Experimental method of fracture toughness of subcritical propagation of fractures in shale reservoirs under soak conditions
By using a deep in-situ hydraulic fracturing experimental system and hollow symmetrical double-fracture samples, the propagation of fractures in shale samples was detected in real time, solving the simulation problem of subcritical propagation in deep shale gas reservoirs and providing a reliable basis for designing well shut-in pressure and time.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- CHINA NAT PETROLEUM CORP
- Filing Date
- 2024-12-11
- Publication Date
- 2026-06-12
AI Technical Summary
Existing technologies are insufficient to simulate the subcritical propagation of shale fractures under high geostress and high geothermal conditions in deep shale gas reservoirs, and cannot provide reliable design basis for well shut-in pressure and time.
A deep in-situ hydraulic fracturing experimental system was used to simulate high geostress and high geothermal conditions. The fracture propagation of shale samples under different well-clogging pressures was monitored using hollow symmetrical double-fracture samples. A acoustic emission probe was used for real-time detection. The subcritical fracture toughness KIC was calculated by combining natural seepage and unloading pore pressure to reduce well-clogging pressure.
It enables accurate determination of whether subcritical expansion has occurred in shale gas reservoirs under well-shutting conditions, and calculates the subcritical expansion fracture toughness, providing a theoretical basis for the construction design of deep shale gas reservoirs and optimizing well-shutting pressure and time.
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Figure CN122192957A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of geological engineering technology, and in particular to an experimental method for assessing the subcritical fracture propagation toughness of fractures under shale reservoir well-clogging conditions. Background Technology
[0002] Well-sealing is an important means to improve shale oil and gas production capacity and reduce extraction costs. During well-sealing, the subcritical propagation of shale fractures plays a crucial role. The well-sealing pressure promotes the full initiation and propagation of micro-fractures at the fracturing front, ultimately leading to subcritical propagation of macro-fractures, which in turn increases the complexity of fractures and the volume of reservoir stimulation. According to relevant studies, when a constant fluid pressure lower than the rock fracturing pressure is continuously applied to fractures, after a period of time, the fractures will undergo subcritical propagation and re-initiate propagation. This "delayed initiation" exhibits a significant time-pressure correlation, meaning that the time required for rock fracturing to occur varies depending on the magnitude of the well-sealing pressure. However, this subcritical fracture propagation is currently mainly found in granite. Whether subcritical propagation of fracturing fractures occurs in shale gas reservoirs remains to be solved. There is an urgent need for an experimental method to determine whether subcritical propagation occurs in shale gas reservoirs under different well-sealing pressures and to calculate the subcritical propagation fracture toughness, thereby more scientifically optimizing the design of well-sealing pressure and well-sealing time.
[0003] For example, a Chinese patent, application number 202110083452.0, application date January 21, 2021, entitled "A Simulation Method for Subcritical Crack Propagation Fracture Energy of a Rock Double-Torsion Specimen," describes the following technical solution: standard rock material specimens are prepared and tested to obtain the rock material's mechanical parameters; double-torsion specimens are prepared and tested to obtain the specimen's load relaxation rate and failure load; an extended finite element model of the double-torsion specimen is established, with pre-defined notches and cracks; the obtained rock material mechanical parameters, load relaxation rate, and failure load are substituted into the extended finite element model; the extended finite element model is loaded, and during loading, concentrated loads are applied through reference point coupling. The load was applied in the form of displacement, and the loading rate was controlled to obtain the crack propagation results of the double-torsion specimen. The correspondence between the maximum principal stress at the crack tip, the loading point force, and time was obtained, and the maximum principal stress-time curve and the loading point force-time curve at the crack tip were obtained. The preset crack initiation time was also read. The interaction between cracks was defined, and the energy release rate at the crack tip of the double-torsion specimen was calculated to obtain the crack tip energy release rate-time curve. An extended finite element model based on the VCCT method was established to calculate the energy release rate. The fracture energy value was preset and made greater than the empirical value. The load was applied in the form of displacement, and the loading rate was controlled to obtain the crack tip energy release rate at the time of pre-crack initiation as the critical energy release rate.
[0004] The aforementioned patent employs a dual-torsion test and numerical simulation method, using the mechanical parameters of rock materials and the parameters obtained from the dual-torsion test for simulation and application, thereby obtaining the energy release rate-time curve at the crack tip and acquiring the fracture energy. This provides data support for the study of avoiding subcritical crack propagation in rocks. However, the dual-torsion test method used in the aforementioned patent to study the fracture law of subcritical crack propagation is difficult to apply to the high geostress and high geothermal environment at depth, and cannot simulate the subcritical propagation of shale cracks in the deep environment. Therefore, the results obtained through the dual-torsion test method cannot provide a reliable theoretical basis and reference for the construction design and scheme optimization of deep shale gas reservoirs. Summary of the Invention
[0005] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide an experimental method for the subcritical propagation fracture toughness of shale reservoirs under well-steaming conditions that can adapt to the high geostress and high geothermal environment applied to deep depths and can simulate the subcritical propagation of shale fractures in deep environments.
[0006] To achieve the above-mentioned technical effects, the technical solution of the present invention is as follows:
[0007] An experimental method for assessing subcritical fracture propagation toughness under shale reservoir well-clogging conditions includes the following specific steps:
[0008] Step 1: Obtain first and second shale samples that can adapt to the high geostress and high geothermal environment at depth;
[0009] Step 2: Place the first shale sample into a deep in-situ hydraulic fracturing test system used to provide in-situ stress and formation temperature. Inject fracturing fluid into the deep in-situ hydraulic fracturing test system and gradually increase the pressure of the fracturing fluid until the first shale sample fractures propagate. Determine the peak pressure P required for the first shale sample fractures to propagate. b ;
[0010] Step 3: Place a portion of the second shale sample into a deep in-situ hydraulic fracturing test system used to provide in-situ stress and formation temperature. Inject fracturing fluid into the deep in-situ hydraulic fracturing test system to the preset well-closing pressure value, then stop injecting fracturing fluid and gradually reduce the well-closing pressure value. During the process of gradually reducing the well-closing pressure value, determine whether the fractures in this portion of the second shale sample have undergone subcritical propagation.
[0011] Step 4: When the fractures in the second shale sample from Step 3 have undergone subcritical propagation, place the remaining second shale sample into a deep in-situ hydraulic fracturing test system. Inject fracturing fluid into the deep in-situ hydraulic fracturing test system to the pre-set well pressure value, and then stop injecting fracturing fluid. Calculate the subcritical fracture toughness K corresponding to the subcritical propagation. ICAnd the time when subcritical expansion was detected corresponding to the well stagnation pressure value.
[0012] Furthermore, the specific steps for obtaining the first and second shale samples, which can adapt to the high geostress and high geothermal environment at depth, in step one are as follows:
[0013] Step a: Obtain multiple shale samples from deep in-situ shale gas reservoirs;
[0014] Step b: Prepare multiple shale samples into multiple hollow symmetrical double-fracture samples;
[0015] Step c: Divide the prepared hollow symmetrical double-fracture samples into a first shale sample for determining the peak pressure required for shale fracture propagation in a deep in-situ shale gas reservoir environment and a second shale sample for determining whether shale fractures have undergone subcritical propagation.
[0016] Furthermore, in step b, the hollow symmetrical double-fracture sample is provided with a pre-made hole, which is a through hole along the central axis of the shale sample; along the radial direction of the circular surface passing through the pre-made hole, a pre-cut crack connected to the pre-made hole is also provided, and the pre-cut crack is symmetrically arranged with equal length on both sides of the pre-made hole in the radial direction.
[0017] Furthermore, the specific structural standard for the hollow symmetrical double-crack specimen in step b is as follows: a cylinder with a height twice the diameter, the diameter of the pre-drilled hole on the hollow symmetrical double-crack specimen being twenty times the width of the pre-cut crack, and the diameter of the pre-drilled hole being 1.5 times the length of the pre-cut crack.
[0018] Furthermore, the specific parameters of the hollow symmetrical double-crack specimen are as follows: a cylinder with a diameter of 25 mm and a height of 50 mm, a pre-drilled hole diameter of 3 mm, a pre-cut crack width of 0.15 mm, and a pre-cut crack length of 2 mm.
[0019] Furthermore, the multiple shale samples obtained in step a are shale core samples or shale outcrop samples.
[0020] Furthermore, in step a, multiple shale samples are obtained using a rock corer.
[0021] Furthermore, multiple acoustic emission probes are installed at both the upper and lower ends of the first and second shale samples to detect crack propagation in the first shale sample and subcritical propagation in the second shale sample.
[0022] Furthermore, four acoustic emission probes were installed at the top and bottom of the first shale sample, and four acoustic emission probes were installed at the top and bottom of the second shale sample.
[0023] Furthermore, in step two, the injection rate of fracturing fluid is no more than 10 ml / min.
[0024] Furthermore, the preset pressure value for well shut-in in step three is less than the peak pressure P required for shale fracture propagation. b .
[0025] Furthermore, the preset well-sealing pressure values in steps three and four include multiple well-sealing pressure values. By repeating step three, it is determined whether subcritical propagation of fractures occurs in the second shale sample under multiple well-sealing pressure values, and in step four, the various subcritical fracture toughness K values corresponding to these different well-sealing pressure values are determined. IC .
[0026] Furthermore, the range of the various well-sealing pressure values is 60% of the peak pressure P. b ~90% peak pressure value P b .
[0027] Furthermore, in step three, after no more fracturing fluid is injected, the specific method for gradually reducing the well stagnation pressure is as follows: by natural seepage or by unloading the pore pressure, the well stagnation pressure is gradually reduced to determine whether the fractures in the second shale sample have undergone subcritical propagation.
[0028] Furthermore, after no more fracturing fluid is injected in step four, the pre-set pressure value for well shut-in is maintained by adjusting the regulating pump pressure of the deep in-situ hydraulic fracturing experimental system, and the subcritical fracture toughness K corresponding to the occurrence of subcritical propagation is obtained. IC And the time when subcritical expansion was detected corresponding to the well stagnation pressure value.
[0029] Furthermore, the specific method of natural seepage is as follows: by allowing fracturing fluid to seep naturally from the pre-cut fractures of the hollow symmetrical double-fracture sample without artificial intervention, the well suffocation pressure is reduced; the specific method of unloading pore pressure is as follows: by adjusting the fracturing fluid flow rate at the injection end and outlet end of the deep in-situ environment hydraulic fracturing experimental system and adjusting the pump pressure, the well suffocation pressure is adjusted.
[0030] Furthermore, the subcritical fracture toughness K corresponding to the occurrence of subcritical propagation is obtained. IC The specific formula is as follows:
[0031]
[0032] In the formula, P injmax The pressure value that causes subcritical propagation of pre-cut fractures in the second shale sample; σ m denoted as the well pressure value; a is the pre-cut fracture length of the shale sample; f is the shape factor of the shale sample.
[0033] Furthermore, deionized purified water or slickwater is used as the fracturing fluid.
[0034] According to the above technical solution, the beneficial effects of the present invention are as follows:
[0035] 1. The experimental method for subcritical fracture toughness of shale reservoirs under suffocation conditions of the present invention enables the determination of whether subcritical expansion of shale gas reservoirs has occurred under suffocation conditions, and the calculation of subcritical expansion fracture toughness, thereby providing important theoretical basis and reference for the construction design and scheme optimization of deep shale gas reservoirs.
[0036] 2. The present invention uses a first shale sample and a second shale sample. Compared with the double torsion test method used in the prior art, it can better adapt to the high ground stress and high ground temperature environment at deep depths, thereby more accurately judging subcritical extension and more precisely calculating the subcritical extension fracture toughness.
[0037] 3. The experimental method adopted in this invention, by preset different well-suppression pressure values, maintains the well-suppression pressure value at a constant pressure until subcritical fracture propagation occurs, thereby obtaining the fracture toughness evolution law of shale fractures in deep in-situ environment. This solves the problem of the relationship between well-suppression pressure and well-suppression time in the prior art, which characterizes the subcritical propagation of fractures in deep shale reservoirs, and thus allows for the scientific optimization of the design of well-suppression pressure and well-suppression time.
[0038] 4. The method used in this invention enables indoor well-sealing simulation experiments, simulating the state where well-sealing pressure changes as ground stress and ground temperature gradually increase. Compared to existing technologies, which do not have this experimental capability in actual wells, this invention can provide a more suitable design for well-sealing pressure and time through experimental simulation.
[0039] 5. The method employed in this invention utilizes two methods to reduce well pressure: natural seepage and unloading pore pressure. By using these two methods to reduce well pressure, it is possible to more accurately determine whether the pre-cut fractures in the shale sample have undergone subcritical propagation. Attached Figure Description
[0040] Figure 1 This is a schematic diagram of the method flow of the present invention.
[0041] Figure 2 This is a schematic diagram of the experimental curve for observing subcritical propagation after injecting fracturing fluid.
[0042] Figure 3 This is a schematic diagram of the curves of well sump pressure versus macroscopic fracturing time. Detailed Implementation
[0043] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of this application, but not all embodiments.
[0044] Example 1
[0045] like Figure 1 As shown, the experimental method for subcritical fracture propagation toughness under shale reservoir well-drainage conditions includes the following specific steps:
[0046] Step 1: Obtain first and second shale samples that can adapt to the high geostress and high geothermal environment at depth;
[0047] Step 2: Place the first shale sample into a deep in-situ hydraulic fracturing test system used to provide in-situ stress and formation temperature. Inject fracturing fluid into the deep in-situ hydraulic fracturing test system and gradually increase the pressure of the fracturing fluid until the first shale sample fractures propagate. Determine the peak pressure P required for the first shale sample fractures to propagate. b The crack in the first shale sample here is the same crack structure as the pre-cut crack in the first shale sample mentioned later in this invention.
[0048] Step 3: Place a portion of the second shale sample into a deep in-situ hydraulic fracturing test system used to provide in-situ stress and formation temperature. Inject fracturing fluid into the deep in-situ hydraulic fracturing test system to the preset well-closing pressure value, then stop injecting fracturing fluid and gradually reduce the well-closing pressure value. During the gradual reduction of the well-closing pressure value, determine whether the fractures in this portion of the second shale sample have undergone subcritical propagation. The fractures in the second shale sample here are the same fracture structure as the pre-cut fractures in the second shale sample described later in this invention.
[0049] Step 4: When the fractures in the second shale sample from Step 3 have undergone subcritical propagation, place the remaining second shale sample into a deep in-situ hydraulic fracturing test system. Inject fracturing fluid into the deep in-situ hydraulic fracturing test system to the pre-set well pressure value, and then stop injecting fracturing fluid. Calculate the subcritical fracture toughness K corresponding to the subcritical propagation. IC And the time when subcritical expansion was detected corresponding to the well stagnation pressure value.
[0050] The specific steps for obtaining the first and second shale samples, which can adapt to the high ground stress and high geothermal environment at depth, in Step 1 are as follows:
[0051] Step a: Obtain multiple shale samples from deep in-situ shale gas reservoirs;
[0052] Step b: Prepare multiple shale samples into multiple hollow symmetrical double-fracture samples;
[0053] Step c: Divide the prepared hollow symmetrical double-fracture samples into a first shale sample for determining the peak pressure required for shale fracture propagation in a deep in-situ shale gas reservoir environment and a second shale sample for determining whether shale fractures have undergone subcritical propagation.
[0054] In step b, a pre-drilled hole is provided on the hollow symmetrical double-fracture sample, and the pre-drilled hole is a through hole along the central axis of the shale sample; along the radial direction of the circular surface passing through the pre-drilled hole, a pre-cut crack connected to the pre-drilled hole is also provided, and the pre-cut crack is symmetrically arranged with equal length on both sides of the pre-drilled hole in the radial direction.
[0055] The specific structural setup standard for the hollow symmetrical double-crack specimen in step b is as follows: a cylinder with a height twice the diameter, the diameter of the pre-drilled hole on the hollow symmetrical double-crack specimen being twenty times the width of the pre-cut crack, and the diameter of the pre-drilled hole being 1.5 times the length of the pre-cut crack.
[0056] The specific parameters of the hollow symmetrical double-fracture specimen are as follows: a cylinder with a diameter of 25 mm and a height of 50 mm; a pre-drilled hole diameter of 3 mm; a pre-cut fracture width of 0.15 mm; and a pre-cut fracture length of 2 mm. Shale specimens were prepared in this manner to create the first and second shale specimens, which have the characteristics of simulating fracture propagation under hydraulic fracturing conditions and easily simulating complex geological conditions of deep high temperature and high pressure. To investigate the subcritical fracture propagation law under deep high temperature and high pressure in-situ environment of shale, the confining pressure fluid and temperature control module of the deep in-situ environment hydraulic fracturing experimental system were used to simulate the in-situ stress and formation temperature of the deep in-situ environment under real conditions. Then, the deep in-situ environment hydraulic fracturing experiment was conducted... The upper and lower hydraulic systems of the testing system inject fracturing fluid into both ends of the shale sample, respectively. To simulate the in-situ stress and temperature of the deep environment in close proximity to real conditions, the stress and temperature of the target reservoir can be determined by consulting relevant data or conducting on-site surveys. For example, the stress and temperature at the target burial depth can be determined by using the average stress gradient and temperature gradient of the shale reservoir. For example, the average minimum horizontal stress of a shale reservoir in a certain area is approximately 0.024 MPa / m, and the formation temperature is T = 18 + 0.0243h, where h is the formation depth in meters. This allows us to determine the stress and temperature at the target reservoir depth.
[0057] The deep in-situ hydraulic fracturing experimental system used in this invention can be a rock high temperature and high pressure environment simulation experimental device and experimental system disclosed in CN219179125U. Therefore, the deep in-situ hydraulic fracturing experimental system used in this invention is the prior art in this field, and will not be described in detail here.
[0058] The multiple shale samples obtained in step a are either shale core samples or shale outcrop samples. The multiple shale samples in step a are obtained using a rock corer. Shale outcrop samples are easier to obtain than shale core samples, and accurate experimental results can be obtained using shale outcrop samples. Shale outcrop samples can also be sampled using existing devices.
[0059] Multiple acoustic emission probes are installed at both the top and bottom of the first and second shale samples to detect crack propagation in the first shale sample and subcritical propagation in the second shale sample. The number of acoustic emission probes that can be installed on the shale sample depends on the size of the shale sample. The purpose is to monitor the crack propagation and subcritical propagation of the shale sample in real time. Therefore, when the subcritical propagation of the shale sample can be monitored completely in real time, the installation of more acoustic emission probes can be stopped. The acoustic emission probes work in conjunction with acoustic emission (AE) to monitor the propagation of the shale sample in real time. The acoustic emission probes are common devices in the prior art.
[0060] Four acoustic emission probes are installed at the top and bottom of the first shale sample, and four acoustic emission probes are installed at the top and bottom of the second shale sample; a total of eight acoustic emission probes are arranged in each first shale sample and each second shale sample.
[0061] Example 2
[0062] Based on Example 1, the injection rate of fracturing fluid in step two is no more than 10 ml / min; the injection rate of fracturing fluid is adjusted according to the size of the shale sample; for example, the specific parameters of the first shale sample are: a cylinder with a diameter of 25 mm and a height of 50 mm, a pre-drilled hole diameter of 3 mm, a pre-cut fracture width of 0.15 mm, and a pre-cut fracture length of 2 mm.
[0063] The preset pressure value for well shut-in in step three is less than the peak pressure P required for shale fracture propagation. b Steps three and four involve setting multiple pre-set well-sealing pressure values. By repeating step three, it is determined whether subcritical propagation of fractures occurs in the second shale sample under various well-sealing pressure values. Additionally, step four involves determining the subcritical fracture toughness K corresponding to various well-sealing pressure values. IC .
[0064] The range of various well-sealing pressure values is 60% of the peak pressure P. b ~90% peak pressure value P bIn step three, after ceasing the injection of fracturing fluid, the specific method for gradually reducing the well-closing pressure is as follows: by naturally flowing through or by unloading the pore pressure, the well-closing pressure is gradually reduced to determine whether subcritical propagation has occurred in the second shale sample fracture. In step four, after ceasing the injection of fracturing fluid, the preset well-closing pressure is maintained by adjusting the regulating pump pressure of the deep in-situ hydraulic fracturing experimental system, and the subcritical fracture toughness K corresponding to subcritical propagation is obtained. IC The specific method of natural seepage is as follows: by allowing fracturing fluid to seep naturally from the pre-cut fractures of the hollow symmetrical double-fracture sample without artificial intervention, the pressure of the well being shut off is reduced. The specific method of unloading pore pressure is as follows: by adjusting the fracturing fluid flow rate at the injection end and the outlet end of the deep in-situ environment hydraulic fracturing experimental system and adjusting the pump pressure, the pressure of the well being shut off is adjusted.
[0065] The subcritical fracture toughness K corresponding to the occurrence of subcritical propagation is obtained. IC The specific formula is as follows:
[0066]
[0067] In the formula, P injmax The pressure value that causes subcritical propagation of pre-cut fractures in the second shale sample; σ m denoted as , where is the well pressure value; 'a' is the pre-cut fracture length of the shale sample; and 'f' is the shape factor of the shale sample. The shape factor of the shale sample is obtained by first conducting a uniaxial compression test on the shale sample to obtain its elastic parameters. Then, a semi-3D model is established in ABAQUS based on the sample size, boundary conditions are set according to the experimental conditions, and the aforementioned elastic parameters of the shale are substituted into the model to calibrate the shale fracture resistance-shape factor f. The specific method for obtaining the shape factor of the shale sample is a prior art in this field.
[0068] Example 3
[0069] Based on Example 2, such as Figure 2 The diagram illustrates a specific implementation of the second step experiment. In this step, a deep in-situ hydraulic fracturing experimental system is used to provide the first shale sample with a geostress and geothermal environment corresponding to actual conditions. Then, according to... Figure 2 As shown, fracturing fluid was injected at a fixed injection rate of 3 ml / min to induce subcritical propagation of the pre-cut fractures in the first shale. The peak pressure P required for subcritical propagation of the pre-cut fractures in the first shale sample under deep in-situ conditions was obtained. b Repeat step two of the experiment, and take the peak pressure P obtained from the two experiments. b The average value of P is taken as the peak pressure P required for shale fracture propagation.b In step two of the experiment, deionized purified water was used as the fracturing fluid. When time t... b After subcritical expansion occurs, such as Figure 2 As shown, the pressure value of the well will drop rapidly when it is sealed.
[0070] Example 4
[0071] Based on Example 3, such as Figure 2 As shown, Figure 2 The experiment in step three was also demonstrated, in which the preset pressure value for well sealing was set to 90% of the peak pressure P. b Similarly, as in step two of the experiment, the geostress and geothermal environment around the second shale sample were kept the same as the actual geostress and geothermal environment. Then, as... Figure 2 As shown, fracturing fluid was injected at a fixed injection rate of 3 ml / min, and fracturing fluid pressure was applied to the second shale sample. When the pressure increased to 90% of the peak pressure P... b In other words, when the preset pressure value for well shut-off is reached, no more fracturing fluid is injected, and the time at this point is t. b1 , in t b1 Two methods were selected: natural seepage and unloading pore pressure. The well pressure was gradually reduced to determine whether subcritical propagation had occurred. In step three, deionized pure water or slickwater was used as the fracturing fluid in the experiment.
[0072] Example 5
[0073] Based on Example 3, such as Figure 3 As shown, this paper illustrates the subcritical propagation time of the pre-cut fracture in the second shale sample corresponding to various well-sealing pressure values obtained in step four. A curve showing the macroscopic fracture time versus well-sealing pressure time is plotted, and the corresponding subcritical fracture toughness KI is calculated. C ;like Figure 3 In this process, different well-clogging pressures were applied to multiple second shale samples, with the specific values being: 90% peak pressure P b1 80% peak pressure P b2 70% peak pressure P b3 and 60% peak pressure P b4 Similarly, using a fixed injection rate of 3 ml / min, after multiple second shale samples reached the four specific values of the aforementioned well-closing pressure, the well-closing pressure was kept constant by adjusting the pump pressure of the deep in-situ hydraulic fracturing experimental system. The subcritical propagation time of the pre-cut fractures in the second shale samples corresponding to each well-closing pressure value was observed, and a curve of macroscopic fracturing time versus well-closing pressure time was plotted. For example... Figure 3 90% of the peak pressure P b1 In tc1 At that time, the pre-cut fractures in the second shale began to propagate subcritically, reaching 80% of the peak pressure P. b2 In t c2 The pre-cut fractures in the second shale began to propagate subcritically, reaching 70% of the peak pressure P. b3 In t c3 The pre-cut fractures in the second shale began to propagate subcritically, reaching 60% of the peak pressure P. b4 In t c4 When the pre-cut fractures in the second shale begin to propagate subcritically, and then the peak pressure P is added... b and peak pressure P b Time t b When the pre-cut fractures of the first shale sample undergo subcritical propagation, a curve of well-clogging pressure versus macroscopic fracturing time is plotted. The macroscopic fracturing time refers to the time it takes for the pre-cut fractures of the shale sample to undergo subcritical propagation. The curve of well-clogging pressure versus macroscopic fracturing time can also be combined with existing fracturing models such as KGD or PKN to determine the critical net pressure for subcritical propagation of fractures in deep in-situ environments and whether a critical depth exists, thus laying a solid foundation for optimizing well-clogging pressure design.
[0074] The above description is a detailed description of the preferred embodiments of this application. However, the embodiments are not intended to limit the scope of the patent application of this application. All equivalent changes or modifications made under the technical spirit of this application should fall within the patent scope covered by this application.
Claims
1. An experimental method for assessing subcritical fracture propagation toughness under shale reservoir well-clogging conditions, characterized in that, The specific steps include the following: Step 1: Obtain first and second shale samples that can adapt to the high geostress and high geothermal environment at depth; Step 2: Place the first shale sample into a deep in-situ hydraulic fracturing test system used to provide in-situ stress and formation temperature. Inject fracturing fluid into the deep in-situ hydraulic fracturing test system and gradually increase the pressure of the fracturing fluid until the first shale sample fractures propagate. Determine the peak pressure P required for the first shale sample fractures to propagate. b ; Step 3: Place a portion of the second shale sample into a deep in-situ hydraulic fracturing test system used to provide in-situ stress and formation temperature. Inject fracturing fluid into the deep in-situ hydraulic fracturing test system to the preset well-closing pressure value, then stop injecting fracturing fluid and gradually reduce the well-closing pressure value. During the process of gradually reducing the well-closing pressure value, determine whether the fractures in this portion of the second shale sample have undergone subcritical propagation. Step 4: When the fractures in the second shale sample from Step 3 have undergone subcritical propagation, place the remaining second shale sample into a deep in-situ hydraulic fracturing test system. Inject fracturing fluid into the deep in-situ hydraulic fracturing test system to the pre-set well pressure value, and then stop injecting fracturing fluid. Calculate the subcritical fracture toughness K corresponding to the subcritical propagation. IC And the time when subcritical expansion was detected corresponding to the well stagnation pressure value.
2. The experimental method for subcritical fracture propagation toughness under shale reservoir well-drainage conditions according to claim 1, characterized in that, The specific steps for obtaining the first and second shale samples, which can adapt to the high ground stress and high geothermal environment at depth, in Step 1 are as follows: Step a: Obtain multiple shale samples from deep in-situ shale gas reservoirs; Step b: Prepare multiple shale samples into multiple hollow symmetrical double-fracture samples; Step c: Divide the prepared hollow symmetrical double-fracture samples into a first shale sample for determining the peak pressure required for shale fracture propagation in a deep in-situ shale gas reservoir environment and a second shale sample for determining whether shale fractures have undergone subcritical propagation.
3. The experimental method for subcritical fracture propagation toughness under shale reservoir well-clogging conditions according to claim 2, characterized in that: In step b, a pre-drilled hole is provided on the hollow symmetrical double-fractured sample, and the pre-drilled hole is a through hole along the central axis of the shale sample; along the radial direction of the circular surface passing through the pre-drilled hole, a pre-cut crack connected to the pre-drilled hole is also provided, and the pre-cut crack is symmetrically arranged with equal length on both sides of the pre-drilled hole in the radial direction.
4. The experimental method for subcritical fracture propagation toughness under shale reservoir well-clogging conditions according to claim 2, characterized in that: The specific structural setup standard for the hollow symmetrical double-crack specimen in step b is as follows: a cylinder with a height twice the diameter, the diameter of the pre-drilled hole on the hollow symmetrical double-crack specimen being twenty times the width of the pre-cut crack, and the diameter of the pre-drilled hole being 1.5 times the length of the pre-cut crack.
5. The experimental method for subcritical fracture propagation toughness under shale reservoir well-clogging conditions according to claim 4, characterized in that: The specific parameters of the hollow symmetrical double-crack specimen are as follows: a cylinder with a diameter of 25 mm and a height of 50 mm, a pre-drilled hole diameter of 3 mm, a pre-cut crack width of 0.15 mm, and a pre-cut crack length of 2 mm.
6. The experimental method for subcritical fracture propagation toughness under shale reservoir well-clogging conditions according to claim 2, characterized in that: The multiple shale samples obtained in step a are shale core samples or shale outcrop samples.
7. The experimental method for subcritical fracture propagation toughness under shale reservoir well-clogging conditions according to claim 2, characterized in that: In step a, multiple shale samples are obtained using a rock corer.
8. The experimental method for subcritical fracture propagation toughness under shale reservoir well-drainage conditions according to claim 1, characterized in that: Multiple acoustic emission probes are installed at both the top and bottom of the first and second shale samples to detect crack propagation in the first shale sample and subcritical propagation in the second shale sample.
9. The experimental method for subcritical fracture propagation toughness under shale reservoir well-drainage conditions according to claim 1, characterized in that: Four acoustic emission probes were installed at the top and bottom of the first shale sample, and four acoustic emission probes were installed at the top and bottom of the second shale sample.
10. The experimental method for subcritical fracture propagation toughness under shale reservoir well-drainage conditions according to claim 1, characterized in that: In step two, the injection rate of fracturing fluid shall not exceed 10 ml / min.
11. The experimental method for subcritical fracture propagation toughness under shale reservoir well-drainage conditions according to claim 1, characterized in that: The preset pressure value for well shut-in in step three is less than the peak pressure P required for shale fracture propagation. b .
12. The experimental method for subcritical fracture propagation toughness under shale reservoir well-drainage conditions according to claim 1, characterized in that: The pre-set well-sealing pressure values in steps three and four include multiple well-sealing pressure values. By repeating step three, it is determined whether subcritical propagation of fractures occurs in the second shale sample under multiple well-sealing pressure values, and the various subcritical fracture toughness K values corresponding to the multiple well-sealing pressure values in step four are also considered. IC .
13. The experimental method for subcritical fracture propagation toughness under shale reservoir well-drainage conditions according to claim 12, characterized in that: The range of the various well-sealing pressure values is 60% of the peak pressure P. b ~90% peak pressure value P b .
14. The experimental method for subcritical fracture propagation toughness under shale reservoir well-drainage conditions according to claim 1, characterized in that: In step three, after the fracturing fluid is no longer injected, the specific method for gradually reducing the well stagnation pressure is as follows: by natural seepage or by unloading the pore pressure, the well stagnation pressure is gradually reduced to determine whether the fractures in the second shale sample have undergone subcritical propagation.
15. The experimental method for subcritical fracture propagation toughness under shale reservoir well-drainage conditions according to claim 1, characterized in that: In step four, after no more fracturing fluid is injected, the pre-set pressure value for well shut-in is maintained by adjusting the regulating pump pressure of the deep in-situ hydraulic fracturing experimental system. The subcritical fracture toughness K corresponding to the occurrence of subcritical propagation is then obtained. IC And the time when subcritical expansion was detected corresponding to the well stagnation pressure value.
16. The experimental method for subcritical fracture propagation toughness under shale reservoir well-drainage conditions according to claim 14, characterized in that: The specific method of natural seepage is as follows: by allowing fracturing fluid to seep out naturally from the pre-cut fractures of the hollow symmetrical double-fracture sample without artificial intervention, the well suffocation pressure is reduced; the specific method of unloading pore pressure is as follows: by adjusting the fracturing fluid flow rate at the injection end and outlet end of the deep in-situ environment hydraulic fracturing experimental system and adjusting the pump pressure, the well suffocation pressure is adjusted.
17. The experimental method for determining the subcritical fracture propagation toughness of fractures under shale reservoir well-drainage conditions according to claim 1, characterized in that... The subcritical fracture toughness K corresponding to the occurrence of subcritical propagation is obtained. IC The specific formula is as follows: In the formula, P injmax The pressure value that causes subcritical propagation of pre-cut fractures in the second shale sample; σ m denoted as the well pressure value; a is the pre-cut fracture length of the shale sample; f is the shape factor of the shale sample.
18. The experimental method for subcritical fracture propagation toughness under shale reservoir well-drainage conditions according to claim 1, characterized in that: The fracturing fluid used is deionized purified water or slickwater.