A method for testing the conductivity of a deep shale unsupported fracture after hydration

Abstract

The application discloses a kind of deep shale unpropped fracture water hydration post-conductivity testing method, including selecting target block rock sample, and rock sample is divided into two parts;The relative surface between two parts of rock sample is treated to obtain rock sample fracture wall surface;Two parts of rock sample are soaked to required hydration time;Two parts of rock sample are combined with silica gel gasket and loaded into core chamber, water is injected into rock sample;Horizontal direction closure stress is applied to rock sample, shear slip occurs in rock sample under set temperature and pressure, injection flow is measured and collected;Unpropped fracture water hydration post-conductivity is calculated according to injection flow, and rock sample conductivity curve with flow variation is drawn.The method fully considers the influence of hydration on the mechanical strength of deep shale unpropped fracture during unpropped fracturing soak well process, and physical experiment is carried out on the mechanical characteristics of shale unpropped fracture under true triaxial stress state and hydration.The experiment is close to the actual situation of the mine, and provides a basis for accurately establishing a shale reservoir mathematical model.

Description

Technical Field

[0001] This invention relates to a method for testing the conductivity of unsupported fractures in deep shale after hydration, belonging to the field of unconventional oil and gas exploration, development, and production enhancement technology. Background Technology

[0002] Deep shale gas development has become a hot topic in oil and gas exploration and development, and an important successor to increasing production. During the post-hydraulic fracturing and well-shutting process in deep shale gas formation, the fracturing fluid soaking weakens the mechanical strength of natural fractures, lowers the critical activation pressure for shear failure, and allows high-pressure fluid within the fracture to further shear failure, forming unsupported fractures. After the natural fractures fail, hydration further deteriorates the mechanical strength of the unsupported fracture wall. Unsupported fractures are a further extension of hydraulic fractures and a key channel connecting supported fractures and the deep shale matrix. Therefore, the conductivity of unsupported fractures is crucial for the design of deep shale gas fracturing processes and the optimization of production regimes. However, the protruding parts of the unsupported fracture surface are prone to collapse after hydration, and the contact mode of the fracture surface gradually changes from point contact to surface contact, resulting in a decrease in fracture aperture and reduced conductivity. Given that the evolution of fracture conductivity caused by hydration during the well-shutdown period after hydraulic fracturing in deep shale gas formation is still unclear, there is an urgent need to propose a test method for the conductivity of unsupported fractures in deep shale after hydration in order to quantitatively characterize the conductivity of fractures after hydraulic fracturing in deep shale gas formation.

[0003] Previous studies have extensively investigated the variation of conductivity in shale gas fracturing fractures. Fredd's research indicates that fracture wall misalignment is a necessary condition for the conductivity of unsupported fractures. Studies by You Lijun et al. show that unsupported fractures in shale are far more stress-sensitive than supported fractures, and hydration further enhances their stress sensitivity. Research by Wang et al. and Wu et al. indicates that after hydration, the mechanical strength of the fracture wall decreases, and as production progresses and the closure stress on the fracture increases, the roughness of the unsupported fracture surface is worn away, significantly reducing conductivity. Farah et al. pointed out that due to the presence of numerous unsupported fractures after shale gas fracturing, more than 50% of the fracturing fluid may be permanently retained in the shale reservoir, making unsupported fractures a major cause of low flowback rates in shale gas fracturing. Studies by Zhou et al. have shown that deep shale reservoirs have high pressure and large geostress differences. Under the action of high closure overburden pressure, and due to hydration, the mechanical properties of shale also change significantly. Proppant-free fractures will quickly fail after fracturing, resulting in a rapid decrease in production after fracturing of deep shale.

[0004] Currently, there are few experimental studies on the dynamic evolution of the conductivity of unsupported fractures after hydration under actual deep shale reservoir conditions. Therefore, it is necessary to carry out relevant research, innovate and establish a testing method for the conductivity of unsupported fractures after hydration in deep shale, reveal the variation law of the conductivity of unsupported fractures under fluid-solid interaction in deep shale, and provide a reference for establishing a multiphase flow seepage model and productivity evaluation method for shale gas, as well as optimizing fracturing flowback and production systems. Summary of the Invention

[0005] In order to overcome the shortcomings of the existing technology, the present invention aims to provide a method for testing the conductivity of unsupported fractures in deep shale after hydration.

[0006] The technical solution provided by this invention to solve the above-mentioned technical problems is: a method for testing the conductivity of unsupported fractures in deep shale after hydration, comprising the following steps:

[0007] S1. Select rock samples from the target block and divide the rock samples into two parts;

[0008] S2. Create a crack in the opposite surface between the two rock samples to obtain the crack wall of the rock sample.

[0009] S3. Soak the two rock samples for the required hydration time;

[0010] S4. Combine the two rock samples with the silicone gasket and put them into the core chamber. Inject water into the rock samples under constant pressure.

[0011] S5. Apply a horizontal closed stress to the rock sample, and cause shear slippage in the rock sample under a set temperature and pressure. Measure and collect the injection flow rate.

[0012] S6. Calculate the conductivity of unsupported fractures after hydration based on the injection flow rate, and plot the conductivity curve of the rock sample as a function of flow rate.

[0013] A further technical solution is that, in step S1, a vertical slit perpendicular to the end face is cut in the center of the two opposite end faces of the rock sample using a cleaver, thereby cutting the rock sample into two parts.

[0014] A further technical solution is that, in step S2, a grinding wheel is used to grind the two walls of the vertical seam to a roughness of 30 grit.

[0015] A further technical solution is that, in step S4, silicone pads are placed at the upper and lower opposite ends of the two rock samples, which are then placed in a thermoplastic tube and then inserted into the core chamber.

[0016] A further technical solution is that, in step S4, silicone oil is injected into the core chamber for sealing and filling to apply confining pressure. After the silicone oil is full, the temperature control unit is turned on to raise the temperature of the rock sample to a preset value.

[0017] A further technical solution is that, in step S5, after the displacement control head just contacts the top of the specimen at a constant speed in the axial direction, the displacement control method is used to continue to compress the rock sample at a constant speed in the axial direction, causing shear slippage, while collecting injection flow data.

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

[0019]

[0020] In the formula: F is the conductivity of the unsupported fracture in the rock sample, D·cm; Q is the injection velocity of the ISCO pump, m. 3 / s; ΔP is the pressure difference between the fluid injection end and the outflow end, MPa; L is the fluid flow distance, m; T is the preset temperature, ℃; h f Let be the crack height, in meters (m).

[0021] This invention offers the following advantages: It fully considers the impact of hydration on the mechanical strength of unsupported fractures in deep shale during the unsupported fracturing and well-closing process, and proposes a method for testing the conductivity of unsupported fractures in deep shale after hydration. This method conducts physical experiments on the mechanical characteristics of unsupported shale fractures under true triaxial stress hydration, closely simulating real-world mining conditions, and providing a foundation and methodology for accurately establishing mathematical models of shale reservoirs. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of seepage through unsupported fractures in shale.

[0023] Figure 2 Schematic diagram of a test device for the mechanical strength of unsupported cracks in shale;

[0024] Figure 3 The variation of rock sample flow rate over time;

[0025] Figure 4 This shows the variation of the rock sample's conductivity with flow rate. Detailed Implementation

[0026] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] The present invention provides a method for testing the mechanical strength of deep shale natural fractures after hydration, specifically comprising the following steps:

[0028] S1. Select rock samples from the target block and divide the rock samples into two parts;

[0029] Large rocks collected from the target area were cut into 50×50×100mm rectangular rock samples for the experiment using a cutting machine; the surface of the rock samples was then polished with a grinding machine to obtain 3 pairs of parallel surfaces. The non-parallelism error of the rock sample surface after processing did not exceed 0.02mm (meeting the requirements of the ISRM test procedure); a vertical slit perpendicular to the end face was cut in the middle of the two opposite end faces of the rock sample using a cleaver.

[0030] S2. Create a crack in the opposite surface between the two rock samples to obtain the crack wall of the rock sample.

[0031] Use a grinding wheel to grind the two walls of the vertical seam to create a roughness of 30 grit to simulate the micromorphology of an unsupported crack.

[0032] S3. Soak the two rock samples for the required hydration time;

[0033] S4. Combine the two rock samples with the silicone gasket and put them into the core chamber. Inject water into the rock samples under constant pressure.

[0034] The rock sample was cut into two parts, A and B, and then placed with a 50×25×10 silicone pad. Figure 2 After the combination of methods, the sample is placed in a thermoplastic tube, inserted into the core chamber, and then silicone oil is injected into the core chamber for sealing and filling to apply confining pressure. After the silicone oil is full, the temperature control unit of GCTS is turned on to raise the temperature of the rock sample to the preset value, and the temperature threshold is set reasonably to avoid the influence of temperature changes on the experimental results. After the temperature stabilizes, the confining pressure is increased, and the ISCO pump injects water into the rock sample in a constant pressure manner.

[0035] S5. Apply a horizontal closed stress to the rock sample, and cause shear slippage in the rock sample under a set temperature and pressure. Measure and collect the injection flow rate.

[0036] A closed stress of 85 MPa was applied to the rock sample in the horizontal direction to fix the rock sample. The temperature of the test system was raised to a preset value of 130℃ to simulate the temperature conditions of deep shale reservoirs. The temperature threshold was set to 0.1℃ to avoid the influence of temperature changes on the experimental results. After the temperature stabilized, the confining pressure was increased to 85.0 MPa.

[0037] Meanwhile, ISCO pumps are 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 well shut-in, making the simulation closer to the real situation of the reservoir in the mine.

[0038] The rock sample was continuously compressed axially at a constant speed of 0.018 mm / min using a displacement control method, causing shear slippage; during this period, the ISCO pump automatically collected the flow rate Q.

[0039] S6. Calculate the conductivity of unsupported fractures after hydration based on the injection flow rate, and plot the conductivity of rock samples as a function of flow rate.

[0040] By substituting formulas to derive the formula for calculating the conductivity of rock samples after hydration of unsupported fractures, we have:

[0041] F=kω f

[0042]

[0043] μ=0.0017921 / (1+0.03368T+0.000221T 2 )

[0044]

[0045] in:

[0046] A=ω f h f

[0047] and then:

[0048]

[0049] In the formula: F is the conductivity of the unsupported fracture in the rock sample, D·cm; k is the permeability of the unsupported fracture in the rock sample, m. 2 ;ω f Q is the crack width, in meters; Q is the injection velocity of the ISCO pump, in meters. 3 / s; A is the unit cross-sectional area, 1m 2 ΔP is the pressure difference between the fluid injection end and the outflow end, in MPa; μ is the viscosity of water, in Pa·s; L is the fluid flow distance, in m; T is the preset temperature, in °C; h f Let be the crack height, in meters (m).

[0050] When the pump flow rate is 0.002098 mL / min, the conductivity of the rock sample fracture is 5.1498 × 10⁻⁶. -7 D·cm. Substituting the pumping flow rate throughout the process into the calculation formula yields the curve showing the change in conductivity of unsupported fractures in deep shale after hydration as a function of pumping flow rate. Comprehensive analysis of the above results demonstrates that this measurement method possesses high feasibility and operability.

[0051] 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 testing the conductivity of unsupported fractures in deep shale after hydration, characterized in that, Includes the following steps: S1. Select rock samples from the target block and divide the rock samples into two parts; S2. Create a crack in the opposite surface between the two rock samples to obtain the crack wall of the rock sample. S3. Soak the two rock samples for the required hydration time; S4. Combine the two rock samples with the silicone gasket and put them into the core chamber. Inject water into the rock samples under constant pressure. S5. Apply a horizontal closed stress to the rock sample, and cause shear slippage in the rock sample under a set temperature and pressure. Measure and collect the injection flow rate. S6. Calculate the conductivity of unsupported fractures after hydration based on the injection flow rate, and plot the conductivity of rock samples as a function of flow rate. In the formula: F The conductivity of unsupported fractures in the rock sample is expressed in D·cm. Q For the injection velocity of the ISCO pump, m 3 / s; ΔP The pressure difference between the fluid injection end and the outflow end, in MPa; L The fluid flow distance is in meters (m). T The preset temperature is ℃; h f Let be the crack height, in meters (m).

2. The method for testing the conductivity of unsupported fractures in deep shale after hydration according to claim 1, characterized in that, In step S1, a vertical slit perpendicular to the end face is cut in the center of the two opposite end faces of the rock sample using a cleaver, thereby cutting the rock sample into two parts.

3. The method for testing the conductivity of unsupported fractures in deep shale after hydration according to claim 2, characterized in that, In step S2, a grinding wheel is used to grind the two walls of the vertical seam to a roughness of 30 grit.

4. The method for testing the conductivity of unsupported fractures in deep shale after hydration according to claim 1, characterized in that, In step S4, silicone pads are placed at the upper and lower opposite ends of the two rock samples, which are then placed in a thermoplastic tube and then inserted into the core chamber.

5. The method for testing the conductivity of unsupported fractures in deep shale after hydration according to claim 4, characterized in that, In step S4, silicone oil is injected into the core chamber for sealing and filling to apply confining pressure. After the silicone oil is full, the temperature control unit is turned on to raise the temperature of the rock sample to a preset value.

6. The method for testing the conductivity of unsupported fractures in deep shale after hydration according to claim 1, characterized in that, In step S5, after the displacement control head just contacts the top of the specimen at a constant speed in the axial direction, the displacement control method is used to continue to compress the rock sample at a constant speed in the axial direction, causing shear slippage. At the same time, the injection flow rate data is collected.

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