Experimental system and method for determining dynamic closure characteristics of cracks

Abstract

The present application provides an experimental system and method for determining the dynamic closure characteristics of cracks, including: a wrapping system for wrapping a specimen to prevent the surface of the specimen from contacting the hydraulic fluid, the specimen having a hydraulic fluid application channel; a true triaxial hydraulic fracturing simulation device for axially pressurizing the wrapped specimen to a first target value and radially pressurizing the wrapped specimen to a second target value, stabilizing the first target value and the second target value for a preset time, and after the preset time, adding hydraulic fluid to the hydraulic fluid application channel at a preset rate until the specimen is ruptured; a scanning system for scanning the damaged specimen to obtain a three-dimensional morphological image of the crack; a detection system for measuring the circumferential deformation information of the specimen in real time; and a processing system for determining the dynamic closure characteristics of the crack based on the circumferential deformation information and the three-dimensional morphological image.

Description

Technical Field

[0001] The present application relates to the field of oil and gas development, and in particular to an experimental system and method for determining dynamic closure characteristics of fractures. Background Art

[0002] The micro-injection pressure drop test in tight reservoirs involves injecting a certain amount of liquid (generally a few cubic meters to more than ten cubic meters) into the reservoir at a constant, small displacement rate to create a hydraulic fracture of a certain size. The well is then shut down and the pump is stopped to monitor the pressure drop. After the pump is stopped, the liquid in the fracture is lost to the formation under the action of the pressure difference. As the fluid pressure in the fracture decreases, the fracture morphology continues to change until it gradually closes. Based on the pressure drop data at different time stages after the pump is stopped, information such as reservoir physical properties and fracture parameters is evaluated. There are various methods for interpreting pressure drop data after pumping is stopped in the prior art, which mainly assume the dynamic closure law of the fracture and then solve the formation and fracture information through pressure drop. The accuracy of the description of the fracture closure law caused by the pressure drop during the pumping process will directly affect the interpretation of the pressure drop during the pumping process. At present, the fracture closure law is mainly obtained based on rock mechanics mechanisms and simulation analysis. Summary of the Invention

[0003] In response to the above problems, the present application provides an experimental system and method for determining the dynamic closure characteristics of cracks.

[0004] This application provides an experimental system for determining the dynamic closure characteristics of a crack, comprising:

[0005] a wrapping system for wrapping a test piece to prevent a surface of the test piece from contacting the hydraulic fluid, wherein the test piece has a hydraulic fluid application channel;

[0006] A true triaxial hydraulic fracturing simulation device is configured to pressurize the wrapped sample axially to a first target value and radially to a second target value, stabilize the first target value and the second target value for a preset time period, and then, after the preset time period, apply channel-added hydraulic fluid at a preset rate until the sample is fractured;

[0007] A scanning system is used to scan the sample after destruction to obtain a three-dimensional image of the crack;

[0008] A detection system, used for measuring the circumferential deformation information of the sample in real time;

[0009] A processing system is used to determine the dynamic closure characteristics of the crack based on the annular deformation information and the three-dimensional morphological image.

[0010] In some embodiments, the specimen is cylindrical, and the packaging system comprises:

[0011] A fluorine and chlorine test piece cover, used for wrapping the side of the test piece;

[0012] an upper pressure head, used for covering the upper end surface of the test piece;

[0013] A lower pressure head, used for covering the lower end surface of the test piece;

[0014] A fastener is used to fasten the fluorine and chlorine test piece sleeve to the side surface.

[0015] In some embodiments, the detection system comprises:

[0016] The circumferential extensometer is arranged on the fluorine and chlorine test piece sleeve and is used for measuring the circumferential deformation information of the test piece in real time.

[0017] In some embodiments, the dynamic closure characteristics of the crack include: a change pattern of the crack opening, and the processing system includes:

[0018] a first determining module, configured to determine a diameter change based on the circumferential deformation information at the first moment and the circumferential deformation information at the second moment, and determine a circumference change based on the diameter change;

[0019] A second determination module is used to determine the angle between the normal direction of the three-dimensional crack surface and the axial direction of the specimen based on the three-dimensional morphological image;

[0020] The third determination module is configured to determine the crack width based on the perimeter change and the angle, so as to determine a change pattern of the crack opening based on the crack width.

[0021] In some embodiments, the scanning system is further configured to determine crack morphology data by tracing the crack edge contour based on the three-dimensional topography image, wherein the crack morphology data includes: crack morphology and crack area.

[0022] In some embodiments, it further includes:

[0023] The sample making system is used to process the rock sample into a cylindrical initial sample and to drill a hole at the center of the end surface of the cylindrical initial sample, wherein the hole has a preset depth and a preset diameter, so as to make a hydraulic fluid application channel based on the hole to obtain the sample.

[0024] In some embodiments, the cylindrical initial sample has a diameter of 100 mm, a height of 200 mm, a preset depth of 150 mm, and a preset diameter of 10 mm.

[0025] The present embodiment provides an experimental method for determining the dynamic closure characteristics of a crack, which is applied to the experimental system for determining the dynamic closure characteristics of a crack in any of the above embodiments, including:

[0026] preparing a test specimen, wherein the test specimen has a hydraulic fluid application channel;

[0027] wrapping the sample;

[0028] After axially pressurizing the wrapped specimen to a first target value and radially pressurizing the wrapped specimen to a second target value, stabilizing the first target value and the second target value for a preset time, and after the preset time, applying channel-adding hydraulic fluid at a preset rate until the specimen ruptures;

[0029] Scan the damaged specimen to obtain a three-dimensional image of the crack;

[0030] The circumferential deformation information of the sample is measured in real time, and the dynamic closure characteristics of the crack are determined based on the circumferential deformation information and the three-dimensional morphological image.

[0031] In some embodiments, preparing the sample comprises:

[0032] Collect rock samples;

[0033] Processing the rock sample to obtain a cylindrical initial sample;

[0034] Punching a hole at a center point of the end surface of the cylindrical initial sample to form a hole with a preset depth and a preset diameter;

[0035] A fiberglass rod was placed in the hole to create a hydraulic fluid application channel to obtain the sample.

[0036] In some embodiments, after axially pressurizing the wrapped sample to a first target value and radially pressurizing the wrapped sample to a second target value, stabilizing the first target value and the second target value for a preset time, and then applying the hydraulic fluid to the channel at a preset rate after the preset time until the sample is ruptured, comprising:

[0037] The wrapped sample is loaded radially at a first rate of confining pressure to a second target value, and the wrapped sample is loaded axially at a second rate to the first target value;

[0038] Stabilize the first target value and the second target value for a preset time period;

[0039] After a predetermined period of time, hydraulic fluid is applied at a predetermined rate to rupture the sample, wherein the first rate is less than the second rate.

[0040] The present application provides an experimental system and method for determining the dynamic closure characteristics of cracks. A wrapping system is set up to wrap the specimen, and the wrapped specimen is pressurized and fractured by a true triaxial hydraulic fracturing simulation device. A scanning system is used to scan the damaged specimen, and a detection system measures the circumferential deformation information of the specimen. A processing system is used to determine the dynamic closure characteristics of the crack based on the circumferential deformation information and three-dimensional morphological images, thereby realizing the acquisition of crack closure characteristics in an experimental manner. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Hereinafter, the present application will be described in more detail based on embodiments with reference to the accompanying drawings.

[0042] Figure 1 A schematic diagram of the structure of an experimental system for determining dynamic closure characteristics of cracks provided in an embodiment of the present application;

[0043] Figure 2 A schematic diagram of the installation structure of a packaging system provided in an embodiment of the present application;

[0044] Figure 3 A schematic diagram of an implementation flow of an experimental method for determining dynamic closure characteristics of a crack provided in an embodiment of the present application;

[0045] Figure 4 A schematic diagram of crack width and pumping pressure provided in an embodiment of the present application;

[0046] Figure 5 A schematic diagram of an implementation process for determining potential particles provided in an embodiment of the present application.

[0047] In the drawings, like components are given like reference numerals, and the drawings are not drawn to scale. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical solutions and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings. The described embodiments should not be regarded as limiting this application. All other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0049] In the following description, reference is made to “some embodiments”, which describes a subset of all possible embodiments, but it will be understood that “some embodiments” may be the same subset or different subsets of all possible embodiments and may be combined with each other without conflict.

[0050] If similar descriptions of "first\second\third" appear in the application documents, the following explanation will be added. In the following description, the terms "first\second\third" are only used to distinguish similar objects and do not represent a specific order for the objects. It can be understood that "first\second\third" can be interchanged with a specific order or sequence where permitted, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein.

[0051] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application pertains. The terms used herein are for the purpose of describing the embodiments of this application only and are not intended to limit this application.

[0052] Before introducing an experimental system for determining the dynamic closure characteristics of fractures provided in an embodiment of the present application, a brief introduction to the prior art is given. The micro-injection pressure drop test of a tight reservoir is to inject a certain amount of liquid (generally a few cubic meters to more than ten cubic meters) into the reservoir at a constant small displacement to create a hydraulic fracture of a certain size, and then shut down the well and stop the pump to monitor the pressure drop. After the pump is stopped, the liquid in the fracture is filtered into the formation under the action of the pressure difference. As the pressure of the fluid in the fracture decreases, the fracture morphology continues to change until it gradually closes. According to the pressure drop data at different time stages after the pump is stopped, information such as reservoir physical properties and fracture parameters is evaluated. There are many methods for interpreting pressure drop data after pump stop in the prior art, which are mainly by assuming the dynamic closure law of the fracture, and then solving the formation and fracture information through pressure drop. The accuracy of the description of the fracture closure law caused by the pressure drop during the pump stop process will directly affect the interpretation result of the pump stop pressure drop. At present, the crack closure law is mainly obtained based on rock mechanics mechanism and simulation analysis. There are few reports describing the dynamic closure law of cracks from an experimental perspective. There is a lack of indoor experimental research on the dynamic closure law of cracks: the related technology provides a method for combining actual measurement and theory to analyze the spatiotemporal evolution law of the generation, development and closure of surface cracks in coal mining. The length, width and depth of the crack life cycle are measured by using a steel ruler distance method and lime spray as a tracer. The method does not involve the influence of fluid and pressure; the related technology provides a method for simulating the closure coefficient of shale oil and gas reservoirs, which is tested through fracture conductivity test experiments. The fracture closure coefficient of the proppant is obtained, but the influence of the pressure change in the fracture is not taken into account (static fracture closure experiment), which is different from the dynamic closure phenomenon of the fracture caused by the pressure drop in the actual process; the related art discloses a dynamic closure seepage simulation test device for fractured rock mass fractures in mining, which realizes the seepage simulation test of the loading fracture opening closure process of fractures with different openings, inclinations, and connection degrees, but the experimental design of the invention is different from the actual situation: the fracture morphology changes dynamically under the action of the fluid pressure and filtration in the fracture during the fracturing pump stop process. Not only will the width change, but the fracture tip will also continue to expand in a short time after the pump is stopped, which will affect the pressure drop after the pump is stopped. It can be seen that there are many methods for interpreting the pressure drop data after the pump is stopped in the prior art, which are mainly based on the assumption of the dynamic closure law of the fracture, and then solve the formation and fracture information through the pressure drop. The accuracy of the description of the fracture closure law caused by the pressure drop during the pump stop process will directly affect the interpretation result of the pump stop pressure drop. However, at present, the fracture closure law is mainly obtained based on rock mechanics mechanism and simulation analysis.

[0053] Based on the problems existing in the related art, an embodiment of the present application provides an experimental system for determining the dynamic closure characteristics of cracks.

[0054] Example 1

[0055] The present invention provides an experimental system for determining the dynamic closure characteristics of cracks. Figure 1 A schematic diagram of the structure of an experimental system for determining the dynamic closure characteristics of a crack provided in an embodiment of the present application is shown in FIG. Figure 1 As shown, an experimental system 100 for determining dynamic closure characteristics of fractures includes: a wrapping system 101, a true triaxial hydraulic fracturing simulator 102, a scanning system 103, a detection system 104, and a processing system 105. The wrapping system 101 is used to wrap a specimen to prevent its surface from contacting hydraulic fluid. The specimen has a hydraulic fluid application channel. The true triaxial hydraulic fracturing simulator 102 is used to pressurize the wrapped specimen axially to a first target value and radially to a second target value, stabilize the first and second target values for a preset time, and then apply hydraulic fluid to the hydraulic fluid application channel at a preset rate until the specimen ruptures. The scanning system 103 is used to scan the specimen after failure to obtain a three-dimensional morphological image of the fracture. The detection system 104 is used to measure the circumferential deformation information of the specimen in real time. The processing system 105 is used to determine the dynamic closure characteristics of the fracture based on the circumferential deformation information and the three-dimensional morphological image.

[0056] In this embodiment, the specimen is cylindrical and has a hydraulic fluid application channel. The specimen is prepared by collecting rock from a target area. The encapsulation system includes a fluorine-chlorine specimen sleeve for encapsulating the sides of the specimen; an upper pressure head for covering the upper end surface of the specimen; a lower pressure head for covering the lower end surface of the specimen; and fasteners for securing the fluorine-chlorine specimen sleeve to the sides. In this embodiment, the upper and lower pressure heads are circular, with a through hole in the upper pressure head connecting to the hydraulic fluid application channel. The hydraulic fluid can be water or another liquid.

[0057] In an embodiment of the present application, the true triaxial hydraulic fracturing simulation device can axially pressurize the wrapped sample to a first target value, and pressurize the wrapped sample to a second target value. The first target value can be set according to the size, shape, etc. of the specific specimen. For example, the first target value is 2.6 MPa.

[0058] In an embodiment of the present application, when pressurizing, axial pressurization can be performed at a second rate, and when radial pressurization is performed, pressurization can be performed at a first rate. In an embodiment of the present application, the preset time period can be 24 hours. The preset rate can be 1 mL / min, and the first rate is less than the second rate. For example, the second rate can be 2.6 MPa / min and the first rate can be 2 MPa / min.

[0059] In this embodiment, the wrapped specimen is subjected to axial pressure to a first target value and radial pressure to a second target value using a true triaxial hydraulic fracturing simulator. The specimen is then stabilized at these first and second target values for a predetermined period of time, allowing stress recovery. Because the specimen undergoes approximately 24 hours of stress recovery prior to hydraulic fracturing, its deformation under both confining and axial pressures can be considered to have reached a stable state.

[0060] In an embodiment of the present application, the scanning system may be a CT scanner, and the scanning system may perform a CT scan on the damaged specimen to obtain a three-dimensional morphological image of the crack.

[0061] In this embodiment of the present application, the detection system may include a circumferential extensometer mounted on the fluorine and chlorine test piece sleeve for real-time measurement of circumferential deformation information of the test piece. In this embodiment of the present application, the detection system records circumferential deformation information in real time during pressurization and injection of fracturing fluid in the true triaxial hydraulic fracturing simulator.

[0062] In some embodiments, the processing system includes: a first determination module, used to determine the diameter change based on the circumferential deformation information at the first moment and the circumferential deformation information at the second moment, and determine the circumference change based on the diameter change; a second determination module, used to determine the angle between the normal of the three-dimensional crack surface and the axial direction of the specimen based on the three-dimensional morphological image; a third determination module, used to determine the crack width based on the circumference change and the angle, and to determine the change law of the crack opening based on the crack width.

[0063] In the embodiments of the present application, the first moment is represented by t0. The second moment is represented by t1. The diameter measured by the circumferential extensometer changes from D0 to D0+ΔD, and the change in circumference can be determined to be π·ΔD. Since the axial pressure and confining pressure remain unchanged during the hydraulic fracturing and closure stages, the deformation of the rock specimen caused by the axial pressure and confining pressure can be ignored. Therefore, the hydraulic fracture width is:

[0064]

[0065] Where w is the crack width (mm), ΔD is the change in specimen diameter measured by the circumferential extensometer, and θ is the angle. In the present embodiment, the interval between the first moment and the second moment can be preset, for example, the interval can be set to 0.1 seconds. By determining the change in crack width at different moments, the change pattern of crack aperture can be determined.

[0066] In some embodiments, the scanning system is further configured to determine crack morphology data by tracing the crack edge contour based on the three-dimensional topography image, wherein the crack morphology data includes: crack morphology and crack area.

[0067] The present application provides an experimental system for determining the dynamic closure characteristics of cracks. The system wraps a specimen by setting up a wrapping system, pressurizes and fractures the wrapped specimen through a true triaxial hydraulic fracturing simulation device, scans the damaged specimen through a scanning system, and measures the circumferential deformation information of the specimen through a detection system. The processing system is used to determine the dynamic closure characteristics of the cracks based on the circumferential deformation information and three-dimensional morphological images, thereby realizing the acquisition of crack closure characteristics in an experimental manner.

[0068] Example 2

[0069] Based on the aforementioned embodiments, the present application further provides an experimental system for determining the dynamic closure characteristics of a crack, comprising: a wrapping system, a true triaxial hydraulic fracturing simulator, a scanning system, a detection system, a processing system, and a sample preparation system. The wrapping system is used to wrap a specimen to prevent the surface of the specimen from contacting the hydraulic fluid, and the specimen has a hydraulic fluid application channel. The true triaxial hydraulic fracturing simulator is used to axially pressurize the wrapped specimen to a first target value and radially pressurize the wrapped specimen to a second target value, stabilize the first target value and the second target value for a preset time, and then add hydraulic fluid to the hydraulic fluid application channel at a preset rate after the preset time until the specimen is ruptured. The scanning system is used to scan the damaged specimen to obtain a three-dimensional morphological image of the crack. The detection system is used to measure the circumferential deformation information of the specimen in real time. The processing system is used to determine the dynamic closure characteristics of the crack based on the circumferential deformation information and the three-dimensional morphological image. The sample making system is used to process the rock sample into a cylindrical initial sample and to drill a hole at the center of the end surface of the cylindrical initial sample, wherein the hole has a preset depth and a preset diameter, so as to make a hydraulic fluid application channel based on the hole to obtain the sample.

[0070] In the embodiment of the present application, the test piece is cylindrical and has a hydraulic fluid application channel. Figure 2 A schematic diagram of the installation structure of a packaging system provided in an embodiment of the present application is shown in FIG. Figure 2 As shown, the wrapping system includes: a fluorine-chlorine test piece sleeve 4 for wrapping the side surface of the test piece 5; an upper pressure head 1 for covering the upper end surface of the test piece 5; a lower pressure head 2 for covering the lower end surface of the test piece 5; and fasteners (not shown) for fastening the fluorine-chlorine test piece sleeve 4 to the side surface. In this embodiment of the application, the upper pressure head 1 and the lower pressure head 2 are also circular, with a through hole (not shown) on the upper pressure head that connects to the hydraulic fluid application channel 6. The hydraulic fluid can be water or other liquids.

[0071] In an embodiment of the present application, the true triaxial hydraulic fracturing simulation device can axially pressurize the wrapped sample to a first target value, and pressurize the wrapped sample to a second target value. The first target value can be set according to the size, shape, etc. of the specific specimen. For example, the first target value is 2.6 MPa.

[0072] In an embodiment of the present application, when pressurizing, axial pressurization can be performed at a second rate, and when radial pressurization is performed, pressurization can be performed at a first rate. In an embodiment of the present application, the preset time period can be 24 hours. The preset rate can be 1 mL / min, and the first rate is less than the second rate. For example, the second rate can be 2.6 MPa / min and the first rate can be 2 MPa / min.

[0073] In this embodiment, a true triaxial hydraulic fracturing simulator is used to apply axial pressure to a first target value and radial pressure to a second target value after wrapping the specimen. The specimen is then stabilized at these first and second target values for a predetermined period of time, allowing stress recovery. Because the rock undergoes approximately 24 hours of stress recovery prior to hydraulic fracturing, deformation under both confining and axial pressure can be considered to have reached a stable state.

[0074] In an embodiment of the present application, the scanning system may be a CT scanner, and the scanning system may perform a CT scan on the damaged specimen to obtain a three-dimensional morphological image of the crack.

[0075] In the present application, see Figure 2 The detection system may include: a circumferential extension, 3, arranged on the fluorine and chlorine test piece sleeve 4, for real-time measurement of the circumferential deformation information of the test piece.

[0076] In some embodiments, the processing system includes: a first determination module, used to determine the diameter change based on the circumferential deformation information at the first moment and the circumferential deformation information at the second moment, and determine the circumference change based on the diameter change; a second determination module, used to determine the angle between the normal of the three-dimensional crack surface and the axial direction of the specimen based on the three-dimensional morphological image; a third determination module, used to determine the crack width based on the circumference change and the angle, and to determine the change law of the crack opening based on the crack width.

[0077] In the embodiments of the present application, the first moment is denoted by t0. The second moment is denoted by t1. The diameter measured by the circumferential extensometer changes from D0 to D0+ΔD, and the change in circumference can be determined to be π·ΔD. Since the axial pressure and confining pressure remain unchanged during the hydraulic fracturing and closure stages, the deformation of the rock specimen caused by the axial pressure and confining pressure can be ignored. Therefore, the hydraulic fracture width is:

[0078]

[0079] Where w is the crack width (mm), ΔD is the change in specimen diameter measured by the circumferential extensometer, and θ is the angle.

[0080] In some embodiments, the scanning system is further configured to determine crack morphology data by tracing the crack edge contour based on the three-dimensional topography image, wherein the crack morphology data includes: crack morphology and crack area.

[0081] In an embodiment of the present application, the sample preparation system may include core drilling tools, cutting tools, grinding tools, drill bits, etc., including processing them into cylindrical samples with a diameter of 100 mm and a height of 200 mm through processes such as core drilling, cutting, and grinding. A hole with a diameter of 10 mm and a depth of about 150 mm is formed at the center point of the end face of the cylindrical sample using a drill bit as an open hole section. In order to reduce the volume of the hole, a fiberglass rod with a diameter of 9 mm is placed inside it, and the steel pipe and the specimen are bonded together using high-strength structural adhesive to serve as a channel for applying fracturing fluid. In an embodiment of the present application, the diameter of the fiberglass rod needs to be smaller than the diameter of the center hole of the end face.

[0082] Example 3

[0083] Based on the above embodiments, the present application further provides an experimental method for determining the dynamic closure characteristics of a crack. The method is applied to any of the above experimental systems for determining the dynamic closure characteristics of a crack. Figure 3 A schematic diagram of the implementation flow of an experimental method for determining the dynamic closure characteristics of a crack provided in an embodiment of the present application is shown as follows: Figure 3 As shown, the method includes:

[0084] Step S301 : preparing a sample, wherein the sample has a hydraulic fluid application channel.

[0085] In the embodiment of the present application, the sample is made from a rock sample, and the sample can be made by a sample making system. In the embodiment of the present application, the sample making system may include core drilling tools, cutting tools, grinding tools, drill bits, etc., including processing it into a cylindrical sample with a diameter of 100 mm and a height of 200 mm through processes such as core drilling, cutting, and grinding. A hole with a diameter of 10 mm and a depth of about 150 mm is formed at the center point of the end face of the cylindrical sample using a drill bit as a naked hole section. In order to reduce the volume of the hole, a fiberglass rod with a diameter of 9 mm is placed inside it, and the steel pipe and the specimen are bonded together using high-strength structural adhesive to serve as a channel for applying fracturing fluid. In the embodiment of the present application, the diameter of the fiberglass rod needs to be smaller than the diameter of the center hole of the end face.

[0086] Step S302: Wrap the sample.

[0087] In an embodiment of the present application, the specimen can be wrapped using a wrapping system to prevent the specimen surface from contacting the hydraulic fluid. In this embodiment, the wrapping system includes: a fluorine-chlorine specimen sleeve for wrapping the side surfaces of the specimen; an upper pressure head for covering the upper end surface of the specimen; a lower pressure head for covering the lower end surface of the specimen; and fasteners for fastening the fluorine-chlorine specimen sleeve to the side surfaces. In this embodiment, the upper and lower pressure heads are also circular, with a through hole in the upper pressure head that communicates with the hydraulic fluid application channel.

[0088] In the embodiment of the present application, when wrapping, the fluorine-chlorine test piece is first put on the side of the test piece, and then the lower pressure head is set on the lower end face of the test piece, and the upper pressure head is set on the upper end face of the test piece. The fasteners fasten the fluorine-chlorine test piece cover to the side face to complete the wrapping of the test piece.

[0089] Step S303, after axially pressurizing the wrapped sample to a first target value and radially pressurizing the wrapped sample to a second target value, stabilizing the first target value and the second target value for a preset time, and then applying hydraulic fluid to the channel at a preset rate after the preset time until the sample is ruptured;

[0090] In an embodiment of the present application, after the wrapped sample is axially pressurized to a first target value and radially pressurized to a second target value by a true triaxial hydraulic fracturing simulation device, the first target value and the second target value are stabilized for a preset time, and after the preset time, the hydraulic fluid is applied to the channel at a preset rate until the sample is ruptured. In an embodiment of the present application, when pressurizing, axial pressurization can be performed at a second rate, and when radial pressurization is performed, pressurization can be performed at a first rate. In an embodiment of the present application, the preset time can be 24 hours. The preset rate can be 1 mL / min, the first rate is less than the second rate, and the preset rate is less than the first rate. For example, the second rate can be 2.6 MPa / min and the first rate can be 2 MPa / min.

[0091] In this embodiment, a true triaxial hydraulic fracturing simulator is used to apply axial pressure to a first target value and radial pressure to a second target value after wrapping the specimen. The specimen is then stabilized at these first and second target values for a predetermined period of time, allowing stress recovery. Because the rock undergoes approximately 24 hours of stress recovery prior to hydraulic fracturing, deformation under both confining and axial pressure can be considered to have reached a stable state.

[0092] Step S304 : Scan the damaged sample to obtain a three-dimensional image of the crack.

[0093] In an embodiment of the present application, the damaged sample can be scanned by a scanning system to obtain a three-dimensional morphological image of the crack. The scanning system can be a CT scanner, and the scanning system can perform a CT scan on the damaged sample to obtain a three-dimensional morphological image of the crack.

[0094] Step S305: measuring the circumferential deformation information of the sample in real time.

[0095] In an embodiment of the present application, the circumferential deformation information of the vehicle specimen can be obtained in real time through a detection system. The detection system may include: a circumferential extensometer, which is arranged on the fluorine-chlorine specimen sleeve and is used to measure the circumferential deformation information of the specimen in real time.

[0096] Step S306: determining the dynamic closure characteristics of the crack based on the circumferential deformation information and the three-dimensional morphological image.

[0097] In an embodiment of the present application, a processing system can be used to determine the dynamic closure characteristics of the crack based on the circumferential deformation information and the three-dimensional morphological image. In an embodiment of the present application, the dynamic closure characteristics of the crack include: the variation pattern of the crack opening. In the case where the dynamic closure characteristics of the crack include: the variation pattern of the crack opening, the processing system can include: a first determination module for determining the diameter change based on the circumferential deformation information at the first moment and the circumferential deformation information at the second moment, and determining the circumference change based on the diameter change; a second determination module for determining the angle between the normal of the three-dimensional crack surface and the axial direction of the specimen based on the three-dimensional morphological image; a third determination module for determining the crack width based on the circumference change and the angle, and determining the variation pattern of the crack opening based on the crack width.

[0098] In the embodiments of the present application, the first moment is denoted by t0. The second moment is denoted by t1. The diameter measured by the circumferential extensometer changes from D0 to D0+ΔD, and the change in circumference can be determined to be π·ΔD. Since the axial pressure and confining pressure remain unchanged during the hydraulic fracturing and closure stages, the deformation of the rock specimen caused by the axial pressure and confining pressure can be ignored. Therefore, the hydraulic fracture width is:

[0099]

[0100] Where w is the crack width (mm), ΔD is the change in specimen diameter measured by the circumferential extensometer, and θ is the angle. Figure 4 A schematic diagram of crack width and pumping pressure provided in an embodiment of the present application is shown in FIG. Figure 4 As shown in Figure 1, at the moment of rupture, pumping is stopped. It can be seen from the figure that the dynamic closing characteristics of the crack have certain fluctuations.

[0101] The present application provides an experimental method for determining the dynamic closure characteristics of cracks. The method wraps a specimen, pressurizes and fractures the wrapped specimen, and then scans the damaged specimen to measure the circumferential deformation information of the specimen. The dynamic closure characteristics of the cracks are determined based on the circumferential deformation information and three-dimensional morphological images, thereby realizing an experimental method for obtaining the closure characteristics of the cracks.

[0102] Example 4

[0103] Based on the above embodiments, the present application further provides an experimental method for determining the dynamic closure characteristics of a crack. The method is applied to any of the above experimental systems for determining the dynamic closure characteristics of a crack. Figure 5 A schematic diagram of a process for determining potential particles provided in an embodiment of the present application is shown in FIG. Figure 5 As shown, the method includes:

[0104] Step S501: collecting rock samples.

[0105] Step S502: Process the rock sample to obtain a cylindrical initial sample.

[0106] In the embodiment of the present application, after the surface weathering layer of the collected rock sample is cleaned, it is processed into a cylindrical sample with a diameter of 100 mm and a height of 200 mm through processes such as core drilling, cutting, and grinding.

[0107] Step S503 : punching a hole at the center point of the end surface of the cylindrical initial sample to form a hole with a preset depth and a preset diameter.

[0108] In the embodiment of the present application, a hole with a diameter of 10 mm and a depth of about 150 mm is formed at the center point of the end surface of the cylindrical specimen using a drill bit as the open hole section.

[0109] Step S504: placing a fiberglass rod into the hole to create a hydraulic fluid application channel to obtain the sample.

[0110] In the embodiment of the present application, in order to reduce the volume of the hole, a fiberglass rod with a diameter of 9 mm is placed inside it, and the steel pipe and the test piece are bonded together using high-strength structural adhesive to serve as a channel for applying fracturing fluid.

[0111] Step S505: wrapping the sample.

[0112] In this example, a fluorine-chlorine specimen sleeve was used to encase the circular specimen and the upper and lower pressure heads, and a hoop was used to secure the sleeve to prevent contact between the confining pressure oil and the specimen. To accurately determine the variation in crack aperture, a circumferential extensometer was placed in the center of the specimen to measure its circumferential deformation, providing real-time deformation measurements throughout the entire process.

[0113] Step S506: After axially pressurizing the wrapped sample to a first target value and radially pressurizing the wrapped sample to a second target value, the first target value and the second target value are stabilized for a preset time, and after the preset time, the hydraulic fluid is channel-added at a preset rate until the sample is ruptured.

[0114] A hydraulic fracturing simulation test and pump-off pressure detection were carried out on cylindrical specimens using a true triaxial hydraulic fracturing simulation device. In order to ensure a smooth fracturing and closure process, after the specimen is installed, when pressurizing, the axial pressure can be applied at a second rate, and when pressurizing radially, the pressure can be applied at a first rate. In an embodiment of the present application, the preset time can be 24 hours. The preset rate can be 1 mL / min, and the first rate is less than the second rate. For example, the second rate can be 2.6 MPa / min, and the first rate can be 2 MPa / min. The radial pressure is loaded to the target value at a rate of 2 MPa / min, and the axial pressure is loaded to 2.6 MPa at a rate of 2.6 MPa / min; in order to minimize the error in the crack width measurement caused by rock deformation, the stress state is stabilized for about 24 hours after the confining pressure and axial pressure are loaded to the target value. After the stress recovery is completed, water pressure is applied at a rate of 1 mL / min until the specimen ruptures, and the hydraulic pump is immediately shut down after the specimen ruptures. During the entire process, the pump pressure, circumferential deformation, and axial deformation were monitored in real time. The experiment was stopped when the water pressure dropped to zero or the clay expanded after absorbing water (the circumferential deformation reversed). Note that during this process, the axial pressure and confining pressure were maintained at constant values.

[0115] Step 507 : Scan the damaged sample to obtain a three-dimensional image of the crack.

[0116] After the hydraulic fracturing test is completed, the damaged specimens are subjected to CT scanning to obtain a 3D image of the hydraulic fracture. Using the reconstructed 3D fracture topography, the fracture edge contours are traced to obtain information on the fracture morphology and area.

[0117] Step S508 : measuring the circumferential deformation information of the sample in real time, and determining the dynamic closure characteristics of the crack based on the circumferential deformation information and the three-dimensional morphological image.

[0118] In the embodiment of the present application, the dynamic closure characteristics of the crack include: the change law of the crack opening. In order to accurately obtain the change law of the crack opening, a circumferential extensometer is used for real-time measurement throughout the process; since the rock has undergone about 24 hours of stress recovery before hydraulic fracturing, the deformation of the rock under the confining pressure and axial pressure can be considered to have all reached a stable state. During the hydraulic fracturing process, the changes in the sensor can be assumed to be all caused by the initiation of the crack. Therefore, the crack width can be obtained by the diameter change measured by the circumferential extensometer. Assuming that from t0 to t1, the diameter measured by the circumferential extensometer changes from D0 to D0+ΔD, then the change in its circumference is π·ΔD. Since the axial pressure and confining pressure remain unchanged during the hydraulic fracturing and closure stages, the deformation of the rock sample itself caused by the axial pressure and confining pressure can be ignored, so the width of its hydraulic crack is:

[0119]

[0120] Where w is the crack width (mm), ΔD is the change in specimen diameter measured by the circumferential extensometer, and θ is the angle.

[0121] The embodiment of the present application provides an experimental method for determining the dynamic closure characteristics of cracks. Through a true triaxial hydraulic fracturing simulation device and a designed experimental process, an integrated test of fracturing crack expansion and pump-off crack dynamic closure is achieved. Combining displacement sensors and three-dimensional CT crack morphology reconstruction, the dynamic width change of the crack is characterized, thereby obtaining the dynamic closure, pressure drop and crack morphology data of the crack after injection cessation (post-pressure), and then clarifying the correlation between the dynamic closure characteristics of the crack after pumping and the pump-off pressure drop, providing a basis for the establishment of a post-pressure crack parameter evaluation method. This method fills the gaps in micro-injection testing and small-scale test fracturing indoor experiments, and provides a basis for the establishment of a post-pressure crack parameter evaluation method.

[0122] Example 5

[0123] Based on the aforementioned embodiments, an embodiment of the present application further provides an experimental method for determining the dynamic closure characteristics of a crack. The method is applied to any of the above experimental systems for determining the dynamic closure characteristics of a crack, including:

[0124] Step S501 , processing a cylindrical sample with a diameter of 100 mm and a height of 200 mm, and forming a hole with a diameter of 10 mm and a depth of about 150 mm at the center point of the end face of the cylindrical sample using a drill bit as an open hole section.

[0125] In step S502, in order to reduce the volume of the hole, a fiberglass rod with a diameter of 9 mm is placed inside the hole, and the steel pipe and the test tube are bonded together using high-strength structural adhesive to serve as a channel for applying fracturing fluid.

[0126] In step S503, the circular specimen and the upper and lower pressure heads are wrapped with a fluorine-chlorine specimen sleeve and fastened with a hoop to prevent the confining pressure oil from contacting the specimen. A circumferential extensometer is placed in the middle of the specimen to measure its circumferential deformation, and real-time deformation measurement is performed throughout the entire process.

[0127] In step S504, a true triaxial hydraulic fracturing simulator was used to conduct hydraulic fracturing simulation tests and pump-off pressure monitoring on the cylindrical specimen. To ensure a smooth fracturing and closure process, after the specimen was assembled, the confining pressure was applied to the target value at a rate of 2 MPa / min, and the axial pressure was applied to 2.6 MPa at a rate of 2.6 MPa / min. To minimize errors in fracture width measurement due to rock deformation, the stress state was stabilized for approximately 24 hours (stress recovery) after the confining and axial pressures reached their target values.

[0128] Step S505: After stress recovery is complete, apply water pressure at a rate of 1 mL / min until the sample ruptures. Immediately after rupture, shut down the hydraulic pump. Throughout the entire process, monitor the pump pressure, circumferential deformation, and axial deformation in real time. The experiment is terminated when the water pressure drops to zero or the clay absorbs water and expands (reversing the circumferential deformation). Note that during this process, the axial and confining pressures are maintained at constant values.

[0129] Step S506: After the hydraulic fracturing test is completed, a CT scan test is performed on the damaged sample to obtain a three-dimensional image of the hydraulic fracture. The reconstructed three-dimensional fracture morphology image is used to obtain the fracture morphology and area information by tracing the fracture edge contour.

[0130] In step S507 , during the hydraulic fracturing process, the sensor changes can be assumed to be entirely caused by crack initiation, and the crack width can be obtained from the diameter change measured by the circumferential extensometer.

[0131] In the embodiment of the present application, the dynamic closure characteristics of the crack include: the change law of the crack opening. In order to accurately obtain the change law of the crack opening, a circumferential extensometer is used for real-time measurement throughout the process; since the rock has undergone about 24 hours of stress recovery before hydraulic fracturing, the deformation of the rock under the confining pressure and axial pressure can be considered to have all reached a stable state. During the hydraulic fracturing process, the changes in the sensor can be assumed to be all caused by the initiation of the crack. Therefore, the crack width can be obtained by the diameter change measured by the circumferential extensometer. Assuming that from t0 to t1, the diameter measured by the circumferential extensometer changes from D0 to D0+ΔD, then the change in its circumference is π·ΔD. Since the axial pressure and confining pressure remain unchanged during the hydraulic fracturing and closure stages, the deformation of the rock sample itself caused by the axial pressure and confining pressure can be ignored, so the width of its hydraulic crack is:

[0132]

[0133] Where w is the crack width (mm), ΔD is the change in specimen diameter measured by the circumferential extensometer, and θ is the angle between the normal of the reconstructed 3D crack surface and the wellbore axis.

[0134] By using a true triaxial hydraulic fracturing simulator and a designed experimental process, an integrated test of fracture expansion and dynamic fracture closure after pumping is stopped is achieved. Combining displacement sensors and 3D CT fracture morphology reconstruction, the dynamic width changes of the fracture are characterized. This allows for the acquisition of data on fracture dynamic closure, pressure drop, and fracture morphology after injection cessation (post-fracturing). Furthermore, the correlation between the dynamic closure characteristics of fractures after pumping is stopped and the pressure drop during pumping is clarified, providing a foundation for the development of a post-fracturing fracture parameter evaluation method. This approach addresses the gaps in micro-injection testing and small-scale test fracturing laboratory experiments, providing a foundation for the development of a post-fracturing fracture parameter evaluation method.

[0135] It should be understood that "one embodiment" or "an embodiment" mentioned throughout the specification means that the specific features, structures or characteristics related to the embodiment are included in at least one embodiment of the present application. Therefore, "in one embodiment" or "in an embodiment" appearing throughout the specification does not necessarily refer to the same embodiment. In addition, these specific features, structures or characteristics can be combined in one or more embodiments in any suitable manner. It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application. The above-mentioned serial numbers of the embodiments of the present application are for description only and do not represent the advantages and disadvantages of the embodiments.

[0136] It should be noted that, in this document, the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus comprising a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or apparatus comprising the element.

[0137] In the several embodiments provided in this application, it should be understood that the disclosed devices and methods can be implemented in other ways. The device embodiments described above are merely schematic. For example, the division of the units is merely a logical function division. In actual implementation, there may be other division methods, such as: multiple units or components can be combined, or can be integrated into another system, or some features can be ignored or not executed. In addition, the coupling, direct coupling, or communication connection between the components shown or discussed can be through some interfaces, and the indirect coupling or communication connection of the devices or units can be electrical, mechanical or other forms.

[0138] The units described above as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units; they may be located in one place or distributed across multiple network units; some or all of the units may be selected according to actual needs to achieve the purpose of the scheme of this embodiment.

[0139] In addition, all functional units in the embodiments of the present application can be integrated into one processing unit, or each unit can be a separate unit, or two or more units can be integrated into one unit; the above-mentioned integrated units can be implemented in the form of hardware or in the form of hardware plus software functional units.

[0140] Those skilled in the art will understand that all or part of the steps of implementing the above-mentioned method embodiments can be completed by hardware related to program instructions, and the aforementioned program can be stored in a computer-readable storage medium. When the program is executed, it executes the steps of the above-mentioned method embodiments; and the aforementioned storage medium includes: mobile storage devices, read-only memories (ROMs), magnetic disks, optical disks, and other media that can store program codes.

[0141] Alternatively, if the above-mentioned integrated unit of the present application is implemented in the form of a software functional module and sold or used as an independent product, it can also be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the embodiment of the present application, or the part that contributes to the prior art, can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes a number of instructions for enabling a controller to execute all or part of the methods described in each embodiment of the present application. The aforementioned storage medium includes: various media that can store program codes, such as mobile storage devices, ROMs, magnetic disks or optical disks.

[0142] The above is merely an embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. An experimental system for determining the dynamic closure characteristics of cracks, characterized by include: a wrapping system for wrapping a test piece to prevent a surface of the test piece from contacting the hydraulic fluid, wherein the test piece has a hydraulic fluid application channel; A true triaxial hydraulic fracturing simulation device is configured to pressurize the wrapped specimen axially to a first target value and radially to a second target value, stabilize the first target value and the second target value for a preset time period, and then, after the preset time period, apply channel hydraulic fluid at a preset rate until the specimen is fractured; Scanning system, used to scan the specimen after destruction to obtain a three-dimensional image of the crack; A detection system, used for measuring the circumferential deformation information of the test piece in real time; a processing system for determining dynamic closure characteristics of the crack based on the circumferential deformation information and the three-dimensional morphological image; The dynamic closure characteristics of the crack include: the change law of the crack opening, and the processing system includes: a first determining module, configured to determine a diameter change based on the circumferential deformation information at the first moment and the circumferential deformation information at the second moment, and determine a circumference change based on the diameter change; A second determination module is used to determine the angle between the normal direction of the three-dimensional crack surface and the axial direction of the specimen based on the three-dimensional morphological image; a third determining module, configured to determine a crack width based on the perimeter change and the included angle, and to determine a change pattern of a crack aperture based on the crack width; The crack width is obtained by the following formula: , where w is the crack width in millimeters; is the diameter change, is the angle.

2. The experimental system for determining dynamic closure characteristics of cracks according to claim 1, characterized in that: The test piece is cylindrical, and the packaging system includes: A fluorine and chlorine test piece cover, used for wrapping the side of the test piece; an upper pressure head, used for covering the upper end surface of the test piece; A lower pressure head, used for covering the lower end surface of the test piece; A fastener is used to fasten the fluorine and chlorine test piece sleeve to the side surface.

3. The experimental system for determining dynamic closure characteristics of cracks according to claim 2, characterized in that: The detection system comprises: The circumferential extensometer is arranged on the fluorine and chlorine test piece sleeve and is used for measuring the circumferential deformation information of the test piece in real time.

4. The experimental system for determining dynamic closure characteristics of cracks according to claim 1, wherein the scanning system is further configured to determine crack morphology data by tracing the crack edge contour based on the three-dimensional topography image, wherein: The crack morphology data includes: crack morphology and crack area.

5. The experimental system for determining dynamic closure characteristics of cracks according to claim 1, characterized in that: Also includes: The specimen making system is used to process the rock sample into a cylindrical initial specimen and to drill a hole at the center of the end face of the cylindrical initial specimen, wherein the hole has a preset depth and a preset diameter, so as to make a hydraulic fluid application channel based on the hole to obtain the specimen.

6. The experimental system for determining dynamic closure characteristics of cracks according to claim 5, characterized in that: The cylindrical initial test piece has a diameter of 100 mm, a height of 200 mm, a preset depth of 150 mm, and a preset diameter of 10 mm.

7. An experimental method for determining the dynamic closure characteristics of a crack, characterized in that: An experimental system for determining dynamic closure characteristics of a crack as claimed in any one of claims 1 to 6, comprising: making a test piece, wherein the test piece has a hydraulic fluid application channel; wrapping the test piece; After axially pressurizing the wrapped specimen to a first target value and radially pressurizing the wrapped specimen to a second target value, stabilizing the first target value and the second target value for a preset time, and after the preset time, applying hydraulic fluid to the channel at a preset rate until the specimen is ruptured; Scan the damaged specimen to obtain a three-dimensional image of the crack; measuring the circumferential deformation information of the specimen in real time, and determining the dynamic closure characteristics of the crack based on the circumferential deformation information and the three-dimensional morphological image; The dynamic closure characteristics of the crack include: the change pattern of the crack opening, and the determination of the dynamic closure characteristics of the crack based on the circumferential deformation information and the three-dimensional morphological image includes: determining a diameter change based on the circumferential deformation information at the first moment and the circumferential deformation information at the second moment, and determining a circumference change based on the diameter change; Determine the angle between the normal direction of the three-dimensional crack surface and the axis of the specimen based on the three-dimensional morphological image; Determining a crack width based on the perimeter change and the angle, and determining a change pattern of a crack opening based on the crack width; The crack width is obtained by the following formula: , where w is the crack width in millimeters; is the diameter change, is the angle.

8. The method according to claim 7, characterized in that The test piece is made by: Collect rock samples; processing the rock sample to produce a cylindrical initial test piece; Punching a hole at a center point of the end surface of the cylindrical initial test piece to form a hole with a preset depth and a preset diameter; A fiberglass rod is placed in the hole to create a hydraulic fluid application channel to obtain the test piece.

9. The method according to claim 8, characterized in that After axially pressurizing the wrapped specimen to a first target value and radially pressurizing the wrapped specimen to a second target value, stabilizing the first target value and the second target value for a preset time, and after the preset time, applying hydraulic fluid to the channel at a preset rate until the specimen is ruptured, comprising: The wrapped specimen is loaded radially at a first rate of confining pressure to a second target value, and the wrapped specimen is loaded axially at a second rate to the first target value; Stabilize the first target value and the second target value for a preset time period; After a predetermined time period, hydraulic fluid is applied at a predetermined rate to rupture the specimen, wherein the first rate is less than the second rate.

Images