Method and system for evaluating stability of carbon dioxide flooding fault

Abstract

The invention provides a carbon dioxide flooding fault stability evaluation method and system. Establishing a fault stability dynamic analysis and evaluation model by adopting a fault surrounding three-dimensional stress model used for representing a fault surrounding stress state before CO2 injection, a fault surrounding four-dimensional dynamic stress model used for representing the influence of CO2 injection amount on reservoir stress characteristics and a Mohr-Coulomb fracture criterion; the CO2 injection amount is input into the fault stability dynamic analysis and evaluation model to obtain a fault apparent friction coefficient k1 and a fault friction coefficient k, and the fault stability is evaluated according to the size relation between the fault apparent friction coefficient k1 and the fault friction coefficient k; according to the method, the critical activation stress state of the fault after CO2 is injected into the stratum can be accurately simulated, the stability evaluation result is obtained, the accurate fault stability evaluation technology can provide technical support for the upper limit operation pressure of CCUS, CCS and a gas storage, and it is guaranteed that operation construction is safe and effective.

Description

Technical Field

[0001] The present invention belongs to the technical field of CCUS (carbon dioxide capture, utilization and storage), CCS (carbon dioxide capture and storage) and gas storage, and specifically belongs to a method and system for evaluating the stability of a carbon dioxide flooding fault. Background Art

[0002] Carbon capture, utilization and storage (CCUS) is one of the key technologies for addressing global climate change. It is highly valued by countries around the world, and they have increased their research and development efforts. Fault stability evaluation is a very important technical means for the construction of CCUS, CCS and gas storage. Fault stability refers to the risk of reactivation of pre-existing faults under stress field conditions. The reactivation of faults within the oil field is mainly due to the increase in fluid pressure in the fault zone.

[0003] CO2 flooding technology can improve the recovery rate of low-permeability reservoirs and achieve geological storage of CO2. In order to ensure that the injected CO2 can be safely buried underground for a long time, it is necessary to analyze and study the stability of oil and gas reservoir faults to ensure that the faults are not activated during the CO2 injection and oil recovery process. Fault stability is an important research part in CO2 flooding operations, which directly affects the CO2 flooding efficiency, the integrity of oil and gas reservoirs, and the safety of life and property of operators. In the actual CO2 flooding construction process, as the amount of injected gas continues to increase, "stress disturbance" caused by increased formation pressure will be generated near the fault of the oil and gas reservoir. If the amount of CO2 injected gas exceeds the "safe injection scale" that the formation can withstand, the fault will experience three stress states: "stable-critical activation-activated"; if the fault is reactivated, the injected CO2 will escape along the activated fault plane to the upper non-target layer or the surface, resulting in a decrease in oil recovery efficiency and affecting the ultimate recovery rate of the oil and gas reservoir; in severe cases, a large amount of oil and gas will leak to the surface or the ocean, causing damage to the ecological environment, and even oil and gas explosion safety accidents, resulting in equipment damage and casualties.

[0004] The commonly used fault stability evaluation methods mainly include the following two categories:

[0005] (i) The strength of fault sealing under hydrostatic conditions depends on the size of the capillary pressure difference between the fault and the reservoir. The fine-grained material in the mud can effectively reduce the porosity of the fault rock and increase the capillary force of the fault zone. Therefore, the greater the mud content, the stronger the fault sealing ability. Based on this, many scholars have proposed calculation methods for predicting the mud content of fault zones based on outcrop data and laboratory research. The capillary sealing pressure of the fault, that is, the pressure difference across the fault, increases with the increase of the mud content in the faulted strata. Therefore, the relationship between the mud content and the pressure difference across the fault is established to evaluate the fault sealing. The most widely used calculation method is the SGR (Shale Gouge Ratio), which derives the relationship between SGR and fault stability:

[0006]

[0007] Where: SGR is the proportion of fault mudstone, V sh is the mud content of the formation, ΔZ is the formation thickness, and D is the fault throw.

[0008] However, this method only considers the fault morphology and the lithological characteristics of the two sides of the fault, without analyzing the stress characteristics around the fault; and does not give a definite fault activation stress condition, and can only qualitatively analyze the mechanical stability characteristics of the fault, resulting in poor practicality of this method on site.

[0009] (ii) Some scholars have proposed fault slip trend analysis based on the analysis of the stress state around the fault. The slip trend analysis is controlled by the normal stress, and the normal stress on the fault depends on the stress field and the attitude of the fault plane. The ability of the crack to expand and transmit fluid is directly related to its pore size, and the pore size is a function of the effective normal stress it is subjected to. The normal stress on any plane in a known stress field can be calculated by the formula. The normal stress on the fault plane is normalized by the differential stress, which is the ratio of the difference between the maximum principal stress and the normal stress on the fault to the differential stress. Therefore, the expansion trend value is between 0-1. As the trend value increases, the risk of tensile reactivation of the fault increases. This parameter can qualitatively characterize the relative trend of tensile rupture of the fault.

[0010]

[0011] Where, T d is the sliding trend (dimensionless); σ1 and σ3 are the maximum and minimum effective principal stresses of the reservoir around the fault, σ n is the effective normal stress on the fault plane.

[0012] This technical approach only considers the initial characteristics of reservoir geostress and cannot characterize the temporal variation of reservoir geostress during the CO2 injection recovery process. It can only be based on assumptions (the reservoir stress state does not change), and the calculation results are too idealized. Summary of the invention

[0013] In order to solve the problems existing in the prior art, the present invention provides a method and system for evaluating the stability of a carbon dioxide flooding fault. Based on the study of the three-dimensional stress distribution characteristics around the fault and the critical stress conditions for fault instability, the influence of injected CO2 gas on reservoir geomechanics is studied, and the changing law of stress characteristics around the fault under different CO2 injection conditions is simulated, thereby establishing three mechanical states (unactivated, critically activated and activated) of the fault plane changing with dynamic ground stress. The research results can accurately simulate the critical activation stress state of the fault after CO2 is injected into the formation; the precise fault stability assessment technology can provide technical support for CCUS, CCS and the upper limit operating pressure of the gas storage reservoir, and ensure safe and effective operation and construction.

[0014] To achieve the above object, the present invention provides the following technical solution: a method for evaluating the stability of a carbon dioxide flooding fault, the specific steps of which are as follows:

[0015] S1 uses a three-dimensional stress model around the fault to characterize the stress state around the fault before CO2 injection, a four-dimensional dynamic stress model around the fault to characterize the effect of CO2 injection on reservoir stress characteristics, and the Mohr-Coulomb rupture criterion to establish a dynamic analysis and evaluation model for fault stability;

[0016] S2 inputs the CO2 injection volume into the dynamic analysis and evaluation model of fault stability to obtain the fault apparent friction coefficient k1 and the fault friction coefficient k. When the fault friction coefficient k> the fault apparent friction coefficient k1, it is assessed that the fault is in a stable state.

[0017] Furthermore, in S1, based on the geological data of the study area, a three-dimensional stress model around the fault was established using the constant strain coefficient method to characterize the stress state around the fault and the formation scale before CO2 injection.

[0018] Furthermore, the geological data include static Young's modulus, static Poisson's ratio, Biot's coefficient, tectonic stress coefficient and original formation pressure.

[0019] Furthermore, in S1, the initial stress conditions are obtained based on the three-dimensional stress model around the fault. The initial stress conditions are combined with the increment of formation pressure caused by different CO2 injection amounts to obtain a four-dimensional dynamic stress model around the fault that characterizes the change law of stress characteristics around the fault under different CO2 injection conditions over time.

[0020] Furthermore, in S1, the four-dimensional dynamic stress model around the fault is combined with the Mohr-Coulomb rupture criterion to obtain a dynamic analysis and evaluation model of fault stability, specifically:

[0021]

[0022]

[0023] A=σ1+σ3-2p i (3)

[0024]

[0025] Where k is the fault friction coefficient, is the internal friction angle, k1 is the apparent friction coefficient of the fault, A, B and c are all mechanical parameters, σ1 and σ3 are the dynamic minimum and maximum horizontal stresses, P i is the formation pressure, and the tan value of the internal friction angle is the internal friction coefficient, which represents the slope of the Mohr circle.

[0026] Furthermore, the stress polygon rule is used to calculate the dynamic minimum and maximum horizontal stresses, as follows:

[0027]

[0028]

[0029] Among them, S v is the overburden pressure, E is the static Young's modulus, μ is the static Poisson's ratio, α is the Biot coefficient, ε h and ε H is the tectonic stress coefficient.

[0030] Furthermore, in S2,

[0031] When the fault friction coefficient k = fault apparent friction coefficient k1, the fault is in a critical stress state;

[0032] When the fault friction coefficient k<fault apparent friction coefficient k1, the fault is in an activated state.

[0033] The present invention also provides a carbon dioxide flooding fault stability evaluation system, comprising:

[0034] A model building module is used to establish a dynamic analysis and evaluation model of fault stability using a three-dimensional stress model around the fault for characterizing the stress state around the fault before CO2 injection, a four-dimensional dynamic stress model around the fault for characterizing the effect of CO2 injection volume on reservoir stress characteristics, and the Mohr-Coulomb rupture criterion;

[0035] The stability evaluation module is used to input the CO2 injection volume into the fault stability dynamic analysis evaluation model to obtain the fault apparent friction coefficient k1 and the fault friction coefficient k. When the fault friction coefficient k> the fault apparent friction coefficient k1, the fault is evaluated to be in a stable state.

[0036] Furthermore, the three-dimensional stress model around the fault is established using the constant strain coefficient method based on the geological data of the study area; the four-dimensional dynamic stress model around the fault is established based on the initial stress conditions obtained from the three-dimensional stress model around the fault and the increment of formation pressure brought about by different CO2 injection amounts.

[0037] Furthermore, in the stability evaluation module:

[0038] When the fault friction coefficient k = fault apparent friction coefficient k1, the fault is in a critical stress state;

[0039] When the fault friction coefficient k is less than the fault apparent friction coefficient k1, the fault is in an activated state.

[0040] Compared with the prior art, the present invention has at least the following beneficial effects:

[0041] The present invention provides a method for evaluating the stability of a carbon dioxide flooding fault, which combines the initial mechanical state of the fault with the change in reservoir stress characteristics simulated by the CO2 injection amount, realizes a four-dimensional dynamic simulation of geomechanics, and proposes a dynamic analysis and evaluation model for fault stability for evaluating the stress state of the fault plane during the CO2 injection operation in combination with the Mohr-Coulomb fracture criterion. The model can accurately simulate the dynamic change characteristics of the stress of the fault plane as the amount of injected CO2 increases, evaluate the fault stability under different CO2 injection conditions, provide a technical reference for optimizing the CO2 safe injection scheme, provide a feasible scheme for CO2 safe injection, and improve the fault stability. Field application results show that the present invention has a good application effect in CO2 flooding operations, reduces construction risks, and ensures operation safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is a flow chart of the method of the present invention;

[0043] Figure 2 This is the single well geomechanical model of the LN reservoir;

[0044] Figure 3 Numerical simulation of fluid distribution characteristics for CO2 flooding;

[0045] Figure 4 The results of fault stability assessment. DETAILED DESCRIPTION

[0046] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0047] As the CO2 flooding operation continues, the amount of CO2 injected into the formation continues to increase, causing the formation pressure to continue to rise, thereby causing the reservoir ground stress to change with the change of the CO2 injection amount. The present invention provides a carbon dioxide flooding fault stability evaluation method, which is based on the stress model around the fault, comprehensively considers the change law of the static (initial) stress characteristics of the fault surface with the CO2 injection amount, and forms the core calculation of the dynamic prediction of the fault stress state. Therefore, it realizes the accurate evaluation of the fault stability under the condition of formation pressurization caused by CO2 injection.

[0048] like Figure 1 As shown, the present invention provides a method for evaluating the stability of a carbon dioxide flooding fault, and the specific steps are as follows:

[0049] S1. Based on the geological engineering and other relevant data of the study area, such as full-wave logging, 3D seismic data interpretation results, rock mechanics testing, formation pressure testing and other information, the three-dimensional stress model around the fault is established using the constant strain coefficient method using parameters such as static Young's modulus, static Poisson's ratio, Biot coefficient, tectonic stress coefficient and original formation pressure. The model includes the maximum and minimum horizontal principal stresses. The model characterizes the original (initial) stress characteristics and formation scale before CO2 injection.

[0050] S2, analyze the stress state around the fault based on the three-dimensional stress model around the fault, and consider the influence of CO2 injection volume on reservoir stress characteristics to obtain a four-dimensional dynamic stress model around the fault;

[0051] Specifically, according to the initial stress conditions, combined with the CO2 injection volume, the increment of formation pressure is simulated, so as to calculate the change law of stress characteristics around the fault with time under different CO2 injection scale conditions, and couple the stress changes on the basis of three-dimensional static geomechanics to obtain a four-dimensional dynamic stress model around the fault;

[0052] Based on the analysis of the stress state around the fault and considering the influence of CO2 injection volume on the reservoir stress characteristics, a dynamic analysis and evaluation model of fault stability was established based on the Mohr-Coulomb failure criterion.

[0053] S3, the four-dimensional dynamic stress model around the fault is combined with the Mohr-Coulomb rupture criterion to establish a dynamic analysis and evaluation model of fault stability in which the apparent friction coefficient of the fault changes dynamically with time. This evaluation model can simulate the apparent friction coefficient of the fault (k1) under different CO2 injection conditions. This coefficient is a variable and is related to the stress change caused by CO2 injection, so as to calculate the numerical change law of the apparent friction coefficient of the fault, specifically:

[0054]

[0055]

[0056] A = σ1 + σ3 - 2p i (3)

[0057]

[0058]

[0059]

[0060]

[0061] In the above formula, k is the fault friction coefficient, is the internal friction angle, k1 is the apparent fault friction coefficient, A, B, and c are all mechanical parameters, σ1 and σ3 are the dynamic minimum and maximum horizontal stresses, S v is the overburden pressure, E is the static Young's modulus, μ is the static Poisson's ratio, α is the Biot coefficient, ε h and ε H are the tectonic stress coefficients, P i is the formation pressure, the tangent value of the internal friction angle is the internal friction coefficient, which represents the slope of the Mohr circle. Δp is the increase in formation pressure caused by the injected gas, P sc and T sc represent the standard atmospheric pressure and the temperature under standard conditions respectively, G in represents the gas volume under standard atmospheric pressure, G d represents the gas volume under actual ground conditions, T d represents the actual ground temperature.

[0062] And this model can evaluate the stress state characteristics of the fault according to the numerical relationship between the apparent fault friction coefficient (k1) and the fault friction coefficient (k), so as to obtain three stress characteristics (stable, critical stress, and activated) of the fault.至此,完成整个CO2注入引起的应力增量条件下的断层稳定性评估,具体为:

[0063] When k > k1, the fault is in a stable state and not activated.

[0064] When k = k1, the fault is in a critical stress state. At this time, if a little additional shear stress is applied on the fault plane, the fault will be activated.

[0065] When k < k1, the fault has been activated, and there is a greater risk of operation construction, and CO2 injection should be stopped.

[0066] According to the three different stress characteristics, risk warnings and operation parameter optimization can be provided for oilfield operators, providing technical guidance for the safe construction plan.

[0067] Example

[0068] The present invention is applied to the fault stability analysis during the CO2 flooding operation in a certain block. The LN oil reservoir in this area is a high-water-content and high-permeability oil reservoir. A three-dimensional gas injection development mode of "vertical-horizontal well combination, area-gravity drive synergy" and "top injection and bottom support" bidirectional displacement development mode test is constructed. A CO2 miscible flooding pilot test is carried out jointly by vertical wells and horizontal wells. Conventional technical means cannot take into account the changing law of the fault stress state with the injection of CO2, and cannot accurately calculate the "CO2 safe injection scale" in the area, resulting in greater safety risks in the operation and construction.

[0069] In this paper, the dynamic stress characteristics of faults during the CO2 miscible flooding operation in the LN reservoir are analyzed. First, different geological and logging data, seismic data, and rock mechanics test data in the area are collected. Mechanical parameter tests, wave velocity anisotropy experiments, geomagnetic orientation experiments, and rock triaxial mechanical tests are carried out in the rock mechanics laboratory to obtain rock mechanics parameters, stress orientation, stress magnitude, stress gradient, and reservoir mechanics parameters. On the basis of laboratory analysis, one-dimensional geomechanical analysis is carried out in combination with logging data, lithology distribution characteristics, pore pressure, and fluid properties (see Figure 2 ), through the borehole instability features observed in the imaging logging data, such as borehole collapse, drilling-induced tension fractures and stress release areas, the maximum horizontal principal stress was back-calculated using the stress polygon rule, and the stress value obtained was used as the correction point to verify the single well stress model. The maximum horizontal principal stress results calculated by the stress polygon in this area are shown in Table 1.

[0070] Table 1 Calculation table of initial geostress of LN group reservoir

[0071]

[0072]

[0073] Based on the single well geomechanical modeling, the regional geomechanical characteristics are studied in combination with the structural plane model, fault spatial distribution characteristics and seismic data, which can better reflect the distribution characteristics of reservoir geomechanics in three-dimensional space.

[0074] A three-dimensional geomechanical related model is established by combining the single well stress model and three-dimensional seismic data with the anisotropy of the reservoir space. On this basis, the regional geomechanical characteristics are studied by combining the structural level model, the spatial distribution characteristics of the fault and the seismic data, which can better reflect the distribution characteristics of reservoir geomechanics in three-dimensional space. The research results show that the minimum horizontal principal stress distribution range of the LN reservoir is 36.4-50.7MPa, with an average value of 42.5MPa; the maximum horizontal principal stress distribution range is 48.1-56.6MPa, with an average value of 52.5MPa; the horizontal stress difference distribution range is 9.3-11.6MPa, with an average value of 10.1MPa.

[0075] Combined with the initial geomechanical model simulation, the gas expansion law and reservoir stress characteristics under different CO2 injection conditions are simulated, and a four-dimensional stress model is established to characterize the change law of stress characteristics around the fault with time under different CO2 injection scales to provide data support for the dynamic evaluation of fault stability; at the same time, combined with the injected CO2 injection rate, the distribution characteristics of the LN reservoir formation fluid are simulated (see Figure 3 ).

[0076] like Figure 4 As shown in the figure, based on the dynamic analysis and evaluation model of fault stability, the effective stress in the middle of the LN reservoir structure is lower. When the in-situ stress increment after CO2 injection exceeds the original formation pressure of about 9MPa, the Mohr circle just touches the damage envelope of the fault friction coefficient k=0.4, but it is still far below the damage envelope of the fault friction coefficient k=0.6. At this time, the apparent friction coefficient of the fault k1=0.4, indicating that the fault is in a critical stress state.

[0077] The effective stress in the northern part of the study area is higher due to the deeper structure. When the stress increment after CO2 injection exceeds the original formation pressure by about 12MPa, the Mohr circle is still lower than the damage envelope of the fault friction coefficient k=0.6, and is only close to the damage envelope of the fault friction coefficient k=0.4. At this time, the apparent friction coefficient k1 of the fault is less than 0.4, indicating that the fault is stable.

[0078] The effective stress in the south is relatively low, but higher than that in the middle. When the in-situ stress increment after CO2 injection exceeds the original formation pressure by about 12 MPa, the Mohr circle is lower than the damage envelope of the fault friction coefficient k = 0.6 and the fault friction coefficient k = 0.4, and there is no risk of fault activation. At this time, the apparent friction coefficient of the fault k1 < 0.4, indicating that the fault is stable.

[0079] In summary: when the injection pressure does not exceed 9MPa in the original formation, the fault is stable and the risk of being activated is low. The research results accurately simulate the dynamic stress change characteristics of the fault surface and effectively guide the construction safety of CO2 flooding operations in LN reservoirs.

Claims

1. A method for evaluating the stability of a carbon dioxide flooding fault, characterized in that: The specific steps are as follows: S1 uses a three-dimensional stress model around the fault to characterize the stress state around the fault before CO2 injection, a four-dimensional dynamic stress model around the fault to characterize the effect of CO2 injection on reservoir stress characteristics, and the Mohr-Coulomb rupture criterion to establish a dynamic analysis and evaluation model for fault stability; S2 inputs the CO2 injection volume into the dynamic analysis and evaluation model of fault stability to obtain the fault apparent friction coefficient k1 and the fault friction coefficient k, and evaluates the fault stability according to the relationship between the fault apparent friction coefficient k1 and the fault friction coefficient k.

2. A method for evaluating fault stability in carbon dioxide flooding according to claim 1, characterized in that: In S1, based on the geological data of the study area, a three-dimensional stress model around the fault was established using the constant strain coefficient method to characterize the stress state around the fault and the scale of the formation before CO2 injection.

3. A method for evaluating fault stability in carbon dioxide flooding according to claim 2, characterized in that: The geological data include static Young's modulus, static Poisson's ratio, Biot's coefficient, tectonic stress coefficient and original formation pressure.

4. A method for evaluating fault stability during carbon dioxide flooding according to claim 1, characterized in that: In S1, the initial stress conditions are obtained based on the three-dimensional stress model around the fault. The initial stress conditions are combined with the increment of formation pressure caused by different CO2 injection amounts to obtain a four-dimensional dynamic stress model around the fault that characterizes the change law of stress characteristics around the fault with time under different CO2 injection conditions.

5. A method for evaluating fault stability during carbon dioxide flooding according to claim 1, characterized in that: In S1, the four-dimensional dynamic stress model around the fault is combined with the Mohr-Coulomb rupture criterion to obtain the dynamic analysis and evaluation model of fault stability, which is as follows: A=σ1+σ3-2p i (3) Where k is the fault friction coefficient, is the internal friction angle, k1 is the apparent friction coefficient of the fault, A, B and c are all mechanical parameters, σ1 and σ3 are the dynamic minimum and maximum horizontal stresses, P i is the formation pressure, and the tan value of the internal friction angle is the internal friction coefficient, which represents the slope of the Mohr circle.

6. A method for evaluating fault stability during carbon dioxide flooding according to claim 5, characterized in that: The dynamic minimum and maximum horizontal stresses are calculated using the stress polygon rule as follows: Among them, S v is the overburden pressure, E is the static Young's modulus, μ is the static Poisson's ratio, α is the Biot coefficient, ε h and ε H is the tectonic stress coefficient.

7. A method for evaluating fault stability during carbon dioxide flooding according to claim 1, characterized in that: In S2, When the fault friction coefficient k>fault apparent friction coefficient k1, the fault is in a stable state; When the fault friction coefficient k = fault apparent friction coefficient k1, the fault is in a critical stress state; When the fault friction coefficient k is less than the fault apparent friction coefficient k1, the fault is in an activated state.

8. A carbon dioxide flooding fault stability evaluation system, characterized in that: include A model building module is used to establish a dynamic analysis and evaluation model of fault stability using a three-dimensional stress model around the fault for characterizing the stress state around the fault before CO2 injection, a four-dimensional dynamic stress model around the fault for characterizing the effect of CO2 injection volume on reservoir stress characteristics, and the Mohr-Coulomb rupture criterion; The stability evaluation module is used to input the CO2 injection volume into the fault stability dynamic analysis evaluation model to obtain the fault apparent friction coefficient k1 and the fault friction coefficient k, and evaluate the fault stability according to the relationship between the fault apparent friction coefficient k1 and the fault friction coefficient k.

9. A carbon dioxide flooding fault stability evaluation system according to claim 8, characterized in that: The three-dimensional stress model around the fault is established based on the geological data of the study area using the constant strain coefficient method; the four-dimensional dynamic stress model around the fault is established based on the initial stress conditions obtained from the three-dimensional stress model around the fault and the increment of formation pressure brought about by different CO2 injection amounts.

10. A carbon dioxide flooding fault stability evaluation system according to claim 8, characterized in that: In the stability evaluation module: When the fault friction coefficient k>fault apparent friction coefficient k1, the fault is in a stable state; When the fault friction coefficient k = fault apparent friction coefficient k1, the fault is in a critical stress state; When the fault friction coefficient k is less than the fault apparent friction coefficient k1, the fault is in an activated state.

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