A method for judging activation tendency of hydraulic fracturing-induced faults considering rock damage
By combining rock damage models with fault stability analysis, a fault activation tendency state function M(D) is established, which solves the problem that existing technologies fail to consider rock damage effects, achieves accurate assessment of fault activation risk, and improves the safety and prediction accuracy of hydraulic fracturing.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- LIAONING UNIVERSITY
- Filing Date
- 2025-03-25
- Publication Date
- 2026-05-15
AI Technical Summary
Existing hydraulic fracturing identification methods fail to fully consider rock damage effects, making it difficult to accurately assess the risk of fault activation and posing a potential threat of geological disasters such as earthquakes.
By combining rock damage models and fault stability analysis, a fault activation tendency state function M(D) is established by simulating the rock damage evolution process and stress redistribution to assess the tendency and risk of fault activation.
It improves the safety of hydraulic fracturing operations, avoids earthquake accidents, provides more accurate fault activation prediction, and reduces environmental pollution and economic losses.
Smart Images
Figure CN119986848B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of petroleum engineering, and in particular to a method for determining the tendency of hydraulic fracturing to activate faults considering rock damage. Background Technology
[0002] With the continuous growth of global energy demand and the gradual depletion of traditional oil and gas resources, the development of unconventional oil and gas resources, such as shale gas, oil sands, and coalbed methane, has become an important energy strategy. Compared with conventional oil and gas reservoirs, unconventional reservoirs are characterized by low permeability and low porosity. Therefore, hydraulic fracturing technology is needed to improve reservoir permeability and oil and gas production during development. However, the injection of high-pressure fluids during hydraulic fracturing may trigger fault activation, leading to geological disasters such as earthquakes, posing threats to the environment and safety, and restricting the exploitation of unconventional oil and gas. Therefore, how to accurately determine whether there is a risk of inducing fault activation during hydraulic fracturing has become an urgent problem to be solved in the development of unconventional oil and gas reservoirs.
[0003] Previous methods typically determined whether hydraulic fracturing-induced fault activation exceeded the critical stress of the fault by calculating the changes in in-situ stress. While simple and easy to implement, this method neglects the significant impact of rock damage during fracturing on fault stability. Rocks undergo complex damage evolution processes during hydraulic fracturing, and the accumulation of damage significantly alters the mechanical properties of the rock mass, thereby affecting fault stability. Therefore, existing methods fail to adequately consider the rock damage effect, potentially leading to significant uncertainties in the results and making it difficult to accurately assess the risk of hydraulic fracturing-induced fault activation. More accurate methods are needed to ensure appropriate fracturing operations. Summary of the Invention
[0004] The purpose of this invention is to provide a method for determining the tendency of hydraulic fracturing to induce fault activation considering rock damage. By combining rock damage models with fault stability analysis, this method can more accurately predict whether a fault will be activated by simulating the damage evolution process and stress redistribution of rocks, thereby effectively improving the safety of hydraulic fracturing operations and avoiding possible earthquake accidents.
[0005] To achieve the above objectives, the technical solution adopted in this invention is: a method for determining the tendency of hydraulic fracturing to induce fault activation considering rock damage.
[0006] 1) Basic parameters were determined using triaxial compression experiments to determine the cohesion c and internal friction angle of the hydraulically fractured formation. The pore water pressure p0, well logging and seismic inversion data determine the first, second and third principal stresses σ1, σ2 and σ3 of the fault, the length L of the hydraulic fracturing fracture with layered flow in the segment closest to the fault, the fault dip angle β and strike.
[0007] Assuming the hanging wall and footwall of the fault are homogeneous elastic bodies, in contact through the fault plane, and subjected to the maximum principal stress in the vertical direction and the minimum principal stress in the horizontal direction, shear motion along the fault plane is considered fault activation. A three-dimensional fault structure diagram is established, and a two-dimensional geomechanical model for fault activation analysis is drawn. The model satisfies the following conditions: the normal direction of the fault plane is contained within the plane defined by the directions of the first principal stress σ1 and the third principal stress σ3; the direction of the second principal stress σ2 is contained within the fault plane; σ1>σ2>σ3; and it is perpendicular to the normal direction n of the fault plane. Therefore, σ2 will not affect the normal stress and shear stress of the fault. The fault dip angle is β. Based on the established model, the normal stress σ2 of the fault plane... n Shear stress τ n The relationship between the maximum and minimum principal stresses is as follows:
[0008]
[0009] In the formula: σ1 is the maximum principal stress in the vertical direction; β is the angle between the fault normal and the maximum principal stress, and when σ1 is in the vertical direction, β is the fault dip angle; σ3 is the minimum principal stress in the horizontal direction.
[0010] 2) Based on Mohr's Coulomb law, establish the fault activation tendency state function M(D) expressed by the damage variable D.
[0011] 2.1) Based on the specific theory of the Mohr-Coulomb failure criterion: under a certain stress state, if the maximum shear stress is greater than or equal to the shear strength of the rock, it can be determined that the fault may be activated. The relationship between the fault activation tendency and the normal and shear stresses of the fault plane and the frictional strength of the fault plane is established, as follows:
[0012] τ max ≥τ f =μ(σ n -αp)+C (3)
[0013] In equation (3), μ is the fault friction coefficient, p is the fluid permeation pressure in the damaged fault zone, C is the cohesion of the fault, and α is the Biot coefficient.
[0014] Calculate rock porosity based on average effective stress:
[0015]
[0016] In equation (4), Porosity under zero stress; The stress sensitivity coefficient for porosity can be taken as 5.0 × 10⁻⁸ Pa⁻¹. This represents the limiting value of porosity under high compressive stress, for rocks. The average effective stress can be calculated according to equation (5):
[0017]
[0018] In the formula: σ2 is the second principal stress; p0 is the pore water pressure when the formation is initially undamaged.
[0019] The effect of damage on permeability is shown in equation (6):
[0020]
[0021] In the formula: k0 is the permeability under zero stress, D is the damage variable, and α D This is the damage sensitivity coefficient. According to Darcy's law, the pressure difference of seepage water between undamaged and damaged rocks can be expressed by equations (7) and (8):
[0022]
[0023] p=p0+Δp-Δp0 (9)
[0024] In equation (8): Δp0 and Δp are the pressure difference of seepage water before and after rock damage, respectively; Q is the amount of fluid passing through the rock pores per unit time; μ0 is the fluid viscosity; and A is the cross-sectional area of the rock pores at the fault.
[0025] As the damage variable D increases, microcracks and defects inside the rock gradually expand, leading to a decrease in cohesion C. Cohesion C can be expressed as a function of the initial cohesion C0 and the damage variable D, as shown in equation (10):
[0026] C = C0(1-D) (10)
[0027] In the formula: C0 is the cohesive force in the initial undamaged state.
[0028] 2.2) Calculate the damage variable D
[0029] Calculate failure criteria F1 and F2 based on the maximum principal stress σ1 and the minimum principal stress σ3.
[0030] F1=σ1-f t (11)
[0031]
[0032] Determine the state of damage
[0033]
[0034] ε t0 ε is the initial tensile strain of the rock; ε1 is the current maximum principal strain; ε c0 ε is the initial compressive strain of the rock; ε3 is the current minimum principal strain; dF1 and dF2 represent the rates of change of F1 and F2, respectively;
[0035] If F1<0 and F2<0, the rock has not reached the tensile or shear failure condition and is in an undamaged state, and the damage variable D=0.
[0036] If F1 = 0 and dF1 > 0, the rock reaches the tensile failure condition, and the tensile stress is still increasing; the damage variable...
[0037] If F2 = 0 and dF2 > 0, the rock reaches the shear failure condition, and the compressive stress is still increasing, resulting in damage variables.
[0038]
[0039] 2.3) Substituting equations (1)(2)(7)(8)(9)(10) into equation (14), a fault activation tendency state function M(D) expressed by the damage variable D is constructed, as shown in equation (15):
[0040] M(D)=τ max -τ f (14)
[0041]
[0042]
[0043] Finally, the fault activation tendency state function M(D) is obtained.
[0044] 3) Establish a fault activation tendency discrimination criterion and assess the tendency and risk of fault activation based on the discriminant.
[0045] When M(D)<0, it means that the frictional resistance of the fault plane is stronger than the shear stress of the fault plane, and the fault has no tendency to activate.
[0046] When M(D)>0, it indicates that the frictional resistance of the fault plane is weaker than the shear stress of the fault plane, and the fault has a tendency to be activated.
[0047] When M(D) = 0, it means that the frictional resistance of the fault plane is equal to the shear stress of the fault plane, and the fault is in a critical activation state.
[0048] The beneficial effects of this invention are as follows:
[0049] 1. The method for determining fault activation in hydraulic fracturing that takes into account rock damage provided by this invention comprehensively considers the influence of rock damage on the stress field, which can more accurately assess the stability of the fault during hydraulic fracturing and provide guidance for accurate prediction of fault activation.
[0050] 2. The method for identifying hydraulic fracturing fault activation based on rock damage provided by this invention offers safer operational guidelines for hydraulic fracturing design, helping to avoid potential risks such as earthquake-induced geological disasters or fault rupture, and improving the safety of oil and gas extraction.
[0051] 3. The method for identifying hydraulic fracturing fault activation that takes into account rock damage provided by this invention can more accurately predict the risk of fault activation, take preventive measures in advance, and avoid seismic activity or rupture events caused by improper hydraulic fracturing, thereby reducing environmental pollution and economic losses. Attached Figure Description
[0052] Figure 1 : Flowchart of the method of this invention;
[0053] Figure 2 : Three-dimensional fault structure diagram;
[0054] Figure 3 Two-dimensional geomechanical model for fault activation analysis. Detailed Implementation
[0055] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments.
[0056] 1) Basic parameters were determined using triaxial compression experiments to determine the cohesion c and internal friction angle of the hydraulically fractured formation. The pore water pressure p0, well logging and seismic inversion data determine the first, second and third principal stresses σ1, σ2 and σ3 of the fault, the length L of the hydraulic fracturing fracture with layered flow in the segment closest to the fault, the fault dip angle β and strike.
[0057] Assuming the hanging wall and footwall of the fault are homogeneous elastic bodies, in contact through the fault plane, and subjected to the maximum principal stress in the vertical direction and the minimum principal stress in the horizontal direction, shear motion along the fault plane is considered fault activation. A three-dimensional fault structure diagram is established, and a two-dimensional geomechanical model for fault activation analysis is drawn. The model satisfies the following conditions: the normal direction of the fault plane is contained within the plane defined by the directions of the first principal stress σ1 and the third principal stress σ3; the direction of the second principal stress σ2 is contained within the fault plane; σ1>σ2>σ3; and it is perpendicular to the normal direction n of the fault plane. Therefore, σ2 will not affect the normal stress and shear stress of the fault. The fault dip angle is β. Based on the established model, the normal stress σ2 of the fault plane... n Shear stress τ nThe relationship between the maximum and minimum principal stresses is as follows:
[0058]
[0059] In the formula: σ1 is the maximum principal stress in the vertical direction; β is the angle between the fault normal and the maximum principal stress, and when σ1 is in the vertical direction, β is the fault dip angle; σ3 is the minimum principal stress in the horizontal direction.
[0060] 2) Based on Mohr's Coulomb law, establish the fault activation tendency state function M(D) expressed by the damage variable D.
[0061] 2.1) Based on the specific theory of the Mohr-Coulomb failure criterion: under a certain stress state, if the maximum shear stress is greater than or equal to the shear strength of the rock, it can be determined that the fault may be activated. The relationship between the fault activation tendency and the normal and shear stresses of the fault plane and the frictional strength of the fault plane is established, as follows:
[0062] τ max ≥τ f =μ(σ n -αp)+C (3)
[0063] In equation (3), μ is the fault friction coefficient, p is the fluid permeation pressure in the damaged fault zone, C is the cohesion of the fault, and α is the Biot coefficient.
[0064] Calculate rock porosity based on average effective stress:
[0065]
[0066] In equation (4), Porosity under zero stress; The stress sensitivity coefficient for porosity can be taken as 5.0 × 10⁻⁸ Pa⁻¹. This represents the limiting value of porosity under high compressive stress, for rocks. The average effective stress can be calculated according to equation (5):
[0067]
[0068] In the formula: σ2 is the second principal stress; p0 is the pore water pressure when the formation is initially undamaged.
[0069] The effect of damage on permeability is shown in equation (6):
[0070]
[0071] In the formula: k0 is the permeability under zero stress, D is the damage variable, and α DThis is the damage sensitivity coefficient. According to Darcy's law, the pressure difference of seepage water between undamaged and damaged rocks can be expressed by equations (7) and (8):
[0072]
[0073] p=p0+Δp-Δp0 (9)
[0074] In equation (8): Δp0 and Δp are the pressure difference of seepage water before and after rock damage, respectively; Q is the amount of fluid passing through the rock pores per unit time; μ0 is the fluid viscosity; and A is the cross-sectional area of the rock pores at the fault.
[0075] As the damage variable D increases, microcracks and defects inside the rock gradually expand, leading to a decrease in cohesion C. Cohesion C can be expressed as a function of the initial cohesion C0 and the damage variable D, as shown in equation (10):
[0076] C = C0(1-D) (10)
[0077] In the formula: C0 is the cohesive force in the initial undamaged state.
[0078] 2.2) Calculate the damage variable D
[0079] Calculate failure criteria F1 and F2 based on the maximum principal stress σ1 and the minimum principal stress σ3.
[0080] F1=σ1-f t (11)
[0081]
[0082] Determine the state of damage
[0083]
[0084] ε t0 ε is the initial tensile strain of the rock; ε1 is the current maximum principal strain; ε c0 ε is the initial compressive strain of the rock; ε3 is the current minimum principal strain; dF1 and dF2 represent the rates of change of F1 and F2, respectively;
[0085] If F1<0 and F2<0, the rock has not reached the tensile or shear failure condition and is in an undamaged state, and the damage variable D=0.
[0086] If F1 = 0 and dF1 > 0, the rock reaches the tensile failure condition, and the tensile stress is still increasing; the damage variable...
[0087] If F2 = 0 and dF2 > 0, the rock reaches the shear failure condition, and the compressive stress is still increasing, resulting in damage variables.
[0088]
[0089] 2.3) Substituting equations (1)(2)(7)(8)(9)(10) into equation (14), a fault activation tendency state function M(D) expressed by the damage variable D is constructed, as shown in equation (15):
[0090] M(D)=τ max -τ f (14)
[0091]
[0092] Finally, the fault activation tendency state function M(D) is obtained.
[0093] 3) Establish a fault activation tendency discrimination criterion and assess the tendency and risk of fault activation based on the discriminant.
[0094] When M(D)<0, it means that the frictional resistance of the fault plane is stronger than the shear stress of the fault plane, and the fault has no tendency to activate.
[0095] When M(D)>0, it indicates that the frictional resistance of the fault plane is weaker than the shear stress of the fault plane, and the fault has a tendency to be activated.
[0096] When M(D) = 0, it means that the frictional resistance of the fault plane is equal to the shear stress of the fault plane, and the fault is in a critical activation state.
Claims
1. A method for determining the tendency of hydraulic fracturing to induce fault activation considering rock damage, characterized in that: 1) Basic parameters were determined using triaxial compression experiments to determine the cohesion within the hydraulically fractured formation. internal friction angle pore water pressure Well logging and seismic inversion data were used to determine the first, second, and third principal stresses of the fault. The length L of the hydraulic fracturing fracture closest to the fault, exhibiting laminar flow, and the fault dip angle. and direction; 2) Establish a fault activation tendency state function expressed by the damage variable D based on Mohr's Coulomb law. ; 2.1) Based on the specific theory of the Mohr-Coulomb failure criterion: under a certain stress state, if the maximum shear stress is greater than or equal to the shear strength of the rock, it can be determined that the fault may be activated; and the relationship between the fault activation tendency and the normal and shear stresses of the fault plane and the frictional strength of the fault plane is established, as follows: (3) In equation (3) The fault friction coefficient is... The fluid permeation pressure in the damaged fault zone. The cohesion of the fault is α; α is the Biot coefficient. Calculate rock porosity based on average effective stress: (4) In equation (4), Porosity under zero stress; The stress sensitivity coefficient for porosity is 5.0 × 10⁻⁶. -8 Pa -1 ; This represents the limiting value of porosity under high compressive stress, for rocks. ; The average effective stress can be calculated according to equation (5): (5) In the formula: This is the second principal stress; This represents the pore water pressure when the formation is initially undamaged. The effect of damage on permeability is shown in equation (6): (6) In the formula: Permeability under zero stress state, As a damage variable, The damage sensitivity coefficient is given by equation (7) and (8). According to Darcy's law, the pressure difference between undamaged and damaged rock can be expressed by equations (7) and (8). (7) (8) (9) In equation (8): Q represents the amount of fluid passing through this rock pore per unit time. denoted as fluid viscosity; A is the cross-sectional area of the rock pores at the fault. With damage variables As cohesion increases, microcracks and defects inside the rock gradually expand, leading to decreased cohesion. Reduce, cohesion It can be represented as the initial cohesion. and damage variables The function is as shown in equation (10): (10) In the formula: It is the cohesive force in the initial undamaged state; 2.2) Calculate the damage variable D Based on the maximum principal stress and minimum principal stress Calculate failure criteria F1 and F2 (11) (12) Determine the state of damage (13) It is the initial tensile strain of the rock; This is the current maximum principal strain; It is the initial compressive strain of the rock; It is the current minimum principal strain; and They represent and The rate of change; if and The rock has not reached the tensile or shear failure condition and is in an undamaged state, with damage variable D=0; if and The rock has reached the tensile failure condition, and the tensile stress is still increasing; damage variables... ; if and The rock has reached the shear failure condition, and the compressive stress is still increasing, resulting in damage variables. ; 2.3） (1) (2) In the formula: The maximum principal stress in the vertical direction; The angle between the fault normal and the maximum principal stress is... When it is vertical The fault dip angle; This represents the minimum principal stress in the horizontal direction; Substituting equations (1)(2)(7)(8)(9)(10) into equation (14), Construct a fault activation tendency state function expressed by the damage variable D , specifically as formula (15): (14) (15) Finally, the fault activation tendency state function is obtained. ; 3) Establish a fault activation tendency discrimination criterion and assess the tendency and risk of fault activation based on the discriminant.
2. The method for determining the tendency of hydraulic fracturing to induce fault activation considering rock damage according to claim 1, characterized in that, In step 1), the specific method is as follows: Assuming the hanging wall and footwall of a fault are homogeneous elastic bodies, in contact through the fault plane, and subjected to the maximum principal stress in the vertical direction and the minimum principal stress in the horizontal direction, shear motion along the fault plane is considered fault activation. A three-dimensional fault structure diagram is established, and a two-dimensional geomechanical model for fault activation analysis is drawn. The model satisfies that the normal direction of the fault plane is included in the first principal stress. and the third principal stress Within the plane defined by the direction, the second principal stress The direction is contained within the fault plane. > > It is perpendicular to the normal direction n of the fault plane, therefore It will not affect the normal stress and shear stress of the fault, and the fault dip angle is... Based on the established model, the normal stress at the fault plane Shear stress The relationship between the maximum and minimum principal stresses is as follows: (1) (2) In the formula: The maximum principal stress in the vertical direction; The angle between the fault normal and the maximum principal stress is... When it is vertical The fault dip angle; This represents the minimum principal stress in the horizontal direction.
3. The method for determining the tendency of hydraulic fracturing to induce fault activation considering rock damage according to claim 1, characterized in that, The specific method in step 3) is as follows: when This indicates that the frictional resistance of the fault plane is stronger than the shear stress of the fault plane, and the fault has no tendency to be activated. when This indicates that the frictional resistance of the fault plane is weaker than the shear stress of the fault plane, and the fault has a tendency to be activated. when This indicates that the frictional resistance of the fault plane is equal to the shear stress of the fault plane, and the fault is in a critical activation state.