Method for judging intersecting extension behavior of hydraulic fracture and arbitrary natural fracture in space

Abstract

The invention discloses a method for discriminating intersecting extension behaviors of a hydraulic fracture and any natural fracture in space. The method comprises the following steps: collecting occurrence parameters of the natural fracture, in-situ stress field parameters and rock strength parameters; establishing a plane Omega rectangular coordinate system perpendicular to the hydraulic fracture and the natural fracture, and calculating a corresponding inclination angle, a trend angle and an intersection angle of the natural fracture in the Omega plane; the principal stress of the in-situ principal stress coordinate system is converted into a coordinate system on an omega plane, and the far-field stress on the plane is calculated; calculating a stress coefficient of the hydraulic fracture penetrating through the natural fracture; calculating a critical friction coefficient; the hydraulic fracture extension behavior is judged by comparing the relative size relation of the material friction coefficient and the critical friction coefficient; and drawing a critical friction coefficient chart of the expansion behavior after the hydraulic fracture intersects with the natural fracture. According to the method, the defect that only the intersection effect judgment method of the hydraulic fracture and the natural fracture on the two-dimensional plane is considered at present is overcome, and the gap of fracture extension behavior after the hydraulic fracture meets any natural fracture in the space is filled.

Description

Technical Field

[0001] This invention relates to a method for determining the intersection and extension behavior of hydraulic fractures and arbitrary natural fractures in space, belonging to the field of unconventional reservoir volumetric fracturing for enhanced production. Background Technology

[0002] Unconventional shale oil and gas reservoirs have extremely low matrix permeability and well-developed natural fractures that may have different dip angles and orientations. The encounter between hydraulically driven fractures and these natural fractures can lead to complex extension behaviors, determining the reservoir stimulation volume (SRV) and the complexity of the fracture network, thus attracting significant attention from the industry. Hydraulic fracturing physics experiments, using artificially created natural fracture rock samples to simulate the extension behavior of hydraulic fractures (HF) and natural fractures (NF) in large-scale specimens, revealed three possible modes: HF passing through NF, HF opening NF and extending along the NF interface, and HF being blocked by NF while NF undergoes shear slip. Preliminary delineation of the respective occurrence regions influenced by the NF intersection angle and biaxial stress difference was also conducted. Renshaw et al. established a criterion for determining whether HF passes through or along the natural fracture interface when encountering orthogonal NFs on a plane, using tensile and shear failure criteria. Gu et al. extended this theory to the range of arbitrary intersection angles in two-dimensional non-orthogonal systems, established a corresponding discrimination model for hydraulic fracture propagation behavior, and presented a graph showing the relationship between the intersection angle, biaxial stress difference, and critical friction coefficient when tensile strength and cohesion are zero. Wang Tao et al., based on the mechanical equilibrium and fracture mechanics theories at the HF-NF interface, proposed an explicit criterion for judging the intersection propagation behavior of HF-NF interfaces with given parameters. However, their criterion is based on the premise that the hydraulic fracture intersects with an angle of... The physical conditions of intersecting planar natural cracks are considered, but they cannot accurately reflect the influence of arbitrary spatial natural cracks (3DNF) with arbitrary dip angles and orientations in actual mines. Summary of the Invention

[0003] The present invention mainly overcomes the shortcomings of the prior art by proposing a method for judging the intersection and extension behavior of hydraulic cracks and arbitrary natural cracks in space. This method overcomes the defects of the current method that only considers the intersection of hydraulic cracks and natural cracks on a two-dimensional plane and fills the gap in the crack extension behavior after hydraulic cracks and arbitrary natural cracks in space meet.

[0004] The technical solution provided by this invention to solve the above-mentioned technical problems is: a method for determining the intersection and extension behavior of hydraulic fractures and arbitrary natural spatial fractures, comprising the following steps: Step S10: Collect natural fracture orientation parameters, in-situ stress field parameters, and rock strength parameters; Step S20: Establish a plane Ω rectangular coordinate system perpendicular to the hydraulic fracture and the natural fracture, and calculate the corresponding dip angle, strike angle and intersection angle of the natural fracture in the Ω plane; Step S30, converting the principal stress of the in-situ stress coordinate system to the coordinate system on the Ω plane, calculating the far-field stress on the plane; Step S40, calculating the stress coefficient of the hydraulic fracture crossing the natural fracture; Step S50, calculating the critical friction coefficient of the hydraulic fracture crossing the natural fracture interface without the in-situ stress and the natural fracture occurrence; Step S60, comparing the relative size relationship between the material friction coefficient and the critical friction coefficient to determine the hydraulic fracture extension behavior; Step S70, drawing a critical friction coefficient chart of the hydraulic fracture extension behavior after intersecting the natural fracture.

[0005] Further technical solutions are that the natural fracture occurrence parameters include the natural fracture geographic orientation and the inclination angle; the in-situ stress field parameters include the in-situ horizontal maximum principal stress geographic orientation and the in-situ stress field value; and the rock strength parameters include the cohesion and the internal friction coefficient.

[0006] Further technical solutions are that the specific process of step S20 is: Step S21, converting the natural fracture occurrence in the geographic coordinate system to the relative strike angle of the natural fracture in the principal stress coordinate system according to the geometric relationship; Step S22, establishing a plane Ω perpendicular to the hydraulic fracture and the natural fracture, and calculating the occurrence parameters of the natural fracture on the Ω plane; Step S23, calculating the intersection angle of the natural fracture on the Ω plane according to the spatial geometry.

[0007] Further technical solutions are that the calculation formula in step S30 is:

[0008] In the formula, σx and σy are the in-situ horizontal maximum and minimum principal stresses in the principal stress coordinate system, respectively, MPa; σz is the vertical principal stress, MPa; σxΩ and σyΩ are the two principal stresses of the natural fracture on the Ω plane, respectively, MPa; and θΩ is the relative strike angle of the natural fracture face on the principal stress coordinate system of the Ω plane. 、 、 , 、 , ,

[0009] Further technical solutions are that the calculation formula in step S40 is:

[0010] In the formula, σx and σy are the in-situ horizontal maximum and minimum principal stresses in the principal stress coordinate system, respectively, MPa; σz is the vertical principal stress, MPa; σxΩ and σyΩ are the two principal stresses of the natural fracture on the Ω plane, respectively, MPa; and θΩ is the relative strike angle of the natural fracture face on the principal stress coordinate system of the Ω plane. is the stress coefficient of the hydraulic fracture crossing the natural fracture; 、 , All are intermediate coefficients.

[0011] A further technical solution is that the specific process of step S50 is as follows: Step S51: Calculate the combined normal stress of the Ω plane acting on the natural crack surface; Step S52: Calculate the combined shear stress of the Ω plane acting on the natural crack surface; Step S53: Calculate the critical friction coefficient of the known natural crack direction angle and dip angle based on the combined normal stress and combined shear stress acting on the natural crack surface by the Ω plane.

[0012] A further technical solution is that the calculation formula in step S51 is:

[0013] In the formula: The normal stress acting on the Ω plane is expressed in MPa. , are the two principal stresses of the natural crack on the Ω plane, respectively, in MPa; The stress coefficient for hydraulic fractures penetrating natural fractures; The angle of intersection of the natural cracks on the Ω plane is denoted as .

[0014] A further technical solution is that the calculation formula in step S52 is:

[0015] In the formula: The shear stress acting on the normal direction of the Ω plane is expressed in MPa. , are the two principal stresses of the natural crack on the Ω plane, respectively, in MPa; The stress coefficient for hydraulic fractures penetrating natural fractures; The angle of intersection of the natural cracks on the Ω plane is denoted as .

[0016] A further technical solution is that the calculation formula in step S53 is:

[0017] In the formula: The shear stress acting on the normal direction of the Ω plane is expressed in MPa. The normal stress acting on the Ω plane is expressed in MPa. The critical internal friction coefficient for hydraulic fracture penetration / slippage is dimensionless; The cohesive force is expressed in MPa.

[0018] A further technical solution is that the discrimination criterion in step S60 is:

[0019] wherein: is the critical internal friction coefficient of the hydraulic fracture crossing / slipping, dimensionless; is the internal friction coefficient, dimensionless.

[0020] The present application can comprehensively reflect the fracture extension behavior after the hydraulic fracture induced by water pressure intersects with the natural fracture with an arbitrary dip angle and strike, fills the blank of the judgment of the hydraulic fracture extension behavior in the spatial three-dimensional natural fracture rock mass medium, and improves the accuracy of the judgment of the hydraulic fracture extension behavior after the hydraulic fracture intersects with the natural fracture. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is the geometric relationship diagram of the hydraulic fracture and the natural fracture in the in-situ stress coordinate system; Figure 2 is the Ω plane and normal vector diagram perpendicular to the hydraulic fracture and the natural fracture at the same time; Figure 3 is the geometric relationship diagram of the hydraulic fracture and the natural fracture in the Ω plane; Figure 4 is the critical friction coefficient judgment curve under different dip angles and strike angles. DETAILED DESCRIPTION

[0022] The technical solutions of the present application will be described clearly and completely below in combination with the drawings. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by the person of ordinary skill in the art without creative labor belong to the protection scope of the present application.

[0023] The present application provides a method for judging the intersection and extension behavior of the hydraulic fracture and the spatial arbitrary natural fracture, comprising the following steps: Step S10, collecting the natural fracture occurrence parameters, in-situ stress field parameters and rock strength parameters; The natural fracture occurrence parameters include the natural fracture geographic orientation and the natural fracture dip angle ; wherein the natural fracture geographic orientation and the natural fracture dip angle are determined by the geophysical method, such as the imaging logging, the seismic multi-stage fracture body interpretation, etc. The in-situ stress field parameters include the in-situ horizontal maximum principal stress geographic orientation, the in-situ stress field values (including the in-situ horizontal maximum principal stress , the minimum principal stress , the vertical principal stress ); wherein the in-situ horizontal maximum principal stress geographic orientation is obtained by core experiment testing or microseismic monitoring method; and the in-situ stress field value is obtained by well logging data interpretation, core experiment testing method, etc. The rock strength parameters include cohesion and internal friction coefficient ; wherein the cohesion and internal friction coefficient are obtained by core experiment; Step S20, establishing a Cartesian coordinate system perpendicular to the plane of the hydraulic fracture and the natural fracture, calculating the corresponding dip angle and strike angle of the natural fracture in the plane Omega, and the intersection angle; Step S21, according to the geometric relationship, converting the natural fracture occurrence in the geographic coordinate system to the relative strike angle of the natural fracture in the principal stress coordinate system;

[0024] In the formula: is the relative strike angle of the natural fracture; Step S22, establishing a plane Omega perpendicular to the hydraulic fracture and the natural fracture, and calculating the occurrence parameters of the natural fracture in the plane Omega according to the relative strike angle of the natural fracture; The occurrence parameters of the natural fracture in the plane Omega include the relative strike angle and dip angle of the natural fracture surface in the principal stress coordinate system of the plane Omega;

[0025] In the formula: β is the relative strike angle of the natural fracture; , respectively, the relative strike angle and dip angle of the natural fracture surface in the principal stress coordinate system of the plane Omega; Step S23, calculating the intersection angle of the natural fracture on the plane Omega according to spatial geometry;

[0026] In the formula: θ is the intersection angle of the hydraulic fracture and the natural fracture on the plane Omega, radian; is the dip angle of the natural fracture.

[0027] Step S30, converting the principal stress of the in-situ principal stress coordinate system to the coordinate system on the plane Omega, and calculating the far-field stress on the plane;

[0028] In the formula: , , respectively, the in-situ horizontal maximum and minimum principal stress, and the vertical principal stress in the principal stress coordinate system, MPa; , respectively, are two principal stresses of the natural fracture on the Ω plane, MPa; is the relative strike angle of the natural fracture surface under the principal stress coordinate system of the Ω plane.

[0029] Step S40, calculate the stress coefficient of the hydraulic fracture crossing the natural fracture; Step S41, calculate the constant determined by the material strength and the far-field stress;

[0030] Step S42, calculate the intermediate coefficient according to the constant determined by the material strength and the far-field stress;

[0031]

[0032]

[0033] Step S43, the stress coefficient of the hydraulic fracture crossing the natural fracture

[0034] In the formula: is the stress coefficient of the hydraulic fracture crossing the natural fracture; , , are all intermediate coefficients; Step S50, calculate the critical friction coefficient of the hydraulic fracture crossing the natural fracture interface without the in-situ stress and the occurrence of the natural fracture; Step S51, calculate the resultant normal stress of the Ω plane acting on the natural fracture surface;

[0035] In the formula: is the normal stress acting on the normal of the Ω plane, MPa; , respectively, are two principal stresses of the natural fracture on the Ω plane, MPa; is the stress coefficient of the hydraulic fracture crossing the natural fracture; is the intersection angle of the natural fracture on the Ω plane.

[0036] Step S52, calculate the resultant shear stress of the Ω plane acting on the natural fracture surface;

[0037] In the formula: is the shear stress acting on the normal of the Ω plane, MPa; , respectively, are two principal stresses of the natural fracture on the Ω plane, MPa; The stress coefficient for hydraulic fractures penetrating natural fractures; The angle of intersection of the natural cracks on the Ω plane is denoted as .

[0038] Step S53: Calculate the critical friction coefficient of the known natural crack direction angle and dip angle based on the combined normal stress and combined shear stress acting on the natural crack surface by the Ω plane.

[0039]

[0040] In the formula: The shear stress acting on the normal direction of the Ω plane is expressed in MPa. The normal stress acting on the Ω plane is expressed in MPa. The critical internal friction coefficient for hydraulic fracture penetration / slippage is dimensionless; The cohesive force is expressed in MPa.

[0041] Step S60: Compare the relative magnitudes of the material friction coefficient and the critical friction coefficient to determine the propagation behavior of hydraulic cracks;

[0042] In the formula: The critical internal friction coefficient for hydraulic fracture penetration / slippage is dimensionless; is the internal friction coefficient, which is dimensionless.

[0043] Step S70: Plot the critical friction coefficient chart for the propagation behavior of hydraulic fractures after they intersect with natural fractures; By successively changing the in-situ stress, the dip angle and orientation of the natural fracture, the critical friction coefficient is calculated using the method described above. Based on these calculation results, a discrimination line is plotted. The left side of each curve represents the non-crossing zone of the hydraulic fracture (i.e., the slip zone), and the right side represents the non-crossing zone of the hydraulic fracture (i.e., the slip zone).

[0044] Example Step 1: Collect natural fracture orientation parameters, in-situ stress field parameters, and strength parameters; In this example, the geographical location of the natural fracture NE was obtained using interpretation methods such as multi-level seismic fault body interpretation. Natural crack dip angle .

[0045] In this example, the geographical azimuth of the maximum horizontal principal stress is NE65 degrees. =52MPa, minimum principal stress =45MPa, vertical principal stress =48MPa.

[0046] Among the rock strength parameters, cohesion and internal friction coefficient Obtained through core experiments. (Value taken as an example.) =0MPa, coefficient of internal friction =0.6.

[0047] Step 2: Establish a plane Ω rectangular coordinate system perpendicular to the hydraulic cracks and natural cracks, and calculate the corresponding dip angle, strike angle, and intersection angle of the natural cracks in the Ω plane; 1. Based on geometric relationships, transform the orientation of natural fractures in the geographic coordinate system to the relative azimuth angle of natural fractures in the principal stress coordinate system;

[0048] 2. Establish a plane Ω perpendicular to the hydraulic fractures and natural fractures, and calculate the orientation parameters of the natural fractures in the Ω plane;

[0049] 3. Calculate the intersection angle of natural cracks on the Ω plane using spatial geometry;

[0050] Step 3: Transform the principal stresses of the in-situ principal stress coordinate system to the coordinate system on the Ω plane, and calculate the "far-field stress" on the plane; Based on the coordinate transformation principle under different rectangular coordinate systems, the principal stresses in the in-situ principal stress coordinate system are transformed to a coordinate system composed of the Ω plane and the normal. According to the hydraulic fracturing principle, the two-dimensional "far-field stresses" are obtained as follows:

[0051] Step 4: Calculate the stress coefficient of the hydraulic fracture passing through the natural fracture; (1) Calculate the material strength and the constants determined by the "far-field stress";

[0052] (2) Calculate the intermediate coefficients;

[0053]

[0054] (3) Calculate the stress coefficient of hydraulic fractures passing through natural fractures;

[0055] Step 5: Calculate the critical friction coefficient at the interface where hydraulic fractures do not cross natural fractures, given the in-situ stress and the natural fracture orientation. (1) Calculate the combined normal stress of the Ω plane acting on the natural crack surface; (2) Calculate the combined shear stress of the Ω plane acting on the natural crack surface;

[0056] (3) Calculate the critical friction coefficient for the known natural crack orientation angle and dip angle;

[0057] Step 6: Determine the propagation behavior of hydraulic fractures after they intersect with natural fractures; By comparing the relative magnitudes of the material's friction coefficient and the critical friction coefficient, the propagation behavior of hydraulic fractures can be determined. For the act of crossing; when This is a slip behavior.

[0058] In this example, Hydraulic fractures do not directly pass through natural fractures after they intersect, which is a slip behavior.

[0059] Step 7: Plot the critical friction coefficient chart for the propagation behavior of hydraulic fractures after they intersect with natural fractures; By successively changing the in-situ stress, the dip angle and orientation of the natural fracture, the critical friction coefficient is calculated using the method described above. Based on these calculation results, a discrimination line is plotted. The left side of each curve represents the non-crossing zone of the hydraulic fracture (i.e., the slip zone), and the right side represents the non-crossing zone of the hydraulic fracture (i.e., the slip zone).

[0060] In this example, under given stress conditions, the dip angle of the natural crack is... The azimuth angles of the natural cracks are respectively , , , , and The calculated results of the critical friction coefficient at the corresponding time are attached. Figure 4 As shown.

[0061] The above description is not intended to limit the present invention in any way. Although the present invention has been disclosed through the above embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall fall within the scope of the present invention.

Claims

1. A method for determining the intersection and extension behavior of hydraulic fractures and arbitrary natural spatial fractures, characterized in that, Includes the following steps: Step S10: Collect natural fracture orientation parameters, in-situ stress field parameters, and rock strength parameters; Step S20: Establish a plane Ω rectangular coordinate system perpendicular to the hydraulic fracture and the natural fracture, and calculate the corresponding dip angle, strike angle and intersection angle of the natural fracture in the Ω plane; Step S30: Transform the principal stresses of the in-situ principal stress coordinate system to the coordinate system on the Ω plane, and calculate the far-field stress on the plane; Step S40: Calculate the stress coefficient of the hydraulic fracture passing through the natural fracture; Step S50: Calculate the critical friction coefficient at the interface where hydraulic fractures do not cross natural fractures, given the in-situ stress and the natural fracture orientation. Step S60: Compare the relative magnitudes of the material friction coefficient and the critical friction coefficient to determine the propagation behavior of hydraulic cracks; Step S70: Draw a chart of the critical friction coefficients for the propagation behavior of hydraulic fractures after they intersect with natural fractures.

2. The method for determining the intersection and extension behavior of hydraulic fractures and arbitrary natural spatial fractures according to claim 1, characterized in that, The natural fracture orientation parameters include the geographical location and dip angle of the natural fracture; the in-situ stress field parameters include the geographical location of the maximum horizontal principal stress and the in-situ stress field value; the rock strength parameters include cohesion and internal friction coefficient.

3. The method for determining the intersection and extension behavior of hydraulic fractures and arbitrary natural spatial fractures according to claim 1, characterized in that, The specific process of step S20 is as follows: Step S21: Based on geometric relationships, transform the orientation of natural fractures in the geographic coordinate system to the relative strike angle of natural fractures in the principal stress coordinate system; Step S22: Establish a plane Ω perpendicular to the hydraulic fracture and the natural fracture, and calculate the orientation parameters of the natural fracture in the Ω plane; Step S23: Calculate the intersection angle of the natural cracks on the Ω plane based on spatial geometry.

4. The method for determining the intersection and extension behavior of hydraulic fractures and arbitrary natural spatial fractures according to claim 1, characterized in that, The calculation formula in step S30 is: In the formula: , , These represent the maximum and minimum horizontal principal stresses and vertical principal stresses in the principal stress coordinate system, respectively, in MPa. , are the two principal stresses of the natural crack on the Ω plane, respectively, in MPa; The angle between the natural crack surfaces is the relative orientation angle in the Ω-plane principal stress coordinate system.

5. The method for determining the intersection and extension behavior of hydraulic fractures and arbitrary natural spatial fractures according to claim 1, characterized in that, The calculation formula in step S40 is: In the formula: The stress coefficient for hydraulic fractures penetrating natural fractures; , , All are intermediate coefficients.

6. The method for determining the intersection and extension behavior of hydraulic fractures and arbitrary natural spatial fractures according to claim 1, characterized in that, The specific process of step S50 is as follows: Step S51: Calculate the combined normal stress of the Ω plane acting on the natural crack surface; Step S52: Calculate the combined shear stress of the Ω plane acting on the natural crack surface; Step S53: Calculate the critical friction coefficient of the known natural crack direction angle and dip angle based on the combined normal stress and combined shear stress acting on the natural crack surface by the Ω plane.

7. The method for determining the intersection and extension behavior of hydraulic fractures and arbitrary natural spatial fractures according to claim 6, characterized in that, The calculation formula in step S51 is: In the formula: The normal stress acting on the Ω plane is expressed in MPa. , are the two principal stresses of the natural crack on the Ω plane, respectively, in MPa; The stress coefficient for hydraulic fractures penetrating natural fractures; The angle of intersection of the natural cracks on the Ω plane is denoted as .

8. The method for determining the intersection and extension behavior of hydraulic fractures and arbitrary natural spatial fractures according to claim 6, characterized in that, The calculation formula in step S52 is: In the formula: The shear stress acting on the normal direction of the Ω plane is expressed in MPa. , are the two principal stresses of the natural crack on the Ω plane, respectively, in MPa; The stress coefficient for hydraulic fractures penetrating natural fractures; The angle of intersection of the natural cracks on the Ω plane is denoted as .

9. The method for determining the intersection and extension behavior of hydraulic fractures and arbitrary natural spatial fractures according to claim 6, characterized in that, The calculation formula in step S53 is: In the formula: The shear stress acting on the normal direction of the Ω plane is expressed in MPa. The normal stress acting on the Ω plane is expressed in MPa. The critical internal friction coefficient for hydraulic fracture penetration / slippage is dimensionless; The cohesive force is expressed in MPa.

10. The method for determining the intersection and extension behavior of hydraulic fractures and arbitrary natural spatial fractures according to claim 6, characterized in that, The discrimination criterion in step S60 is: In the formula: The critical internal friction coefficient for hydraulic fracture penetration / slippage is dimensionless; is the internal friction coefficient, which is dimensionless.

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