Method for testing mechanical strength of deep shale natural fracture after hydration

Abstract

The application discloses a kind of deep shale natural fracture hydration post-mechanical strength test methods, including selecting target block rock sample, and carry out fracture treatment and obtain rock sample fracture wall surface;Rock sample fracture wall surface is soaked to required hydration time;The elastic coefficient k of silica gel gasket is measured;Rock sample and silica gel gasket are combined and placed in thermoplastic tube, and the mechanical strength test of shale natural fracture is carried out;Determination, collection test data under different closure stress;According to test data, the stress analysis of rock sample shear slip process is carried out, and the linear relationship of F2-F1 and effective closure stress is obtained by fitting, the slope and intercept in linear relationship are the friction coefficient μ of deep shale natural fracture wall surface and cohesion C.The application solves the deficiencies in the prior art, explores the mechanical strength change rule test method of considering deep shale reservoir natural fracture under hydration, realizes the accurate characterization of deep shale oil and gas reservoir reservoir parameter.

Description

Technical Field

[0001] The invention relates to a method for testing the mechanical strength of natural fractures in deep shale after hydration, and belongs to the technical field of unconventional oil and gas exploration, development and production increase and transformation. Background Art

[0002] Shale gas, an unconventional natural gas with abundant reserves in my country, is a key component in optimizing the country's energy mix. Most of my country's shale gas resources are buried in formations deeper than 3,500 meters. Deep shale gas development has become a hotspot in oil and gas exploration and development, and a crucial alternative for increasing production. Deep shale reservoirs often contain microscopic fractures, which are much weaker than the rock itself and more susceptible to damage. Especially during the post-hydraulic fracturing well shut-in process, the mechanical strength of natural fractures in deep shale is weakened by the fracturing fluid, reducing the critical activation pressure for shear failure. The high-pressure fluid within the fractures makes them more susceptible to shear failure, which in turn has complex implications for tectonic stress fields, fracture prediction, and reservoir conductivity. Given the complex dynamics of rock mechanical strength after hydraulic fracturing in deep shale reservoirs, a method for testing the dynamics of natural fractures in deep shale after hydration is urgently needed to quantitatively characterize the fracture mechanical characteristics in target reservoirs.

[0003] Currently, there are few studies on the weakening mechanism of mechanical properties of natural fractures in deep shale under the hydration of fracturing fluid. Therefore, it is necessary to carry out relevant research work to explore testing methods that consider the change law of mechanical strength of natural fractures in deep shale reservoirs under the action of hydration. Through physical experiments on the response of mechanical properties of natural fractures in deep shale under hydration, data support can be provided for accurately characterizing the dynamic evolution process of basic physical fields during the soaking period of deep shale reservoirs, realizing accurate characterization of reservoir parameters of deep shale oil and gas reservoirs and facilitating the efficient development of deep shale oil and gas reservoirs. Summary of the Invention

[0004] In order to overcome the defects in the prior art, the present invention aims to provide a method for testing the mechanical strength of natural fractures in deep shale after hydration.

[0005] The present invention solves the above technical problems and provides a technical solution: a method for testing the mechanical strength of natural fractures in deep shale after hydration, comprising the following steps:

[0006] S1. Select rock samples from the target area and perform fracture processing to obtain fracture walls of the rock samples;

[0007] S2. Soaking the crack wall of the rock sample for the required hydration time;

[0008] S3. Measure the elastic coefficient k of the silicone gasket;

[0009] S4. The rock sample is combined with a silicone gasket and placed in a thermoplastic tube to perform a mechanical strength test on the natural fractures of the shale;

[0010] S5. Measure and collect test data under different closing stresses;

[0011] S6. Based on the test data, the shear slip process of the rock sample was subjected to stress analysis, and a linear relationship between F2-F1 and the effective closure stress was obtained by fitting, where F2 is the shear force, F1 is the elastic force, and the slope and intercept of the linear relationship are the friction coefficient μ and cohesion C of the natural fracture wall of deep shale, respectively.

[0012] A further technical solution is that the specific process of step S1 is as follows: using a cutting machine to cut the large rock collected from the target block into 50×50×100 mm rectangular rock samples; then grinding the surface of the rock sample, and obtaining three pairs of mutually parallel faces after polishing with a grinder; using a splitter to cut vertical cracks perpendicular to the end faces in the middle of two opposite end faces of the rock sample, and using a grinding wheel to grind the two walls of the crack to a roughness of 30 mesh to simulate the microscopic morphology of natural cracks.

[0013] A further technical solution is that the specific process of step S2 is: placing the two parts of the cut rock sample into a constant temperature drying oven at 90°C and baking them for 24 hours until the quality no longer changes; then applying 704 silica gel to the remaining five end faces of the rock sample outside the cut section; finally, immersing the rock sample coated with silica gel in a constant temperature water bath to allow the crack wall to fully contact distilled water, and taking it out for use after the hydration time set in the experiment.

[0014] A further technical solution is that the measurement process in step S3 is: the rectangular silicone gasket is installed into the pressure chamber, the silicone gasket is continuously compressed at a constant axial speed, and the axial stress and axial displacement of the process are automatically collected by the computer; the fitting equation is obtained by measuring the axial stress and axial displacement of the silicone gasket during the compression process, and the slope in the fitting equation is the elastic coefficient k.

[0015] A further technical solution is that the specific process of step S4 is as follows: the two parts of the cut rock sample and the silicone gasket are combined and placed in a thermoplastic tube, and then loaded into the pressure chamber according to the process, an initial closing stress is applied to the rock sample in the horizontal direction to fix the rock sample, and the pressure head is controlled axially at a constant speed by displacement control so that it just contacts the top of the specimen; silicone oil is then injected into the true triaxial cavity for sealing and filling to apply confining pressure. After the silicone oil is full, the temperature control unit of the GCTS is turned on, the temperature of the test system is raised to a preset value, and the temperature threshold is reasonably set. After the temperature stabilizes, the confining pressure is increased; and at the same time, distilled water is continuously injected at a constant pressure using an ISCO pump.

[0016] A further technical solution is that in step S4, a displacement control method is used to continuously compress the rock sample in the axial direction at a constant speed, causing shear slip, until the pressure-time curve shows that the axial pressure is stable, and the test under the current closing stress condition is completed; then a force control method is used to increase the horizontal closing stress at a constant speed to the next closing stress condition set in the experimental plan, thereby gradually increasing the horizontal closing stress until the test under all closing stress conditions is completed.

[0017] A further technical solution is that the test data includes closing stress F3, shear force F2, time t, axial compression velocity v, and fluid pressure F4.

[0018] A further technical solution is that the calculation formula of the elastic force F1 in step S6 is:

[0019] F1=k·vt

[0020] Where: v is the axial compression velocity; t is the time; k is the elastic coefficient; F1 is the elastic force.

[0021] A further technical solution is that the effective closing stress in step S6 is the difference between the closing stress F3 and the fluid pressure F4.

[0022] The present invention has the following beneficial effects: the present invention conducts physical experiments on the mechanical characteristics of natural fractures in deep shale under hydration in a true triaxial stress state. The experiments are close to the actual mining site and can more accurately reflect the influence of hydration on the mechanical characteristics of natural fractures in the well soaking process after fracturing construction, characterize the mechanical strength of natural fractures after hydration, and provide a basis and method for accurately establishing a mathematical model of deep shale reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a diagram of the mechanical strength test device for natural fractures in shale and the force applied to the rock sample;

[0024] Figure 2 The elastic force of the silicone pad changes with the amount of compression;

[0025] Figure 3 Schematic diagram of stress loading path in crack shear experiment;

[0026] Figure 4 This is a linear relationship diagram between the experimental results F2-3.4329×0.018t and F3-F4. DETAILED DESCRIPTION

[0027] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0028] The present invention provides a method for testing the mechanical strength of natural fractures in deep shale after hydration, which specifically comprises the following steps:

[0029] S1. Select the rock sample from the target area, cut, grind and polish it to the required size for the experiment, and perform seam treatment;

[0030] Large rocks collected from the target area were cut into 50×50×100 mm rectangular rock samples required for the experiment using a cutting machine. The surface of the rock sample was then polished to obtain three pairs of parallel faces. After processing, the non-parallelism error of the rock sample surface did not exceed 0.02 mm (meeting the requirements of the ISRM experimental procedures).

[0031] A splitter is used to cut vertical cracks perpendicular to the end faces in the middle of the two opposite end faces of the core, and a grinding wheel is used to grind the two walls of the crack to a roughness of 30 mesh to simulate the microscopic morphology of natural cracks; after the cracking treatment, the core's external size and morphology are not changed, and closing stress can be applied in the horizontal direction, and shear stress can also be applied in the axial direction.

[0032] S2. Soaking the crack wall of the rock sample for the required hydration time;

[0033] After the rock sample is cut, the two parts need to be placed in a constant temperature drying oven at 90℃ and baked for 24 hours until the quality no longer changes. Then, the remaining five end faces of the rock sample outside the cut section are coated with 704 silica gel to prevent water from seeping into the matrix from the five end faces of the rock sample, thereby causing damage to the shale during the shear process. The fracture walls can be fully soaked to simulate the well soaking condition after fracturing. Finally, the silica gel-coated rock sample is immersed in a constant temperature water bath to ensure that the fracture walls are fully exposed to distilled water. After the hydration time set in the experiment is reached, it is removed and set aside.

[0034] S3. Measure the elastic modulus of the silicone gasket;

[0035] Measurement Figure 1 The elastic coefficient of the silicone gasket in the device is measured by placing a 50×50×10 mm rectangular silicone gasket into the pressure chamber and continuously compressing the silicone gasket at an axial speed of 0.018 mm / min. The axial stress and axial displacement of the process are automatically collected by a computer.

[0036] The elastic force F1 and the compression displacement x generated by the silicone gasket during compression increase approximately linearly. By measuring the stress and displacement of the silicone gasket during compression (such asFigure 2 As shown) the fitting equation is obtained:

[0037] y=3.4329x

[0038] From Hooke's law equation F1 = k·Δx, we know that the elastic coefficient k of the silicone pad is 3.4329. The obtained elastic coefficient k can be used to facilitate the subsequent analysis of the stress conditions of the rock sample.

[0039] S4. Combine the rock sample with the silicone gasket and conduct a mechanical strength test on the natural fracture of the shale, wherein the horizontal closing stress is gradually increased and the rock sample is subjected to shear slip under the set temperature and pressure;

[0040] The rock sample is cut into two parts A and B and the 50×25×10 size silicone gasket is pressed Figure 1 After the combination is completed, it is placed in a thermoplastic tube and loaded into the pressure chamber according to the process. An initial closing stress of 85 MPa is applied to the rock sample in the horizontal direction to fix the rock sample. The displacement control method is used to control the pressure head in the axial direction at a constant speed of 0.018 mm / min to just touch the top of the specimen. Silicone oil is then injected into the true triaxial cavity for sealing and filling to apply confining pressure. After the silicone oil is filled, the temperature control unit of the GCTS is turned on, and the temperature of the test system is raised to a preset value of 130°C to simulate the temperature conditions of deep shale reservoirs. The temperature threshold is set at 0.1°C to avoid the influence of temperature changes on the experimental results. After the temperature stabilizes, the confining pressure is increased to 85.0 MPa.

[0041] At the same time, an ISCO pump was used to continuously inject distilled water at a constant pressure of 82.0 MPa to more realistically simulate the water filling state of the fracture wall, the fluid flow process in the fracture, and the actual stress state of the fracture during the soaking period, making the simulation closer to the actual situation of the mine reservoir.

[0042] The displacement control method is used to continuously compress the rock sample in the axial direction at a constant speed of 0.018mm / min, causing shear slippage until the pressure-time curve shows that the axial pressure is stable. The test under the current closing stress condition A: 85.0MPa is completed. Then the force control method is used to increase the horizontal closing stress at a constant speed of 5MPa / min to the next closing stress condition B: 86.5MPa set in the experimental plan. At the same time, under continuous axial compression, the pressure-time curve shows that the axial pressure is stable. The test of the second closing stress condition B: 86.5MPa is completed. The force control method is used again to increase the horizontal closing stress at a constant speed of 5MPa / min to the next closing stress condition C: 87.0MPa. Under continuous axial compression, the pressure-time curve shows that the axial pressure is stable. The test of the second closing stress condition B is completed. Similarly, the test of subsequent closing stress conditions (D: 89.5MPa, E: 91.0MPa, F: 94.0MPa) is completed (as shown in the figure). Figure 3 shown).

[0043] During this process, the instrument controls the pressure head in the axial direction at a constant speed of 0.018mm / min. The compression displacement of the silicone gasket can be expressed as:

[0044] Δx=vt=0.018t

[0045] Where: v is the axial compression velocity; t is the time.

[0046] The elastic force F1 generated by the silicone pad can be further expressed as:

[0047] F1=kΔx=k·vt=3.4329×0.018t

[0048] S5. Measure and collect experimental data under different closing stresses;

[0049] The computer can automatically collect experimental data such as closing stress and axial shear stress.

[0050] S6: Analyze the shear slip process of the rock sample and calculate the friction coefficient μ and cohesion C of the natural fracture wall of deep shale;

[0051] right Figure 2 The rock sample was analyzed and it was found that shear slip occurred in part A and part B remained stationary. The stress analysis of part A showed that (such as Figure 1 During the experiment, the main forces acting on the crack are the upward elastic force F1 generated by the silicone gasket, the downward shear stress F2 generated by the pressure head, the closing stress F3 in the direction perpendicular to the wall, the fluid pressure F4, the cohesive force C within the crack wall that hinders relative motion, and the friction force f between the crack walls. When the axial pressure is stable during the experiment, that is, the force balance is achieved, then:

[0052] F2=F1+f+C

[0053] f=μ(F3-F4)

[0054] Furthermore, by combining the above equations, we can obtain:

[0055] F2=k·vt+μ(F3-F4)+C=3.4329×0.018t+μ(F3-82)+C→

[0056] Where: μ is the friction coefficient of shale fracture.

[0057] The values ​​of F2, t, and F3 can be determined experimentally and are known quantities. Therefore, the only unknown quantities in the equation are the friction coefficient μ and the cohesive force C. After transforming the above equation, we get:

[0058] F2-3.4329×0.018t=μ(F3-F4)+C

[0059] It can be seen that there is a linear relationship between F2-3.4329×0.018t and F3-F4. The slope of this linear relationship is the crack wall friction coefficient μ, and the intercept is the cohesive force C.

[0060] According to the test results obtained in the above test steps (Table 1), the linear relationship between F2-3.4329×0.018t and F3-F4 was fitted (e.g. Figure 4 ) obtained the slope (friction coefficient μ) = 1.3707, and the intercept (cohesion C) = 2.7781 (MPa), thereby realizing the measurement and characterization of the mechanical strength of deep shale natural fractures after hydration.

[0061] Table 1 Rock sample shear test data

[0062]

[0063]

[0064] Comprehensive analysis of the above results shows that this measurement method has high feasibility and operability.

[0065] The above description does not limit the present invention in any form. 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 use the technical content disclosed above to make some changes or modifications to equivalent embodiments without departing from the scope of the technical solution of the present invention. However, 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 content of the technical solution of the present invention are within the scope of the technical solution of the present invention.

Claims

1. A method for testing the mechanical strength of natural fractures in deep shale after hydration, characterized in that: The following steps are involved: S1. Select rock samples from the target area and perform fracture processing to obtain fracture walls of the rock samples; S2. Soaking the crack wall of the rock sample for the required hydration time; S3. Measure the elastic coefficient of the silicone gasket k ; S4. The rock sample is combined with a silicone gasket and placed in a thermoplastic tube to perform a mechanical strength test on the natural fractures of the shale; S5. Measure and collect test data under different closing stresses; S6. According to the test data, the shear slip process of the rock sample is subjected to stress analysis and fitted to obtain F 2- F 1 is the linear relationship between the effective closing stress, where F 2 is the shear force, F 1 is the elastic force, and the slope and intercept in the linear relationship are the friction coefficients of the natural fracture wall of deep shale. μ and cohesion C ; The test data includes closing stress F 3. Shear force F 2. Time t , axial compression speed v , fluid pressure F 4; The elastic force F The calculation formula for 1 is: Where: v is the axial compression velocity; t For time; k is the elastic coefficient; F 1 is elasticity.

2. The method for testing the mechanical strength of natural fractures in deep shale after hydration according to claim 1, characterized in that: The specific process of step S1 is as follows: a large rock block collected from the target block is cut into a 50×50×100 mm rectangular rock sample using a cutting machine; the surface of the rock sample is then polished to obtain three pairs of parallel faces; a vertical crack perpendicular to the end faces is cut in the middle of two opposite end faces of the rock sample using a splitter, and a 30-mesh roughness is polished on both walls of the crack using a grinding wheel to simulate the microscopic morphology of a natural crack.

3. The method for testing the mechanical strength of natural fractures in deep shale after hydration according to claim 1, characterized in that: The specific process of step S2 is as follows: the two cut rock samples are placed in a constant temperature drying oven at 90° C. and baked for 24 hours until the mass no longer changes; then, the remaining five end faces of the rock sample outside the cut section are coated with 704 silica gel; finally, the rock sample coated with silica gel is immersed in a constant temperature water bath to ensure that the crack wall is fully exposed to distilled water, and the rock sample is taken out for use after the hydration time set in the experiment.

4. The method for testing the mechanical strength of natural fractures in deep shale after hydration according to claim 1, characterized in that: The measurement process in step S3 is as follows: a rectangular silicone gasket is placed in a pressure chamber, the silicone gasket is continuously compressed at a constant axial speed, and the axial stress and axial displacement of the process are automatically collected by a computer; a fitting equation is obtained by measuring the axial stress and axial displacement of the silicone gasket during compression, and the slope of the fitting equation is the elastic coefficient k .

5. The method for testing the mechanical strength of natural fractures in deep shale after hydration according to claim 1, characterized in that: The specific process of step S4 is as follows: the two cut parts of the rock sample and the silicone gasket are combined and placed in a thermoplastic tube, and then loaded into the pressure chamber according to the process. An initial closing stress is applied to the rock sample in the horizontal direction to fix the rock sample. The pressure head is controlled axially at a constant speed using a displacement control method to just contact the top of the specimen. Silicone oil is then injected into the true triaxial cavity to seal and fill the cavity to apply confining pressure. After the silicone oil is filled, the temperature control unit of the GCTS is turned on, the test system temperature is raised to a preset value, and the temperature threshold is reasonably set. After the temperature stabilizes, the confining pressure is increased; and distilled water is continuously injected at a constant pressure using an ISCO pump.

6. The method for testing the mechanical strength of natural fractures in deep shale after hydration according to claim 5, characterized in that: In step S4, the rock sample is continuously compressed in the axial direction at a constant speed using a displacement control method, so as to cause shear slippage, until the pressure-time curve shows that the axial pressure is stable, and the test under the current closed stress condition is terminated; Then, the force control method is used to increase the horizontal closing stress at a constant speed to the next closing stress condition set in the experimental plan, so as to gradually increase the horizontal closing stress until the test under all closing stress conditions is completed.

7. The method for testing the mechanical strength of natural fractures in deep shale after hydration according to claim 1, characterized in that: The effective closing stress in step S6 is the closing stress F 3 and fluid pressure F A difference of 4.

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