Test method for simulating rock hydraulic fracturing in true triaxial stress state

Abstract

The invention discloses a test method for simulating rock hydraulic fracturing in a true triaxial stress state, and relates to the technical field of hydraulic fracturing, the test method comprises the following steps: S1, a sample preparation stage: processing a rock block into a standard square rock sample, and processing a plurality of hydraulic fracturing holes according to a test target size; s2, a sample mounting stage; s3, an equipment inspection stage; s4, a pre-load applying stage; s5, a hydraulic pressure applying stage of the hydraulic fracturing model: injecting hydraulic fracturing fluid with different flow rates into the hydraulic fracturing holes at different flow rates by utilizing a constant-flow constant-pressure pump through the hydraulic fracturing hydraulic pressure applying pipeline, and recording data and closing the constant-flow constant-pressure pump after the square rock sample is damaged; s6, an unloading stage; and S7, ending the test. According to the invention, a single hydraulic fracturing hole or a plurality of hydraulic fracturing holes are processed on a square rock sample to simulate single well and multi-well model samples, and different hydraulic pressurization tests are realized, so that the qualitative analysis of the reservoir rock crack evolution law under the true triaxial stress condition is completed.

Description

Technical Field

[0001] The invention belongs to the technical field of hydraulic fracturing and relates to a test method for simulating rock hydraulic fracturing under a true triaxial stress state. Background Art

[0002] With the continuous advancement of oil and gas field development technology, the exploration of unconventional oil and gas resources with low permeability and low density has received increasing attention. Among them, hydraulic fracturing has become an effective technology for improving the recovery rate of unconventional oil and gas resources. With the increase in formation depth and the influence of human factors such as hydraulic grooving, the reservoir rock is in a complex three-dimensional stress state, namely the true triaxial stress state (σ1>σ2>σ3). Therefore, it is of great practical significance to explore the evolution of hydraulic fracturing crack propagation under the true triaxial stress state. Secondly, the influence of the location of hydraulic fracturing wells and the sequence of fluid injection into hydraulic fracturing wells on crack propagation also requires further research.

[0003] Current hydraulic fracturing tests on reservoir rock specimens typically use conventional triaxial or true triaxial testing systems to apply external loads to the model specimens, followed by fluid injection to induce fractures and study crack evolution. However, there is a lack of experimental methods to understand the effects of fluid injection sequence, and the effects of fluid injection sequence between multiple hydraulic fracturing wells, on crack evolution. Summary of the Invention

[0004] In view of this, the present invention provides a test method for simulating rock hydraulic fracturing under a true triaxial stress state to solve the problems raised in the above background technology, and specifically discloses the following contents:

[0005] A test method for simulating rock hydraulic fracturing under true triaxial stress state specifically comprises the following steps:

[0006] S1. Sample preparation stage: Process the rock block into a standard square rock sample and process a number of hydraulic fracturing holes according to the target test size. The diameter, position arrangement and multi-hole spacing of the hydraulic fracturing holes are adjusted according to the actual working conditions.

[0007] S2. Sample installation stage: First, place the square rock sample on the Y-direction pressure head. Make sure that the square rock sample is placed at the center of the Y-direction pressure head. Raise the Y-direction pressure head until the center of the square rock sample is consistent with the centers of the four XZ-direction pressure heads to avoid eccentric loading. Then, place the four XZ-direction pressure heads close to the square rock sample with a gap to ensure that no pressure is applied to the square rock sample. Finally, connect the hydraulic fracturing hole to the hydraulic fracturing water pressure application pipeline and place the Y+ direction pressure head close to the square rock sample to complete the sample installation stage.

[0008] S3, Equipment inspection stage: Turn on the hydraulic source, computer and oscilloscope, and observe whether there are any abnormalities in all displacement sensors, force sensors and hydraulic fracturing water pressure application pipelines. If there are no abnormalities, proceed to the next step;

[0009] S4, preload application stage: the servo controls the six indenters in the XYZ directions to symmetrically and slowly apply force to 10kN. After loading is completed, stabilize for one minute before proceeding to the next step;

[0010] S5, hydraulic fracturing model water pressure application stage: using a constant flow and constant pressure pump to inject hydraulic fracturing fluid into the hydraulic fracturing hole through the hydraulic fracturing water pressure application pipeline until the square rock sample is destroyed, recording data and turning off the constant flow and constant pressure pump;

[0011] S6, Unloading stage: After the test is completed, stop the hydraulic loading, and then slowly, symmetrically and synchronously reduce the stress in the X, Y and Z directions to zero to avoid the square rock specimen being damaged by the deviation stress caused by unloading too quickly or unsynchronized in the three directions; record the stress and strain data in the X, Y and Z directions of the true triaxial testing machine;

[0012] S7. End of test: After unloading is completed, the square rock sample is taken out and the shape of the square rock sample is observed to complete the test.

[0013] Furthermore, in step S2, after the pressure head in the Y+ direction is brought close to the square rock sample, the positions of each pressure head and the hydraulic fracturing water pressure application pipeline are checked.

[0014] Furthermore, the preload application stage in step S4 is divided into two loading stages;

[0015] The first loading stage adopts the force control mode, and different loading speeds and loading paths are used according to the test plan to load to the first preset stress state, where the stress in the Y direction is σ1, the stress in the Z direction is σ2, and the stress in the X direction is σ3, and σ1≥σ2≥σ3;

[0016] The second loading stage adopts the force control mode. According to the actual construction conditions of the hydraulic fracturing project, the stress changes after the excavation of the hydraulic fracturing well are simulated. According to the test plan design, different loading speeds and loading paths are used to load to the second preset stress state.

[0017] Furthermore, when processing a hydraulic fracturing hole in a square rock sample, step S5 adopts single-well pressurization to maintain the stress state in the three directions of XYZ, and uses a constant flow and constant pressure pump to inject hydraulic fracturing fluid of different flow rates into the hydraulic fracturing hole through the hydraulic fracturing water pressure application pipeline at different flow rates until the square rock sample is destroyed, and then records the data and turns off the constant flow and constant pressure pump.

[0018] Furthermore, when processing multiple hydraulic fracturing holes in a square rock sample, step S5 uses multiple wells to pressurize sequentially to maintain the stress state in the three directions of XYZ, and uses a constant flow and constant pressure pump to inject hydraulic fracturing fluid of different flow rates into different hydraulic fracturing holes in sequence through the hydraulic fracturing water pressure application pipeline until the square rock sample is destroyed, record the data and turn off the constant flow and constant pressure pump.

[0019] Furthermore, when processing multiple hydraulic fracturing holes in a square rock sample, step S5 adopts synchronous pressurization of multiple wells to maintain the stress state in the three directions of XYZ, and uses a constant flow and constant pressure pump to synchronously inject hydraulic fracturing fluids of different flow rates into different hydraulic fracturing holes through the hydraulic fracturing water pressure application pipeline until the square rock sample is destroyed, record the data and turn off the constant flow and constant pressure pump.

[0020] Furthermore, after the experiment is completed in step S7, the debris of each pressure head and the square rock sample in the pressure chamber are cleaned, and the water and electricity are turned off.

[0021] The beneficial effects of the present invention are as follows: the present invention simulates single-well and multi-well model samples by machining a single hydraulic fracturing hole or multiple hydraulic fracturing holes on a square rock sample, and realizes experiments with different hydraulic pressurization time sequences, thereby completing a qualitative analysis of the evolution law of reservoir rock cracks under true triaxial stress conditions, thereby optimizing the location layout of hydraulic fracturing wells and the sequence of water pressure application, and improving the recovery rate of unconventional oil and gas resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0023] Figure 1 This is a schematic diagram of the structure of processing a hydraulic fracturing hole in a square rock sample in the present invention.

[0024] Figure 2 Schematic diagram of the structure of processing two hydraulic fracturing holes in a square rock sample in the present invention Figure 1 .

[0025] Figure 3 Schematic diagram of the structure of processing two hydraulic fracturing holes in a square rock sample in the present invention Figure 2 .

[0026] Figure 4 This is a schematic diagram of the structure of processing n hydraulic fracturing holes in a square rock sample in the present invention.

[0027] Figure 5This is a structural schematic diagram of the first arrangement position of the hydraulic fracturing holes in a square rock sample in the present invention.

[0028] Figure 6 This is a structural schematic diagram of the second arrangement position of the hydraulic fracturing holes in the square rock sample in the present invention.

[0029] Figure 7 This is a structural schematic diagram of the third arrangement position of the hydraulic fracturing holes in the square rock sample in the present invention.

[0030] Figure 8 Schematic diagram of the three-dimensional stress state of the square rock sample in step S7 of the present invention. DETAILED DESCRIPTION

[0031] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. 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 making any creative efforts are within the scope of protection of the present invention.

[0032] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequential order. It should be understood that the data used in this way can be interchanged where appropriate to describe the embodiments of the present application here. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or components is not necessarily limited to those steps or components clearly listed, but may include other steps or components that are not clearly listed or inherent to these processes, methods, products or devices.

[0033] In this application, terms such as "upper," "lower," "left," "right," "front," "back," "top," "bottom," "inner," "outer," "center," "vertical," "horizontal," "transverse," and "longitudinal" indicate positions or locations based on the positions or locations shown in the accompanying drawings. These terms are primarily intended to better describe this application and its embodiments and are not intended to limit the devices, elements, or components indicated to having a specific orientation, or to being constructed or operated in a specific orientation.

[0034] Furthermore, some of the above terms may be used to express other meanings besides indicating a position or location. For example, the term "on" may also be used to indicate a dependency or connection in certain circumstances. Those skilled in the art will understand the specific meanings of these terms in this application based on the specific circumstances.

[0035] Furthermore, the terms "installed," "disposed," "provided with," "connected," "connected," and "socketed" should be interpreted broadly. For example, they can refer to fixed connections, removable connections, or integral structures; mechanical connections or electrical connections; direct connections, indirect connections through an intermediary, or internal communication between two devices, elements, or components. Those skilled in the art will understand the specific meanings of these terms in this application based on the specific circumstances.

[0036] The present invention discloses a test method for simulating rock hydraulic fracturing under a true triaxial stress state, which specifically comprises the following steps:

[0037] S1. Sample preparation stage: Process the rock block into a standard square rock sample and process a number of hydraulic fracturing holes according to the target test size. The diameter, position arrangement and multi-hole spacing of the hydraulic fracturing holes are adjusted according to the actual working conditions.

[0038] Reference Attachment Figure 1-4 ,In this embodiment, the hydraulic fracturing holes can be divided into two types: single hole and multi-hole;

[0039] S2. Sample installation stage: First, place the square rock sample on the Y-direction pressure head. Make sure that the square rock sample is placed at the center of the Y-direction pressure head. Raise the Y-direction pressure head until the center of the square rock sample is consistent with the centers of the four XZ-direction pressure heads to avoid eccentric loading. Then, place the four XZ-direction pressure heads close to the square rock sample with a gap to ensure that no pressure is applied to the square rock sample. Finally, connect the hydraulic fracturing hole to the hydraulic fracturing water pressure application pipeline and place the Y+ direction pressure head close to the square rock sample to complete the sample installation stage.

[0040] Reference Attachment Figure 5-7 ,In this embodiment, the hydraulic fracturing holes have three relative ,setting positions in the X, Y, and Z directions, which are adjusted according to ,the experimental scheme;

[0041] S3, Equipment inspection stage: Turn on the hydraulic source, computer and oscilloscope, and observe whether there are any abnormalities in all displacement sensors, force sensors and hydraulic fracturing water pressure application pipelines. If there are no abnormalities, proceed to the next step;

[0042] S4, preload application stage: the servo controls the six indenters in the XYZ directions to symmetrically and slowly apply force to 10kN. After loading is completed, stabilize for one minute before proceeding to the next step;

[0043] S5, hydraulic fracturing model water pressure application stage: using a constant flow and constant pressure pump to inject hydraulic fracturing fluid into the hydraulic fracturing hole through the hydraulic fracturing water pressure application pipeline until the square rock sample is destroyed, recording data and turning off the constant flow and constant pressure pump;

[0044] S6, Unloading stage: After the test is completed, stop the hydraulic loading, and then slowly, symmetrically and synchronously reduce the stress in the X, Y and Z directions to zero to avoid the square rock specimen being damaged by the deviation stress caused by unloading too quickly or unsynchronized in the three directions; record the stress and strain data in the X, Y and Z directions of the true triaxial testing machine;

[0045] S7. End of test: After unloading is completed, the square rock sample is taken out and the shape of the square rock sample is observed to complete the test.

[0046] In step S2, after the pressure head in the Y+ direction is brought close to the square rock sample, the positions of each pressure head and the hydraulic fracturing water pressure application pipeline are checked.

[0047] The preload application stage in step S4 is divided into two loading stages;

[0048] The first loading stage adopts the force control mode, and different loading speeds and loading paths are used according to the test plan to load to the first preset stress state, where the stress in the Y direction is σ1, the stress in the Z direction is σ2, the stress in the X direction is σ3, and σ 1≥ σ 2≥ σ3; see attached Figure 8 , is a schematic diagram of the three-dimensional stress state of a square rock specimen;

[0049] The second loading stage adopts the force control mode. According to the actual construction conditions of the hydraulic fracturing project, the stress changes after the excavation of the hydraulic fracturing well are simulated. According to the test plan design, different loading speeds and loading paths are used to load to the second preset stress state.

[0050] When processing a hydraulic fracturing hole in a square rock sample, step S5 uses single-well pressurization to maintain the stress state in the three directions of X, Y, and Z. A constant flow and constant pressure pump is used to inject hydraulic fracturing fluid of different flow rates (pressures) into the hydraulic fracturing hole at different flow rates through the hydraulic fracturing water pressure application pipeline until the square rock sample is destroyed. The data is recorded and the constant flow and constant pressure pump is turned off.

[0051] When processing multiple hydraulic fracturing holes in a square rock specimen, step S5 employs sequential pressurization of multiple wells, maintaining stress states in the X, Y, and Z directions. A constant-flow, constant-pressure pump is then used to sequentially inject hydraulic fracturing fluids of varying flow rates (pressures) into each hydraulic fracturing hole (first, pressurizing the first hydraulic fracturing hole until it reaches a predetermined pressure; then, pressurizing the second hydraulic fracturing hole; and finally, pressurizing the nth hydraulic fracturing hole) at varying flow rates until the square rock specimen is destroyed. The data is then recorded and the constant-flow, constant-pressure pump is turned off. In this embodiment, the data represents the water pressure, flow rate, and other relevant data for each hydraulic fracturing hole.

[0052] When processing multiple hydraulic fracturing holes in a square rock specimen, step S5 uses simultaneous pressurization of multiple wells to maintain stress states in the X, Y, and Z directions. A constant-flow, constant-pressure pump is used to simultaneously inject hydraulic fracturing fluid at different flow rates through the hydraulic fracturing pressure application pipeline into each well (the first, second, and nth wells are pressurized simultaneously) until the square rock specimen is destroyed. The data is then recorded and the constant-flow, constant-pressure pump is turned off. In this embodiment, the data includes relevant data such as the water pressure and flow rate of each hydraulic fracturing well.

[0053] After the experiment is completed in step S7, the debris of each pressure head and the square rock sample in the pressure chamber are cleaned up, and the water and electricity are turned off.

[0054] The above are merely preferred embodiments of the present application and are not intended to limit the present application. Those skilled in the art will readily appreciate that various modifications and variations are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A test method for simulating rock hydraulic fracturing under true triaxial stress state, characterized in that: The specific steps include: S1. Sample preparation stage: Process the rock block into a standard square rock sample and process a number of hydraulic fracturing holes according to the target test size. The diameter, position arrangement and multi-hole spacing of the hydraulic fracturing holes are adjusted according to the actual working conditions. S2. Sample installation stage: First, place the square rock sample on the Y-direction pressure head. Make sure that the square rock sample is placed at the center of the Y-direction pressure head. Raise the Y-direction pressure head until the center of the square rock sample is consistent with the centers of the four XZ-direction pressure heads to avoid eccentric loading. Then, place the four XZ-direction pressure heads close to the square rock sample with a gap to ensure that no pressure is applied to the square rock sample. Finally, connect the hydraulic fracturing hole to the hydraulic fracturing water pressure application pipeline and place the Y+ direction pressure head close to the square rock sample to complete the sample installation stage. S3, Equipment inspection stage: Turn on the hydraulic source, computer and oscilloscope, and observe whether there are any abnormalities in all displacement sensors, force sensors and hydraulic fracturing water pressure application pipelines. If there are no abnormalities, proceed to the next step; S4, preload application stage: the servo controls the six indenters in the XYZ directions to symmetrically and slowly apply force to 10kN. After loading is completed, stabilize for one minute before proceeding to the next step; S5, hydraulic fracturing model water pressure application stage: using a constant flow and constant pressure pump to inject different flow rates of hydraulic fracturing fluid into the hydraulic fracturing hole through the hydraulic fracturing water pressure application pipeline at different flow rates until the square rock sample is destroyed, recording the data and turning off the constant flow and constant pressure pump; S6, Unloading stage: After the test is completed, stop the hydraulic loading, and then slowly, symmetrically and synchronously reduce the stress in the X, Y and Z directions to zero to avoid the square rock specimen being damaged by the deviation stress caused by unloading too quickly or unsynchronized in the three directions; record the stress and strain data in the X, Y and Z directions of the true triaxial testing machine; S7. End of test: After unloading is completed, the square rock sample is taken out and the shape of the square rock sample is observed to complete the test.

2. The test method for simulating rock hydraulic fracturing under true triaxial stress state according to claim 1, characterized in that: In step S2, after the pressure head in the Y+ direction is brought close to the square rock sample, the positions of each pressure head and the hydraulic fracturing water pressure application pipeline are checked.

3. The test method for simulating rock hydraulic fracturing under true triaxial stress state according to claim 1, characterized in that: The preload application stage in step S4 is divided into two loading stages; The first loading stage adopts the force control mode, and different loading speeds and loading paths are used according to the test plan to load to the first preset stress state, where the stress in the Y direction is σ1, the stress in the Z direction is σ2, the stress in the X direction is σ3, and σ 1≥ σ 2≥ σ3; The second loading stage adopts the force control mode. According to the actual construction conditions of the hydraulic fracturing project, the stress changes after the excavation of the hydraulic fracturing well are simulated. According to the test plan design, different loading speeds and loading paths are used to load to the second preset stress state.

4. The test method for simulating rock hydraulic fracturing under true triaxial stress state according to claim 1, characterized in that: When processing a hydraulic fracturing hole in a square rock sample, step S5 uses single-well pressurization to maintain the stress state in the three directions of X, Y, and Z. A constant flow and constant pressure pump is used to inject hydraulic fracturing fluid of different flow rates into the hydraulic fracturing hole through the hydraulic fracturing water pressure application pipeline until the square rock sample is destroyed. The data is recorded and the constant flow and constant pressure pump is turned off.

5. The test method for simulating rock hydraulic fracturing under true triaxial stress state according to claim 1, characterized in that: When processing multiple hydraulic fracturing holes in a square rock sample, step S5 uses sequential pressurization of multiple wells to maintain the stress state in the three directions of XYZ. A constant flow and constant pressure pump is used to inject hydraulic fracturing fluid of different flow rates into different hydraulic fracturing holes through the hydraulic fracturing water pressure application pipeline until the square rock sample is destroyed. The data is recorded and the constant flow and constant pressure pump is turned off.

6. The test method for simulating rock hydraulic fracturing under true triaxial stress state according to claim 1, characterized in that: When processing multiple hydraulic fracturing holes in a square rock sample, step S5 uses synchronous pressurization of multiple wells to maintain the stress state in the three directions of XYZ. A constant flow and constant pressure pump is used to synchronously inject hydraulic fracturing fluids of different flow rates into different hydraulic fracturing holes through the hydraulic fracturing water pressure application pipeline until the square rock sample is destroyed. The data is recorded and the constant flow and constant pressure pump is turned off.

7. The test method for simulating rock hydraulic fracturing under true triaxial stress state according to claim 1, characterized in that: After the experiment is completed in step S7, the debris of each pressure head and the square rock sample in the pressure chamber are cleaned up, and the water and electricity are turned off.

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