A method for calculating the friction coefficient in high-angle natural fractures in formations

Abstract

The present invention discloses a method for calculating the internal friction coefficient of high-angle natural fractures in formations, including: collecting the natural fracture direction, horizontal maximum principal stress direction, horizontal maximum principal stress, horizontal minimum principal stress, pore elastic constant and formation fluid pressure of natural fracture reservoirs; calculating the natural fracture approach angle; calculating the normal stress and shear stress of the natural fracture surface; calculating the effective normal stress and effective shear stress in the natural fracture; and calculating the internal friction coefficient of the natural fracture. The present invention applies mechanical principles and strength criteria to obtain a method for calculating the internal friction angle of high-angle natural fractures in rock masses containing natural fracture media through mathematical methods; thereby filling the gap in the method for obtaining the internal friction coefficient of high-angle natural fractures, and solving the difficulty of obtaining the internal friction coefficient of natural fractures involved in the volume pressure design of unconventional reservoirs in recent years.

Description

Technical Field

[0001] The invention relates to a method for calculating the internal friction coefficient of high-angle natural cracks in a stratum, and belongs to the technical field of volume fracturing exploration and development of unconventional reservoirs. Background Art

[0002] The mechanical parameters of rocks have very important applications in the field of engineering technology. The "Standard for Engineering Rock Test Methods" gives a recommended method for testing the mechanical parameters of continuous media. The tests on the shear properties of rock masses mainly include triaxial compression tests and direct shear tests. Liu Jianfeng, Xie Heping, etc. invented "A rock structural surface shear test method and its implementation device" (201210223787). The upper and lower parts of the rock sample block are fixed in the upper and lower open shear boxes with opposite mouth surfaces through filling, and the mouth surfaces of the upper and lower shear boxes are kept at a certain distance so that the shear test structural surface of the rock sample block is exposed between the upper and lower shear box mouth surfaces; a pair of shear loads in opposite directions are applied to the upper and lower shear boxes, which act on the rock sample block through the filler in the shear box, and then act on the rock sample block structure surface, so as to obtain the shear strength parameters of the rock sample block structure surface. In order to solve the problem that the shear strength of rocks cannot be obtained when the shear surface is subjected to non-uniform stress using traditional testing methods and specimens, Xu Rongchao and others invented "A Testing Method and Process for Rock Shear Strength" (201710937345), which can be used to test the shear strength of different types of rocks under non-uniform stress conditions on the shear surface. Obviously, these methods and devices are experimental testing methods for uniform continuum media, and the mechanical parameters such as the internal friction coefficient of the continuum are obtained.

[0003] In recent years, in order to meet the needs of the national energy strategic security situation, the implementation of fracture network volume fracturing transformation technology for unconventional reservoirs has developed rapidly, leading the development direction of oil and gas reservoir production enhancement transformation technology, especially prompting the industry to pay great attention to the fracture mechanism and extension evolution process of natural fracture media. Studies have shown that the internal friction coefficient of natural fractures significantly restricts the network fracture expansion behavior of the mutual restriction condition of hydraulic fractures and artificial fractures, thereby determining the scope of fracturing production enhancement transformation and significantly affecting the volume fracturing transformation effect. Therefore, it is very important to obtain the internal friction coefficient of the natural fracture wall. However, it is very difficult to obtain rock samples containing natural fracture media, and the reservoir is full of formation fluids. Therefore, the aforementioned experimental test method cannot meet the engineering requirements for obtaining the internal friction coefficient of the natural fracture wall in natural fracture media. Summary of the invention

[0004] In order to overcome the problems in the prior art, the present invention provides a method for calculating the friction coefficient within a high-angle natural fracture in a formation.

[0005] The technical solution provided by the present invention to solve the above technical problems is: a method for calculating the internal friction coefficient of high-angle natural fractures in a formation, comprising the following steps:

[0006] A. Collect the natural fracture direction, horizontal maximum principal stress direction, horizontal maximum principal stress, horizontal minimum principal stress, pore elastic constant and formation fluid pressure of the natural fracture reservoir;

[0007] B. Calculate the natural fracture approach angle based on the natural fracture trend and the horizontal maximum principal stress direction;

[0008] C. Calculate the normal stress and shear stress on the natural fracture surface based on the horizontal maximum principal stress, horizontal minimum principal stress, and natural fracture approach angle;

[0009] D. Calculate the effective normal stress and effective shear stress in natural fractures based on the poroelastic constant, formation fluid pressure, normal stress and shear stress;

[0010] E. Calculate the friction coefficient inside the natural fracture based on the effective normal stress and effective shear stress in the natural fracture.

[0011] A further technical solution is that in step A, the natural fracture direction is obtained by microscopic observation using imaging logging or directional coring; the horizontal maximum principal stress orientation is obtained by microseismic monitoring or core experimental testing method; the horizontal maximum principal stress and the horizontal minimum principal stress are determined by testing fracturing interpretation or Kaiser effect core testing or corrected logging interpretation method; and the rock poroelastic constant is determined by core experimental testing.

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

[0013] β = natural fracture direction - horizontal maximum principal stress orientation

[0014] Where: β is the natural fracture approach angle.

[0015] A further technical solution is that the calculation formula in step C is:

[0016]

[0017]

[0018] Where: σ1, σ3 are the maximum horizontal principal stress and the minimum horizontal principal stress respectively; σ is the normal stress; τ is the shear stress.

[0019] A further technical solution is that the calculation formula in step D is:

[0020] σ n,e =σ-α.p s

[0021] τ e =τ

[0022] Where: α is the poroelastic constant; p s is the formation fluid pressure; τ e is the effective shear stress; n,e is the effective normal stress.

[0023] A further technical solution is that the calculation formula in step E is:

[0024] μ=τ e / σ n,e

[0025] Where: μ is the friction coefficient inside the natural fracture.

[0026] The present invention has the following beneficial effects: the present invention considers the combined effect of the in situ stress field and the formation fluid, and proposes a method for calculating the internal friction coefficient of high-angle natural fractures in the formation. The method applies mechanical principles and strength criteria to obtain a method for calculating the internal friction angle of high-angle natural fractures in a rock mass containing a natural fracture medium through a mathematical method; thereby filling the gap in the method for obtaining the internal friction coefficient of high-angle natural fractures, and solving the difficulty in obtaining the internal friction coefficient of natural fractures involved in the volume pressure design of unconventional reservoirs in recent years. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a geometric relationship diagram between natural fractures and principal stress directions;

[0028] Figure 2 This is the dominant trend map of natural fractures in Well XJ-J102 (1380-1403m well section) interpreted by imaging logging;

[0029] Figure 3 It is the horizontal maximum principal stress azimuth map of the target work area determined by microseismic testing;

[0030] Figure 4 It is a graph showing the relationship between the friction coefficient and the approach angle in a natural fracture. DETAILED DESCRIPTION

[0031] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0032] A method for calculating the internal friction coefficient of a high-angle natural fracture in a formation according to the present invention comprises the following steps:

[0033] A. Interpret or collect the direction of high-angle natural fractures in the formation, the direction of the maximum horizontal principal stress, the maximum horizontal principal stress, the minimum horizontal principal stress, the pore elastic constant and the formation fluid pressure based on the logging data;

[0034] The orientation of natural fractures can usually be obtained by imaging logging or directional coring observation;

[0035] The orientation of the maximum horizontal principal stress can be obtained by microseismic monitoring or core experimental testing;

[0036] The values ​​of horizontal biaxial principal stresses can be determined by testing fracturing interpretation or Kaiser effect core testing or corrected well logging interpretation;

[0037] The rock poroelastic constants are obtained through core test or other methods;

[0038] B. Calculate the approach angle based on the natural fracture trend and the horizontal maximum principal stress direction;

[0039] β = natural fracture direction - horizontal maximum principal stress orientation

[0040] Where: β is the natural fracture approach angle;

[0041] C. Calculate the normal stress and shear stress on the natural fracture surface based on the horizontal maximum principal stress, horizontal minimum principal stress, and natural fracture approach angle;

[0042] The top view of the high-angle natural fractures formed during the accumulation of oil and gas reservoirs is shown in the attached figure. Figure 1 According to the stress decomposition principle in engineering mechanics or rock mechanics, the calculation formulas for the normal stress and shear stress of the two-way horizontal principal stress acting on the natural fracture surface in the rock medium containing natural fractures can be derived;

[0043]

[0044]

[0045] Where: σ1, σ3 are the maximum horizontal principal stress and the minimum horizontal principal stress, respectively, in MPa; σ is the normal stress, in MPa; τ is the shear stress, in MPa;

[0046] D. Calculate the effective normal stress and effective shear stress in natural fractures based on the poroelastic constant, formation fluid pressure, normal stress and shear stress;

[0047] Before being mined, the reservoir is always filled with formation fluid, and the formation fluid pressure is recorded as p sSince rock mass failure is induced under the action of effective stress, the effective normal stress and effective shear stress of the structural weak surface under the action of formation fluid pressure are calculated according to the Terzaghi principle, and its expression is:

[0048] σ n,e =σ-α.p s

[0049] τ e =τ

[0050] Where: α is the poroelastic constant, dimensionless; p s is the formation fluid pressure, MPa; τ e is the effective shear stress, MPa; σ n,e is the effective normal stress, MPa;

[0051] E. Calculate the friction coefficient in natural fractures based on the effective normal stress and effective shear stress in natural fractures;

[0052] Considering that natural cracks in the rock mass under the original formation conditions will not produce new fracture surfaces, that is, natural cracks are in a static mechanical equilibrium state. Therefore, the natural crack wall satisfies the Coulomb shear strength theory (that is, when the shear force on the crack surface is greater than the shear strength, the crack will shear and rupture). The shear strength condition that should be satisfied under critical conditions is expressed as:

[0053] τ e =c+μ.σ n,e

[0054] Therefore, the internal friction coefficient of the natural fracture wall can be expressed as

[0055] μ=(τ e -c) / σ n,e

[0056] It is generally believed that the cohesive force of natural cracks can be approximately taken as zero. So:

[0057] μ=τ e / σ n,e

[0058] Where: μ is the friction coefficient within the natural fracture, dimensionless; c is the cohesion of the natural fracture surface, MPa.

[0059] In the 1380-1403m section of the XJ-J102 well in a western oilfield, natural fractures are well developed in the reservoir. During the well construction process, imaging logging, core experiment testing of in-situ stress field, and fracturing microseismic monitoring were carried out.

[0060] The method of the present invention is now used to calculate the friction coefficient in natural fractures, and the steps are as follows:

[0061] A. Collect parameters such as the natural fracture direction, horizontal maximum principal stress direction, horizontal maximum principal stress, horizontal minimum principal stress, and pore elastic constant of natural fracture reservoirs;

[0062] In this example, the imaging logging interpretation principle is used to interpret the 15 high-conductivity natural fractures in the well section of 1403.0-1380.0 m. The natural fractures are mainly oriented in the SN direction. Figure 2 ;

[0063] In this embodiment, the maximum horizontal principal stress orientation is NE45° obtained by microseismic fracturing crack monitoring. Figure 3 ;

[0064] In this embodiment, based on the core test method such as Kaiser effect, the minimum horizontal principal stress σ3 = 25.0 MPa and the maximum horizontal principal stress σ1 = 29.5 MPa are obtained;

[0065] This experiment tests the rock mass poroelastic constant α = 0.7, the formation pressure is p s =18.5MPa;

[0066] B. Calculate the natural fracture approach angle based on the natural fracture trend and the horizontal maximum principal stress direction;

[0067] β=|natural fracture strike-horizontal maximum principal stress orientation|=|AZM(SN)-AZM(NE45)|=45°

[0068] C. Calculate the normal stress and shear stress of the natural fracture surface;

[0069]

[0070]

[0071] D. Calculate the effective stress in natural fractures;

[0072] σ n,e =27.25-0.7×18.5=14.3MPa

[0073] τ e =2.25MPa

[0074] E. Determine the friction coefficient in natural fractures;

[0075] μ=2.25 / 14.3=0.1573

[0076] Then, the friction coefficient in the natural fracture of the reservoir in the 1380-1403 m section of the XJ-J102 well in a certain oil field in the west of this embodiment is 0.1573.

[0077] Assuming that the in-situ stress field, formation pressure and pore elastic constant remain constant, similar calculations can be used to obtain the relationship between the friction coefficient of the natural fracture wall and the approach angle when the dominant orientation of the natural fracture changes. Figure 4 .

[0078] The above description is not intended to impose any form of limitation on the present invention. Although the present invention has been disclosed through the above embodiments, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A method for calculating the internal friction coefficient of high-angle natural fractures in a formation, characterized in that: The following steps are involved: A. Collect the natural fracture direction, horizontal maximum principal stress direction, horizontal maximum principal stress, horizontal minimum principal stress, pore elastic constant and formation fluid pressure of the natural fracture reservoir; B. Calculate the natural fracture approach angle based on the natural fracture trend and the horizontal maximum principal stress direction; β=|natural fracture direction-horizontal maximum principal stress orientation| Where: β is the natural fracture approach angle; C. Calculate the normal stress and shear stress on the natural fracture surface based on the horizontal maximum principal stress, horizontal minimum principal stress, and natural fracture approach angle; Where: σ1, σ3 are the maximum horizontal principal stress and the minimum horizontal principal stress respectively; σ is the normal stress; τ is the shear stress; D. Calculate the effective normal stress and effective shear stress in natural fractures based on the poroelastic constant, formation fluid pressure, normal stress and shear stress; s n,e =σ-α.p s t e =t Where: α is the poroelastic constant; p s is the formation fluid pressure; τ e is the effective shear stress; n,e is the effective normal stress; E. Calculate the friction coefficient in natural fractures based on the effective normal stress and effective shear stress in natural fractures; μ = τ e / s n,e Where: μ is the friction coefficient inside the natural fracture.

2. A method for calculating the internal friction coefficient of high-angle natural fractures in a formation according to claim 1, characterized in that: In the step A, the natural fracture orientation is obtained by imaging logging or directional coring microscopic observation; the horizontal maximum principal stress orientation is obtained by microseismic monitoring or core experimental testing method; the horizontal maximum principal stress and the horizontal minimum principal stress are determined by testing fracturing interpretation or Kaiser effect core testing or corrected logging interpretation method; and the rock poroelastic constant is determined by core experimental testing.

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