Efficient gas extraction method for stacked thin coal seam

Abstract

The invention discloses an efficient gas extraction method for stacked thin coal seams, which comprises the following steps of: firstly, simultaneously fracturing a plurality of thin coal seams by using a high-pressure hydraulic fracturing technology to realize a crack expansion process of a coal-rock mass; the method comprises the following steps of: soaking a well, injecting alkali liquor mixed with a magnetic proppant into the cracks, distributing the magnetic proppant in each crack, performing chemical corrosion action on the high-pressure alkali liquor and mineral components in a coal rock layer after soaking, further improving the roughness and the space volume of the fractured cracks, and ensuring that the magnetic proppant plays a role in supporting and preventing the cracks from being closed; 2, a positioning effect of detecting the development range and width of the crack is achieved; obtaining magnetic flux data of different positions of each thin coal seam again, comparing the two magnetic flux change conditions of the same position, evaluating the effect of alkali liquor expansion crack width, determining the distribution range of the magnetic proppant according to the next magnetic flux data of different positions of each thin coal seam, and further obtaining the crack expansion and development range. And finally, efficient gas extraction is realized.

Description

Technical Field

[0001] The invention relates to a method for efficient gas extraction from superimposed thin coal seams, belonging to the technical field of gas disaster prevention and efficient extraction from superimposed coal seams. Background Art

[0002] Superimposed thin coal seams generally have the characteristics of high gas content and close distance between coal seams. The stress disturbance caused by the mining of a certain layer of coal seams will affect the stress state of the adjacent coal seams. If gas pre-extraction measures are not taken, the gas of the adjacent coal seams will migrate to the mining space, causing accidents such as gas exceeding the limit in the construction area. If dense drilling is used to pre-extract gas from superimposed coal seams, there will be technical limitations such as limited effective extraction range and rapid attenuation of gas extraction flow due to ground stress. Hydraulic fracturing technology is currently a commonly used coal seam fracturing and permeability enhancement technology measure. It uses high-pressure water to fracture and enhance the permeability of coal seams. By destroying the overall structural integrity of the coal body and generating a complex fracture network, it can effectively improve the porosity and permeability characteristics of the coal seams.

[0003] For superimposed thin coal seams, the parameter selection of hydraulic fracturing technology is of great significance to the overall crack initiation and expansion. Affected by ground stress, the fracturing cracks generally expand along the direction of the maximum principal stress. In order to increase the effective fracturing area, hydraulic jets, hydraulic cutting and other measures can be used to induce the expansion of cracks, but these methods face the problems of low induction efficiency and rock expansion when hydraulic fracturing cracks encounter water-absorbent rock layers between coal seams, which has an adverse effect on the crack expansion path. At the same time, the bottom water needs to be discharged after fracturing. Once the water is discharged from the cracks, the fracturing cracks will be closed due to the influence of ground stress, thereby greatly reducing the fracturing efficiency; and after fracturing, it is impossible to timely evaluate the expansion and development of the cracks, and additional professional detection equipment is required for detection and determination. As a result, once the evaluation finds that the fracturing effect does not meet expectations, it is necessary to re-deploy the fracturing equipment and perform fracturing construction again, which will ultimately further reduce the fracturing efficiency.

[0004] Therefore, how to provide a new method that can achieve efficient fracturing of superimposed thin coal seams, and effectively reduce the closure of cracks due to ground stress after cracks are generated, and timely evaluate the fracturing effect after fracturing, and quickly carry out subsequent fracturing work as needed, and ultimately achieve efficient gas extraction, is the research direction required by the present invention. Summary of the invention

[0005] In view of the problems existing in the above-mentioned prior art, the present invention provides a method for efficient gas extraction from superimposed thin coal seams, which can achieve efficient fracturing of superimposed thin coal seams, and after the cracks are generated, it can effectively reduce the closure of the cracks due to ground stress, and timely evaluate the fracturing effect after fracturing, and quickly carry out subsequent fracturing work as needed, ultimately achieving efficient gas extraction.

[0006] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a method for efficient gas extraction from stacked thin coal seams, the specific steps are:

[0007] A. First, geological exploration is conducted on the superimposed thin coal seams. The geological exploration methods include existing methods such as seismic wave detection and geophysical exploration to obtain the stratigraphic distribution, coal seam thickness, coal seam boundary and coal seam spatial distribution information of the superimposed thin coal seams, and statistical analysis is conducted on the mechanical parameters and gas occurrence parameter data of the top and bottom plate rock layers and each thin coal seam by taking cores on the ground;

[0008] B. According to the gas storage parameter data of different positions of each thin coal seam obtained in step A, the position with more gas storage parameters in the superimposed thin coal seam is selected to be divided into an area, and the area is used as the superimposed coal seam fracturing extraction area; then, the mechanical parameters of the area are determined according to the data of step A, and then the horizontal drilling construction layer and its construction parameters, perforation parameters and hydraulic fracturing parameters of the area are determined;

[0009] C. The first horizontal drilling is carried out from the ground to the top rock layer of the thin coal seam closest to the ground, and the second horizontal drilling is carried out to the bottom rock layer of the thin coal seam farthest from the ground. Both the first horizontal drilling and the second horizontal drilling are carried out according to the horizontal well construction parameters;

[0010] D. Perform segmented perforation on the first horizontal wellbore. After the perforation is completed, the first horizontal wellbore is continuously subjected to segmented hydraulic fracturing, and the fractures formed by the fracturing are extended in the fracturing and extraction area of ​​the superimposed coal seam;

[0011] E. After the water pressure is detected in the second horizontal well, the hydraulic fracturing process of the first horizontal well is stopped; then, a magnetic detector is respectively sent to the first horizontal well and the second horizontal well, and an alkali solution mixed with a magnetic proppant is injected into the first horizontal well, so that the alkali solution flows along each crack formed in step D and flows into each thin coal seam in turn, and at the same time, the magnetic proppant is distributed in each crack;

[0012] F. When alkali solution is detected in the second horizontal drilling well, stop injecting alkali solution, and first obtain magnetic flux data of different positions of each thin coal seam through the magnetic detector; then continue to inject alkali solution to increase pressure, and when the injection pressure reaches a certain pressure value, close the first horizontal drilling well and the second horizontal drilling well for a period of time, and obtain magnetic flux data of different positions of each thin coal seam again through the magnetic detector. According to the change of magnetic flux at the same position before and after twice, evaluate the effect of alkali solution on expanding the width of cracks, and according to the magnetic flux data of different positions of each thin coal seam in the latter time, determine the distribution range of magnetic proppant, and then obtain the range of crack expansion and development;

[0013] G. According to the alkali solution expansion crack width and crack expansion development range determined in step F, if the desired fracturing effect is achieved, the pressure water and alkali solution are gradually drained out through the second horizontal drilling, and the magnetic proppant is left in the crack to support the crack and prevent it from closing. Then, the gas in the superimposed coal seam fracturing and extraction area is efficiently extracted through the first horizontal drilling and the second horizontal drilling; if the desired fracturing effect is not achieved, the hydraulic fracturing and alkali hydraulic fracturing process from D to F is repeated until the desired fracturing effect is achieved, the fracturing process is completed, and gas extraction is performed;

[0014] H. After completing the extraction work in the current superimposed coal seam fracturing and extraction area, along the direction of the superimposed thin coal seam, according to the gas storage parameter data of different positions of each thin coal seam obtained in step A, reselect the position with more gas storage parameters in the superimposed thin coal seam and divide it into an area, and use this area as the new superimposed coal seam fracturing and extraction area, then repeat steps B to G to realize the extraction work in the current superimposed coal seam fracturing and extraction area, and repeat this process until the gas extraction work of the entire superimposed thin coal seam is completed.

[0015] Furthermore, the distance between the first horizontal drilling well and the thin coal seam closest to the ground is 3 to 5 m, and the distance between the second horizontal drilling well and the thin coal seam farthest from the ground is 3 to 5 m.

[0016] Furthermore, the segmented perforating in the first horizontal drilling is carried out using large-diameter bullets, with 3 clusters per segment, 10 holes per meter, and 1 meter per cluster. The holes are arranged vertically downward, and the penetration depth of the perforating bullets reaches the thin coal seam closest to the ground. The perforating hole diameter is 15 mm.

[0017] Furthermore, the hydraulic fracturing adopts pulsating hydraulic fracturing, the pulsation frequency is controlled at 500-2000 Hz, and the fracturing pressure is determined according to the mechanical strength and ground stress of the superimposed thin coal seams.

[0018] Furthermore, the alkali solution is a sodium hydroxide solution with a concentration of more than 85%. The use of this solution can further improve the crack expansion effect of the solution on the crack.

[0019] Furthermore, a certain amount of surfactant and viscosity enhancer is added to the alkali solution to ensure that the alkali solution has a good ability to carry the magnetic support agent.

[0020] Furthermore, the magnetic proppant is a magnetic support particle, which includes a ceramic shell and an iron core, wherein the ceramic shell is wrapped around the outside of the iron core, and the magnetic support particle has a compressive strength of 20 MPa and a particle size of 3 mm to 10 mm. The use of such magnetic support particles can not only meet the required support compressive strength, but also have a good response to the magnetic detector, thereby achieving the purpose of monitoring the expansion and development of cracks.

[0021] Furthermore, the first horizontal drilling and the second horizontal drilling are respectively sent into N-level and S-level magnetic detectors, the detection width of the magnetic detectors is 20-25m, and the spatial distribution characteristics of the fracturing cracks in the superimposed coal seams are characterized according to the intensity of the magnetic flux change between the two magnetic detectors.

[0022] Compared with the prior art, the present invention firstly utilizes high-pressure hydraulic fracturing technology to simultaneously fracture multiple thin coal seams, and relies on perforation to induce directional expansion and extension of the fracturing cracks, thereby realizing the crack expansion behavior of the coal rock structure and improving the targeting of crack expansion; then, an alkali solution mixed with a magnetic proppant is injected into the fracturing cracks, so that the magnetic proppant is distributed in each fracturing crack, and after the alkali solution is injected, the magnetic flux data of different positions of each thin coal seam is first obtained through a magnetic detector, and the distribution range of the cracks formed by hydraulic fracturing can be known according to the data, as a preliminary evaluation; then, the alkali solution pressure in the fracturing and extraction area of ​​the superimposed coal seams is increased to a certain value, and then the well is shut down, at which time the high-pressure alkali solution and the mineral components in the coal rock layer undergo chemical dissolution, further improving the coarseness of the fracturing cracks. Roughness and spatial volume, magnetic proppants can continue to accumulate in newly added cracks or spaces, and through this supporting effect, the degree of opening of the fracturing cracks is ensured; the magnetic proppants play the role of supporting the cracks to prevent them from closing, and secondly, they play the role of positioning to detect the development range and width of the cracks; after completion, the magnetic flux data of different positions of each thin coal seam are obtained again, and the changes of magnetic flux at the same position before and after are compared to evaluate the effect of alkali solution on expanding the crack width, and according to the magnetic flux data of different positions of each thin coal seam at the latter time, the distribution range of magnetic proppants is determined, and then the development range of crack expansion is obtained; if the desired fracturing effect is achieved, subsequent gas extraction work is carried out, if not, the fracturing process can be repeated in time to ultimately achieve efficient gas extraction. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is an overall construction schematic diagram of the present invention.

[0024] In the figure: 1-roof rock layer, 2-first thin coal seam, 3-second thin coal seam, 4-third thin coal seam, 5-bottom rock layer, 6-first horizontal drilling, 7-second horizontal drilling, 8-fracture. DETAILED DESCRIPTION

[0025] The present invention will be further described below.

[0026] like Figure 1 As shown, the specific steps of the present invention are:

[0027] A. First, geological exploration is conducted on the superimposed thin coal seams. The geological exploration methods include existing methods such as seismic wave detection and geophysical exploration to obtain the stratigraphic distribution, coal seam thickness, coal seam boundary and coal seam spatial distribution information of the superimposed thin coal seams, and statistical analysis is conducted on the mechanical parameters and gas occurrence parameter data of the top and bottom plate rock layers and each thin coal seam by taking cores on the ground;

[0028] B. According to the gas occurrence parameter data of different positions of each thin coal seam obtained in step A, the position with more gas occurrence parameters in the superimposed thin coal seams is selected to be divided into an area, and the area is used as the superimposed coal seam fracturing extraction area, and the thin coal seams distributed in sequence from the ground to the underground in the area are numbered sequentially, namely, the first thin coal seam 2, the second thin coal seam 3 and the third thin coal seam 4; then, the mechanical parameters of the area are determined according to the data of step A, and then the horizontal drilling construction layer and its construction parameters, perforation parameters and hydraulic fracturing parameters of the area are determined;

[0029] C. Construct a first horizontal drilling well 6 from the ground toward the roof rock layer 1 of the first thin coal seam 2, and construct a second horizontal drilling well 7 toward the bottom rock layer 5 of the third thin coal seam 4. Both the first horizontal drilling well 6 and the second horizontal drilling well 7 are constructed according to the horizontal well construction parameters; the distance between the first horizontal drilling well 6 and the first thin coal seam 2 is 3 to 5 m, and the distance between the second horizontal drilling well 7 and the third thin coal seam 4 is 3 to 5 m.

[0030] D. Perform segmented perforation on the first horizontal well 6. After the perforation is completed, perform continuous segmented hydraulic fracturing on the first horizontal well 6. The fractures 8 formed by the fracturing extend in the fracturing and extraction area of ​​the superimposed coal seam. The segmented perforation in the first horizontal well 6 uses large-aperture bullets for perforation operations, with 3 clusters per segment, 10 holes per meter, and 1 meter per cluster. The holes are arranged vertically downward, and the penetration depth of the perforating bullet reaches the first thin coal seam 2. The perforation aperture is 15 mm. The hydraulic fracturing uses pulsating hydraulic fracturing, and the pulsation frequency is controlled at 500-2000 Hz. The fracturing pressure is determined according to the mechanical strength and ground stress of the superimposed thin coal seam.

[0031] E. After the water pressure is monitored in the second horizontal well 7, the hydraulic fracturing process of the first horizontal well 6 is stopped; then, a magnetic detector is respectively sent to the first horizontal well 6 and the second horizontal well 7, and an alkali solution mixed with a magnetic proppant is injected into the first horizontal well, so that the alkali solution flows along each crack formed in step D and flows into each thin coal seam in turn, and at the same time, the magnetic proppant is distributed in each crack 8; the magnetic proppant is a magnetic support particle, and the magnetic support particle includes a ceramic shell and an iron core, and the ceramic shell is wrapped around the outside of the iron core, and the two are in close contact through high-temperature forging, and the compressive strength of the magnetic support particle is 20MPa, and the particle size is 3mm to 10mm. The use of such magnetic support particles can not only meet the required support compressive strength, but also have a good response to the magnetic detector, thereby achieving the purpose of monitoring the expansion and development of the cracks.

[0032] F. When alkali solution is detected in the second horizontal wellbore 7, stop injecting alkali solution, and first obtain magnetic flux data of different positions of each thin coal seam through the magnet detector; then continue to inject alkali solution to increase pressure, and when the injection pressure reaches a certain pressure value, close the first horizontal wellbore 6 and the second horizontal wellbore 7 for a period of time, and obtain magnetic flux data of different positions of each thin coal seam again through the magnet detector. According to the change of magnetic flux at the same position twice before and after, evaluate the effect of alkali solution on expanding the width of cracks, and according to the magnetic flux data of different positions of each thin coal seam in the latter time, determine the distribution range of magnetic proppant, and then derive the range of crack expansion and development.

[0033] G. According to the width of the alkali solution expansion crack 8 and the expansion development range of the crack 8 determined in step F, if the desired fracturing effect is achieved, the pressure water and alkali solution are gradually discharged through the second horizontal drilling 7, and the magnetic proppant is left in the crack 8 to support the crack and prevent it from closing. Then, the gas in the superimposed coal seam fracturing and extraction area is efficiently extracted through the first horizontal drilling and the second horizontal drilling; if the desired fracturing effect is not achieved, the hydraulic fracturing and alkali hydraulic fracturing process from D to F is repeated until the desired fracturing effect is achieved, the fracturing process is completed and gas extraction is carried out.

[0034] H. After completing the extraction work in the current superimposed coal seam fracturing and extraction area, along the direction of the superimposed thin coal seam, according to the gas storage parameter data of different positions of each thin coal seam obtained in step A, reselect the position with more gas storage parameters in the superimposed thin coal seam and divide it into an area, and use this area as the new superimposed coal seam fracturing and extraction area, then repeat steps B to G to realize the extraction work in the current superimposed coal seam fracturing and extraction area, and repeat this process until the gas extraction work of the entire superimposed thin coal seam is completed.

[0035] As an improvement of the present invention, the alkali solution is a sodium hydroxide solution with a concentration of more than 85%. The use of this solution can further improve the crack expansion effect of the solution. A certain amount of surfactant and viscosity enhancer is added to the alkali solution to ensure that the alkali solution has a good ability to carry the magnetic proppant.

[0036] As another improvement of the present invention, the first horizontal drilling 6 and the second horizontal drilling 7 are respectively sent into N-level and S-level magnetic detectors. The detection width of the magnetic detectors is 20 to 25 meters. When working, the two magnetic detectors are started synchronously to obtain magnetic flux data at different positions. Since the iron core in the magnetic support particles is a magnetic body, the magnetic flux at the position where the magnetic support particles are gathered is different from the magnetic flux of the surrounding coal body, and the magnetic support particles are in the cracks 8. According to this characteristic, the spatial distribution characteristics of the cracks 8 can be obtained. At the same time, for the same position, the magnetic flux data before and after alkali-hydraulic fracturing are obtained. According to its change, the change in the number of magnetic support particles at this position can be obtained. The more magnetic support particles there are, the larger the expansion width of the crack 8 is and the more particles it can accommodate. Therefore, according to this characteristic, the change in the expansion width of the crack 8 at the same position can be monitored.

[0037] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for efficient gas extraction from stacked thin coal seams, characterized in that: The specific steps are: A. First, geological exploration is conducted on the superimposed thin coal seams to obtain the stratigraphic distribution, coal seam thickness, coal seam boundary and coal seam spatial distribution information of the superimposed thin coal seams, and statistical analysis is conducted on the mechanical parameters and gas occurrence parameter data of the roof and floor rock layers and each thin coal seam by taking cores on the ground; B. According to the gas storage parameter data of different positions of each thin coal seam obtained in step A, the position with more gas storage parameters in the superimposed thin coal seam is selected to be divided into an area, and the area is used as the superimposed coal seam fracturing extraction area; then, the mechanical parameters of the area are determined according to the data of step A, and then the horizontal drilling construction layer and its construction parameters, perforation parameters and hydraulic fracturing parameters of the area are determined; C. The first horizontal drilling is carried out from the ground to the top rock layer of the thin coal seam closest to the ground, and the second horizontal drilling is carried out to the bottom rock layer of the thin coal seam farthest from the ground. Both the first horizontal drilling and the second horizontal drilling are carried out according to the horizontal well construction parameters; D. Perform segmented perforation on the first horizontal wellbore. After the perforation is completed, the first horizontal wellbore is continuously subjected to segmented hydraulic fracturing, and the fractures formed by the fracturing are extended in the fracturing and extraction area of ​​the superimposed coal seam; E. After the water pressure is detected in the second horizontal well, the hydraulic fracturing process of the first horizontal well is stopped; then, a magnetic detector is respectively sent to the first horizontal well and the second horizontal well, and an alkali solution mixed with a magnetic proppant is injected into the first horizontal well, so that the alkali solution flows along each crack formed in step D and flows into each thin coal seam in turn, and at the same time, the magnetic proppant is distributed in each crack; F. When alkali solution is detected in the second horizontal drilling well, stop injecting alkali solution, and first obtain magnetic flux data of different positions of each thin coal seam through the magnetic detector; then continue to inject alkali solution to increase pressure, and when the injection pressure reaches a certain pressure value, close the first horizontal drilling well and the second horizontal drilling well for a period of time, and obtain magnetic flux data of different positions of each thin coal seam again through the magnetic detector. According to the change of magnetic flux at the same position before and after twice, evaluate the effect of alkali solution on expanding the width of cracks, and according to the magnetic flux data of different positions of each thin coal seam in the latter time, determine the distribution range of magnetic proppant, and then obtain the range of crack expansion and development; G. According to the alkali solution expansion crack width and crack expansion development range determined in step F, if the desired fracturing effect is achieved, the pressure water and alkali solution are gradually drained out through the second horizontal drilling, and the magnetic proppant is left in the crack to support the crack and prevent it from closing. Then, the gas in the superimposed coal seam fracturing and extraction area is efficiently extracted through the first horizontal drilling and the second horizontal drilling; if the desired fracturing effect is not achieved, the hydraulic fracturing and alkali hydraulic fracturing process from D to F is repeated until the desired fracturing effect is achieved, the fracturing process is completed, and gas extraction is performed; H. After completing the extraction work in the current superimposed coal seam fracturing and extraction area, along the direction of the superimposed thin coal seam, according to the gas storage parameter data of different positions of each thin coal seam obtained in step A, reselect the position with more gas storage parameters in the superimposed thin coal seam and divide it into an area, and use this area as the new superimposed coal seam fracturing and extraction area, then repeat steps B to G to realize the extraction work in the current superimposed coal seam fracturing and extraction area, and repeat this process until the gas extraction work of the entire superimposed thin coal seam is completed.

2. The method for efficient gas extraction from stacked thin coal seams according to claim 1, characterized in that: The distance between the first horizontal drilling well and the thin coal seam closest to the ground is 3 to 5 m, and the distance between the second horizontal drilling well and the thin coal seam farthest from the ground is 3 to 5 m.

3. The method for efficient gas extraction from stacked thin coal seams according to claim 1 is characterized in that: The segmented perforating in the first horizontal drilling construction uses large-diameter bullets for perforating operations, with 3 clusters per segment, 10 holes per meter, and each cluster of 1 meter. The holes are arranged vertically downward, and the penetration depth of the perforating bullets reaches the thin coal seam closest to the ground. The perforating hole diameter is 15mm.

4. The method for efficient gas extraction from stacked thin coal seams according to claim 1 is characterized in that: The hydraulic fracturing adopts pulsating hydraulic fracturing, the pulsation frequency is controlled at 500-2000 Hz, and the fracturing pressure is determined according to the mechanical strength and ground stress of the superimposed thin coal seams.

5. The method for efficient gas extraction from stacked thin coal seams according to claim 1 is characterized in that: The alkali solution is a sodium hydroxide solution with a concentration of more than 85%.

6. The method for efficient gas extraction from stacked thin coal seams according to claim 1, characterized in that: A certain amount of surfactant and viscosity enhancer is added into the alkali solution to ensure that the alkali solution has a good ability to carry the magnetic support agent.

7. The method for efficient gas extraction from stacked thin coal seams according to claim 1, characterized in that: The magnetic support agent is magnetic support particles, which include a ceramic shell and an iron core. The ceramic shell is wrapped around the outside of the iron core. The magnetic support particles have a compressive strength of 20 MPa and a particle size of 3 mm to 10 mm.

8. The method for efficient gas extraction from stacked thin coal seams according to claim 1, characterized in that: The first horizontal drilling well and the second horizontal drilling well are respectively sent into N-level and S-level magnetic detectors, and the detection width of the magnetic detectors is 20 to 25 meters. The spatial distribution characteristics of the fracturing cracks in the superimposed coal seams are characterized according to the intensity of the magnetic flux change between the two magnetic detectors.

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