Method for pre-extracting coal seam gas by combining high-density well spacing with small-scale hydraulic fracturing

Abstract

The invention discloses a method for pre-extracting coal seam gas by combining high-density well spacing with small-scale hydraulic fracturing, and belongs to the technical field of coal seam gas ground pre-extraction outburst elimination. The method comprises the following steps: deploying a plurality of ground gas extraction vertical wells and directional wells with two-opening well bore structures in a coal face according to a square well pattern with the distance between targets being 50-60m, tripping in a sleeve after drilling a two-opening well section, and packing an annular channel between the lower part of the sleeve and a well wall by using a packer outside the sleeve, so that a closed space is formed at the lower part of the gas extraction well; after small-scale hydraulic sand fracturing, overflow releasing and well flushing of a pre-exploited coal seam are carried out on the ground in a light casing pipe injection mode through small fracturing equipment, drainage and mining equipment such as an electric submersible pump and an underground pressure meter are put into a casing pipe, and drainage gas production and negative pressure gas extraction operation are carried out in sequence. By means of the method, low-cost and rapid pre-drainage of coal seam gas of the coal face can be achieved, the gas outburst risk in the coal roadway tunneling and working face stoping process is remarkably reduced, and economic, environmental and social benefits are achieved.

Description

Technical Field

[0001] The present invention relates to a method for pre-extracting coal seam gas by combining high-density well arrangement with small-scale hydraulic fracturing in a coal mining face, and in particular to a method for pre-extracting coal seam gas by utilizing ground high-density gas extraction vertical wells and directional wells to carry out small-scale hydraulic fracturing transformation, drainage gas extraction and negative pressure gas extraction in pre-extracted coal seams, thereby realizing pre-extraction and elimination of coal seam gas outbursts and development and utilization of coal seam gas resources in coal mining faces in coal mining areas, and belongs to the technical field of ground pre-extraction and elimination of coal seam gas outbursts. Background Art

[0002] In the process of coal mining, the extraction and management of coal seam gas is a crucial link, which is directly related to the safe production and mining efficiency of coal mines. With the continuous advancement of coal mining technology and the increasing requirements for safe production, how to pre-extract coal seam gas efficiently, safely and at low cost has become a key technical problem that needs to be solved in the underground mining of coal resources.

[0003] Traditional coal seam gas extraction methods, such as drilling extraction and ground extraction, can achieve gas extraction to a certain extent, but they often have disadvantages such as low efficiency, high cost, and reduced excavation and coal mining efficiency. As a result, traditional gas control methods are not only difficult to meet the needs of efficient and safe production in modern coal mines, but may also have an adverse impact on the sustainable mining of coal mines. Therefore, it is particularly important to explore a new, efficient, and low-cost coal seam gas extraction method.

[0004] In order to comprehensively promote coal seam gas control work within the scope of my country's coal mining areas, it is necessary to solve the problems of high cost, low efficiency and impact on coal mine production of traditional gas control methods, and actively explore new methods and technologies for ground pre-extraction of coal seam gas. New coal seam gas extraction methods need to be able to improve gas extraction efficiency and reduce extraction costs while ensuring safe production in coal mines, while minimizing the impact on the integrity of the coal seam roof and floor. This will not only improve the effect of coal mine gas control and the overall production efficiency of coal mines, but also maximize the exploitation and utilization of coal seam gas resources, generating comprehensive benefits in many aspects such as economy, environment and safety. Summary of the invention

[0005] Technical problem: The purpose of the present invention is to overcome the problems of large amount of drilling work for pre-extraction of coal seam gas by traditional underground drilling along the layer and through the layer in the coal mining working face, low gas extraction efficiency, long extraction cycle and slow elimination of gas outburst effects of surface gas extraction wells, and provide a method for pre-extraction of coal seam gas by high-density well layout combined with small-scale hydraulic fracturing, which utilizes multiple surface gas pre-extraction vertical wells and directional wells combined with small-scale hydraulic fracturing to improve the permeability of pre-mined coal seams, realize drainage and gas extraction in pre-mined coal seams first, and then negative pressure extraction of gas, thereby improving the gas extraction effect of pre-mined coal seams.

[0006] Technical solution: In order to achieve the above-mentioned purpose, a method for pre-extracting coal seam gas by combining high-density well arrangement with small-scale hydraulic fracturing of the present invention comprises the following steps:

[0007] (a) According to the coal mine planning mining area and coal mining face design drawings, multiple ground gas extraction vertical wells are deployed in a high-density "square well pattern" type in the coal mining face. When the ground does not have the conditions for deploying gas extraction vertical wells, ground gas extraction directional wells are deployed to keep the target points of each well in the plane to form a "square well pattern";

[0008] (b) The vertical wells or directional wells used for gas extraction on the ground are all of "two-opening" wellbore structure. The surface casing is cemented in the first opening wellbore, and the production casing is lowered after the second opening wellbore is drilled. The annular channel between the second opening wellbore and the production casing is sealed by the packer outside the production casing to form a closed space at the bottom of the gas extraction well; the J55 steel grade surface casing is sealed and connected to the production casing by welding an annular steel plate at the wellhead;

[0009] (c) Carry out hydraulic fracturing construction, connect small-scale hydraulic fracturing equipment to the ground, use small-scale hydraulic fracturing equipment to continuously inject fracturing fluid and proppant into the production casing, and the fracturing fluid and proppant enter the pre-mined coal seam through the pre-perforated screen pipe at the bottom of the production casing, complete the small-scale hydraulic sand fracturing of the pre-mined coal seam, and improve the permeability of the pre-mined coal seam around the wellbore;

[0010] (d) After the fracturing construction is completed, the overflow is released and the well is cleaned, and the drainage equipment in the well is placed in the production casing. The drainage equipment in the well includes a sand settling pipe, a suction port screen pipe, an electric submersible pump and a drainage pipe arranged from bottom to top. A downhole thermometer and pressure gauge for real-time monitoring of temperature and bottom hole flow pressure changes is provided on one side of the drainage pipe. The downhole thermometer and pressure gauge transmits the temperature and bottom hole flow pressure data to the ground through the thermometer and pressure gauge cable; a positive and negative pressure gauge, a first rotary valve and a gas flow meter arranged on the gas pipeline in sequence, and a second rotary valve and a liquid flow meter arranged on the water pipeline in sequence are provided at the wellhead;

[0011] (e) Installing ground equipment for drainage and gas production at the wellhead, the ground equipment for drainage and gas production includes a water ring vacuum pump and a motor connected to the water ring vacuum pump, opening the second rotary valve on the ground to control the drainage and gas production operation, and gradually reducing the gas content and gas pressure of the pre-mined coal seam; the drainage and gas production operation includes six stages: test pumping, initial drainage, casing pressure control gas production, flow pressure reduction and production increase, flow pressure control and stable production, and gas production attenuation. The duration of each stage is determined according to the stability of the liquid level drop during the drainage and pressure reduction process, the casing pressure, the daily gas production, and the daily water production. The total duration of the drainage and gas production process is 180 to 360 days; when the drainage and gas production is completed, the wellbore liquid level gradually drops below the suction port screen position;

[0012] (f) When the wellbore liquid level drops to the position of the suction port screen, close the second rotary valve, connect the gas pipeline to the water ring vacuum pump and the motor, and extract the pre-mined coal seam gas under negative pressure to further reduce the pre-mined coal seam gas content and gas pressure. The ground water pipeline is connected to the underground drainage pipe to transport the fracturing fluid and formation water lifted by the electric submersible pump to the drainage pool on the ground.

[0013] In step (a), the distance between the target coal seam points of two adjacent surface gas extraction vertical wells and directional wells in the "square well network" is 50 to 60 m, the diagonal sides of the square formed by four adjacent surface gas extraction wells are consistent with the direction of the horizontal principal stress in the mining area, the maximum well inclination angle of the surface gas extraction directional well is ≤30°, and the horizontal displacement of the bottom of the surface gas extraction directional well is ≤30 m.

[0014] In step (b), the first wellbore is drilled to 10 to 15 m below the bedrock, and a J55 steel grade surface casing is installed and then G grade oil well cement is injected to form a first cementing cement ring; the second wellbore is drilled to 8 to 10 m below the pre-mined coal seam and then completed, and an N80 steel grade production casing is installed, and the packer outside the production casing is pressurized to expand the packer, so that a closed space is formed in the lower part of the gas extraction well; the packer is sealed at a thick, hard, undeveloped crack and low permeability roof position within a range of 1 to 5 m above the pre-mined coal seam to ensure safety in subsequent fracturing and gas pre-extraction.

[0015] The production casing includes, from bottom to top, a wire plug, an ordinary N80 steel grade casing, an N80 steel grade pre-perforated screen pipe and an ordinary N80 steel grade casing; the N80 steel grade pre-perforated screen pipe is located at the pre-mining coal seam position, the length of the pre-perforated section on the pre-perforated screen pipe is ≤ the coal seam thickness, the pre-perforated hole density on the pre-perforated screen pipe is 12 holes / m, and the hole arrangement method is a positive spiral type.

[0016] In step (c), the small-scale hydraulic fracturing equipment includes a fracturing construction pressure gauge, an injection pump, a sand mixer, a vertical sand tank, a first gate valve, a second gate valve, a fracturing fluid tank and high and low pressure pipelines; the fracturing construction pressure gauge is arranged at the closed wellhead, the injection pump is connected to the outlet pipeline of the sand mixer, the outlets of the vertical sand tank and the fracturing fluid tank are respectively connected to the inlet pipeline of the sand mixer, the first gate valve is arranged on the outlet pipeline of the vertical sand tank, the second gate valve is arranged on the outlet pipeline of the fracturing fluid tank, and the maximum injection displacement of the fracturing injection pump is 0.7-1.0m 3 / min, the maximum injection pressure is 30-40MPa; the sand mixer can fully mix the fracturing fluid and proppant in different proportions according to the actual needs of the site, and then input the mixed fracturing fluid and proppant into the fracturing injection pump; the volume of the vertical sand tank is 2-4m 3 The outlet of the vertical sand tank is 2 to 3 meters higher than the wellhead; the volume of the fracturing fluid tank is 40 to 50 m 3 .

[0017] In step (c), the fracturing fluid is an active water fracturing fluid mixed with 98.8% clean water, 1.0% anti-swelling agent, 0.1% bactericide and 0.1% drainage agent, and the proppant is a mixture of 70-100 mesh fine-grained pottery and 100-140 mesh ultra-fine-grained pottery in a volume ratio of 1:1; when the pre-mined coal seam is shallowly buried and the original permeability is high, a mixture of 70-100 mesh fine-grained quartz sand and 100-140 mesh ultra-fine quartz sand in a volume ratio of 1:1 is used as the proppant.

[0018] In step (c), the small-scale hydraulic sand fracturing process of the pre-mined coal seam includes three stages: injecting pre-fluid to create fractures, injecting sand-carrying fluid to add sand support, and injecting displacement fluid for displacement; in the pre-fluid injection fracture creation stage: the proportion of pre-fluid is not less than 30%, the fracturing injection pump is quickly increased and reaches the maximum displacement or maximum construction pressure, and more fractures extending around the wellbore are generated in the pre-mined coal seam; in the sand-carrying fluid injection and sand support stage: proppant is injected into the fracturing fracture of the pre-mined coal seam to extend the proppant to the front end of the fracture, and the sand ratio is increased in a step-by-step manner during the injection of the sand-carrying fluid, and the average sand ratio in the support stage is ≤10%; in the displacement fluid injection displacement stage: the fracturing fluid without proppant of the same volume as the production casing space is injected to replace the production casing space and reduce sand deposition during the fracturing process; during the whole process of fracturing construction, the cumulative injection amount of fracturing fluid is 60-120m 3 / m coal seam, sand adding strength ≥1.5~2.0m 3 / m coal seam.

[0019] In step (d), the lower end of the sand settling pipe is ≥6m away from the wire plug position at the bottom of the artificial well, the surface of the suction screen pipe is wrapped with copper wire to reduce the entry of coal powder and proppant into the electric submersible pump, the head of the electric submersible pump is greater than the distance from the suction screen pipe to the wellhead, and it has the ability to continuously adjust the drainage rate.

[0020] In step (d), the range of the positive and negative pressure gauges is -0.1 to 5 MPa, with an accuracy of ±0.01 MPa; the range of the gas flow meter is 0 to 200 m 3 / h, accuracy is ±0.1m 3 / h; the range of the liquid flow meter is 0~0.5m 3 / h, accuracy is ±0.01m 3 / h.

[0021] In step (e), when the water ring vacuum pump extracts gas under negative pressure, the negative pressure formed at the wellhead is maintained at -0.08 to -0.06 MPa; when the pure amount of gas extracted under negative pressure in the pre-mined coal seam is ≤50m 3 / d, or when the extracted gas concentration is lower than 10%, the pre-extraction of coal seam gas is completed.

[0022] Beneficial effects: Due to the adoption of the above technical solution, the present invention overcomes the problems of large drilling workload and low gas extraction efficiency of traditional underground gas pre-extraction drilling in the coal seam along the layer and through the layer, long extraction cycle and slow elimination of gas outburst effects of ground gas extraction wells, etc. Multiple ground gas pre-extraction vertical wells and directional wells are used in combination with small-scale hydraulic fracturing to improve the permeability of the pre-mined coal seam, realize the pre-mined coal seam drainage gas extraction first, and then negative pressure gas extraction, and improve the gas extraction effect of the pre-mined coal seam. First, the high-density deployment of gas extraction vertical wells and directional wells can achieve rapid extraction of pre-mined coal seams in the coal mining face, reducing the gas content and gas pressure of the pre-mined coal seams in the short term; second, the use of packers outside the production casing to isolate the annular channel between the second wellbore and the production casing reduces the cementing of the production casing and the perforation operations at the pre-mined coal seam position, greatly reducing the investment in ground gas extraction well projects; third, the use of small-scale hydraulic fracturing to transform the pre-mined coal seams not only reduces the number of fracturing construction equipment and the total displacement requirements, but also avoids the damage to the integrity of the pre-mined coal seam roof caused by hydraulic fracturing, reducing the difficulty of roof management during the face recovery process; finally, the drainage gas extraction and negative pressure extraction methods are successively used for pre-mined coal seam gas extraction, and the steam-water seepage and fluid pressure transmission channels formed by small-scale fracturing can significantly improve the efficiency and extraction rate of gas extraction, and greatly reduce the gas content and gas pressure of the pre-mined coal seam (9). The main advantages compared with the existing technology are:

[0023] 1) The “square well pattern” type is used to deploy vertical wells and directional wells at a high density, which reduces the control area of ​​a single well and significantly shortens the time for pre-extraction of gas from the coal mining face without affecting the tunneling and mining of the coal mining face;

[0024] 2) Use small-scale hydraulic fracturing to transform the pre-mined coal seams, significantly improve the permeability of the pre-mined coal seams, and significantly improve the efficiency and extraction rate of gas extraction through the steam-water seepage and fluid pressure transmission channels formed by fracturing;

[0025] 3) During the fracturing construction process, by controlling the fracturing scale and construction pressure, the risk of damage to the integrity of the pre-mined coal seam roof is reduced, and the difficulty of roof management during the working face mining process is reduced;

[0026] 4) During the fracturing construction process, by controlling the amount of proppant added, the coal quality of the coal mined from the working face can be avoided from being significantly reduced due to the mixing of proppant, thereby improving the economic benefits of coal mining enterprises.

[0027] 5) The technical process is simple, the project implementation cost is low, and the economic, environmental and social benefits are good. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1It is a schematic diagram of the small-scale hydraulic fracturing transformation method of the pre-mining coal seam after high-density well arrangement of the present invention.

[0029] Figure 2 It is a schematic diagram of the method for gas production by draining water and reducing pressure after small-scale hydraulic fracturing transformation of a pre-mined coal seam according to the present invention.

[0030] Figure 3 It is a schematic diagram of the method for negative pressure gas extraction after small-scale hydraulic fracturing transformation of pre-mined coal seams in a coal mining working face of the present invention, drainage and pressure reduction.

[0031] In the figure: 1-ground; 2-first wellbore; 3-surface casing; 4-first wellbore cement ring; 5-second wellbore; 6-production casing; 7-annular passage between second wellbore and production casing; 8-packer; 9-pre-mined coal seam; 10-pre-perforated screen; 11-fracturing fluid; 12-fracturing construction pressure gauge; 13-fracturing injection pump; 14-sand mixer; 15-vertical sand tank; 16-proppant; 17-first gate valve; 18-second gate valve; 19-fracturing fluid tank; 20-annular Steel plate; 21-wellhead; 22-sand settling during fracturing; 23-thread plug; 24-wellbore liquid level; 25-electric submersible pump; 26-inlet screen; 27-sand settling pipe; 28-sand settling during drainage and production; 29-positive and negative pressure gauge; 30-first rotary valve; 31-gas flow meter; 32-gas pipeline; 33-second rotary valve; 34-liquid flow meter; 35-water pipeline; 36-downhole temperature and pressure gauge; 37-temperature and pressure gauge cable; 38-drainage pipe; 39-water ring vacuum pump; 40-motor. DETAILED DESCRIPTION

[0032] The present invention will be further described below with reference to the embodiments in the accompanying drawings:

[0033] The method of pre-extracting coal seam gas by high-density well arrangement combined with small-scale hydraulic fracturing of the present invention comprises the following specific steps:

[0034] (a) According to the planned mining area and coal mining face design drawings of the coal mine, multiple surface gas extraction vertical wells are deployed in a high-density "square well network" type in the coal mining face. When the ground 1 does not meet the conditions for deploying gas extraction vertical wells, surface gas extraction directional wells are deployed to maintain the target points of each well in the plane to form a "square well network"; the distance between the target points of two adjacent surface gas extraction vertical wells and directional wells in the square well network is 50 to 60 meters, the diagonal sides of the square formed by the four surface gas extraction wells are consistent with the direction of the horizontal principal stress in the mining area, the maximum well inclination angle of the surface gas extraction directional well is ≤30°, and the horizontal displacement of the well bottom is ≤30 meters.

[0035] (b) The surface gas extraction well has a "two-opening" wellbore structure. The first opening wellbore 2 is cemented by lowering the surface casing 3. After the second opening wellbore 5 is drilled, the production casing 6 is lowered. The annular channel 7 between the second opening wellbore 5 and the production casing 6 is sealed by using the packer 8 outside the production casing 6 to form a closed space at the bottom of the gas extraction well. The first opening wellbore 2 is drilled to 10 to 15 meters below the bedrock, and the J55 steel grade surface casing 3 is lowered and then G grade oil well cement is injected for cementing to form a first opening cementing cement ring (4). The second opening wellbore 5 is drilled to the pre-mined coal seam 9. After drilling 8 to 10 meters, the N80 steel grade production casing 6 is lowered, and the packer 8 outside the production casing 5 is pressurized to expand, thereby forming a closed space at the bottom of the gas extraction well; the packer 8 is sealed at the top plate position with large thickness, hardness, undeveloped cracks and low permeability within the range of 1 to 5 meters above the pre-mined coal seam 9 to ensure the safety of subsequent fracturing and gas pre-extraction; at the wellhead 21, the J55 steel grade surface casing 3 is sealed and connected with the N80 steel grade production casing 6 by welding with an annular steel plate 20. The N80 steel grade production casing 6 is composed of a wire plug 23, an ordinary N80 steel grade casing, an N80 steel grade pre-perforated screen pipe 10, and an ordinary N80 steel grade casing from bottom to top; the N80 steel grade pre-perforated screen pipe 10 is located at the pre-mined coal seam 9, the length of the pre-perforated section is ≤ the thickness of the coal seam, the pre-perforated hole density is 12 holes / m, and the hole arrangement method is a positive spiral.

[0036] (c) A small-scale hydraulic fracturing device is connected to the ground, and a fracturing injection pump 13 is used to continuously inject the fracturing fluid 11 and the proppant 16 into the production casing 6. The fracturing fluid 11 and the proppant 16 enter the pre-mined coal seam 9 through the pre-perforated screen pipe 10 at the bottom of the production casing 6, so as to realize a small-scale hydraulic sand fracturing of the pre-mined coal seam 9 and improve the permeability of the pre-mined coal seam 9 around the wellbore; the small-scale hydraulic fracturing device includes a fracturing injection pump 13, a sand mixer 14, a vertical sand tank 15, a first gate valve 17, a second gate valve 18, a fracturing fluid tank 19 and high and low pressure pipelines; the maximum injection displacement of the fracturing injection pump 13 is 0.7-1.0m 3 / min, the maximum injection pressure is 30-40MPa; the sand mixer 14 can achieve full mixing of fracturing fluid 11 and proppant 16 in different proportions, and input the mixed fracturing fluid 11 and proppant 16 into the fracturing injection pump 13; the volume of the vertical sand tank 15 is 2-4m 3 The outlet of the vertical sand tank 15 is 2 to 3 meters higher than the wellhead (21); the volume of the fracturing fluid tank 19 is 40 to 50 m 3The fracturing fluid is an active water fracturing fluid, which is mixed with 98.8% clean water, 1.0% anti-swelling agent, 0.1% bactericide, and 0.1% drainage agent. The proppant is a mixture of 70-100 mesh fine-grained pottery and 100-140 mesh ultra-fine-grained pottery in a volume ratio of 1:1; when the pre-mined coal seam 9 is shallowly buried and has a high original permeability, a mixture of 70-100 mesh fine-grained quartz sand and 100-140 mesh ultra-fine-grained quartz sand in a volume ratio of 1:1 can also be used as a proppant. The hydraulic sand fracturing process includes three stages: injecting pre-fluid to create fractures, injecting sand-carrying fluid to add sand support, and injecting displacement fluid for displacement. In the pre-fluid injection stage, the fracturing injection pump 13 is quickly increased and reaches the maximum displacement or maximum construction pressure, and more fractures extending to the periphery of the wellbore are generated in the pre-mined coal seam 9, and the proportion of pre-fluid is not less than 30%. In the sand-carrying fluid injection and sand support stage, proppant is injected into the fracturing fracture of the pre-mined coal seam 9, and the proppant is extended to the front end of the fracture. The sand ratio is increased in a step-by-step manner during the injection of the sand-carrying fluid, and the average sand ratio in the support stage is ≤10%. In the displacement fluid injection stage, the fracturing fluid without proppant of the same volume as the space of the production casing 6 is injected to replace the space of the production casing 6 and reduce the sand settling 22 during the fracturing process. In the whole fracturing construction process, the cumulative injection volume of the fracturing fluid is 60-120m 3 / m coal seam, sand adding strength ≥1.5~2.0m 3 / m coal seam.

[0037] (d) After the fracturing construction is completed, the overflow is released and the well is cleaned. The drainage equipment in the well is placed in the production casing 6, and drainage and gas production operations are carried out on the ground in a controlled manner to gradually reduce the gas content and gas pressure of the pre-mined coal seam 9; the drainage equipment in the well includes a sand settling pipe 27, a suction port screen pipe 26, an electric submersible pump 25, and a drainage pipe 38 from bottom to top; the lower end of the sand settling pipe 27 is ≥6m away from the position of the artificial well bottom wire plug 23, and the surface of the suction port screen pipe 26 is wrapped with copper wire to reduce the entry of coal powder and proppant into the electric submersible pump 25. The head of the electric submersible pump 25 is greater than the distance from the suction port screen pipe 26 to the wellhead 21, and it has the ability to adjust the drainage rate steplessly; the downhole temperature and pressure gauge 36 can monitor the changes in temperature and bottom hole flow pressure in real time, and transmit the temperature and bottom hole flow pressure data to the ground 1 through the temperature and pressure gauge cable 37. The ground equipment for drainage and gas production operation includes a positive and negative pressure gauge (29), a first rotary valve 30, a gas flow meter 31, a gas pipeline 32, a second rotary valve 33, a liquid flow meter 34, and a water pipeline 35; the positive and negative pressure gauge 29 has a range of -0.1 to 5 MPa and an accuracy of ±0.01 MPa; the gas flow meter 31 has a range of 0 to 200 m 3 / h, accuracy is ±0.1m 3 / h; Liquid flow meter 34 has a range of 0 to 0.5m 3 / h, accuracy is ±0.01m 3 / h. The drainage gas production operation includes six stages: test pumping, initial drainage, casing pressure control and gas production, flow pressure reduction and production increase, flow pressure control and stable production, and gas production attenuation. The duration of each stage is determined according to the stability of the liquid level drop during the drainage and pressure reduction process, the casing pressure, the daily gas production, and the daily water production. The total duration of the drainage gas production process is 180 to 360 days. At the end of the drainage gas production, the wellbore liquid level 24 gradually drops below the position of the suction port screen pipe 26.

[0038] (e) When the wellbore liquid level 24 drops to the position of the suction port screen 26, the second rotary valve 33 is closed, and the gas pipeline 32 is connected to the water ring vacuum pump 39 and the motor 40 to perform negative pressure extraction of the pre-mined coal seam 9 gas, thereby further reducing the gas content and gas pressure of the pre-mined coal seam 9; when the water ring vacuum pump 39 extracts gas under negative pressure, the negative pressure formed at the wellhead 21 is maintained at -0.08 to -0.06 MPa; when the pure amount of gas extracted under negative pressure in the pre-mined coal seam 9 is ≤50m 3 / d, or when the extracted gas concentration is lower than 10%, the pre-extraction of coal seam gas is completed.

Claims

1. A method for pre-extracting coalbed gas by combining high-density well arrangement with small-scale hydraulic fracturing, characterized in that: The following steps are involved: (a) According to the coal mine planning mining area and coal mining face design drawings, multiple ground gas extraction vertical wells are deployed in a high-density "square well pattern" type in the coal mining face. When the ground (1) does not meet the conditions for deploying gas extraction vertical wells, ground gas extraction directional wells are deployed to keep the target points of each well in the plane to form a "square well pattern"; (b) The vertical wells or directional wells for gas extraction on the ground are all constructed with a "two-opening" wellbore structure. The surface casing (3) is cemented in the first opening wellbore (2). After the second opening wellbore (5) is drilled, the production casing (6) is lowered. The annular channel (7) between the second opening wellbore (5) and the production casing (6) is sealed by a packer (8) outside the production casing (6), so as to form a closed space at the bottom of the gas extraction well. The J55 steel grade surface casing (3) and the production casing (6) are sealed and connected by welding at the wellhead (21) through an annular steel plate (20); (c) carrying out hydraulic fracturing construction, connecting a small-scale hydraulic fracturing equipment to the ground, and continuously injecting a fracturing fluid (11) and a proppant (16) into a production casing (6) using the small-scale hydraulic fracturing equipment. The fracturing fluid (11) and the proppant (16) enter the pre-mined coal seam (9) through a pre-perforated screen pipe (10) at the bottom of the production casing (6), completing a small-scale hydraulic sand fracturing of the pre-mined coal seam (9), and improving the permeability of the pre-mined coal seam (9) around the wellbore; (d) after the fracturing construction is completed, the overflow is released and the well is cleaned, and the drainage equipment in the well is placed in the production casing (6), wherein the drainage equipment in the well includes a sand settling pipe (27), a suction port screen pipe (26), an electric submersible pump (25) and a drainage pipe (38) arranged from bottom to top, and a downhole thermometer (36) for real-time monitoring of temperature and bottom hole flow pressure changes is provided on one side of the drainage pipe (38), and the downhole thermometer (36) transmits the temperature and bottom hole flow pressure data to the ground (1) through a thermometer cable (37); a positive and negative pressure gauge (29), a first rotary valve (30) and a gas flow meter (31) arranged in sequence on a gas transmission pipeline (32), and a second rotary valve (33) and a liquid flow meter (34) arranged in sequence on a water transmission pipeline (35) are provided at the wellhead (21); (e) installing drainage and gas production ground equipment at the wellhead, the drainage and gas production ground equipment including a water ring vacuum pump (39) and a motor (40) connected to the water ring vacuum pump (39), opening the second rotary valve (33) on the ground to control the drainage and gas production operation, and gradually reducing the gas content and gas pressure of the pre-mined coal seam (9); the drainage and gas production operation includes six stages: test pumping, initial drainage, casing pressure control gas production, flow pressure reduction and production increase, flow pressure control and stable production, and gas production attenuation. The duration of each stage is determined according to the stability of the liquid level drop during the drainage and pressure reduction process, the casing pressure, the daily gas production, and the daily water production. The total duration of the drainage and gas production process is 180 to 360 days; at the end of the drainage and gas production, the wellbore liquid level (24) gradually drops below the position of the suction port screen pipe (26); (f) When the wellbore liquid level (24) drops to the position of the suction port screen (26), the second rotary valve (33) is closed, and the gas pipeline (32) is connected to the water ring vacuum pump (39) and the motor (40) to extract the gas from the pre-mined coal seam (9) under negative pressure, thereby further reducing the gas content and gas pressure of the pre-mined coal seam (9). The water pipeline (35) on the ground is connected to the drainage pipe (38) under the well, and the fracturing fluid (11) and formation water lifted by the electric submersible pump (25) are transported to the drainage pool on the ground.

2. The method for pre-extraction of coal seam gas by high-density well arrangement combined with small-scale hydraulic fracturing according to claim 1 is characterized in that: In step (a), the distance between the target coal seam points of two adjacent surface gas extraction vertical wells and directional wells in the "square well network" is 50 to 60 m, the diagonal sides of the square formed by four adjacent surface gas extraction wells are consistent with the direction of the horizontal principal stress in the mining area, the maximum well inclination angle of the surface gas extraction directional well is ≤30°, and the horizontal displacement of the bottom of the surface gas extraction directional well is ≤30 m.

3. The method for pre-extraction of coal seam gas by high-density well arrangement combined with small-scale hydraulic fracturing according to claim 1 is characterized in that: In step (b), the first wellbore (2) is drilled to 10 to 15 m below the bedrock, and a J55 steel grade surface casing (3) is installed and then G grade oil well cement is injected to form a first cementing cement ring (4); the second wellbore (5) is drilled to 8 to 10 m below the pre-mined coal seam (9) and then completed, and an N80 steel grade production casing (6) is installed, and the packer (8) outside the production casing (6) is pressurized to expand the packer (8), so that a closed space is formed at the bottom of the gas extraction well; the packer (8) is sealed at a thick, hard, undeveloped, and low-permeability roof position within a range of 1 to 5 m above the pre-mined coal seam (9) to ensure safety in subsequent fracturing and gas pre-extraction.

4. The method for pre-extraction of coal seam gas by high-density well arrangement combined with small-scale hydraulic fracturing according to claim 1 or 3, characterized in that: The production casing (6) comprises, from bottom to top, a wire plug (23), an ordinary N80 steel grade casing, an N80 steel grade pre-perforated screen pipe (10) and an ordinary N80 steel grade casing; the N80 steel grade pre-perforated screen pipe (10) is located at the pre-mining coal seam (9) position, the length of the pre-perforated section on the pre-perforated screen pipe (10) is ≤ the thickness of the coal seam, the density of the pre-perforated holes on the pre-perforated screen pipe (10) is 12 holes / m, and the hole arrangement method is a positive spiral type.

5. The method for pre-extraction of coal seam gas by high-density well arrangement combined with small-scale hydraulic fracturing according to claim 1 is characterized in that: In step (c), the small-scale hydraulic fracturing equipment comprises a fracturing construction pressure gauge (12), an injection pump (13), a sand mixer (14), a vertical sand tank (15), a first gate valve (17), a second gate valve (18), a fracturing fluid tank (19) and high and low pressure pipelines; the fracturing construction pressure gauge (12) is arranged at a closed wellhead (21), the injection pump (13) is connected to the outlet pipeline of the sand mixer (14), the outlets of the vertical sand tank (15) and the fracturing fluid tank (19) are respectively connected to the inlet pipeline of the sand mixer (14), the first gate valve (17) is arranged on the outlet pipeline of the vertical sand tank (15), the second gate valve (18) is arranged on the outlet pipeline of the fracturing fluid tank (19), and the maximum injection displacement of the fracturing injection pump (13) is 0.7 to 1.0 m 3 / min, the maximum injection pressure is 30-40MPa; the sand mixer (14) fully mixes the fracturing fluid (11) and the proppant (16) in different proportions according to the actual needs on site, and then inputs the mixed fracturing fluid (11) and the proppant (16) into the fracturing injection pump (13); the volume of the vertical sand tank (15) is 2-4m 3 The outlet of the vertical sand tank (15) is 2 to 3 meters higher than the wellhead (21); the volume of the fracturing fluid tank (19) is 40 to 50 m 3 .

6. The method for pre-extraction of coal seam gas by high-density well arrangement combined with small-scale hydraulic fracturing according to claim 1 is characterized in that: In step (c), the fracturing fluid (11) is an active water fracturing fluid mixed with 98.8% clean water, 1.0% anti-swelling agent, 0.1% bactericide and 0.1% drainage agent, and the proppant (16) is a mixture of 70-100 mesh fine-grained ceramic powder and 100-140 mesh ultra-fine-grained ceramic powder in a volume ratio of 1:1; when the pre-mined coal seam (9) is shallowly buried and has a high original permeability, a mixture of 70-100 mesh fine-grained quartz sand and 100-140 mesh ultra-fine-grained quartz sand in a volume ratio of 1:1 is used as the proppant.

7. The method for pre-extraction of coal seam gas by high-density well arrangement combined with small-scale hydraulic fracturing according to claim 1 is characterized in that: In step (c), the small-scale hydraulic sand fracturing process of the pre-mined coal seam (9) includes three stages: injecting pre-fluid to create fractures, injecting sand-carrying fluid to add sand support, and injecting displacement fluid for displacement; in the pre-fluid injection fracture creation stage: the proportion of pre-fluid is not less than 30%, the fracturing injection pump (13) is quickly increased and reaches the maximum displacement or maximum construction pressure, and more fractures extending to the periphery of the wellbore are generated in the pre-mined coal seam (9); in the sand-carrying fluid injection and sand support stage: proppant is injected into the fracturing fracture of the pre-mined coal seam (9), so that the proppant extends to the front end of the fracture, and the sand ratio is increased in a stepwise manner during the injection of the sand-carrying fluid, and the average sand ratio in the support stage is ≤10%; in the displacement fluid injection displacement stage: a fracturing fluid without proppant having the same volume as the space of the production casing (6) is injected to replace the space of the production casing (6) and reduce sand deposition (22) during the fracturing process; during the whole process of fracturing construction, the cumulative injection amount of the fracturing fluid is 60-120m 3 / m coal seam, sand adding strength ≥1.5~2.0m 3 / m coal seam.

8. The method for pre-extraction of coal seam gas by high-density well arrangement combined with small-scale hydraulic fracturing according to claim 1 is characterized in that: In step (d), the lower end of the sand settling pipe (27) is ≥6 m away from the position of the artificial well bottom wire plug (23), the surface of the suction port screen pipe (26) is wound with copper wire to reduce the coal powder and proppant entering the electric submersible pump (25), the head of the electric submersible pump (25) is greater than the distance from the suction port screen pipe (26) to the wellhead (21), and has the ability to steplessly adjust the drainage rate.

9. The method for pre-extraction of coal seam gas by high-density well arrangement combined with small-scale hydraulic fracturing according to claim 1, characterized in that: In step (d), the range of the positive and negative pressure gauge (29) is -0.1 to 5 MPa, and the accuracy is ±0.01 MPa; the range of the gas flow meter (31) is 0 to 200 m 3 / h, accuracy is ±0.1m 3 / h; the range of the liquid flow meter (34) is 0 to 0.5m 3 / h, accuracy is ±0.01m 3 / h.

10. The method for pre-extraction of coal seam gas by high-density well arrangement combined with small-scale hydraulic fracturing according to claim 1, characterized in that: In step (e), when the water ring vacuum pump (39) extracts gas under negative pressure, the negative pressure formed by the wellhead (21) is maintained at -0.08 to -0.06 MPa; when the pure amount of gas extracted under negative pressure from the pre-mined coal seam (9) is ≤50m 3 / d, or when the extracted gas concentration is lower than 10%, the pre-extraction of coal seam gas is completed.

Images