Low-permeability coal bed gas drainage and mining method

Abstract

The invention relates to a low-permeability coal bed gas drainage and mining method which comprises the following steps: 1) drilling a drainage and mining vertical well which is drilled below a bottom plate; (2) drilling a high-position horizontal well, enabling a horizontal section of the high-position horizontal well to be located above a coal seam roof, and enabling the high-position horizontal well to be in butt joint with the drainage and mining (3) a plurality of low-level horizontal wells are drilled and distributed in parallel, and a low-level horizontal well set is formed below the high-level horizontal wells; the low-level horizontal well at the near end of the drainage and mining vertical well is in butt joint with the drainage and mining vertical well; (4) the low-position horizontal well set is subjected to directional perforation and borehole opening, segmented sand blasting and hole forming and hydraulic fracturing; performing directional perforation and borehole opening on the high-position horizontal well, and performing segmented hydraulic fracturing; (5) fracturing water is flowback through the flushing well, the drainage and mining vertical well and the low-position horizontal well group; and (6) coal bed gas is discharged through the discharge and mining vertical well. The method can improve the single well yield and stable production period of the low-permeability coal bed gas, reduces the power consumption of coal bed gas extraction, and is beneficial to commercial exploitation of the low-permeability coal bed gas.

Description

Technical Field

[0001] The invention relates to a low-permeability coalbed methane drainage method, belonging to the technical field of coalbed methane mining. Background Art

[0002] my country has huge coalbed methane resources, but the degree of development and utilization is low. Among them, the commercial mining value of coalbed methane in high-rank coal seams is large, and the daily output of a single well can reach more than 50,000 cubic meters, and it can maintain a stable high production for more than 3 years. However, the mining of coalbed methane in medium and low-rank coal seams is difficult. The structure of medium and low-rank coal seams is mostly broken and soft coal seams with low permeability, and the effect of fracturing and permeability enhancement is poor. The daily output of a single well of conventional drilling and drainage is less than 5,000 cubic meters, and the stable production cycle is short. After one year, the output declines severely and has no commercial value.

[0003] Patent CN202011437397.2 discloses a method for efficiently mining coalbed methane in a broken low-permeability coal seam by a short horizontal well group. It uses a short horizontal well group to increase permeability, and uses multiple groups of short horizontal wells with roof fracturing to produce gas. The same well field drainage method is used to drain the wells, so that the well group can continuously produce high-quality coalbed methane. Electricity costs account for a quarter of coalbed methane mining. In this solution, vertical wells and short horizontal wells need to be drained at the same time. Multi-well drainage will significantly increase electricity costs, and the commercial value of a single well is still relatively low.

[0004] Patent CN202110915049 discloses a method for extracting coalbed methane from the top and bottom layers of a broken soft coal seam by using a group of bottom plate U-shaped wells and a top plate L-shaped well to jointly extract coalbed methane from the broken soft coal seam. The bottom plate U-shaped well is a combination of a bottom plate L-shaped well and a vertical well. The vertical well is used for drainage and the top plate L-shaped well is used for gas extraction, which can increase the extraction of coalbed methane from the broken soft coal seam. This solution can improve the vertical permeability of the broken soft coal seam, but it has no significant improvement on the permeability of the coal seam plane, and the production increase effect is limited. Summary of the invention

[0005] In view of the above problems, the present invention provides a low-permeability coalbed methane drainage method, the specific scheme is:

[0006] A low-permeability coalbed methane drainage method comprises the following steps:

[0007] 1) Drilling vertical wells, which penetrate below the bottom plate;

[0008] 2) Drilling high-level horizontal wells, the horizontal section of which is located above the coal seam roof, with a horizontal section length of 300-500m, and the high-level horizontal well is connected to the vertical well for drainage and production;

[0009] 3) Drill multiple low-level horizontal wells, which are distributed in parallel to form a low-level horizontal well group below the high-level horizontal well. The horizontal section of the low-level horizontal well is located below the coal seam floor, and the length of the horizontal section is 50-100m; a low-level horizontal well located at the proximal end of the drainage and production vertical well is connected to the drainage and production vertical well;

[0010] 4) Directed perforation for low-level horizontal wells, segmented sandblasting and cavitation, and hydraulic fracturing; Directed perforation for high-level horizontal wells, and segmented hydraulic fracturing;

[0011] 5) Well cleaning, drainage of vertical wells and low-level horizontal well groups, and return of fracturing water;

[0012] 6) Extraction and drainage of coalbed methane from vertical wells.

[0013] In this scheme, the low-level horizontal well and the high-level horizontal well form a large range of permeability enhancement area in the plane, and after the high-level horizontal well and the low-level horizontal well are fractured, the cracks are cross-linked in the vertical plane and in the plane to form a fracture network. Water flows downward into the low-level horizontal well and migrates from the far end to the near end of the vertical well. Gas flows upward into the high-level horizontal well. Therefore, coalbed methane can be produced and discharged through a single vertical well, reducing the electricity cost of coalbed methane extraction.

[0014] Furthermore, in step 3), the horizontal section of the low-level horizontal well and the horizontal section of the high-level horizontal well form an angle of 60-90°.

[0015] Furthermore, in step 6), when the daily gas production of the vertical wells decays to less than 80% of the peak value, the low-level horizontal wells are pumped and drained. In the later stage of the production, the cracks in the coal seam are partially blocked due to the migration of fine coal particles, the permeability decreases, and the water permeability efficiency between adjacent low-level horizontal wells decreases, affecting the analysis and diffusion of coalbed methane. This can be improved by pumping and draining the low-level horizontal wells in the later stage.

[0016] Furthermore, in step 6), the low-level horizontal well is pumped and drained in sequence from the proximal end to the distal end of the vertical well. When the vertical well is drained, the water flow migrates from the distal end to the proximal end of the vertical well, and pumping and draining in sequence from the proximal end to the distal end is conducive to clearing the cracks.

[0017] Furthermore, in step 4), wellbores are drilled on the upper part of the wellbore of the low-level horizontal well with a spacing of 20-30 m, and wellbores are drilled symmetrically on both sides of the lower part of the wellbore of the high-level horizontal well with a spacing of 1 m.

[0018] Furthermore, in step 3), the spacing between low-level horizontal wells is 30-50m.

[0019] Furthermore, in step 4), when hydraulic fracturing a high-level horizontal well, the ratio of coarse sand to medium sand in the proppant is 1:3; when hydraulic fracturing a low-level horizontal well, the ratio of coarse sand to medium sand in the proppant is 3:1. Since the water flow migrates downward, when hydraulic fracturing a high-level horizontal well, increasing the ratio of medium sand is beneficial to increasing the fracturing depth, and when hydraulic fracturing a low-level horizontal well, increasing the ratio of coarse sand can reduce the return of sand.

[0020] The present invention combines a high-level horizontal well set on the roof with a low-level horizontal well set on the floor, which can improve the coalbed methane drainage effect of low-permeability coal seams. Compared with the prior art, the present invention uses multiple low-level horizontal wells to form a low-level horizontal well group below the high-level horizontal wells, and the low-level horizontal wells and the high-level horizontal wells are drilled to intersect each other horizontally, which can not only improve the vertical permeability enhancement effect in the low-permeability coal seam, but also improve the horizontal permeability enhancement effect.

[0021] The benefit of the present invention is that it can increase the single-well production and stable production period of low-permeability coalbed methane, reduce the power consumption of coalbed methane extraction, and is conducive to the commercial exploitation of low-permeability coalbed methane. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a plan view schematic diagram of the high-position horizontal well and the low-position horizontal well in the present invention.

[0023] Figure 2 It is a schematic elevation diagram of a high-position horizontal well and a low-position horizontal well in the present invention. DETAILED DESCRIPTION

[0024] The scheme of the present invention is described in detail below with reference to specific examples.

[0025] When coalbed methane is drained, there is diffusion and migration of water and gas. When extraction is carried out near the drainage point, the internal pressure of the proximal coal seam decreases, and a pressure gradient difference is generated toward the distal end. Under the action of the pressure difference, the water and coalbed methane in the distal coal seam diffuse and migrate toward the proximal end, so coalbed methane can be continuously produced at the drainage point. The permeability of high-permeability coal seams is above 20mD, which is conducive to the diffusion and migration of coalbed methane in a larger range to the drainage point, so that sustained high production can be achieved. The permeability of low-permeability coal seams is lower than 5mD, and the diffusion efficiency of water and gas in coal seams is very low, resulting in low gas production and a short stable production cycle. In addition, when hydraulic fracturing is performed to increase permeability, the extension range of cracks in high-permeability coal seams can reach 90m, while the extension range of cracks in low-permeability coal seams can only reach 30m, which is also a reason that affects gas production and stable production cycle. Taking the 3# coal seam in Qinshui Basin as an example, the permeability of the coal reservoir is 0.03-3.25mD. Conventional U-shaped wells are used for drainage and production. Short horizontal wells are opened in the roof. The daily output of a single well is 5,000 cubic meters, which decays to 3,000 cubic meters after one year. By using the method of the present invention for mining, the daily output of a single well is expected to increase to 10,000 cubic meters, and the production will be stable for more than 2 years.

[0026] Specific as Figure 1 and Figure 2 Drilling is performed as shown:

[0027] 1. Drilling of vertical well 1: First drilling into the bedrock layer 10m, lowering the surface casing, and returning cement to the ground. Second drilling to 5m above the top of the target coal seam, lowering the technical casing to 5m above the top of the target coal seam, and returning cement to the ground.

[0028] 2. Drill high-level horizontal well 2. Drill to 10m below the stable bedrock, lower the surface casing, and cement the well back to the hole mouth. Drill to 5m above the coal seam roof, lower the technical casing, and cement the well back to the hole mouth. Drill into the target coal seam roof within 0.5-2m, reach the target point range, and connect with the vertical well. The horizontal section length is designed to be 300-500m, lower the production casing, and cement the well back to the ground.

[0029] 3. Drill low-level horizontal well 3. Drill multiple low-level horizontal wells in sequence from the proximal end to the distal end of the vertical well. Each low-level horizontal well is distributed in parallel with a spacing of 30-50m. The first low-level horizontal well is connected to the vertical well. When drilling a low-level horizontal well, the first well is drilled to 10m below the stable bedrock, the surface casing is lowered, and the cementing is returned to the hole mouth. The second well is drilled to 5m below the bottom plate of the coal seam, the technical casing is lowered, and the cementing is returned to the hole mouth. The second well is drilled to a 60° angle with the high-level horizontal well. The third well is drilled within 0.5-2m of the top plate of the target coal seam to reach the target range. The horizontal section length of the low-level horizontal well is designed to be 100-200m, the production casing is lowered, and the cementing cement is returned to the ground.

[0030] 4. First, directional perforation is performed on the low-level horizontal wells. During construction, directional perforation is performed on the upper part of the horizontal section of the well wall of the low-level horizontal well located at the near end of the vertical well for drainage and production, with a spacing of 20-30m, and then sandblasting is performed to create holes with a hole diameter of 1.5-2m. Then, hydraulic fracturing is performed with sand-carrying active fracturing fluid, with a ratio of coarse sand to medium sand of 3:1. Blowout prevention is performed on the remaining wellheads; then directional perforation is performed to open the wellbore, sandblasting is performed to create holes, and hydraulic fracturing is performed in turn to the low-level horizontal well at the far end of the vertical well for drainage and production.

[0031] 5. Then carry out directional perforation to open the wellbore for the high-level horizontal well. Open wellbores symmetrically on both sides of the lower part of the wellbore wall of the high-level horizontal well with a spacing of 1m. Then use sand-carrying active fracturing fluid for hydraulic fracturing. The ratio of coarse sand to medium sand is 1:3. Prepare blowout prevention measures for the other wellheads.

[0032] 6. All well bores are cleaned and drained, the water level in the vertical wells is lowered, the coal seam pressure is reduced to below the analysis pressure, and the coal seam gas is precipitated.

[0033] 7. The vertical wells are connected to the gas gathering pipeline for gas production.

[0034] 8. After two years of high production, the daily gas production of vertical wells will decline. When the gas production drops to less than 80% of the peak value, such as when the maximum daily production reaches 10,000 cubic meters and the daily production drops to 8,000 cubic meters, the low-level horizontal wells will be cleaned again, pumped out water, and blocked to improve gas production efficiency. When cleaning and pumping out water, each low-level horizontal well will be cleaned in turn from the proximal end to the distal end of the vertical well.

Claims

1. A method for draining low-permeability coalbed methane, characterized in that: The following steps are involved: 1) Drilling vertical wells, which penetrate below the bottom plate; 2) Drilling high-level horizontal wells, the horizontal section of which is located above the coal seam roof, with a horizontal section length of 300-500m, and the high-level horizontal well is connected to the vertical well for drainage and production; 3) Drill multiple low-level horizontal wells, which are distributed in parallel to form a low-level horizontal well group below the high-level horizontal well. The horizontal section of the low-level horizontal well is located below the coal seam floor, and the length of the horizontal section is 50-100m; a low-level horizontal well located at the proximal end of the drainage and production vertical well is connected to the drainage and production vertical well; 4) Directed perforation for low-level horizontal wells, segmented sandblasting and cavitation, and hydraulic fracturing; Directed perforation for high-level horizontal wells, and segmented hydraulic fracturing; 5) Well cleaning, drainage of vertical wells and low-level horizontal well groups, and return of fracturing water; 6) Extraction and drainage of coalbed methane from vertical wells.

2. The low-permeability coalbed methane drainage method according to claim 1, characterized in that: In step 3), the horizontal section of the low-level horizontal well and the horizontal section of the high-level horizontal well form an angle of 60-90°.

3. The low-permeability coalbed methane drainage method according to claim 1, characterized in that: In step 6), when the daily gas production of the vertical well decreases to less than 80% of the peak value, the low-level horizontal well is pumped out.

4. The low-permeability coalbed methane drainage method according to claim 3, characterized in that: In step 6), the low-level horizontal wells are pumped and drained in sequence from the proximal end to the distal end of the vertical drainage well.

5. The low-permeability coalbed methane drainage method according to claim 3, characterized in that: In step 4), wellbores are drilled on the upper part of the wellbore of the low-level horizontal well with a spacing of 20-30 m, and wellbores are drilled symmetrically on both sides of the lower part of the wellbore of the high-level horizontal well with a spacing of 1 m.

6. The low-permeability coalbed methane drainage method according to claim 3, characterized in that: In step 3), the spacing between low-level horizontal wells is 30-50m.

7. The low-permeability coalbed methane drainage method according to claim 3, characterized in that: In step 4), when hydraulic fracturing a high-level horizontal well, the ratio of coarse sand to medium sand in the proppant is 1:3; when hydraulic fracturing a low-level horizontal well, the ratio of coarse sand to medium sand in the proppant is 3:1.

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