Evaluation method of critical injection parameters of CO2 flooding for low permeability long fracture well group

Abstract

The application provides a kind of low permeability long fracture fracturing well group CO2 drive critical injection parameter evaluation method, comprising: carrying out CO2 injection well group reservoir characteristics fine description;Carrying out CO2 injection well group geomechanics parameter description;Establish low permeability long fracture fracturing well group CO2 drive reservoir matrix-fracture coupling numerical simulation model;Carrying out long fracture fracturing well group CO2 drive numerical simulation, evaluating reservoir pressure change;Carrying out long fracture fracturing well group CO2 drive disturbance stress field change evaluation;Carrying out long fracture fracturing well group CO2 drive fracture propagation determination, evaluating gas channeling and CO2 leakage risk, determining critical injection parameter limit.The low permeability long fracture fracturing well group CO2 drive critical injection parameter evaluation method combines the dynamic change evaluation CO2 drive critical injection parameter of low permeability long fracture fracturing well characteristics and reservoir physical parameter, and has important significance for improving CO2 drive development effect and realizing CO2 drive reservoir scientific management.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oil reservoir development, in particular to a method for evaluating critical CO2 injection parameters of a low-permeability long-fracture fractured well group. BACKGROUND

[0002] CO2 flooding in low-permeability oil reservoirs is an effective method for improving oil recovery. CO2 can reach a supercritical state under the temperature and pressure conditions in deep low-permeability oil reservoirs, has the density of a liquid and the viscosity of a gas, and is easy to inject into low-permeability oil reservoirs to supplement the energy of the formation; at the same time, CO2 is easy to mix with crude oil, causing the volume of the crude oil to expand and the viscosity to decrease, thereby greatly improving oil displacement efficiency.

[0003] CO2 flooding in low-permeability oil reservoirs is an effective method for improving oil recovery. CO2 can reach a supercritical state under the temperature and pressure conditions in deep low-permeability oil reservoirs, has the density of a liquid and the viscosity of a gas, and is easy to inject into low-permeability oil reservoirs to supplement the energy of the formation; at the same time, CO2 is easy to mix with crude oil, causing the volume of the crude oil to expand and the viscosity to decrease, thereby greatly improving oil displacement efficiency.

[0004] At present, research on CO2 flooding in low-permeability oil reservoirs at home and abroad mainly focuses on the mechanism of CO2 miscible injection and the improvement of oil recovery by CO2 flooding, and lacks quantitative evaluation methods for critical CO2 injection parameters in long-fracture fractured wells.

[0005] In the Chinese patent application with the application number CN201711128892.3, a method for determining CO2 flooding injection parameters of a separate-layer injection well is disclosed. The method comprises: establishing a single-pipe separate-layer injection mathematical model or a concentric double-pipe separate-layer injection mathematical model according to the CO2 flooding separate-layer injection process; solving the mathematical model by using a node system analysis method; for single-pipe separate-layer injection, drawing a gas nozzle outflow curve and a formation inflow curve of a single layer, and solving by using an iterative method to determine the reasonable injection amount and injection pressure of each oil layer under different gas nozzle conditions; for concentric double-pipe separate-layer injection, drawing a single-layer inflow curve, a total inflow curve, and a ground pipeline outflow curve to determine the reasonable injection amount and injection pressure of each oil layer.

[0006] In the Chinese patent application No. CN201910561096.1, a two-dimensional CO2 non-miscible displacement mathematical simulation method for low-permeability oil reservoirs is involved. The method comprises: establishing a two-dimensional CO2 non-miscible displacement mathematical model for low-permeability oil reservoirs; processing parameters and boundary conditions required in the solving process of the two-dimensional CO2 non-miscible displacement for low-permeability oil reservoirs; solving the two-dimensional CO2 non-miscible displacement mathematical model for low-permeability oil reservoirs, which is solved by using a numerical method, i.e. by using an implicit pressure and explicit saturation method; selecting a low-permeability oil reservoir and obtaining its geological parameters, calculating by using the two-dimensional CO2 non-miscible displacement mathematical simulation method for low-permeability oil reservoirs, and analyzing the results.

[0007] In the Chinese patent application No. CN201510501873.5, a method for determining the oil component of CO2 foam flooding and a simulation method of CO2 flooding are involved. The method for determining the oil component of CO2 foam flooding comprises: a parameter determining step of determining the thermodynamic parameters, relative permeability and molar percentage of each component and gas bubbles of the oil of foam flooding; and a component determining step of determining the content of each component and gas bubbles based on a preset component model according to the parameters obtained in the parameter determining step.

[0008] The above prior art has great difference from the present application and cannot solve the technical problems we want to solve. Therefore, we have invented a new evaluation method for critical gas injection parameters of CO2 flooding for low-permeability long-fracture fractured well groups. SUMMARY

[0009] The purpose of the present application is to provide an evaluation method for critical gas injection parameters of CO2 flooding for low-permeability long-fracture fractured well groups, which fully considers the coupling effect of CO2 gas injection parameters and fracturing fractures in the CO2 gas injection process, evaluates the CO2 gas injection potential of long-fracture fractured well groups in combination with the dynamic change of reservoir physical parameters, and determines the critical gas injection parameters.

[0010] The purpose of the present application can be achieved by the following technical measures: the evaluation method for critical gas injection parameters of CO2 flooding for low-permeability long-fracture fractured well groups, which comprises:

[0011] Step 1: carrying out fine description of reservoir characteristics of CO2 gas injection well groups;

[0012] Step 2: carrying out description of geomechanical parameters of CO2 gas injection well groups;

[0013] Step 3: establishing a CO2 flooding reservoir matrix-fracturing fracture coupling numerical simulation model for low-permeability long-fracture fractured well groups;

[0014] Step 4: carrying out numerical simulation of CO2 flooding for long-fracture fractured well groups and evaluating the change of reservoir pressure;

[0015] Step 5, carry out long fracture fracturing well group CO2 flooding disturbance stress field change evaluation;

[0016] Step 6, carry out long fracture fracturing well group CO2 flooding fracture propagation determination, evaluate gas channeling and CO2 leakage risk, and determine critical injection parameter limit.

[0017] The object of the application can also be achieved by the following technical measures:

[0018] In step 1, collect the structure data, drilling data, logging data, oil testing and production testing and core analysis test data of the long fracture fracturing well group, determine the basic parameters of the injection layer section, such as reservoir depth, temperature, pressure, porosity and permeability.

[0019] In step 2, the geomechanical parameter description includes rock mechanics parameters of long fracture fracturing well injection layer section, geostress logging interpretation and evaluation, and determination of fracture length and orientation of injection layer section.

[0020] In step 2, according to the core mechanical property test or logging data interpretation of the long fracture fracturing well injection layer section, the rock mechanics strength parameters of the injection layer section, such as Young's modulus, Poisson's ratio, tensile strength, cohesion and internal friction angle, are determined; the fracture length and main fracture orientation are determined according to the fracture construction data of the long fracture fracturing well.

[0021] In step 2, the fracture construction data of the long fracture fracturing well is used to determine the fracture pressure and instantaneous pump shutdown pressure of the fracturing layer section, and the maximum horizontal stress and minimum horizontal stress of the fracturing layer section are interpreted; further, according to the logging data of the long fracture fracturing well, stratified stress interpretation is carried out to obtain the stress and fracture pressure of the CO2 injection layer section.

[0022] In step 3, according to the long fracture fracturing well fracture morphology obtained in step 2, a deterministic modeling method is used to establish a CO2 flooding reservoir matrix-fracture coupling model of low permeability long fracture fracturing well group; and the coupling simulation evaluation of CO2 flooding reservoir pressure change and CO2 flooding induced stress field change is carried out.

[0023] In step 3, the established CO2 flooding reservoir matrix-fracture coupling numerical simulation model of low permeability long fracture fracturing well group, the fracture length in the model is consistent with the fracture length in step 2, and the fracture orientation is consistent with the maximum horizontal stress orientation.

[0024] In step 4, set the initial CO2 injection parameters, and use the matrix-fracture coupling model established in step 3 to numerically simulate and analyze the reservoir pressure change of CO2 injection well group under different parameters.

[0025] In step 4, the reservoir pressure change is obtained by solving the following low-permeability reservoir matrix seepage field and hydraulic fracture seepage field equations and corresponding boundary conditions and initial conditions:

[0026] The low-permeability reservoir matrix seepage field is described as follows:

[0027]

[0028] The hydraulic fracture internal flow is described as follows:

[0029]

[0030] Wherein, φ is the porosity of the low-permeability reservoir; C w is the matrix compression coefficient; k is the matrix permeability; μ is the fluid viscosity; ρ is the fluid density; u is the low-permeability reservoir matrix skeleton deformation; f w is the fluid source and sink term; α is the Biot consolidation coefficient; t is the time; z is the vertical depth; v is the CO2 and crude oil miscible fluid velocity; C f is the hydraulic fracture compression coefficient; k f is the fracture permeability; d f is the fracture width; is the tangential derivative along the hydraulic fracture; Q f is the flow exchange between the low-permeability reservoir matrix and the hydraulic fracture surface; n is the normal direction of the fracture surface.

[0031] The equations (1)-(4) are solved by numerical calculation methods such as finite difference method and finite element method.

[0032] In step 5, the CO2 flooding disturbance geostress field change is analyzed according to the reservoir pressure change obtained in step 4; wherein the CO2 flooding disturbance geostress field change is obtained by solving the low-permeability reservoir matrix deformation equation considering the effect of pore fluid pressure and the boundary conditions.

[0033] In step 5, the low-permeability reservoir matrix deformation is described as follows:

[0034]

[0035] Wherein, G is the shear modulus; ν is the Poisson's ratio; α is the Biot coefficient; u i is the rock displacement; f i is the body force; p is the reservoir pressure.

[0036] The boundary conditions are as follows:

[0037]

[0038] Wherein, σ′ ij is the effective stress of the rock; n jis the cosine of the normal direction outside the boundary; T i (t) are the known displacement and stress boundary conditions on the boundary, respectively; δ ij is the Kronecker symbol.

[0039] The formula (5), (6) can be solved by numerical calculation using finite element method.

[0040] In step 6, according to the well group geostress field dynamic change data obtained in step 5, combined with the low permeability reservoir rock fracture criterion, the well group CO2 drive crack propagation is determined, and the gas channeling and CO2 leakage risk is evaluated; the low permeability reservoir rock fracture criterion includes two types of crack tensile fracture and crack shear fracture.

[0041] In step 6, the crack tensile fracture is to the maximum tensile critical stress F t As a test standard of CO2 drive induced crack tensile fracture, that is,

[0042] F t =σ1-f t0 (7)

[0043] The crack shear fracture is to the maximum shear failure stress F s As a test standard of CO2 drive induced crack shear fracture, that is,

[0044]

[0045] Wherein, σ1, σ3 are the first and third principal stress of geostress field, respectively, obtained in step 5; the reservoir pressure p is obtained in step 4; f t0 is the tensile strength of rock, is the rock cohesion, and c is the rock internal friction angle, which are obtained in step 2.

[0046] In step 6, if the evaluation F t ≥0 or F s ≥0, it indicates that the long crack fractured well reaches the critical fracture condition under the current gas injection parameter, and the gas channeling and CO2 leakage risk is easy to produce; the corresponding gas injection parameter in step 4 is determined as the critical gas injection parameter.

[0047] In step 6, if the evaluation F t ≤0 and F

[0048] The low-permeability long-fracture fracturing well group CO2 flooding critical injection parameter evaluation method in the application fully considers the coupling influence of CO2 injection parameters and fracturing fractures in the CO2 injection process, combines with the dynamic change evaluation of the physical parameters of the reservoir to evaluate the CO2 injection potential of the long-fracture fracturing well group, and determines the critical injection parameter. The low-permeability long-fracture fracturing well group CO2 flooding critical injection parameter evaluation method in the application is based on geological data, logging data and fracturing microseismic monitoring data, establishes a long-fracture fracturing well matrix-fracture coupling model, comprehensively utilizes field test and numerical simulation calculation to accurately and intuitively reflect the influence of CO2 flooding on the reservoir pressure and the stress field, carries out the fracture expansion determination, evaluates the gas channeling and leakage risk, determines the critical injection parameter, and has wide application prospect in improving the development effect of low-permeability reservoirs. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 The flow chart of a specific embodiment of the low-permeability long-fracture fracturing well group CO2 flooding critical injection parameter evaluation method in the application;

[0050] Figure 2 The schematic diagram of the CO2 flooding reservoir matrix-fracture coupling simulation model of the low-permeability long-fracture fracturing well group in a specific embodiment of the application;

[0051] Figure 3 The schematic diagram of the CO2 flooding reservoir pressure distribution of the long-fracture fracturing well in a specific embodiment of the application;

[0052] Figure 4 The schematic diagram of the CO2 flooding tensile fracture prediction and evaluation of the long-fracture fracturing well in a specific embodiment of the application;

[0053] Figure 5 The schematic diagram of the CO2 flooding shear fracture prediction and evaluation of the long-fracture fracturing well in a specific embodiment of the application. DETAILED DESCRIPTION

[0054] It should be noted that the following detailed description is exemplary in nature and is intended to provide further description of the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0055] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments in accordance with the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups thereof.

[0056] The method for evaluating the CO2 flooding critical gas injection parameters of a low-permeability long-fractured well group according to the application comprises the following steps:

[0057] Step one, fine description of the reservoir characteristics of the CO2 injection well group

[0058] Collect the structure data, drilling data, logging data, oil testing and production testing data and core analysis testing data of the long-fractured well group to determine the basic parameters of the injection interval, such as reservoir depth, temperature, pressure, porosity and permeability.

[0059] Step two, description of the geomechanical parameters of the CO2 injection well group

[0060] The geomechanical parameter description includes the rock mechanics parameters of the injection interval of the long-fractured well and the geostress logging interpretation and evaluation to determine the fracture length and orientation of the injection interval.

[0061] According to the core mechanics property testing or logging data interpretation of the injection interval of the long-fractured well, the rock mechanics strength parameters of the injection interval, such as Young's modulus, Poisson's ratio, tensile strength, cohesion and internal friction angle, are determined; the fracture length and main fracture orientation of the injection interval are determined according to the fracture construction data of the long-fractured well.

[0062] The fracture length and main fracture orientation of the injection interval are determined according to the fracture construction data of the long-fractured well.

[0063] Step three, establishment of the CO2 flooding reservoir matrix-fracture coupling numerical simulation model of the long-fractured well group

[0064] According to the fracture shape (fracture length and fracture orientation) of the long-fractured well obtained in step two, a deterministic modeling method is adopted to establish the CO2 flooding reservoir matrix-fracture coupling model of the low-permeability long-fractured well group; and the coupling simulation evaluation of the CO2 flooding reservoir pressure change and the CO2 flooding induced stress field change is carried out.

[0065] The CO2 flooding reservoir matrix-fracture coupling numerical simulation model of the low-permeability long-fractured well group is established, in which the fracture length is consistent with the fracture length in step two and the fracture orientation is consistent with the maximum horizontal stress orientation.

[0066] Step four, CO2 flooding numerical simulation of the long-fractured well group and evaluation of the reservoir pressure change

[0067] Set the initial CO2 injection parameters, using the matrix-fracture coupling model established in step three, numerical simulation analysis of the changes in reservoir pressure under different parameters CO2 injection well group.

[0068] Wherein the reservoir pressure change can be obtained by solving the following low permeability reservoir matrix seepage field and hydraulic fracture within the seepage field equation and the corresponding boundary conditions, initial conditions.

[0069] Low permeability reservoir matrix seepage field is described as:

[0070]

[0071] Hydraulic fracture within the flow is described as:

[0072]

[0073] Where, φ is the low permeability reservoir porosity; C w is the matrix compression coefficient; k is the matrix permeability; μ is the fluid viscosity; ρ is the fluid density; u is the low permeability reservoir matrix skeleton deformation; f w is the fluid source and sink term; α is the Biot consolidation coefficient; t is the time; z is the vertical depth; v is the CO2 and crude oil miscible fluid velocity; C f is the hydraulic fracture compression coefficient; k f is the fracture permeability; d f is the fracture width; is the hydraulic fracture along the tangential derivative; Q f is the low permeability reservoir matrix and hydraulic fracture surface flow exchange; n is the fracture surface normal direction.

[0074] Equations (1)-(4) can be solved by numerical calculation methods such as finite difference method, finite element method, etc.

[0075] Step five, long fracture fracturing well group CO2 drive disturbance stress field change evaluation

[0076] Further according to the reservoir pressure change obtained in step four, the CO2 drive disturbance stress field change analysis is carried out. Wherein the CO2 drive disturbance stress field change can be obtained by solving the low permeability reservoir matrix deformation equation and the boundary conditions considering the effect of pore fluid pressure.

[0077] In step 5, the low permeability reservoir matrix deformation is described as follows:

[0078]

[0079] Where, G is the shear modulus; ν is the Poisson's ratio; α is the Biot coefficient; u i is the rock displacement; f i is the body force; p is the reservoir pressure.

[0080] The boundary conditions are as follows:

[0081]

[0082] where σ' ij is the effective stress of the rock formation; n j is the normal direction cosine outside the boundary; T i (t) are respectively the displacement and stress boundary conditions known on the boundary; δ ij is the Kronecker symbol.

[0083] The equations (5) and (6) can be solved by numerical calculation using the finite element method.

[0084] Step six, CO2 flooding fracture extension determination of long fracture fractured well group, evaluation of gas channeling and CO2 leakage risk

[0085] Further according to the well group stress field dynamic change data obtained in step five, combined with the low permeability reservoir rock fracture criterion, the well group CO2 flooding fracture extension determination is carried out, and the gas channeling and CO2 leakage risk is evaluated. The low permeability reservoir rock fracture criterion includes two types of crack tensile fracture and crack shear fracture.

[0086] The crack tensile fracture takes the maximum tensile critical stress F t as the standard for testing the CO2 flooding induced crack tensile fracture, that is,

[0087] F t = σ1-f t0 (7)

[0088] The crack shear fracture takes the maximum shear failure stress F s as the standard for testing the CO2 flooding induced crack shear fracture, that is,

[0089]

[0090] where σ1 and σ3 are respectively the first and third principal stresses of the stress field, obtained in step five; the reservoir pressure p is obtained in step four; f t0 is the tensile strength of the rock formation, is the cohesion of the rock formation, and c is the internal friction angle of the rock formation, all obtained in step two.

[0091] If F t ≥ 0 or F s ≥ 0, it indicates that the long fracture fractured well reaches the critical fracture condition under the current gas injection parameters, and is prone to gas channeling and CO2 leakage risk. The corresponding gas injection parameters in step four are determined as the critical gas injection parameters.

[0092] If F t ≤0 and F s ≤0, it indicates that the crack propagation condition has not been reached, and the CO2 injection parameters can be further increased.

[0093] Further return to step four, carry out the evaluation of the reservoir pressure after optimizing the injection parameters;

[0094] Further carry out step five, evaluate the change of the CO2 drive disturbed stress field;

[0095] Further carry out step six, evaluate the risk of CO2 drive gas channeling and leakage; through cyclic optimization, determine the critical injection parameters.

[0096] In a specific embodiment 1 of the application, a CO2 drive in a long-fractured well group in a low-permeability reservoir in Shengli Oilfield is taken as a demonstration engineering research object, and detailed description is made as shown in the accompanying drawings.

[0097] Figure 1 A flow chart of a method for evaluating the critical injection parameters of a CO2 drive in a long-fractured well group in a low-permeability reservoir according to the application.

[0098] Step one, carry out fine description of the geological horizon and permeability of the CO2 injection well group; collect the structure data, drilling data, logging data, oil testing and production data, and core analysis and testing data of the research area to which the long-fractured well belongs, and determine the basic parameters such as the reservoir depth, temperature, pressure, porosity, and permeability of the injection layer as shown in Table 1.

[0099] Table 1 Basic parameters of the CO2 injection layer in the long-fractured well group

[0100]

[0101] Step two, carry out description of the geomechanical parameters of the injection well group, including rock mechanics parameters and geostress logging interpretation and evaluation, and determine the fracture length and orientation of the fractured well.

[0102] According to the core mechanical property test of the injection layer of the long-fractured well, determine the rock mechanics strength parameters such as the Young's modulus, Poisson's ratio, tensile strength, cohesion, and internal friction angle of the injection layer as shown in Table 2.

[0103] Table 2 Rock mechanics parameters of the CO2 injection layer in the long-fractured well

[0104]

[0105] The rock mechanics parameters of the well are evaluated by using the fracturing operation data, logging interpretation data and other basic data of the long fracture fracturing well, and the magnitude of the gas injection layer stress is interpreted; the maximum horizontal stress of the gas injection layer is 81.1 MPa, and the minimum horizontal stress is 75.0 MPa. According to the microseismic monitoring record of the fracturing well group, the fracture length and fracture azimuth of the fracturing well group are determined as shown in Table 3.

[0106] Table 3 Fracture parameter table of long fracture fracturing well group

[0107]

[0108] Step three, long fracture fracturing well CO2 flooding reservoir matrix-fracture coupling numerical simulation model establishment

[0109] According to the fracture shape (fracture length, main fracture direction, etc.) of the long fracture fracturing well obtained in step two, a deterministic modeling method is used to establish a CO2 flooding reservoir matrix-fracture coupling model of the low permeability long fracture fracturing well group; and the coupling simulation evaluation of the CO2 flooding reservoir pressure change and the CO2 flooding induced stress field change is carried out.

[0110] Figure 2 For the long fracture fracturing well CO2 flooding reservoir matrix-fracture coupling simulation model established by using the finite element software COMSOL, the length and direction of the fracture are consistent with the field data of the long fracture fracturing well.

[0111] Step four, long fracture fracturing well group CO2 flooding numerical simulation, evaluation of reservoir pressure change

[0112] The CO2 flooding reservoir pressure of the low permeability long fracture fracturing well is evaluated by using the reservoir matrix-fracture coupling model established in step three. In the long fracture fracturing well group, Z23-X416 and Z23-X417 are gas injection wells. The initial gas injection parameters of the two wells are set to 100 t / d, and the numerical simulation analyzes the low permeability long fracture near-wellbore reservoir pressure change under the gas injection parameters.

[0113] The low permeability long fracture near-wellbore reservoir pressure change can be obtained by simultaneously solving the low permeability reservoir matrix seepage field and the hydraulic fracture seepage field equation group and the corresponding boundary conditions and initial conditions.

[0114] The low permeability reservoir matrix seepage field is described as:

[0115]

[0116] The flow in the hydraulic fracture is described as:

[0117]

[0118] Where, φ is the porosity of the low permeability reservoir; C wis the matrix compressibility; k is the matrix permeability; μ is the fluid viscosity; ρ is the fluid density; u is the low permeability reservoir matrix skeleton deformation; f w is the fluid source-sink term; α is the Biot consolidation coefficient; t is time; z is the vertical depth; v is the CO2 and crude oil miscible fluid velocity; C f is the hydraulic fracture compressibility; k f is the fracture permeability; d f is the fracture width; is the tangential derivative along the hydraulic fracture; Q f is the low permeability reservoir matrix and hydraulic fracture face flow exchange; n is the fracture face normal direction.

[0119] Figure 3 is the CO2 flooding reservoir pressure distribution of long fracture well group calculated according to formula (1)-(4) by using finite element software COMSOL.

[0120] Step five, CO2 flooding disturbance geostress field change evaluation of long fracture well group

[0121] Further according to the reservoir pressure change obtained in step four, the CO2 flooding disturbance geostress field change analysis is carried out. The CO2 flooding disturbance geostress field change can be obtained by simultaneously solving the low permeability reservoir matrix skeleton deformation equation and the boundary condition considering the action of pore fluid pressure.

[0122] The low permeability reservoir matrix skeleton deformation is described as follows:

[0123]

[0124] Wherein, G is the shear modulus; ν is the Poisson's ratio; α is the Biot coefficient; u i is the rock displacement; f i is the body force; p is the reservoir pressure.

[0125] The boundary condition is as follows:

[0126]

[0127] Wherein, σ' ij is the rock effective stress; n j is the boundary outer normal direction cosine; T i (t) are the displacement, stress boundary conditions known on the boundary respectively; δ ij is the Kronecker symbol.

[0128] Formula (5), (6) can be solved by numerical calculation using finite element software COMSOL.

[0129] Step six, long fracture fracturing well group CO2 drive fracture propagation determination, evaluation of gas channeling and CO2 leakage risk

[0130] Further according to the long fracture fracturing well stress field dynamic change data obtained in step five, combined with low permeability reservoir rock failure criterion, long fracture fracturing well CO2 drive fracture propagation determination is carried out, and the risk of gas channeling and CO2 leakage is evaluated. The low permeability reservoir rock failure criterion includes two types of fracture tensile failure and fracture shear failure.

[0131] The fracture tensile failure is based on the maximum tensile critical stress F t As a test standard for CO2 drive induced fracture tensile failure, that is,

[0132] F t =σ1-f t0 (7)

[0133] The fracture shear failure is based on the maximum shear failure stress F s As a test standard for CO2 drive induced fracture shear failure, that is,

[0134]

[0135] Wherein, σ1, σ3 are the first and third principal stresses of the stress field, obtained in step five; the reservoir pressure p is obtained in step four; f t0 is the tensile strength of the rock layer, is the rock cohesion, and c is the rock internal friction angle, all obtained in step two.

[0136] Figure 4 is the long fracture fracturing well CO2 drive tensile failure prediction and evaluation, Figure 5 is the long fracture fracturing well CO2 drive shear failure prediction and evaluation, the tensile failure standard F t ≤0, and the shear failure standard F s ≤0; it shows that the fracture propagation condition has not been reached, and the CO2 injection parameters can be further adjusted.

[0137] Further return to step four, carry out reservoir pressure evaluation after optimizing injection parameters;

[0138] Further carry out step five, evaluate CO2 drive disturbance stress field change;

[0139] Further carry out step six, evaluate CO2 drive gas channeling and leakage risk; through cyclic optimization, determine the critical injection parameters.

[0140] Table 4 is the statistical long fracture fracturing well bottom pressure and induced fracture propagation determination under different CO2 drive parameters. According to the calculation results, when the injection parameter reaches 120 t / d, the near-fracture area reaches the fracture propagation determination condition, and 105 t / d is determined as the critical injection parameter.

[0141] Table 4 Evaluation table of bottom hole pressure induced fracture under different CO2 flooding parameters

[0142]

[0143] In a specific embodiment 2 of the application, the low-permeability long-fracture fractured well group CO2 flooding critical gas injection parameter evaluation method according to the application has the same evaluation steps, implementation principle and beneficial effects as those of embodiment 1, except that the initial gas injection parameters of the two wells are set to 80 t / d, and the numerical simulation analyzes the low-permeability long-fracture near-wellbore region reservoir pressure change under the gas injection parameters.

[0144] Table 5 is a statistical evaluation of long-fracture bottom hole pressure and induced fracture expansion under different CO2 flooding parameters. According to the calculation results, the gas channeling and leakage risk is low under the injection rate of 80 t / d, 90 t / d and 100 t / d, and the wellhead gas injection rate can be increased. However, when the injection rate is increased to 110 t / d, the gas channeling risk is high, and the wellhead CO2 injection rate needs to be reduced. According to the evaluation steps of the application, the critical gas injection parameter is determined to be 105 t / d.

[0145] Table 5 Evaluation table of bottom hole pressure induced fracture under different CO2 flooding parameters

[0146]

[0147]

[0148] In a specific embodiment 3 of the application, the low-permeability long-fracture fractured well group CO2 flooding critical gas injection parameter evaluation method according to the application has the same evaluation steps, implementation principle and beneficial effects as those of embodiment 1, except that the gas injection interval position of Z23-X416 well is adjusted to 3822.5-3828.8 m. According to the CO2 injection well group geomechanical parameter description in step 2, the rock mechanical parameters of Z23-X416 well 3822.5-3828.8 m are shown in Table 6.

[0149] Table 6 Rock mechanical parameters of Z23-X416 well gas injection interval

[0150]

[0151] Step 2, carry out geomechanical parameter description of gas injection well group, including rock mechanical parameter, geostress logging interpretation and evaluation, and determine the fracture length and orientation of fractured well.

[0152] The rock mechanics parameters of the well are evaluated by using the fracturing operation data, logging interpretation data and other basic data of the long fracture fracturing well, and the magnitude of the stress in the gas injection interval is interpreted. The maximum horizontal stress in the gas injection interval is 82.3 MPa, and the minimum horizontal stress is 75.7 MPa. According to the microseismic monitoring record of the fracturing well, the fracture length and fracture azimuth of the fracturing well group are determined as shown in Table 7.

[0153] Table 7 Fracture parameter table of Z23-X416 well

[0154]

[0155] Step three, long fracture fracturing well CO2 flooding reservoir matrix-fracture coupling numerical simulation model establishment

[0156] According to the fracture shape change (fracture length, main fracture, etc.) of Z23-X416 well obtained in step two, a deterministic modeling method is used to establish a CO2 flooding reservoir matrix-fracture coupling model of the low permeability long fracture fracturing well group in the gas injection interval of Z23-X416 well after updating. The coupling simulation evaluation of CO2 flooding reservoir pressure change and CO2 flooding induced stress field change is carried out.

[0157] Step four, long fracture fracturing well group CO2 flooding numerical simulation, evaluation of reservoir pressure change

[0158] Using the reservoir matrix-fracture coupling model established in step three, the CO2 flooding reservoir pressure evaluation of the low permeability long fracture fracturing well is carried out. In the long fracture fracturing well group, the initial gas injection parameter of Z23-X417 is 100 t / d, and the initial gas injection parameter of Z23-X416 is 110 t / d. The numerical simulation analyzes the low permeability long fracture near-wellbore region reservoir pressure change under this gas injection parameter.

[0159] Step five, CO2 flooding disturbance stress field change evaluation of long fracture fracturing well group

[0160] Further according to the reservoir pressure change obtained in step four, the CO2 flooding disturbance stress field change analysis is carried out.

[0161] Step six, evaluation of CO2 flooding gas channeling and leakage risk; through cyclic optimization, the critical gas injection parameter is determined.

[0162] Further according to the long fracture fracturing well stress field dynamic change data obtained in step five, combined with the low permeability reservoir rock failure criterion, the long fracture fracturing well CO2 flooding fracture extension determination is carried out, and the gas channeling and CO2 leakage risk is evaluated.

[0163] Table 8 is the statistical Z23-X417 well bottom hole pressure and induced fracture extension determination, according to the calculation results, determine the 105t / d as the critical injection parameters. Table 9 is the statistical Z23-X416 well bottom hole pressure and induced fracture extension determination, according to the calculation results, determine the 115t / d as the critical injection parameters.

[0164] Table 8 Z23-X417 well bottom hole pressure induced fracture evaluation table

[0165]

[0166] Table 9 Z23-X416 well bottom hole pressure induced fracture evaluation table

[0167]

[0168] The evaluation method of the critical injection parameters of the CO2 flooding of the low-permeability long-fracture fracturing well group according to the present application fully considers the coupling influence of the injection parameters and the fracturing fractures in the CO2 injection process, combines the evaluation of the critical injection parameters of the CO2 flooding with the dynamic change of the low-permeability long-fracture fracturing well features and the reservoir physical parameters, and has important significance for improving the development effect of the CO2 flooding and realizing the scientific management of the CO2 flooding reservoir.

[0169] Finally, it should be noted that: the above only for the preferred embodiments of the present application, and not for the purpose of limiting the present application, although the present application has been described in detail with reference to the foregoing examples, for those skilled in the art, it still can be modified, or part of the technical features of the equivalent replacement. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application, should be included in the protection scope of the present application.

[0170] In addition to the technical features described in the specification, they are known to those skilled in the art.

Claims

1. A method for evaluating critical CO2 injection parameters in low-permeability long-fractured well groups, characterized in that, The evaluation methods for the critical CO2 injection parameters of this low-permeability long-fractured well group include: Step 1: Conduct detailed description of reservoir characteristics in CO2 injection well groups; Step 2: Describe the geomechanical parameters of the CO2 injection well group; Step 3: Establish a coupled numerical simulation model of matrix-fracture fracture in CO2-driven reservoirs of low-permeability long-fractured well groups; Step 4: Perform numerical simulation of CO2 flooding in long-fractured well groups to evaluate reservoir pressure changes; Step 5: Conduct an evaluation of the changes in the in-situ stress field caused by CO2 disturbance in the long-fractured well group; Step 6: Conduct CO2-driven fracture propagation assessment for long-fractured well groups, evaluate the risk of gas channeling and CO2 leakage, and determine the critical injection parameter limits. In step 6, the crack tensile fractures at the maximum tensile critical stress F. t As a standard for testing the tensile rupture of CO2-induced cracks, i.e. F t =σ1-f t0 (7) Crack shear fracture at maximum shear failure stress F s As a standard for testing CO2-induced crack shear failure, i.e. Where σ1 and σ3 are the first and third principal stresses of the geostress field, respectively, obtained from step 5; reservoir pressure p is obtained from step 4; f t0 For the tensile strength of rock strata, c represents the cohesion of the rock strata, and c represents the internal friction angle of the rock strata, both of which are obtained from step 2; In step 6, if F is evaluated t ≥0 or F s If ≥0, it indicates that under the current gas injection parameters, the long-fractured well has reached the critical fracture condition and is prone to gas channeling and CO2 leakage risks; the gas injection parameters in step 4 are determined as the critical gas injection parameters. In step 6, if F is evaluated t ≤0 and F s If the value is ≤0, it indicates that the crack propagation condition has not yet been met, so the CO2 injection parameters are further increased; the process returns to step 4; through iterative optimization, the critical injection parameters are determined.

2. The evaluation method for critical gas injection parameters of CO2 flooding in low-permeability long-fractured well groups according to claim 1, in step 1, collect structural data, drilling data, logging data, oil testing and production testing data and core analysis data of the study area to which the long-fractured well group belongs, and determine the basic parameters such as reservoir depth, temperature, pressure, porosity and permeability of the gas injection zone.

3. The evaluation method for critical gas injection parameters of CO2 flooding in low-permeability long-fractured well groups according to claim 1, in step 2, the description of geomechanical parameters includes rock mechanical parameters of the gas injection section of the long-fractured well, geostress logging interpretation and evaluation, and determination of the length and orientation of the fracturing fractures in the gas injection section.

4. The evaluation method for critical gas injection parameters of CO2 flooding in low-permeability long-fractured well groups according to claim 3, in step 2, based on the core mechanical property test or logging data interpretation of the gas injection section of the long-fractured well, the rock mechanical strength parameters such as Young's modulus, Poisson's ratio, tensile strength, cohesion, and internal friction angle of the gas injection section are determined; the magnitude of in-situ stress and fracture pressure of the gas injection section are evaluated using the fracturing construction data of the long-fractured well; and the length of the fracturing fracture and the orientation of the main fracture are determined based on the microseismic monitoring records of the fracturing well.

5. The evaluation method for critical CO2 injection parameters of low-permeability long-fractured well groups according to claim 3, in step 2, using the fracturing construction data of the long-fractured well, the fracturing pressure and instantaneous pump shutdown pressure of the fracturing section of the well are determined, and the maximum and minimum horizontal in-situ stress of the fracturing section are interpreted; further, based on the logging data of the long-fractured well, layered in-situ stress interpretation is carried out to obtain the in-situ stress and fracturing pressure of the CO2 injection section.

6. The evaluation method for critical gas injection parameters of CO2 flooding in low-permeability long-fractured well groups according to claim 1, in step 3, based on the fracture morphology of the long-fractured well obtained in step 2, a deterministic modeling method is used to establish a matrix-fracture coupling model of CO2 flooding reservoir in low-permeability long-fractured well groups; and a coupled simulation evaluation of pressure changes in CO2 flooding reservoirs and CO2 flooding-induced geostress field changes is carried out.

7. The evaluation method for critical gas injection parameters of CO2 flooding in low-permeability long-fractured well groups according to claim 6, in step 3, the established numerical simulation model of matrix-fractured fracture coupling in CO2 flooding reservoirs of low-permeability long-fractured well groups, the fracture length in the model is consistent with the fracture length monitored by microseismic monitoring in step 2, and the fracture orientation is consistent with the orientation of the maximum horizontal geostress.

8. The evaluation method for critical CO2 injection parameters of low-permeability long-fractured well groups according to claim 1, in step 4, initial CO2 injection parameters are set, and the matrix-fracture coupling model established in step 3 is used to numerically simulate and analyze the reservoir pressure changes of CO2 injection well groups under different parameters.

9. The evaluation method for critical CO2 injection parameters of low-permeability long-fractured well groups according to claim 8, in step 4, the reservoir pressure change is obtained by simultaneously solving the following equations of the low-permeability reservoir matrix seepage field and the seepage field in the hydraulic fracture, along with the corresponding boundary conditions and initial conditions: The flow field of the matrix in a low-permeability reservoir is described as follows: The flow within the hydraulic fracture is described as follows: in, φ represents the porosity of low-permeability reservoirs; C w ρ is the matrix compressibility coefficient; k is the matrix permeability; μ is the fluid viscosity; ρ is the fluid density; u is the matrix skeleton deformation in low-permeability reservoirs; f w For fluid source and sink terms; α is the Biot consolidation coefficient; t is time; z is vertical depth; v is the velocity of the CO2 and crude oil miscible fluid; C f k is the hydraulic fracture compressibility coefficient. f d represents the crack permeability. f The width of the crack; To differentiate along the tangential direction of the hydraulic fracture; Q f This represents the flow exchange between the low-permeability reservoir matrix and the hydraulic fracture surface; n is the normal direction of the fracture surface. Equations (1)–(4) are solved using numerical calculation methods such as the finite difference method and the finite element method.

10. The evaluation method for critical gas injection parameters of CO2 drive in low-permeability long-fractured well groups according to claim 1, in step 5, the change of reservoir pressure obtained in step 4 is used to conduct a CO2 drive disturbance stress field change analysis; wherein the change of CO2 drive disturbance stress field is obtained by solving the low-permeability reservoir matrix deformation equation and boundary conditions simultaneously, considering the effect of pore fluid pressure.

11. The evaluation method for critical CO2 injection parameters of low-permeability long-fractured well groups according to claim 10, in step 5, the matrix deformation of low-permeability reservoirs is described by the following formula: in, G is the shear modulus; ν is Poisson's ratio; α is the Biot coefficient; f i p represents physical strength; p represents reservoir pressure. The boundary conditions are as follows: Where, σ′ ij n represents the effective stress of the rock strata. j The cosine of the direction of the outer normal to the boundary; T i (t) represents the known displacement and stress boundary conditions on the solution boundary; δ ij Kronecker symbol; Equations (5) and (6) are solved numerically using the finite element method.

12. The evaluation method for critical gas injection parameters of CO2 flooding in low-permeability long-fractured well groups according to claim 1, in step 6, based on the dynamic change data of the geostress field of the well group obtained in step 5, and combined with the low-permeability reservoir rock fracture criteria, the CO2 flooding fracture propagation determination of the well group is carried out, and the risk of gas channeling and CO2 leakage is assessed; the low-permeability reservoir rock fracture criteria include two types: fracture tensile fracture and fracture shear fracture.

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