Method for testing flow conductivity of hydrated unsupported fracture of deep shale

Abstract

The invention discloses a method for testing the flow conductivity of deep shale after support-free fracture hydration, which comprises the following steps: selecting a target block rock sample, and dividing the rock sample into two parts; performing fracture forming treatment on opposite surfaces between the two parts of rock samples to obtain rock sample fracture wall surfaces; soaking the two parts of rock samples for required hydration time; combining the two parts of the rock sample and the silica gel gasket, loading into a rock core chamber, and injecting water into the rock sample; applying horizontal closing stress to the rock sample, enabling the rock sample to generate shear slippage at set temperature and pressure, and measuring and collecting injection flow; and calculating the flow conductivity of the support-free fracture after hydration according to the injection flow, and drawing a curve of the flow conductivity of the rock sample changing along with the flow. According to the method, the influence of the hydration effect on the mechanical strength of the deep shale unsupported fracture in the unsupported fracturing soaking process is fully considered, a physical experiment is carried out according to the mechanical characteristics of the shale unsupported fracture under the hydration effect in the true triaxial stress state, the experiment is close to the practice of a mine field, and a foundation is provided for accurately establishing a shale reservoir mathematical model.

Description

Technical Field

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

[0002] Deep shale gas development has become a hotspot in oil and gas exploration and development, and a crucial alternative to increasing production. During the post-hydraulic fracturing process, the soaking of the fracturing fluid weakens the mechanical strength of the natural fractures in the deep shale, reducing the critical activation pressure for shear failure. The high-pressure fluid within the fractures then causes further shear failure, forming unsupported fractures. After natural fractures fail, hydration further degrades the mechanical strength of the unsupported fracture walls. Unsupported fractures are extensions of hydraulic fractures and serve as critical pathways connecting supported fractures with the deep shale matrix. Therefore, the conductivity of unsupported fractures is crucial for the design and optimization of deep shale gas fracturing processes. However, the protruding support portion of the unsupported fracture surface is prone to collapse after hydration, and the contact pattern of the fracture surfaces gradually shifts from point contact to surface contact, resulting in a decrease in fracture aperture and conductivity. Since the evolution law of fracture conductivity caused by hydration during the soaking period after hydraulic fracturing of deep shale gas reservoirs is still unclear, it is urgent to propose a test method for the post-hydration conductivity of unsupported fractures in deep shale to quantitatively characterize the fracture conductivity of deep shale gas reservoirs after hydraulic fracturing.

[0003] Previous research has investigated the changing patterns of conductivity in shale gas fracturing fractures. Fredd et al. have shown that fracture wall misalignment is a necessary condition for the conductivity of unsupported fractures. You Lijun et al. have shown that the stress sensitivity of unsupported fractures in shale is far greater than that of supported fractures, and that hydration further enhances the stress sensitivity of unsupported fractures. Wang et al. and Wu et al. have shown that after hydration of unsupported fractures, the mechanical strength of the fracture walls decreases. As production progresses and the closure stress on the fractures increases, the roughness of the unsupported fracture surfaces is worn away, significantly reducing the conductivity. Farah et al. have pointed out that after shale gas fracturing, due to the presence of a large number of unsupported fractures, more than 50% of the fracturing fluid may be permanently retained in the shale reservoir. Unsupported fractures are the primary cause of the low flowback rate of shale gas fracturing. Research by Zhou et al. shows that deep shale reservoirs have high pressure and large ground stress differences. Under the action of high closing overburden pressure, the mechanical properties of shale will also change significantly due to hydration. Proppant-free fractures will fail quickly after fracturing, resulting in a rapid decrease in production after deep shale fracturing.

[0004] Currently, there are few experimental studies on the dynamic evolution of the conductivity of unsupported fractures after hydration under actual reservoir conditions in deep shale. Therefore, it is necessary to carry out relevant research work and establish an innovative test method for the conductivity of unsupported fractures in deep shale after hydration, revealing the changing law of the conductivity of unsupported fractures under the interaction of flow and solid in deep shale, and providing reference for establishing shale gas multiphase flow seepage models and production capacity evaluation methods, as well as optimizing fracturing backflow systems and production systems. Summary of the Invention

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

[0006] The present invention solves the above technical problems and provides a technical solution: a method for testing the conductivity of unsupported fractures in deep shale after hydration, comprising the following steps:

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

[0008] S2. performing a cracking process on the opposite surfaces of the two rock samples to obtain a crack wall surface of the rock sample;

[0009] S3, soaking the two rock samples for the required hydration time;

[0010] S4, placing the two rock samples and the silicone gasket into the core chamber, and injecting water into the rock sample at a constant pressure;

[0011] S5. Applying horizontal closing stress to the rock sample to cause shear slippage of the rock sample under set temperature and pressure, and measuring and collecting the injection flow rate;

[0012] S6. Calculate the conductivity of the unsupported fracture after hydration based on the injection flow rate, and draw a curve of the rock sample's conductivity changing with the flow rate.

[0013] A further technical solution is that in step S1 , a splitter is used to cut a vertical seam perpendicular to the end faces in the middle of two opposite end faces of the rock sample, 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 wall surfaces of the vertical seam to a roughness of 30 mesh.

[0015] A further technical solution is that in step S4, the upper and lower opposite ends of the two parts of the rock sample are padded with silicone gaskets respectively, and then placed in a thermoplastic tube, and then loaded 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, and after the silicone oil is filled, the temperature control unit is turned on to increase the temperature of the rock sample to a preset value.

[0017] A further technical solution is that in step S5, after the pressure head is controlled axially at a constant speed to just contact the top of the specimen by using a displacement control method, the displacement control method is continued to be used to continuously compress in the axial direction at a constant speed to cause shear slip of the rock sample, while collecting injection flow rate data.

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

[0019]

[0020] Where: F is the conductivity of the unsupported fracture of the rock sample, D·cm; Q is the injection speed 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, °C; h f is the crack height, m.

[0021] This invention has the following beneficial effects: This method fully considers the impact of hydration on the mechanical strength of unsupported fractures in deep shale during unsupported fracturing and soaking, and proposes a method for testing the conductivity of hydrated unsupported fractures in deep shale. This method conducts physical experiments on the mechanical characteristics of unsupported fractures in shale under true triaxial stress and hydration. These experiments, closely aligned with field conditions, provide a foundation and methodology for accurately establishing mathematical models of shale reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of unsupported fracture seepage in shale;

[0023] Figure 2 This is a diagram of the mechanical strength test device for unsupported shale fractures;

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

[0025] Figure 4 The law of change of rock sample conductivity with flow rate. DETAILED DESCRIPTION

[0026] 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.

[0027] 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:

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

[0029] 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 rock samples were then polished to obtain three pairs of parallel faces. The parallelism error of the processed rock samples did not exceed 0.02mm (meeting the requirements of the ISRM experimental protocol). A splitter was used to cut vertical seams perpendicular to the end faces in the middle of the two opposite end faces of the rock samples.

[0030] S2. performing a cracking process on the opposite surfaces of the two rock samples to obtain a crack wall surface of the rock sample;

[0031] A grinding wheel was used to grind the two walls of the vertical crack to a 30-grit roughness to simulate the micromorphology of an unsupported crack;

[0032] S3, soaking the two rock samples for the required hydration time;

[0033] S4, placing the two rock samples and the silicone gasket into the core chamber, and injecting water into the rock sample at a constant pressure;

[0034] The rock sample is cut into two parts A and B and the 50×25×10 size silicone gasket is pressed Figure 2 After the combination is assembled, it is placed in a thermoplastic tube and loaded into the core chamber. Silicone oil is then injected into the core chamber to seal and apply confining pressure. After the silicone oil is filled, the temperature control unit of the GCTS is turned on to raise the rock sample temperature to the preset value. The temperature threshold is reasonably set to avoid the influence of temperature changes on the experimental results. After the temperature stabilizes, the confining pressure is increased and the ISCO pump is used to inject water into the rock sample at a constant pressure.

[0035] S5. Applying horizontal closing stress to the rock sample to cause shear slippage of the rock sample under set temperature and pressure, and measuring and collecting the injection flow rate;

[0036] A closing stress of 85 MPa was applied horizontally to the rock sample to fix it. The temperature of the test system was raised to a preset value of 130°C to simulate the temperature conditions of deep shale reservoirs. The temperature threshold was set at 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.

[0037] 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.

[0038] The displacement control method was used to continuously compress the sample in the axial direction at a constant speed of 0.018 mm / min, causing shear slippage of the rock sample. During this period, the ISCO pump automatically collected the flow rate Q.

[0039] S6. Calculate the conductivity of the unsupported fracture after hydration based on the injection flow rate, and draw a curve of the change of the conductivity of the rock sample with the flow rate;

[0040] By replacing the formula, the calculation formula for the conductivity of the rock sample after hydration of unsupported fractures is derived:

[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] Where: F is the conductivity of the unsupported fracture of the rock sample, D·cm; k is the permeability of the unsupported fracture of the rock sample, m 2 ;ω f is the crack width, m; Q is the injection rate of the ISCO pump, m 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, MPa; μ is the viscosity of water, Pa·s; L is the fluid flow distance, m; T is the preset temperature, ℃; h f is the crack height, m.

[0050] When the pump flow rate is 0.002098 mL / min, the conductivity of the rock sample fracture is 5.1498×10 -7 D·cm. Substituting the pumping flow rate throughout the entire process into the calculation formula yielded a curve showing how the conductivity of unsupported fractures in deep shale changes with pumping flow rate after hydration. Comprehensive analysis of these results demonstrates that this measurement method is highly feasible and operational.

[0051] 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 conductivity of unsupported fractures in deep shale after hydration, characterized in that: The following steps are involved: S1. Select the target block rock sample and divide it into two parts; S2. performing a cracking process on the opposite surfaces of the two rock samples to obtain a crack wall surface of the rock sample; S3, soaking the two rock samples for the required hydration time; S4, placing the two rock samples and the silicone gasket into the core chamber, and injecting water into the rock sample at a constant pressure; S5. Applying horizontal closing stress to the rock sample to cause shear slippage of the rock sample under set temperature and pressure, and measuring and collecting the injection flow rate; S6. Calculate the conductivity of the unsupported fracture after hydration based on the injection flow rate, and draw a curve of the rock sample's conductivity changing with the flow rate.

2. A method for testing the conductivity of unsupported fractures in deep shale after hydration according to claim 1, characterized in that: In step S1, a splitter is used to cut a vertical seam perpendicular to the end faces in the middle of two opposite end faces of the rock sample, 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 wall surfaces of the vertical seam to a roughness of 30 mesh.

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, the upper and lower opposite ends of the two rock samples are padded with silicone gaskets, which are then placed in a thermoplastic tube and then loaded into the core chamber.

5. A 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 to seal and fill the chamber to apply confining pressure. After the silicone oil is filled, the temperature control unit is turned on to increase 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 pressure head is controlled to just contact the top of the specimen in the axial direction at a constant speed using the displacement control method, the displacement control method is continued to be used to continuously compress the specimen in the axial direction at a constant speed to cause shear slippage of the rock sample, while collecting injection flow rate data.

7. The method for testing the conductivity of unsupported fractures in deep shale after hydration according to claim 1, characterized in that: The calculation formula in step S6 is: Where: F is the conductivity of the unsupported fracture of the rock sample, D·cm; Q is the injection speed 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, °C; h f is the crack height, m.

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