WO2021179334A1

WO2021179334A1 - Integrated test method for combined fracturing, pressure relief and drainage of multiple coal seams

Info

Publication number: WO2021179334A1
Application number: PCT/CN2020/079419
Authority: WO
Inventors: 王刚; 刘义鑫; 倪冠华; 孙路路; 刘震; 于岩斌; 秦相杰; 刘志远; 韩冬阳
Original assignee: 山东科技大学
Priority date: 2020-03-09
Filing date: 2020-03-15
Publication date: 2021-09-16
Also published as: CN111323307A; CN111323307B

Abstract

An integrated test method for combined fracturing, pressure relief and drainage of multiple coal seams, comprising: preparing a composite coal rock sample (3), and placing a sand rock sample between two raw coal samples to form a composite coal rock sample having a fracturing hole (16) and a drainage hole (15); mounting the sample on a true triaxial test device; applying a triaxial stress; adding gas pressure; performing primary gas drainage; fracturing a lower coal seam; and performing secondary gas drainage and the like in a total of nine steps. In the test method, the composite coal rock sample is placed in the true triaxial test device, and the effective control of primary drainage, fracturing and secondary drainage is combined, so that the influence of the setting of technical parameters for the combined mining of multiple coal seams on the efficiency of gas drainage can be reproduced.

Description

Integrated test method for combined fracturing, pressure relief and drainage of multiple coal seams

Technical field

The invention belongs to the technical field of coal seam mining simulation test methods, and specifically relates to a simulation test method for fracturing and extracting a composite coal roof under true three-dimensional stress.

Background technique

With the increase of mining depth, the original rock stress changes from shallow tectonic stress to deep two-way or three-way equal stress state. The deep ground stress increases, and the mechanical properties of the surrounding rock show a nonlinear change, which easily induces the mutual change of rock burst, mine shock and gas energy. At the same time, the porosity of coal seams decreases with the increase of in-situ stress. Therefore, the porosity of coal and rock mass under high in-situ stress is lower. High in-situ stress not only increases the difficulty of desorption of CBM in the adsorbed state, but also makes the channel of CBM released into the free state narrower, that is, the high in-situ stress in the deep causes the difficulty of CBM extraction. In the actual production process, due to the difficulty of deep coalbed methane extraction and the longer extraction time, it is difficult to meet the production needs of coal seams with poor permeability using a single extraction method.

For deep, high-stress and low-permeability coal seams, the combined mining of multiple coal seams has become a development trend. By hydraulic fracturing the deep coal seams first, the upper coal seam is wetted while the upper coal seam is wetted and the upper coal seam has the effect of depressurization and permeability enhancement, so that the inner part of the upper coal seam is produced. The fissures are conducive to the analysis of internal gas, improve the extraction efficiency, and enhance the safety of coal seam mining.

At present, most of the research in this area focuses on the use of cross-bed drilling for multi-coal drainage. There are few studies on the combined fracturing, pressure relief, and drainage integration of multi-coal seams, and there are few reports through laboratory tests and theoretical studies.

Summary of the invention

The present invention intends to provide a multi-coal joint fracturing, pressure relief, and drainage integrated test method, which is used to study the best technical parameters of multi-coal joint mining under true triaxial high stress, and to scientifically select relevant processes in engineering practice The parameters provide a scientific basis.

To this end, the technical solution adopted by the present invention is: an integrated test method for combined fracturing, pressure relief, and drainage of multiple coal seams, which includes the following steps:

Step 1. Sample preparation;

(1) Preparation of two raw coal samples;

Select two pieces of raw coal to cut and polish to make the section flatness within ±0.02mm, punch a central blind hole on one surface of the raw coal sample, dry it at 105±15° for 24±4 hours, then cool it down Stand by at room temperature;

(2) Preparation of sandstone samples;

Select the sandstone block to cut and polish to make its section flatness within ±0.02mm, dry it at 105 ± 15° for 24 ± 4 hours, and then cool it to room temperature for later use;

(3) Preparation of composite coal and rock samples;

Place the sandstone sample in the middle, and evenly apply high-strength epoxy resin adhesive on its upper and lower surfaces. In addition, apply high-strength epoxy resin evenly on the opposite sides of the two raw coal samples with the central blind hole, and then apply the raw coal separately. The sample is coated with high-strength epoxy resin adhesive to compactly fit the top and bottom of the sandstone sample. The central blind hole at the top is used as a drainage hole, and the central blind hole at the bottom is used as a fracturing hole;

Step 2: The sample is installed on the test device of the true triaxial test;

The true three-axis test device includes a host, a host support component, a slide rail, a slide rail support component, and a servo cylinder. The lower part extends forward and backward, and after passing through the mainframe, it is supported on the ground by the sliding rail support assembly. The mainframe includes a cast-shaped integral ring frame. The outer side is equipped with a cover plate. The integral ring frame and cover plate enclose the main body shell. The inner cavity of the main body is used to place the composite coal and rock samples. Each is equipped with sample pads, and a sample moving bracket that can move forward and backward on the slide rail is set under the sample pad on the lower side; the servo cylinders on the front and rear sides are both set to move forward and backward on the slide rail. The cylinder moving bracket, the cover plate can move together with the servo cylinders on the corresponding side. The servo cylinders on the upper, lower, left, and right sides are fixed on the corresponding sides of the integral ring frame, and the front end of the piston rod of the servo cylinder is provided with a load sensor at the center position , A pressure head is installed after the front end of the load sensor passes through the main casing;

Put the composite coal and rock sample into a cavity surrounded by six sample pads and install it through the quick lock combination, and then combine the edge sealant at the joint of the sample pad to seal it into a sample gasket, thereby sealing The composite coal and rock sample is sealed therein; the edge sealant is applied with liquid silicone rubber on the edge that needs to be sealed, and the sealing between the sample pads can be realized after the silicone rubber is cured;

A suction pipe facing the extraction hole is fixed under the upper pressure head, and a fracturing pipe facing the fracturing hole is fixed above the lower pressure head, and the outer walls of the suction pipe and the fracturing pipe are evenly coated For silicone rubber, first install the sample gasket on the lower indenter and insert the fracturing tube into the fracturing hole to bond and seal, and then control the upper indenter to move down so that the upper indenter is in contact with the upper surface of the sample gasket. The extraction tube is inserted into the extraction hole to be glued and sealed, and finally the front, back, left and right four indenters are controlled to move so that the corresponding indenters are attached to the corresponding surfaces of the sample gasket;

Step 3: Apply triaxial stress;

Apply stress to the composite coal and rock sample to a predetermined value through six indenters from front, back, left, right, up and down;

Step 4. Gas pressure;

Simultaneously inject gas into composite coal and rock samples through drainage holes and fracturing holes, so that the gas pressure reaches a predetermined value and remains stable for 24±4 hours;

Step 5. Gas drainage for the first time;

Close the fracturing hole, open the drainage hole, and record the change of gas flow rate and the pressure and displacement of each pressure head until the end of gas drainage;

Step Six: Fracture of the lower coal seam;

Repeat step 4, then open the fracturing hole for high-pressure water injection to allow high-pressure water to enter the lower coal sample for hydraulic fracturing. After the water pressure is reduced by 50%, close the fracturing hole to stop fracturing, and record the front and back of the process , Left, right, upper and lower six pressure head pressure and displacement changes;

Step 7. Secondary gas drainage;

Open the drainage hole for gas drainage, and record the change of gas flow rate and the change of pressure and displacement of the pressure head in all directions;

Step 8. Other tests in the same group;

Replace the composite coal and rock samples, change the thickness of the raw coal test and the sandstone sample, or change the rock sample, triaxial pressure, gas pressure, and repeat steps one to seven;

Step Nine, organize the test data.

As a preference of the above scheme, the composite coal and rock sample is a cube, the size of the composite coal and rock sample is 200×200×200mm, the size of the raw coal sample is 200×200×70mm, and the size of the sandstone sample is 200×200×60mm .

It is further preferred that the suction hole and the fracturing hole are equal in size, with a diameter of 10 mm and a depth of 40 mm; the length of the suction pipe and the fracturing pipe is 30 mm.

It is further preferred that universal sealing joints are installed at the outer ends of the drainage holes and fracturing holes.

The beneficial effects of the present invention:

(1) The new true triaxial test test device is adopted. Compared with the traditional cavity structure enclosed by the inner and outer frames, the main body of the test device is only provided with a cast-shaped integral ring frame. The sample spacer block encloses a sample gasket to contain the sample, thereby omitting the separate pressure-resistant cavity formed between the traditional inner and outer layers. The indenter directly touches the sample pad on the corresponding side after passing through the main chassis. On the block, it can free up more space to arrange the overall ring frame with larger size and thickness, so that the cavity can withstand greater pressure and can meet the simulation test of more complex environments;

(2) Since the inner frame is omitted, the servo cylinder directly exerts force on each surface of the sample without passing through the pressure chamber. When the servo cylinder passes through the pressure chamber, dynamic sealing needs to be considered, thus simplifying the structure and reducing The cost and reliability are higher. At the same time, because the traditional inner frame is thinner than the outer frame, the inner frame is prone to expansion and deformation when the pressure is high, which further affects the sealing between the inner frame and the servo cylinder;

(3) In this test device, the pressure head and the sample pad are designed to be separated. The joints of the sample pads are coated with liquid silicone rubber and then cured and sealed, so that the injected fluid will not penetrate into the external area;

(4) Holes are provided on the front and rear sides of the integral ring frame, and a cover plate is equipped on the outside of each opening position, which together enclose the main frame, which is more convenient for the installation of the components on the front side of the sample; while the traditional structure is only in the rear The side opening is equipped with a cover plate, and the front parts need to be overhauled or the sample is assembled and disassembled. The sample needs to be moved out of the overall ring frame through the sample moving bracket, which is very troublesome;

(5) Place the sandstone sample between two raw coal samples to form a composite coal sample with fracturing holes and drainage holes, and place it in a true triaxial test device for multi-coal combined fracturing and decompression , The integrated test method of drainage, combined with the effective control of first drainage, fracturing, and secondary drainage, can truly reproduce the combined mining of multiple coal seams. The influence of the setting of technical parameters on the efficiency of gas drainage is scientific in engineering practice. Select the relevant process parameters to provide a scientific basis.

Description of the drawings

Figure 1 is a schematic diagram of the structure of the true triaxial test device used in the present invention (including two states of sample loading and removal).

Figure 2 is a left side view of the host and the host supporting assembly in Figure 1;

Figure 3 is a perspective view of a sample gasket surrounded by six sample gaskets.

Fig. 4 is a front view of the cross-sectional state of Fig. 3.

Figure 5 shows the state after the edge seal of the sample pad is cured with silicone rubber.

Detailed ways

In the following, the present invention will be further explained through the embodiments in combination with the drawings:

An integrated test method for combined fracturing, pressure relief, and drainage of multiple coal seams, including the following steps:

Step 1. Sample preparation;

(1) Preparation of two raw coal samples;

Select two pieces of raw coal to cut and polish to make the section flatness within ±0.02mm, punch a central blind hole on one surface of the raw coal sample, dry it at 105±15° for 24±4 hours, then cool it down Stand by at room temperature.

(2) Preparation of sandstone samples;

Select the sandstone block to cut and polish to make its section flatness within ±0.02mm, dry it at 105 ± 15° for 24 ± 4 hours, and then cool it to room temperature for later use.

(3) Preparation of composite coal and rock samples;

Place the sandstone sample in the middle, and evenly apply high-strength epoxy resin adhesive on its upper and lower surfaces. In addition, apply high-strength epoxy resin evenly on the opposite sides of the two raw coal samples with the central blind hole, and then apply the raw coal separately. The sample is coated with high-strength epoxy resin adhesive to compactly fit the top and bottom of the sandstone sample. The central blind hole at the top is used as the drainage hole 15, and the central blind hole at the bottom is used as the fracturing hole 16.

Preferably, the composite coal and rock sample is a cube, the size of the composite coal and rock sample is 200×200×200mm, among which the size of the raw coal sample is 200×200×700mm, and the size of the sandstone sample is 200×200×60mm. The mining hole 15 and the fracturing hole 16 are equal in size, with a diameter of 10 mm and a depth of 40 mm, but they are not limited to this.

Step 2: The sample is installed on the test device of the true triaxial test;

As shown in Figure 1-Figure 4, the true three-axis test device is mainly composed of the main engine A, the main engine support component B, the slide rail C, the slide rail support component D and the servo cylinder E. The host A is supported on the ground by the host support assembly B, and six sets of servo cylinders E are arranged in the up and down, left and right, and front and rear directions outside the host A (namely, the three directions of XYZ). The slide rail C extends forward and backward below the host A, and the slide rail C passes through the host A and is supported on the ground by the slide rail support assembly D.

The integral annular frame 1 is formed by casting. The integral annular frame 1 has holes on the front and rear sides, and a cover plate 2 is provided on the outside of each hole position. The integral ring frame 1 and the two cover plates 2 jointly enclose the main frame. The internal cavity of the main engine is used to place the composite coal and rock sample 3. The upper, lower, left, right, front and rear sides of the composite coal and rock sample 3 are equipped with sample pads 4, and a total of six sample pads are required. 4. A sample moving bracket 5 that can move back and forth on the slide rail C is provided under the sample pad 4 on the lower side.

The front and rear servo cylinders E are arranged outside the cover plate 2 on the corresponding side, and the front and rear servo cylinders E are equipped with cylinder moving brackets 6 that can move back and forth on the slide rail C. The cover plate 2 can follow the corresponding side Servo cylinder E moves together. The servo cylinders E on the upper, lower, left and right sides are arranged outside the corresponding sides of the integral ring frame.

A load sensor 8 is provided at the center of the front end of the piston rod 7 of the servo cylinder E, and the load sensor 8 is preferably embedded in installation. The front end of the load sensor 8 is provided with an indenter 9, and the front end of the load sensor 8 passes through the main casing and is installed with an indenter 9. When the composite coal and rock sample 3 is loaded, the indenter 9 directly abuts the sample pad 4 on the corresponding side. Before the test, install the sample pad 4 outside the composite coal sample 3, and then seal the joint of the sample pad 4, after the completion of the sealing, place the composite coal sample 3 on the sample moving support 5, and push the sample moving bracket 5 and the cylinder moving bracket 6 on the rear side into the inner cavity of the main engine and fix them so that all the indenters 9 are directly against the sample pad 4 on the corresponding side before the test .

Put the composite coal and rock sample into the cavity enclosed by six sample pads 4 and install it through the quick lock 14, then combine with the edge sealant at the joint of the sample pad 4 to seal a sample seal The composite coal rock sample 3 is sealed in it; the edge sealant is applied with liquid silicone rubber on the edge that needs to be sealed. After the silicone rubber is cured, the sealing between the sample cushion blocks 4 can be realized (as shown in the figure). 5). The pre-sealing is achieved after the silicone rubber is cured. During the test, the silicone rubber is tightly attached to the sample through the confining pressure of the inner cavity of the host, which can not only achieve the sealing between the adjacent surfaces of the sample pad 4, but also weaken the edges. The boundary effect at the place. Preferably, the servo cylinder E is provided with a cylinder displacement sensor 10, the upper, lower, left and right sides of the servo cylinders E are fixedly installed on the integral ring frame 1 through the end cover 11, and the front and rear servo cylinders E are fixedly installed on the integral ring through the cover plate 2. On the frame 1, all the piston rods 7 are provided with bushings at the positions where the piston rods 7 pass through the main casing to ensure the airtightness of the inner cavity of the main body.

Preferably, in the XYZ direction outside the sealed cavity surrounded by the six sample pads 4, the sample deformation displacement sensor 12 is provided in pairs, and the sample deformation displacement sensor 12 is installed on the sample through the extension rod 13 of the displacement sensor. Outside the edge of the cushion block 4, and a pair of sample deformation displacement sensors 12 in the same direction are arranged diagonally staggered, which can realize the measurement of unbalanced and uneven deformation under true triaxial conditions.

Preferably, it is also equipped with two sets of electro-hydraulic servo boosters, which provide confining pressure for the internal cavity of the host, and water injection pressure or osmotic pressure for the sample, so that the confining pressure, water injection pressure or osmotic pressure can be controlled separately. Complete complex test conditions. The working pressure of the control high-pressure valve in the electro-hydraulic servo booster is greater than the pressure of the highest output of the booster, in order to ensure high reliability and long service life.

Preferably, it is also equipped with an axial piston pump hydraulic pressure source, and the hydraulic pressure source has high and low pressure conversion, which is convenient for the smooth switching of high and low pressure during the test.

First put the composite coal and rock sample into the sample gasket surrounded by six sample pads 4, and then brush the liquid silicone rubber on the edges. After the silicone rubber is cured, move the sample through the sample moving bracket 5 Push into the inner cavity of the main engine, and finally install the front and rear cover plates 2 on the integral ring frame 1 through the cylinder moving bracket 6, and ensure the sealing of the inner cavity of the main engine during installation, and then start the test.

A suction pipe facing the extraction hole 15 is fixedly arranged below the upper pressure head 9, and a fracturing pipe facing the fracturing hole 16 is fixed above the lower pressure head 9, respectively. Spread the silicone rubber uniformly, first install the sample gasket on the lower indenter 9 and insert the fracturing tube into the fracturing hole 16 for adhesion and sealing, and then control the upper indenter 9 to move down so that the upper indenter 9 is sealed with the sample While the upper surface of the pad is attached, the extraction tube is inserted into the extraction hole 15 to be glued and sealed. Finally, the four indenters 9 in the front, rear, left and right are controlled to move so that the corresponding indenters 9 are attached to the sample gasket. The corresponding surface.

Preferably, the length of the extraction pipe and the fracturing pipe are 30 mm, and the outer ends of the extraction hole 15 and the fracturing hole 16 are equipped with a universal sealing joint 17.

Step 3: Apply triaxial stress;

The stress is applied to the composite coal and rock sample to a predetermined value through the six indenters 9 at the front, rear, left, right, upper and lower sides.

Step 4. Gas pressure;

Gas is injected into the composite coal and rock sample simultaneously through the extraction hole 15 and the fracturing hole 16, so that the gas pressure reaches a predetermined value and remains stable for 24±4 hours.

Step 5. Gas drainage for the first time;

Close the fracturing hole 16, open the drainage hole 15, and record the change of gas flow rate and the pressure and displacement of each pressure head until the end of gas drainage.

Step Six: Fracture of the lower coal seam;

Repeat step 4, then open the fracturing hole 16 for high-pressure water injection, so that high-pressure water enters the lower raw coal sample for hydraulic fracturing. After the water pressure is reduced by 50%, close the fracturing hole 16 to stop fracturing, and record the previous , Back, left, right, up and down six pressure head pressure and displacement changes.

Step 7. Secondary gas drainage;

Open the drainage hole 15 for gas drainage, and record the gas flow changes and the pressure and displacement changes of the pressure head in all directions.

Step 8. Other tests in the same group;

Replace the composite coal and rock sample, change the thickness of the raw coal test and sandstone sample, or change the rock sample, triaxial pressure, gas pressure, and repeat steps one to seven.

Step Nine, organize the test data. The following table shows the recorded data during the test.

Claims

An integrated test method for combined fracturing, pressure relief, and drainage of multiple coal seams, which is characterized in that it comprises the following steps:

Step 1. Sample preparation;

(1) Preparation of two raw coal samples;

Select two pieces of raw coal to cut and polish to make the section flatness within ±0.02mm, punch a central blind hole on one surface of the raw coal sample, dry it at 105±15° for 24±4 hours, then cool it down Stand by at room temperature;

(2) Preparation of sandstone samples;

Select the sandstone block to cut and polish to make its section flatness within ±0.02mm, dry it at 105 ± 15° for 24 ± 4 hours, and then cool it to room temperature for later use;

(3) Preparation of composite coal and rock samples;

Place the sandstone sample in the middle, and evenly apply high-strength epoxy resin adhesive on its upper and lower surfaces. In addition, apply high-strength epoxy resin evenly on the opposite sides of the two raw coal samples with the central blind hole, and then apply the raw coal separately. The sample is coated with high-strength epoxy resin adhesive to compactly fit the top and bottom of the sandstone sample. The central blind hole at the top is used as the drainage hole (15), and the central blind hole at the bottom is used as the fracturing hole (16) ；

Step 2: The sample is installed on the test device of the true triaxial test;

The true three-axis test device includes a host (A), a host support component (B), a slide rail (C), a slide rail support component (D) and a servo cylinder (E), six sets of the servo cylinder (E) Arranged in the up and down, left and right, front and rear directions outside the host (A), the slide rail (C) extends back and forth below the host (A), and passes through the host (A) and is supported by the slide rail support assembly (D) On the ground, the host (A) includes a cast-shaped integral ring frame (1), the front and rear sides of the integral ring frame (1) have holes, and a cover plate (2) is provided on the outside of each hole position , The integral annular frame (1) and the cover plate (2) enclose the main frame, and the internal cavity of the main frame is used to place the composite coal and rock sample (3). The upper, lower, left and right sides of the composite coal and rock sample (3) The right, front and rear sides are respectively equipped with sample pads (4), and a sample moving bracket ( 5); below the servo cylinders (E) on the front and rear sides are provided with a cylinder moving bracket (6) that can move back and forth on the slide rail (C), and the cover plate (2) can follow the servo cylinder (E) on the corresponding side ) Move together, the servo cylinders (E) on the upper, lower, left and right sides are fixed outside the corresponding sides of the integral ring frame (1), and the front end of the piston rod (7) of the servo cylinder (E) is provided with a load sensor (8), A pressure head (9) is installed after the front end of the load sensor (8) passes through the main casing;

Put the composite coal and rock sample into the cavity surrounded by six sample pads (4) and install it through the quick lock (14) combination, and then seal it with the edge sealant at the joint of the sample pad (4) The composite coal and rock sample (3) is sealed into a sample gasket; the edge sealant adopts brushing liquid silicone rubber on the edges that need to be sealed, and the sample gasket can be realized after the silicone rubber is cured The seal between the blocks (4);

A drainage pipe facing the drainage hole (15) is fixed under the upper pressure head (9), and a fracturing pipe facing the fracturing hole (16) is fixed above the lower pressure head (9), Coat the outer walls of the extraction tube and fracturing tube uniformly with silicone rubber. First, install the sample gasket on the lower head (9) and insert the fracturing tube into the fracturing hole (16) for adhesion and sealing, and then control the upper pressure Move the head (9) down so that the upper pressure head (9) is attached to the upper surface of the sample gasket while the extraction tube is inserted into the extraction hole (15) to be glued and sealed, and finally the front, back, left and right four are controlled separately Move the two indenters (9) so that the corresponding indenters (9) are respectively attached to the corresponding surfaces of the sample gasket;

Step 3: Apply triaxial stress;

Apply stress to the composite coal and rock sample to a predetermined value through six indenters (9) from the front, back, left, right, up and down;

Step 4. Gas pressure;

Simultaneously inject gas into the composite coal and rock samples through the extraction holes (15) and fracturing holes (16), so that the gas pressure reaches a predetermined value and remains stable for 24±4 hours;

Step 5. Gas drainage for the first time;

Close the fracturing hole (16), open the drainage hole (15), and record the change of gas flow rate and the pressure and displacement of each pressure head until the end of gas drainage;

Step Six: Fracture of the lower coal seam;

Repeat step 4, and then open the fracturing hole (16) for high-pressure water injection, so that high-pressure water enters the lower coal sample for hydraulic fracturing. After the water pressure is reduced by 50%, close the fracturing hole (16) to stop fracturing, and record The pressure and displacement changes of the front, back, left, right, upper and lower six pressure heads during the process;

Step 7. Secondary gas drainage;

Open the drainage hole (15) for gas drainage, and record the change of gas flow rate and the change of pressure and displacement of the pressure head in all directions;

Step 8. Other tests in the same group;

Replace the composite coal and rock samples, change the thickness of the raw coal test and the sandstone sample, or change the rock sample, triaxial pressure, gas pressure, and repeat steps one to seven;

Step Nine, organize the test data.
The integrated test method for combined fracturing, pressure relief, and drainage of multiple coal seams according to claim 1, wherein the composite coal and rock sample is a cube, and the size of the composite coal and rock sample is 200×200×200mm, The size of the raw coal sample is 200×200×70mm, and the size of the sandstone sample is 200×200×60mm.
The integrated test method for combined fracturing, pressure relief, and drainage of multiple coal seams according to claim 1, wherein the drainage holes (15) and fracturing holes (16) are equal in size, with a diameter of 10 mm and a depth of 10 mm. It is 40mm; the length of the drainage pipe and fracturing pipe is 30mm.
The multi-coal joint fracturing, pressure relief, and drainage integrated test method according to claim 1, characterized in that: the outer ends of the drainage hole (15) and the fracturing hole (16) are installed with universal seals Connector (17).