Method for testing mechanical strength of hydrated natural fractures of deep shale

Abstract

The invention discloses a method for testing the mechanical strength of a hydrated natural fracture of deep shale, which comprises the following steps: selecting a target block rock sample, and carrying out fracture forming treatment to obtain a rock sample fracture wall surface; soaking the wall surface of the rock sample crack for required hydration time; measuring the elastic coefficient k of the silica gel gasket; the rock sample and the silica gel gasket are combined and then placed in a thermoplastic pipe, and shale natural crack mechanical strength testing is carried out; testing and collecting test data under different closing stresses; stress analysis is conducted on the rock sample in the shear slip process according to test data, a linear relation between F2-F1 and the effective closing stress is obtained through fitting, and the slope and the intercept in the linear relation are the friction coefficient mu and the cohesion C of the wall face of the natural fracture of the deep shale respectively. According to the method, the defects in the prior art are overcome, the testing method considering the mechanical strength change rule of the natural fracture of the deep shale reservoir under the hydration effect is explored, and accurate characterization of reservoir parameters of the deep shale oil and gas reservoir is achieved.

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 and development and production enhancement. Background Art

[0002] Shale gas, as an unconventional natural gas, is abundant in my country and is an important part of optimizing my country's energy structure. Most of my country's shale gas resources are buried in formations deeper than 3,500 meters. The development of deep shale gas has become a hot spot for oil and gas exploration and development and an important successor area for increasing production. Deep shale reservoirs often contain microscopic cracks, whose strength is far less than that of the rock itself and are more susceptible to damage. Especially in the process of well shut-in after hydraulic fracturing, the mechanical strength of natural fractures in deep shale will be weakened after being soaked in fracturing fluid, and the critical activation pressure for shear failure of natural fractures will be reduced. The high-pressure fluid in the fracture makes natural fractures more susceptible to shear failure, which in turn has a complex impact on the tectonic stress field, fracture prediction and reservoir conductivity. In view of the complex law of rock mechanical strength changes after hydraulic fracturing in deep shale reservoirs, it is urgent to propose a test method for the law of mechanical strength changes after hydration of natural fractures in deep shale to quantitatively characterize the mechanical characteristics of fractures in the target reservoir.

[0003] At present, 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 the testing method of mechanical strength change law 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 well soaking in deep shale reservoirs, so as to achieve accurate characterization of reservoir parameters of deep shale oil and gas reservoirs and help 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 technical solution provided by the present invention to solve the above technical problems is: 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 the silicone gasket and placed in a thermoplastic tube to perform a mechanical strength test on natural fractures of shale;

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

[0011] S6. According to the test data, the shear slip process of the rock sample is subjected to stress analysis, and the linear relationship between F2-F1 and the effective closure stress is fitted, where F2 is the shear force, F1 is the elastic force, and the slope and intercept in the linear relationship are the friction coefficient μ and cohesion C of the wall of natural fractures in deep shale, respectively.

[0012] A further technical solution is that the specific process of step S1 is as follows: the large rock collected from the target block is cut into 50×50×100 mm rectangular rock samples 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 face is cut in the middle of the two opposite end faces of the rock sample with a splitter, and a 30-mesh roughness is polished on the two walls of the crack with a grinding wheel 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 90°C constant temperature drying oven 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: loading the rectangular silicone gasket into the pressure chamber, continuously compressing the silicone gasket at a constant axial speed, and automatically collecting the axial stress and axial displacement of the process by a computer; obtaining a fitting equation 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 rock sample cut and the silicone gasket are combined and placed in a thermoplastic tube, and 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 to just contact the top of the specimen; silicone oil is then injected into the true triaxial cavity for sealing and filling to apply confining pressure, and after the silicone oil is filled, the temperature control unit of the GCTS is turned on, the temperature of the test system is increased to a preset value, and the temperature threshold is reasonably set, and the confining pressure is increased after the temperature stabilizes; at the same time, distilled water is continuously injected at a constant pressure by using an ISCO pump.

[0016] A further technical solution is that in step S4, the displacement control method is used to continuously compress the rock sample in the axial direction at a constant speed to cause shear slip, until the pressure-time curve shows that the axial pressure is stable, and the test under the current closed stress condition is completed; 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.

[0017] A further technical solution is that the test data includes closing stress F3, shear force F2, time t, axial compression speed 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 field 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 schematic diagram of the mechanical strength test device for natural fractures in shale and the force on 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 test;

[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 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.

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

[0029] S1. Select the rock samples in the target area, cut, grind and polish them to the required size for the experiment, and perform seam processing;

[0030] The large rocks collected from the target area were cut into 50×50×100mm 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 0.02mm (meeting the requirements of the ISRM experimental procedures).

[0031] A splitter is used to cut a vertical crack perpendicular to the end face 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 crack-making 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 of the cut rock sample need to be placed in a 90℃ constant temperature drying oven for 24 hours until the quality no longer changes; then, 704 silica gel is applied to the remaining five end faces of the rock sample outside the cut section to prevent water from infiltrating from the five end faces of the rock sample into the matrix and causing damage to the shale during the shearing process, so that the fracture wall can be fully soaked to simulate the well soaking condition after fracturing; finally, the rock sample coated with silica gel is immersed in a constant temperature water bath to make the fracture wall fully contact with distilled water, and is taken out for use after the hydration time set in the experiment is reached.

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

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

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

[0037] y=3.4329x

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

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

[0040] The A and B parts of the rock sample are cut and placed on a 50×25×10 silicone gasket. Figure 1 The rock sample was placed in a thermoplastic tube after being combined in the manner and loaded into a pressure chamber according to the process. An initial closing stress of 85 MPa was applied to the rock sample in the horizontal direction to fix the rock sample. The displacement control method was 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 was then injected into the true triaxial cavity for sealing and filling to apply confining pressure. After the silicone oil was filled, the temperature control unit of the GCTS was turned on to increase the temperature of the test system to a preset value of 130 °C to simulate the temperature conditions of deep shale reservoirs. The temperature threshold was set to 0.1 °C to avoid the influence of temperature changes on the experimental results. After the temperature stabilized, the confining pressure was 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; the force control method is then 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 (such as Figure 3 shown).

[0043] In this process, the instrument controls the pressure head in the axial direction at a constant speed of 0.018mm / min, and 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. Determine 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: Perform force analysis on 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 shear slip occurred in part A, while part B remained stationary. The stress analysis of part A shows that (e.g. Figure 1 ), during the experiment, it is mainly subjected to the upward elastic force F1 generated by the silicone pad, the downward shear stress F2 of the pressure head, the closing stress F3 in the vertical wall direction, the fluid pressure F4, the cohesive force C of the crack wall that hinders relative movement, 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, combining the above equations, we can get:

[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] Among them, the values ​​of F2, t, and F3 can be determined through the experimental process and are known quantities. Therefore, there are only two unknown quantities in the equation: the friction coefficient μ and the cohesive force C. After transforming the above formula, 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, in which the slope 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 is fitted (such as Figure 4 ) obtained the slope (friction coefficient μ) = 1.3707, the intercept (cohesion C) = 2.7781 (MPa), and thus achieved 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 is not intended to 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 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 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 k of the silicone gasket; S4, the rock sample is combined with the silicone gasket and placed in a thermoplastic tube to perform a mechanical strength test on natural fractures of shale; S5. Determine 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 the linear relationship between F2-F1 and the effective closure stress is fitted, where F2 is the shear force, F1 is the elastic force, and the slope and intercept in the linear relationship are the friction coefficient μ and cohesion C of the wall of natural fractures in deep shale, respectively.

2. A method for testing the mechanical strength of deep shale natural fractures after hydration according to claim 1, characterized in 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 a 50×50×100 mm rectangular rock sample; then grinding the surface of the rock sample to obtain three pairs of parallel faces after polishing by a grinder; using a splitter to cut a vertical crack perpendicular to the end face in the middle of the 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.

3. A method for testing the mechanical strength of deep shale natural fractures after hydration according to claim 1, characterized in that: The specific process of step S2 is: placing the two parts of the cut rock sample in a 90°C constant temperature drying oven for baking 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.

4. A 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: install the rectangular silicone gasket into the pressure chamber, continuously compress the silicone gasket at a constant axial speed, and automatically collect the axial stress and axial displacement of the process by the computer; obtain the fitting equation 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.

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 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 in the axial direction at a constant speed by displacement control so as to just contact the top of the specimen; silicone oil is then injected into the true triaxial cavity for sealing and filling to apply confining pressure, and after the silicone oil is filled, the temperature control unit of the GCTS is turned on, the temperature of the test system is increased to a preset value, and the temperature threshold is reasonably set, and the confining pressure is increased after the temperature is stabilized; at the same time, distilled water is continuously injected at a constant pressure by using an ISCO pump.

6. A 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 by using a displacement control method, so that shear slip occurs, until the pressure-time curve shows that the axial pressure is stable, and the test under the current closed stress condition is completed; 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. A method for testing the mechanical strength of natural fractures in deep shale after hydration according to claim 1, characterized in that: The test data include closing stress F3, shear force F2, time t, axial compression velocity v, and fluid pressure F4.

8. A method for testing the mechanical strength of natural fractures in deep shale after hydration according to claim 7, characterized in that: The calculation formula of the elastic force F1 in step S6 is: F1=k·vt Where: v is the axial compression velocity; t is the time; k is the elastic coefficient; F1 is the elastic force.

9. A method for testing the mechanical strength of natural fractures in deep shale after hydration according to claim 7, characterized in that: The effective closing stress in step S6 is the difference between the closing stress F3 and the fluid pressure F4.

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