A method and device for testing the generation and expansion of hydraulic fracturing cracks in coal seams on site

Abstract

The present invention discloses a method and device for testing the generation and expansion of hydraulic fracturing cracks in a coal seam on site, including a pulsating water injection system for injecting water into the coal seam, a three-dimensional fluorescence imaging system for observing the generation and expansion of cracks, and a radioactive tracer imaging system. Fluorescein isothiocyanate and radioactive material uranyl acetate are mixed in the fracturing fluid, and the three-dimensional fluorescence imaging system and the radioactive tracer imaging system are used to realize the visualization of the liquid flow path of the generation and expansion of hydraulic fracturing cracks in the coal seam. The three-dimensional fluorescence imaging system is used to observe the crack initiation area and image the crack during the hydraulic fracturing process, and the radioactive tracer imaging system is used to observe the flow trajectory of the liquid when it flows inside the coal seam. By combining the two, the generation and expansion of cracks in the hydraulic fracturing process of the coal seam on site can be observed. The experimental device and method can provide a scientific basis for guiding the permeability enhancement of the coal body on site and predicting the amount of gas in the coal seam.

Description

Technical Field

[0001] The invention relates to the technical field of coal body hydraulic fracturing, in particular to an on-site coal seam hydraulic fracturing crack generation and expansion test experimental method and device. Background Art

[0002] my country's coalbed methane resources are rich and the occurrence conditions are complex. The poor fluidity of gas inside the coal body makes it impossible to efficiently extract gas. Hydraulic fracturing technology is an important technology for increasing the production of gas-containing coal seams. Due to its simple use conditions and low cost, it is used to increase the permeability of coal bodies in underground coal mines, so as to achieve the purpose of co-mining coal and gas and reducing the danger of gas outburst accidents. Under the action of hydraulic fracturing, the generation and expansion of coal body cracks are complex. If the generation and expansion paths of hydraulic fracturing cracks in underground coal bodies can be monitored, the generation and expansion mechanism of cracks under hydraulic fracturing can be further explored, providing a scientific basis for the development of hydraulic fracturing coal body permeability increase technology.

[0003] At present, there are few technologies that can observe the generation and expansion of hydraulic fracturing cracks in coal seams on site, which can be mainly divided into the following two categories:

[0004] The first type is to observe the generation and expansion of cracks by simulating on-site coal seams in the laboratory, but this method has obvious disadvantages. For example, indoor tests generally simulate wellbores by drilling manually, in which case there is no stress loading. However, in on-site coal seams, the coal seams are affected by the original stress field during drilling, and deviations from the actual situation occur during the drilling stage. Therefore, the results obtained by experimental research using simulation technology cannot accurately describe the actual situation underground.

[0005] The second type is to use acoustic emission technology to observe the generation and expansion of hydraulic fracturing cracks, but this method also has certain disadvantages. For example, during the on-site coal seam hydraulic fracturing process, the coal seam will have weak surface damage, but this phenomenon does not belong to the cracks caused by hydraulic fracturing. At the same time, acoustic emission technology cannot distinguish between the fractures caused by hydraulic fracturing and the fracture signals caused by weak surface damage. Therefore, this method cannot effectively monitor the generation and expansion of on-site coal seam hydraulic fracturing cracks.

[0006] From the above analysis, it can be seen that the existing research results cannot meet the current actual needs for on-site observation of coal seam crack initiation and expansion. In order to accurately realize the monitoring of on-site coal seam hydraulic fracturing crack initiation and expansion, new experimental equipment and experimental methods are needed. Summary of the invention

[0007] The purpose of the present invention is to provide a test device that can monitor the initiation and expansion of cracks in real time during the on-site coal seam hydraulic fracturing process, so as to improve the deficiencies of the above-mentioned prior art. The device can monitor the initiation and expansion of cracks caused by hydraulic fracturing in real time during the coal body hydraulic fracturing process, and the present invention also proposes an experimental method for the experimental device.

[0008] To achieve the above-mentioned purpose, the present invention provides an on-site coal seam hydraulic fracturing crack generation and expansion test experimental device, which consists of a pulsating water injection system for injecting water into the coal seam, a three-dimensional fluorescence imaging system for observing the generation and expansion of cracks, and a radioactive tracer imaging system.

[0009] The pulsating water injection system for injecting water into the coal seam includes a liquid storage tank, a pulsating pressure pump, and a stainless steel metal hose. A specific fracturing fluid is added to the liquid storage tank, and the fracturing fluid is mixed with fluorescent substances and radioactive substances. The left side of the pulsating pressure pump is connected to the liquid storage tank through an adapter, which is convenient for subsequent disassembly and storage. The stainless steel metal hose is connected to the right side of the pulsating pressure pump through an adapter, and the pulsating pressure pump can adjust the pressure through a button. The surface of the pump is provided with a display screen, which can display the pressure and the pressurized time in real time. The pressure pump drives the fracturing fluid in the liquid storage tank into the stainless steel metal hose; the tail end of the stainless steel metal hose is connected to the fracturing pipe through an adapter, and the fracturing pipe is pre-placed in the fracturing hole and the hole is sealed; a transparent tube is embedded in the fracturing pipe, and a small hole is opened at the upper end of the transparent pipe for the image acquisition device to move into the transparent pipe and observe the generation and expansion of cracks.

[0010] The three-dimensional fluorescence imaging system for observing the generation and expansion of cracks includes a fluorescence signal detector, a wire, and a computer. The fluorescence signal detector is placed at the bottom of the transparent tube through the opening of the transparent tube and is connected to the computer through a wire. The fluorescence signal detector can receive fluorescence signals, and the signals emitted by the fluorescent substance are monitored by the fluorescence signal detector to obtain parameter information such as the excitation wavelength, the emission wavelength, and the fluorescence intensity. The relevant parameters are corrected through the three-dimensional fluorescence spectrum data software platform in the computer, and the liquid flow trajectory in the small-scale cracking area inside the coal body can be three-dimensionally reconstructed through the Origin software in the computer.

[0011] The radioactive tracer imaging system for observing the generation and expansion of cracks includes a radioactive detector, a wire, and a computer. Four positioning holes are drilled at symmetrical positions 10 cm away from the fracturing hole, and the radioactive detectors are placed in the positioning holes. When the fracturing fluid mixed with radioactive tracers flows in the cracks, the signals emitted by the radioactive substances are monitored by the radioactive detectors, and the data are processed by the least squares method and the Gauss-Seidel iterative algorithm, and the radiation field is reconstructed based on the source activity inversion calculation to achieve three-dimensional reconstruction of the liquid flow trajectory inside the coal body.

[0012] The embedded transparent tube is designed as an integral unit with the fracturing tube. The length of the transparent tube can be adjusted according to the actual length of the fracturing tube. The lower end of the transparent tube is designed as an arc, which facilitates the fluorescent signal detector to smoothly enter the bottom of the transparent tube and move inside the tube.

[0013] The pulsating pressure pump can adjust the pressure by a button, and the pump surface is provided with a display screen which can display the pressure and the pressurizing time in real time. Some devices are connected via an adapter to facilitate installation, disassembly and storage of the device, and to facilitate maintenance in case of failure.

[0014] The fluorescent substance is fluorescein isothiocyanate, and the radioactive substance is uranyl acetate.

[0015] An experimental method for an on-site coal seam hydraulic fracturing crack generation and expansion test experimental device comprises the following steps:

[0016] Step 1: Drill holes in the coal seam that needs hydraulic fracturing. The hole size depends on the size of the fracturing pipe. After the drilling is completed, four positioning holes are drilled at a position 10 cm away from the fracturing pipe. The four holes are symmetrical in shape, and the distance between adjacent holes is kept equal.

[0017] Step 2: After drilling, place the fracturing pipe in the fracturing hole in advance and use the corresponding sealing material to seal the hole to ensure good sealing.

[0018] Step 3: Fill the liquid storage tank with an appropriate amount of clean water, add the fluorescent substance and the radioactive substance into the liquid storage tank in corresponding proportions and stir to mix them thoroughly.

[0019] Step 4: Connect the liquid storage tank, pulsating pressure pump, stainless steel metal hose and fracturing pipe through the adapter, place the fluorescent signal detector inside the fracturing pipe, place the radioactive detector in the four positioning holes, and connect it to the computer through wires.

[0020] Step 5: Check whether all devices are tightly connected. After confirming that the connections are correct, start the computer, fluorescent signal detector and radioactive detector. After the signal receiving software in the computer is opened, start the pulsating pressure pump. When the fracturing fluid enters the positioning hole, stop the experiment.

[0021] Step 6: Save the signal received in the computer, take out the radioactive detector in the positioning hole and the fluorescent signal detector in the fracturing pipe, disassemble each device through the adapter and place them in the corresponding area.

[0022] Step 7: Through the signal processing software and related algorithms in the computer, the fracture initiation area and the flow trajectory of the fracturing fluid flowing inside the coal seam are imaged in three dimensions.

[0023] The present invention adopts the above structure and has the following beneficial effects compared with the common coal seam hydraulic fracturing crack observation device:

[0024] The present invention optimizes the existing problems in the test of the generation and expansion of hydraulic fracturing cracks in underground coal seams, and proposes to add fluorescent substance fluorescein isothiocyanate and radioactive substance uranyl acetate to the fracturing fluid. The radioactive substance has weak radioactivity and can greatly reduce the harm to the experimenter. The three-dimensional fluorescence imaging system and the radioactive tracer imaging system are used to realize the visualization of the liquid flow path of the generation and expansion of hydraulic fracturing cracks in coal seams. The three-dimensional fluorescence imaging system is used to observe the crack initiation area and image the crack during the hydraulic fracturing process, and the radioactive tracer imaging system is used to observe the flow trajectory of the liquid when it flows inside the coal seam. By combining the two, the generation and expansion of cracks in the on-site coal seam hydraulic fracturing process can be observed. The lower end of the transparent tube embedded in the fracturing tube is designed as an arc, which facilitates the camera to smoothly enter the bottom of the transparent tube and move inside the tube. The components of the test system are connected through a transfer interface, which is convenient for installation and disassembly. The imaging machine and computer used in the experiment are explosion-proof. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic diagram of the structure of the on-site coal seam hydraulic fracturing crack initiation and expansion experimental device.

[0026] Figure 2 A partial enlarged schematic diagram of the coal seam area

[0027] In the figure:

[0028] 1-computer; 2-water pressure regulating button; 3-pulsating pressure pump; 4-liquid storage tank; 5-adapter 1; 6-display screen; 7-adapter 2; 8-stainless steel metal hose; 9-adapter 3; 10-fracturing pipe; 11-conductor wire; 12-sealing material; 13-positioning hole; 14-fracturing hole; 15-transparent tube; 16-fluorescent signal detector; 17-radioactivity detector; 18-on-site coal seam. DETAILED DESCRIPTION

[0029] like Figure 1 and Figure 2As shown, the purpose of the present invention is to provide a test device that can monitor the initiation and expansion of cracks in real time during the on-site coal seam hydraulic fracturing process, including a pulsating water injection system for injecting water into the coal seam, including a liquid storage tank 4, a pulsating pressure pump 3, and a stainless steel metal hose 8. A specific fracturing fluid is added to the liquid storage tank 4, and the fracturing fluid is mixed with fluorescent substances and radioactive substances. The left side of the pulsating pressure pump is connected to the liquid storage tank through an adapter 5, which is convenient for subsequent disassembly and storage. The stainless steel metal hose is connected to the right side of the pulsating pressure pump through an adapter 7, and the pulsating pressure pump 3 can adjust the pressure through a button 2. The pump surface is provided with a display screen 6, which can display the pressure and pressurization time in real time. The booster pump 3 drives the fracturing fluid in the liquid storage tank 4 to enter the stainless steel metal hose 8; the tail end 8 of the stainless steel metal hose is connected to the fracturing tube 10 through the adapter 9, and the fracturing tube 10 is pre-placed in the fracturing hole 14 and the hole is sealed; a transparent tube 15 is embedded in the fracturing tube 10, and a small hole is opened at the upper end of the transparent tube 15 for the image acquisition device to move into the transparent tube 15 and observe the generation and expansion of cracks.

[0030] The three-dimensional fluorescence imaging system for observing the generation and extension of cracks includes a fluorescence signal detector 16, a wire 11, and a computer 1. The fluorescence signal detector 16 is placed at the bottom of the transparent tube 15 through the opening of the transparent tube 15, and is connected to the computer 1 through the wire 11. The fluorescence signal detector 16 can receive fluorescence signals, obtain parameter information such as excitation wavelength, emission wavelength, and fluorescence intensity, and correct relevant parameters through the three-dimensional fluorescence spectrum data software platform in the computer. The liquid flow trajectory in a small-scale cracking area inside the coal body can be three-dimensionally reconstructed through the Origin software in the computer 1.

[0031] The radioactive tracer imaging system for observing the generation and expansion of cracks includes a radioactive detector 17, a wire 11, and a computer 1. Four positioning holes 13 are drilled at symmetrical positions 10 cm away from the fracturing hole, and the radioactive detectors 17 are placed in the positioning holes 13. When the fracturing fluid mixed with radioactive tracers flows in the cracks, the signals emitted by the radioactive substances are monitored by the radioactive detectors 17. The data are processed using the least squares method and the Gauss-Seidel iterative algorithm, and the radiation field reconstruction based on the source activity inversion calculation is realized to achieve three-dimensional reconstruction of the liquid flow trajectory inside the coal body.

[0032] The embedded transparent tube 15 is integrally designed with the fracturing tube 10 . The length of the transparent tube 15 can be adjusted according to the actual length of the fracturing tube 10 . The lower end of the transparent tube 15 is designed in an arc shape, which facilitates the fluorescent signal detector 16 to smoothly enter the bottom of the transparent tube 15 and move inside the tube.

[0033] The pulsating pressure pump 3 can adjust the pressure through the button 2, and the surface of the pump 3 is provided with a display screen 6, which can display the pressure and the pressurized time in real time. Some devices are connected through an adapter, which is convenient for installation, disassembly and storage of the device, and is easy to repair when a fault occurs.

[0034] The fluorescent substance is fluorescein isothiocyanate, and the radioactive substance is uranyl acetate.

[0035] An experimental method for an on-site coal seam hydraulic fracturing crack generation and expansion test experimental device comprises the following steps:

[0036] Step 1: Drill holes in the coal seam 18 that needs hydraulic fracturing. The hole size is determined according to the size of the fracturing pipe 10. After the drilling is completed, four positioning holes 13 are drilled at a position 10 cm away from the fracturing pipe. The four holes 13 are symmetrical in shape, and the distance between adjacent holes is kept equal.

[0037] Step 2: After drilling is completed, the fracturing pipe 10 is pre-placed in the fracturing hole 14, and the corresponding sealing material 12 is used to seal the hole to ensure good sealing performance.

[0038] Step 3: Fill the liquid storage tank 4 with an appropriate amount of clean water, add the fluorescent substance and the radioactive substance into the liquid storage tank 4 in corresponding proportions and stir to mix them fully.

[0039] Step 4: Connect the liquid storage tank 4, the pulsating pressure pump 3, the stainless steel metal hose 8, and the fracturing pipe 10 through the adapter, place the fluorescent signal detector 16 inside the fracturing pipe 10, place the radioactive detector 17 in the four positioning holes 13, and connect it to the computer 1 through the wire 11.

[0040] Step 5: Check whether all devices are tightly connected. After confirming that the connections are correct, start the computer 1, the fluorescent signal detector 16 and the radioactive detector 17. After the signal receiving software in the computer 1 is opened, start the pulsating pressure pump 3. When the fracturing fluid enters the positioning hole 13, stop the experiment.

[0041] Step 6: Save the signal received in the computer 1, take out the radioactive detector 17 in the positioning hole 13 and the fluorescent signal detector 16 in the fracturing pipe 10, disassemble each device through the adapter and place them in the corresponding area.

[0042] Step 7: The signal processing software and related algorithms in the computer 1 are used to perform three-dimensional imaging of the fracture initiation area and the flow trajectory of the fracturing fluid when it flows inside the coal seam.

Claims

1. An on-site coal seam hydraulic fracturing crack generation and expansion test experimental device, characterized by: The invention comprises a three-dimensional fluorescence imaging system for observing the generation and expansion of cracks, a radioactive tracer imaging system, and a pulsating water injection system for injecting water into the coal seam, wherein the pulsating water injection system comprises a liquid storage tank (4), a pulsating pressure pump (3), and a stainless steel metal hose (8). A specific fracturing fluid is added into the liquid storage tank (4), and the fracturing fluid contains a fluorescent substance and a radioactive substance. The left side of the pulsating pressure pump is connected to the liquid storage tank through a transfer interface (5) for easy subsequent disassembly and storage. The stainless steel metal hose is connected to the right side of the pulsating pressure pump through a transfer interface (7). ) is connected, and the pulsating pressure pump (3) adjusts the pressure through a button (2). The pump surface is provided with a display screen (6) which can display the pressure and the pressurization time in real time; the pressure pump (3) drives the fracturing fluid in the liquid storage tank (4) to enter the stainless steel metal hose (8); the tail end of the stainless steel metal hose (8) is connected to the fracturing pipe (10) through a transfer interface (9), and the fracturing pipe (10) is pre-placed in the fracturing hole (14) and the hole is sealed; a transparent tube (15) is embedded in the fracturing pipe (10), and the transparent tube (15) ) is provided with a small hole at the upper end thereof for allowing an image acquisition device to move into the transparent tube (15) and observe the generation and expansion of cracks; the embedded transparent tube (15) and the fracturing tube (10) are designed as an integrated unit; the length of the transparent tube (15) can be adjusted according to the actual length of the fracturing tube (10); and the lower end of the transparent tube (15) is designed as an arc, so that the fluorescent signal detector (16) can smoothly enter the bottom of the transparent tube (15) and move inside the tube; a three-dimensional fluorescent imaging system for observing the generation and expansion of cracks comprises a fluorescent signal detector (16), a wire (11), and a computer (1); the fluorescent signal detector (16) is placed at the bottom of the transparent tube (15) through the opening of the transparent tube (15) and is connected to the computer (1) through the wire (11); the fluorescent signal detector (16) can receive fluorescent signals, obtain information on excitation wavelength, emission wavelength, and fluorescence intensity parameters, and correct relevant parameters through a three-dimensional fluorescent spectrum data software platform in the computer; and the liquid flow trajectory in a small-scale cracking area inside the coal body can be three-dimensionally reconstructed through Origin software in the computer (1).

2. The on-site coal seam hydraulic fracturing crack generation and expansion test experimental device according to claim 1, characterized in that: The radioactive tracer imaging system for observing the generation and expansion of cracks comprises a radioactive detector (17), a wire (11), and a computer (1); four positioning holes (13) are drilled at symmetrical positions 10 cm away from the fracturing hole, and the radioactive detectors (17) are placed in the positioning holes (13); when the fracturing fluid mixed with the radioactive tracer flows in the cracks, the signal emitted by the radioactive material is monitored by the radioactive detector (17); the data is processed using the least square method and the Gauss-Seidel iterative algorithm, and the radiation field reconstruction based on the source activity inversion calculation is realized to achieve three-dimensional reconstruction of the liquid flow trajectory inside the coal body.

3. The on-site coal seam hydraulic fracturing crack generation and expansion test experimental device according to claim 2, characterized in that: The fluorescent substance is fluorescein isothiocyanate, and the radioactive substance is uranyl acetate.

4. An experimental method for the on-site coal seam hydraulic fracturing crack generation and expansion test experimental device as claimed in claim 1, characterized in that: The following steps are involved: Step 1: drilling holes in the coal seam (18) to be hydraulically fractured, the size of the holes being determined according to the size of the fracturing pipe (10), after the drilling is completed, four positioning holes (13) are drilled at a position 10 cm away from the fracturing pipe, the four holes (13) being symmetrical in shape, and the distance between two adjacent holes being kept equal; Step 2: After drilling is completed, the fracturing pipe (10) is pre-placed in the fracturing hole (14), and the corresponding sealing material (12) is used to seal the hole to ensure good sealing performance; Step 3: Put an appropriate amount of clean water into the liquid storage tank (4), add the fluorescent substance and the radioactive substance into the liquid storage tank (4) in corresponding proportions and stir to mix them thoroughly; Step 4: Connect the liquid storage tank (4), the pulsating pressure pump (3), the stainless steel metal hose (8), and the fracturing pipe (10) through the adapter, place the fluorescent signal detector (16) inside the fracturing pipe (10), place the radioactive detector (17) in the four positioning holes (13), and connect them to the computer (1) through the wire (11); Step 5: Check whether all devices are tightly connected. After confirming that the connection is correct, start the computer (1), the fluorescent signal detector (16) and the radioactive detector (17). After the signal receiving software in the computer (1) is opened, start the pulsating pressure pump (3). When the fracturing fluid enters the positioning hole (13), stop the experiment; Step 6: Save the signal received in the computer (1), take out the radioactive detector (17) in the positioning hole (13) and the fluorescent signal detector (16) in the fracturing pipe (10), disassemble each device through the adapter and place them in the corresponding area; Step 7: By using the signal processing software and related algorithms in the computer (1), the fracture initiation area and the flow trajectory of the fracturing fluid flowing in the coal seam are respectively imaged in three dimensions.

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