking a deep methane-bearing coal seam as the evaluation object, the carbon dioxide adsorption and sequestration capacity was determined using the evaluation method proposed in the example. The specific steps are as follows: Step S1: Obtaining full-parameter indoor experiments Representative core samples were collected from the target coal seam and its roof and floor, and the following laboratory experiments were conducted: S1.1, Basic physical property tests of coal seams The porosity φ1 of the coal seam was determined to be 0.05 using a helium porosimeter; the density ρ of the coal was determined using the density bottle method. c 1.35t / m 3 .

[0062] S1.2 Rock mechanics and breakthrough pressure experiments on the top and bottom plates Triaxial compression tests, Brazilian splitting tests, and breakthrough pressure tests were conducted on core samples from the top and bottom plates, respectively.

[0063] Triaxial compression experiment: Obtaining the cohesion C of the top plate c The internal friction angle φ of the top plate is 3MPa. c The angle is 30° and the cohesion of the base plate is C. f The internal friction angle φ of the base plate is 2.6 MPa. f It is 28°.

[0064] Brazilian splitting test: Obtaining the tensile strength σ of the top plate tc The tensile strength σ of the base plate is 2 MPa. tf It is 1.8 MPa.

[0065] Breakthrough pressure test: CO2 was used as the test medium to simulate the formation temperature and pressure conditions. The breakthrough pressure P of the roof was obtained using a breakthrough pressure gauge. dcThe breakthrough pressure P of the base plate is 18 MPa. df It is 17 MPa.

[0066] Adsorption experiments of S1.3, CO2, and CH4 gases Under the condition of coal seam temperature T=25℃, the adsorption isotherms of pure CO2 and CH4 gases of coal sample were measured, and at least 5 pressure measuring points were set up.

[0067] By regressing data, the parameters of the extended Langmuir adsorption model were obtained: CO2 Langmuir volume VL CO2 25m 3 / t、CO2 Langmuir pressure PL CO2 2.5 MPa, CH4 Langmuir volume VL CH4 15m 3 / t、CH4 Langmuir pressure PL CH4 It is 3.0 MPa.

[0068] Extraction of geological parameters of S1.4 coal seam Extracted from the geological report: coal seam thickness h is 5m, coal seam burial depth is 420m, formation temperature T is 25℃, original coal seam pore pressure (CH4 pore pressure within the coal seam) P0 is 3MPa, and gas-bearing area A is 1.0×10⁻⁶. 6 m 2 .

[0069] Obtaining the ground stress parameters of the top and bottom plates of S1.5 Obtain the vertical principal stress σ of the roof from the geological report. vc The maximum horizontal principal stress σ of the top plate is 14 MPa. Hc The minimum horizontal principal stress σ of the top plate is 18 MPa. hc The vertical principal stress σ of the base plate is 12 MPa. vf The maximum horizontal principal stress σ of the base plate is 14.07 MPa. Hf The minimum horizontal principal stress σ of the base plate is 18.5 MPa. hf It is 12.3 MPa.

[0070] Step S2: Determining the sealing pressure of the roof and floor and the upper limit pore pressure of the coal seam. S2.1 Determination of sealing pressure Based on rock mechanics theory, the fracture pressures of the top and bottom plates are determined separately: S2.1.1, maximum principal stress, minimum principal stress The maximum principal stress P of the top plate is determined based on the magnitudes of the vertical principal stress, the maximum horizontal principal stress, and the minimum horizontal principal stress of the top and bottom plates. maxc The minimum principal stress P of the top plate minc The maximum principal stress P of the base platemaxf The minimum principal stress P of the base plate minf .

[0071] P maxc =σ Hc =18MPa, P minc =σ hc =12MPa; P maxf =σ Hf =18MPa, P minf =σ hf =12.3MPa.

[0072] S2.1.2, Tensile rupture pressure Tensile fracture occurs when the pore pressure exceeds the sum of the rock's tensile strength and the minimum principal stress. P tc =P minc +σ tc =14MPa; where P tc P represents the tensile fracture pressure of the roof. minc σ represents the minimum principal stress of the top plate. tc This indicates the tensile strength of the top plate.

[0073] P tf =P minf +σ tf =14.1MPa; where, P tf P represents the tensile fracture pressure of the base plate. minf σ represents the minimum principal stress of the base plate. tf This indicates the tensile strength of the base plate.

[0074] S2.1.3, Critical pore pressure at shear failure Based on the maximum and minimum principal stresses of the top plate, the maximum and minimum principal stresses of the bottom plate determined in S2.1.1, and using the Mohr-Coulomb criterion, the formula for calculating the critical pore pressure at shear failure is as follows: P f =(P max -K×P min -2C×K 1 / 2 ) / (1-K), where P f P represents the critical pore pressure at which shear fracture occurs. max P represents the maximum principal stress, K represents the stress coefficient, and P represents the maximum principal stress. min Minimum principal stress, where C represents cohesion.

[0075] K = (1 + sinφ) / (1 - sinφ), where φ represents the internal friction angle.

[0076] If the calculated P fA negative value indicates that CO2 injection will not cause shear fracture under the current stress conditions, and shear fracture does not constitute a constraint.

[0077] It should be noted that: When determining the critical pore pressure for shear failure of the top plate, it is determined according to the following formula.

[0078] P fc =(P maxc -K c ×P minc -2C c ×K c 1 / 2 ) / (1-K c ), where P fc K represents the critical pore pressure at which the top plate undergoes shear failure. c K represents the stress coefficient of the top plate. c =(1+sinφ c ) / (1-sinφ c ), φ c This indicates the internal friction angle of the top plate.

[0079] When determining the critical pore pressure for shear failure of the base plate, it is determined according to the following formula.

[0080] P ff =(P maxf -K f ×P minf -2C f ×K f 1 / 2 ) / (1-K f ), where P ff K represents the critical pore pressure at which the base plate fractures under shear stress. f K represents the stress coefficient of the base plate. f =(1+sinφ f ) / (1-sinφ f ), φ f This indicates the internal friction angle of the base plate.

[0081] Based on calculations, the stress coefficient K of the top plate is determined. c =3, then the critical pore pressure P for shear failure of the top plate fc ≈14.20MPa; Determine the stress coefficient K of the base plate. f If the value is approximately 2.77, then the critical pore pressure P for shear failure of the base plate is... ff ≈13.69MPa.

[0082] S2.1.4. Determination of Top Plate Sealing Pressure and Bottom Plate Sealing Pressure The minimum absolute value of the tensile fracture pressure, shear fracture pressure, and breakthrough pressure is taken as the sealing pressure of the formation. P pc =Min(abs(P tc ), abs(P fc ), abs(P dc =14MPa; where P pc P represents the sealing pressure of the top plate. tc P represents the tensile fracture pressure of the roof. fc P represents the shear failure pressure of the top plate. dc This indicates the pressure required to break through the top plate.

[0083] P pf =Min(abs(P tf ), abs(P ff ), abs(P df =13.69 MPa; where P pf P represents the sealing pressure of the base plate. tf P represents the tensile fracture pressure of the base plate. ff P represents the shear fracture pressure of the base plate. df This indicates the breaking pressure of the base plate.

[0084] S2.2, Upper limit pore pressure of coal seam P max =min(P pc ,P pf =13.69 MPa, where P max P represents the upper limit of pore pressure in the coal seam. pc P represents the sealing pressure of the top plate. pf This indicates the sealing pressure of the bottom plate; compared to the original methane pore pressure of the coal seam: P max =13.69MPa>P0=3MPa, where P0 represents the original coal seam pore pressure (i.e., the original methane pore pressure of the coal seam). The block has the conditions for CO2 sequestration, and subsequent calculations will continue.

[0085] Step S3: Determine the CO2 adsorption equilibrium considering competitive adsorption. Assume that the pore pressure of the coal seam reaches P after CO2 injection. max (i.e., the upper limit pore pressure of the coal seam). At this point, CO2 and CH4 coexist in the coal seam, and they reach equilibrium in the adsorbed and free phases. Based on the extended Langmuir model and the principle of mass balance, the partial pressure of CO2 P under equilibrium conditions is solved simultaneously. CO2 And CH4 partial pressure P CH4 .

[0086] S3.1, Adsorbed Phase Composition Equation VCO2 =VL CO2 ×P CO2 / PL CO2 / (1+P CO2 / PL CO2 +P CH4 / PL CH4 ); V CH4 =VL CH4 ×P CH4 / PL CH4 / (1+P CO2 / PL CO2 +P CH4 / PL CH4 ).

[0087] Among them, V CO2 VL represents the amount of CO2 adsorbed per unit mass of coal. CO2 P represents the Langmuir volume of CO2. CO2 PL represents the partial pressure of CO2 in equilibrium. CO2 P represents the Langmuir pressure of CO2. CH4 PL represents the partial voltage of CH4 in equilibrium. CH4 Indicates CH4 Langmuir pressure; V CH4 VL represents the amount of CH4 adsorbed per unit mass of coal. CH4 This represents the Langmuir volume of CH4.

[0088] S3.2, Total Pressure Constraint Equation P CO2 +P CH4 =P max =13.69MPa. Where, P CO2 P represents the partial pressure of CO2 in equilibrium. CH4 P represents the partial pressure of CH4 in equilibrium. max This indicates the upper limit of the pore pressure in the coal seam.

[0089] S3.3, CH4 mass balance equation The total amount of original CH4 (based on standard volume) includes both the original adsorbed CH4 and the original free CH4, and can be expressed as: M 0CH4 =V 0CH4 ×ρ c ×A×h+(P0 / RT)×φ1×A×h; where M 0CH4 V represents the original total amount of CH4; 0CH4 This represents the initial CH4 adsorption capacity (determined by the initial coal seam pore pressure P0 and the initial composition, assuming the initial composition consists only of CH4); ρ cφ represents the density of coal and rock; A represents the gas-bearing area of ​​the coal seam; h represents the thickness of the coal seam; P0 represents the original pore pressure of the coal seam; RT is a combination term used for gas equation of state conversion, with units of J / mol; R represents the universal gas constant; T represents the formation thermodynamic temperature; φ1 represents the porosity of the coal seam.

[0090] After CO2 is injected into the coal seam, the total amount of CH4 is conserved: V CH4 ×ρ c ×A×h+(P0 / RT)×φ1×A×h=M 0CH4 .

[0091] V CH4 Substituting the expression, we get: VL CH4 ×P CH4 / PL CH4 / (1+P CO2 / PL CO2 +P CH4 / PL CH4 )×ρc×A×h+(P CH4 / RT)×φ×A×h=M 0CH4 With P CO2 +P CH4 =P max Solve the equations simultaneously to find P. CO2 and P CH4 .

[0092] Finally, the partial pressure in equilibrium state is obtained: P CO2 ≈10.36MPa, P CH4 ≈3.33MPa.

[0093] Step S4: Determining the maximum CO2 adsorption and sequestration capacity of the coal seam S4.1, Maximum CO2 adsorption capacity The P obtained in step S3.3 CO2 and P CH4 Substituting into the CO2 adsorption equation, determine the amount of CO2 adsorbed per unit mass of coal: V CO2 =VL CO2 ×P CO2 / PL CO2 / (1+P CO2 / PL CO2 +P CH4 / PL CH4 ), find V CO2 ≈16.57m 3 / t.

[0094] S4.2, Maximum CO2 adsorption and sequestration capacity Mass of coal seam: m=ρ c ×A×h=6.75×106 t. Where m represents the mass of the coal seam, ρ c denoted by ρ, where A represents the gas-bearing area of ​​the coal seam, and h represents the thickness of the coal seam.

[0095] Maximum CO2 adsorption and storage capacity: V = V CO2 ×m≈1.12×10 8 m 3 Where V represents the maximum CO2 adsorption and sequestration capacity, V CO2 This represents the amount of CO2 adsorbed per unit mass of coal, where m represents the mass of the coal seam.

[0096] V. Conclusion Under safe injection pressure constraints and competitive adsorption conditions between CO2 and CH4, the equilibrium CO2 adsorption capacity per unit mass of coal in this block is approximately 16.57 m³. 3 / t; the total mass of the coal seam is 6.75×10 6 The final determined total CO2 adsorption and sequestration volume of the coal seam in this block was approximately 1.12 × 10⁻⁶. 8 m 3 .

[0097] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and do not limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the protection scope of the present invention.

Claims

1. A method for assessing the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams, characterized in that, include: To obtain the basic physical properties of the coal seam, the competitive adsorption parameters of CO2 and CH4, the geological parameters of the coal seam, as well as the rock mechanics parameters, breakthrough pressure parameters, and geostress parameters of the roof and floor; The sealing pressures of the roof and floor are determined by using the rock mechanics parameters, breakthrough pressure parameters, and geostress parameters of the roof and floor. The sealing pressures of the roof and floor include the sealing pressures of the roof and floor. The upper limit pore pressure of the coal seam is then determined by using the sealing pressures of the roof and floor. Under the constraint of the upper limit pore pressure of the coal seam, and by combining the competitive adsorption parameters of CO2 and CH4 with the basic physical properties of the coal seam, the partial pressures of CO2 and CH4 under equilibrium conditions are determined. The CO2 adsorption capacity per unit coal rock is determined based on the CO2 partial pressure and the CH4 partial pressure. The total mass of the coal seam is determined based on the basic physical properties and geological parameters of the coal seam. The carbon dioxide adsorption and storage capacity of the coal seam is determined based on the CO2 adsorption capacity per unit coal rock and the total mass of the coal seam.

2. The method for evaluating the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams as described in claim 1, characterized in that, The rock mechanical parameters of the top plate and the bottom plate include the rock mechanical parameters of the top plate and the rock mechanical parameters of the bottom plate. The rock mechanical parameters of the top plate include the cohesion, internal friction angle and tensile strength of the top plate. The rock mechanical parameters of the bottom plate include the cohesion, internal friction angle and tensile strength of the bottom plate. The breakthrough pressure parameters of the top plate and the bottom plate include the breakthrough pressure parameters of the top plate and the breakthrough pressure parameters of the bottom plate. The breakthrough pressure parameter of the top plate is the breakthrough pressure of the top plate; the breakthrough pressure parameter of the bottom plate is the breakthrough pressure of the bottom plate. The geostress parameters of the top plate and the bottom plate include the geostress parameters of the top plate and the geostress parameters of the bottom plate; the geostress parameters of the top plate include the vertical geostress of the top plate, the maximum horizontal principal stress of the top plate, and the minimum horizontal principal stress of the top plate; the geostress parameters of the bottom plate include the vertical geostress of the bottom plate, the maximum horizontal principal stress of the bottom plate, and the minimum horizontal principal stress of the bottom plate. The specific steps for determining the sealing pressure of the top plate and the sealing pressure of the bottom plate are as follows: The maximum and minimum principal stresses of the top and bottom plates are determined based on the values ​​of the vertical ground stress, the maximum horizontal principal stress, and the minimum horizontal principal stress, respectively. The tensile rupture pressures of the top and bottom plates are determined based on the sum of the minimum principal stresses and tensile strengths of the top and bottom plates, respectively. According to formula P respectively f =(P max -K×P min -2C×K 1 / 2 The critical pore pressure for shear failure of the top and bottom plates is determined by P / (1-K); where P f P represents the critical pore pressure at which shear fracture occurs. max P represents the maximum principal stress, K represents the stress coefficient, and P represents the maximum principal stress. min Minimum principal stress, C represents cohesion; stress coefficient K is determined by the internal friction angle; The minimum absolute value among the tensile rupture pressure, shear rupture critical pore pressure, and breakthrough pressure of the top and bottom plates is determined as the sealing pressure of the top and bottom plates, respectively.

3. The method for evaluating the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams as described in claim 2, characterized in that, The stress coefficient is determined according to the formula K=(1+sinφ) / (1-sinφ), where K represents the stress coefficient and φ represents the internal friction angle.

4. The method for evaluating the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams as described in claim 2, characterized in that, The upper limit pore pressure of the coal seam is determined based on the sealing pressure of the roof and the sealing pressure of the floor. The smaller of the two pressure values ​​is selected as the candidate pressure. If the candidate pressure is greater than the original formation pressure, the candidate pressure is determined as the upper limit pore pressure of the coal seam.

5. The method for evaluating the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams as described in claim 1, characterized in that, The adsorption parameters for the competition between CO2 and CH4 include CO2 Langmuir volume, CO2 Langmuir pressure, CH4 Langmuir volume, and CH4 Langmuir pressure. Determining the partial pressure of CO2 and the partial pressure of CH4 specifically includes: Combining the Langmuir volume of CO2, the Langmuir pressure of CO2, the Langmuir volume of CH4, and the Langmuir pressure of CH4, an adsorption phase composition equation for CO2 and CH4 is established based on the extended Langmuir model. Establish the total pressure constraint equation, establish the methane conservation equation, and combine the adsorption phase composition equation with the total pressure constraint equation to determine the partial pressures of CO2 and CH4 under equilibrium conditions.

6. The method for evaluating the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams as described in claim 5, characterized in that, The composition equation of the adsorbed phase is as follows: V CO2 =VL CO2 ×P CO2 / PL CO2 / (1+P CO2 / PL CO2 +P CH4 / PL CH4 ); V CH4 =VL CH4 ×P CH4 / PL CH4 / (1+P CO2 / PL CO2 +P CH4 / PL CH4 ); Among them, V CO2 VL represents the amount of CO2 adsorbed per unit mass of coal. CO2 P represents the Langmuir volume of CO2. CO2 PL represents the partial pressure of CO2 in equilibrium. CO2 P represents the Langmuir pressure of CO2. CH4 PL represents the partial voltage of CH4 in equilibrium. CH4 Indicates CH4 Langmuir pressure; V CH4 VL represents the amount of CH4 adsorbed per unit mass of coal. CH4 This represents the Langmuir volume of CH4.

7. The method for evaluating the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams as described in claim 6, characterized in that, The total pressure constraint equation is: P CO2 +P CH4 =P max ; Among them, P CO2 P represents the partial pressure of CO2 in equilibrium. CH4 P represents the partial pressure of CH4 in equilibrium. max This indicates the upper limit of the pore pressure in the coal seam.

8. The method for evaluating the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams as described in claim 7, characterized in that, The basic physical properties of the coal seam include coal seam porosity and coal-rock density; the geological parameters of the coal seam include coal seam thickness, coal seam gas-bearing area, and original coal seam pore pressure. The conservation equation for methane is: V 0CH4 ×ρ c ×A×h+(P0 / RT)×φ1×A×h=M 0CH4 ; Among them, M 0CH4 V represents the original total amount of CH4; 0CH4 This represents the initial CH4 adsorption capacity, determined by the initial coal seam pore pressure; ρ c φ represents the coal and rock density; A represents the gas-bearing area of ​​the coal seam; h represents the coal seam thickness; P0 represents the original coal seam pore pressure; R represents the universal gas constant; T represents the formation thermodynamic temperature; φ1 represents the coal seam porosity.

9. The method for evaluating the carbon dioxide adsorption and sequestration capacity of methane-containing coal seams as described in claim 8, characterized in that, Determining the carbon dioxide adsorption and sequestration capacity of the coal seam specifically includes: Based on the partial pressures of CO2 and CH4, the CO2 adsorption capacity per unit mass of coal rock is determined according to the extended Langmuir adsorption equation. The total mass of the coal seam is determined based on the coal rock density, the gas-bearing area of ​​the coal seam, and the coal seam thickness. The amount of CO2 adsorbed per unit mass of coal rock is multiplied by the total mass of the coal seam to obtain the amount of carbon dioxide adsorbed and stored in the coal seam.

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