Thin coal seam directional long drill hole pressure relief and efficient gas extraction method

Abstract

The invention discloses a thin coal seam directional long drill hole pressure relief and efficient gas extraction method, which is characterized in that a horizontal well is constructed in a rock stratum near a top plate and a bottom plate of a thin coal seam respectively, so that a certain pressure relief effect can be achieved on the coal seam, and the gas desorption efficiency of the coal seam is improved; meanwhile, the phenomena of hole collapse and the like of the horizontal well in the subsequent extraction process after fracturing is completed can be reduced, and the effectiveness of a gas drainage channel is improved; a plurality of perforations which are distributed in a crossed manner are arranged in two horizontal wells, so that bidirectional crossed fracturing can be carried out on the thin coal seam during subsequent fracturing, cracks can be promoted to penetrate through the whole thin coal seam to a great extent, coal seam gas desorption is better promoted, and the gas extraction effect is improved; alkali liquor is selected as fracturing fluid, the thin coal seam can be fractured during high-pressure fracturing, the alkali liquor flows into the thin coal seam through cracks to generate chemical corrosion action with mineral components in the thin coal seam during soaking, the roughness and the space volume of the fractured cracks are further improved, and finally the anti-reflection effect on the thin coal seam is improved.

Description

Technical Field

[0001] The invention relates to a thin coal seam directional long drilling pressure relief and gas efficient extraction method, belonging to the technical field of deep thin coal seam gas disaster prevention and efficient extraction. Background Art

[0002] Deep thin coal seams usually have the characteristics of high coal seam gas content, high gas pressure, and strong coal seam structural integrity. Before the safe mining of thin coal seams, it is necessary to advance the extraction of gas in the coal seams. Only after the extraction meets the standards, the thin coal seams can be allowed to be mined. Generally, coal seam gas pre-extraction is mostly carried out by dense drilling, but there are limitations such as long extraction cycle, fast attenuation rate, and poor extraction effect. In addition, the coal-seeing section of conventional through-layer drilling is short, and it is difficult to maintain the stability of the negative pressure of the drilling coal seam section. At present, the method of hydraulic fracturing in ground wells is often used to transform the volume of thin coal seams. The structure of the coal seams is destroyed by high-pressure water, thereby generating a complex fracture network to improve the porosity and permeability characteristics of the coal seams.

[0003] The successful construction of hydraulic fracturing is often affected by a combination of factors such as the geological conditions of the coal seam, ground stress, and construction technical parameters. It is well known in the industry that the cracks produced by hydraulic fracturing mainly extend along the direction of the maximum principal stress, and the direction of ground stress directly affects whether the hydraulic fractures can penetrate the coal seam. At present, some cracks are prefabricated by combining cutting, punching, jetting, etc., and the cracks are initially extended along the prefabricated cracks with the help of high-pressure water. If the horizontal well is arranged in the coal seam, there will usually be a collapse of the horizontal well structure due to excessive stress concentration, which seriously affects the rapid and effective drainage of gas in the coal seam. Therefore, the existing technology will construct the horizontal well in the top rock layer of the coal seam, and combine the hydraulic fracturing technology to carry out coal seam fracturing and gas extraction. However, the existing technology often cannot ensure that the hydraulic fractures completely penetrate the entire coal seam, and the coal seam fracturing is mostly localized, and it is impossible to achieve sufficient fracturing and pressure relief of the entire coal seam, which ultimately leads to poor gas extraction effect.

[0004] Therefore, how to provide a new method to ensure that the fracturing cracks completely penetrate the entire coal seam, achieve sufficient fracturing and pressure relief of the entire coal seam, and ultimately effectively improve the gas extraction effect 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 directional long drilling pressure relief and efficient gas extraction in thin coal seams, which can ensure that the fracturing cracks completely penetrate the entire coal seam, achieve sufficient fracturing and pressure relief for the entire coal seam, and ultimately effectively improve the gas extraction effect.

[0006] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a method for directional long drilling pressure relief and efficient gas extraction in thin coal seams, the specific steps of which are:

[0007] A. First, geological exploration is carried out on thin coal seams deep underground. Coal rock cores of thin coal seams and their top and bottom rock layers are obtained by core drilling, and the geological parameters of the coal rock cores are measured.

[0008] B. Determine the construction parameters of the horizontal well based on the effective stress principle and the coal rock geological parameters of the coal rock core, wherein a ground drilling rig is used to construct a vertical well in the thin coal seam. When approaching the top rock layer of the thin coal seam, the drilling rig turns to construct the first horizontal well in the top rock layer of the thin coal seam, and withdraws the drill after the construction is completed; then continue to drill downward along the vertical drilling well to pass through the thin coal seam, and turn to construct the second horizontal well in the bottom rock layer of the thin coal seam. After the construction is completed, withdraw the drill and keep the second horizontal well parallel to the first horizontal well.

[0009] C. After the first horizontal well and the second horizontal well are washed, perforating bullets are used to vertically perforate the thin coal seam at different positions in the first horizontal well and at different positions in the second horizontal well, and the perforations in the first horizontal well and the second horizontal well are cross-arranged, that is, the perforations in the two horizontal wells are not on the same straight line; this cross-arrangement method can improve the fracturing effect at different positions of the thin coal seam during subsequent fracturing, so that a complex fracture network can be formed in the thin coal seam.

[0010] D. After the perforation of the first horizontal well and the second horizontal well is completed, the first high-pressure resistant packer is sent into the first horizontal well to seal the entire first horizontal well; then the high-pressure fracturing fluid is injected into the second horizontal well along the vertical drilling to fracture the thin coal seam. Since there are perforations in the second horizontal well, the cracks generated after the second horizontal well is pressurized will extend and develop along the perforations to the thin coal seam.

[0011] E. After the second horizontal well is fracturing, the second high-pressure resistant packer is sent into the coal-rock interface between the thin coal seam and the roof rock layer along the vertical drilling to seal the vertical drilling entering the thin coal seam and the entire second horizontal well; the maximum pressure resistance value of the second high-pressure resistant packer is greater than the maximum pressure resistance value of the first high-pressure resistant packer; then the high-pressure fracturing fluid is injected into the vertical drilling, and the injection pressure of the fracturing fluid is continuously increased until it penetrates the first high-pressure resistant packer, and the pressure value is maintained so that the high-pressure fracturing fluid is injected into the first horizontal well to fractur the thin coal seam. Since there are perforations in the first horizontal well, the cracks generated after the first horizontal well is pressurized will extend and develop along each perforation to the thin coal seam.

[0012] F. After completing the fracturing of the first horizontal well, increase the injection pressure of the fracturing fluid until it penetrates the second high-pressure packer and keep the well shut down for a period of time. After completion, drain the high-pressure water in the first and second horizontal wells. Finally, connect the vertical well to the gas drainage system to drain gas from the thin coal seam.

[0013] Furthermore, the geological parameters of the coal rock core in step A include mechanical parameters, gas content and pressure, and geostress environment information.

[0014] Furthermore, the fracturing fluid is an alkali solution. Using the alkali solution as the fracturing fluid can not only fracture the thin coal seam during high-pressure fracturing, but also, during well soaking, the high-pressure alkali solution can flow into the thin coal seam through the cracks generated by fracturing and chemically dissolve the mineral components inside the thin coal seam, further improving the roughness and spatial volume of the fracturing cracks, and further improving the permeability enhancement effect on the thin coal seam.

[0015] Furthermore, the trends of the first horizontal well and the second horizontal well are both parallel to the trend of the thin coal seam; the distance between the first horizontal well and the top plate of the thin coal seam is 0.5-1m, and the distance between the second horizontal well and the bottom plate of the thin coal seam is 0.5-1m.

[0016] Furthermore, the thickness of the thin coal seam is 1.5 to 2 m, and the firmness coefficient f value is greater than 0.8.

[0017] Furthermore, the perforating bullet adopts a large-diameter bullet, the penetration depth of the perforating bullet is 600 mm, the perforation diameter is 20 mm, and the perforation penetrates 0.2 to 0.5 m into the thin coal seam.

[0018] Furthermore, the maximum pressure resistance value of the second high-pressure resistant packer is 25 MPa, and the maximum pressure resistance value of the second high-pressure resistant packer is 30 MPa.

[0019] Furthermore, the distance between adjacent perforations in each of the first horizontal well and the second horizontal well is 10 m.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] 1. The present invention constructs a horizontal well in the rock layer near the top and bottom plates of the thin coal seam respectively. By forming horizontal wells above and below the thin coal seam, it can have a certain pressure relief effect on the coal seam and improve the coal seam gas desorption efficiency; at the same time, two horizontal wells are respectively arranged in the rock layer above and below the thin coal seam, which can reduce the occurrence of hole collapse and the like during the subsequent extraction process after the horizontal well is fractured, thereby extending the effectiveness of the gas drainage channel.

[0022] 2. The present invention arranges multiple perforations in two horizontal wells, and the perforation positions in different horizontal wells are cross-distributed, so that bidirectional cross-fracture can be performed on the thin coal seam during subsequent fracturing, which can greatly promote the cracks to penetrate the entire thin coal seam and form a complex crack network. At the same time, the cracks after penetration can also provide sufficient flow channels for the removal of high-pressure water, reduce the water content in the coal seam, better promote coal seam gas desorption, and improve the gas extraction effect.

[0023] 3. The fracturing fluid of the present invention is alkaline solution, which can not only fracture thin coal seams during high-pressure fracturing, but also, when the well is soaked, the high-pressure alkaline solution flows into the thin coal seams through the cracks generated by fracturing and chemically dissolves the mineral components inside the thin coal seams, further increasing the roughness and spatial volume of the fracturing cracks, and ultimately improving the permeability-enhancing effect on the thin coal seams. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the construction layout of the present invention.

[0025] In the figure: 1-thin coal seam, 2-vertical drilling, 3-first horizontal well, 4-second horizontal well, 5-first high-pressure resistant packer, 6-second high-pressure resistant packer, 7-perforation. DETAILED DESCRIPTION

[0026] The present invention will be further described below.

[0027] like Figure 1 As shown, the specific steps of the present invention are:

[0028] A. First, geological exploration is conducted on the thin coal seam 1 deep underground, and the coal rock core of the thin coal seam 1 and its top and bottom rock layers is obtained by core drilling, and the geological parameters of the coal rock core are measured; the geological parameters include mechanical parameters, gas content and pressure, and ground stress environment information.

[0029] B. According to the effective stress principle and the coal rock geological parameters of the coal rock core, the horizontal well construction parameters are determined, wherein a vertical well 2 is constructed toward the thin coal seam 1 using a ground drilling rig. When approaching the top rock layer of the thin coal seam 1, the drilling rig turns to construct the first horizontal well 3 in the top rock layer of the thin coal seam 1, and the drill is withdrawn after the construction is completed; then, the vertical well 2 continues to drill downward through the thin coal seam 1, and turns to construct the second horizontal well 4 in the bottom rock layer of the thin coal seam 1. After the construction is completed, the drill is withdrawn, and the second horizontal well 4 is kept parallel to the first horizontal well 3.

[0030] C. After the first horizontal well 3 and the second horizontal well 4 are washed, perforating bullets are used to vertically construct perforations 7 at different positions in the first horizontal well 3 and the second horizontal well 4, respectively, and the distance between adjacent perforations 7 in each of the first horizontal well 3 and the second horizontal well 4 is 10m, and the perforations 7 in the first horizontal well 3 and the second horizontal well 4 are cross-arranged, that is, the perforations 7 in the two horizontal wells are not on the same straight line; this cross-arrangement method can improve the fracturing effect at different positions of the thin coal seam 1 during subsequent fracturing, so that a complex fracture network can be formed in the thin coal seam 1; the perforating bullets are large-aperture bullets, the penetration depth of the perforating bullets is 600mm, and the perforation diameter is 20mm, so that the perforations 7 penetrate 0.2-0.5m into the thin coal seam 1.

[0031] D. After the perforation of the first horizontal well 3 and the second horizontal well 4 is completed, the first high-pressure resistant packer 5 is sent into the first horizontal well 3 to seal the entire first horizontal well 3; then the high-pressure fracturing fluid is injected into the second horizontal well 4 along the vertical drilling 2 to fractur the thin coal seam 1. Since there are perforations in the second horizontal well 4, the cracks generated after the second horizontal well 4 is pressurized will extend and develop along each perforation 7 toward the thin coal seam 1.

[0032] E. After the fracturing of the second horizontal well 4 is completed, the second high-pressure resistant packer 6 is sent along the vertical drilling 2 into the coal-rock interface between the thin coal seam 1 and the roof rock layer to seal the vertical drilling 2 entering the thin coal seam 1 and the entire second horizontal well 4; the maximum pressure resistance value of the second high-pressure resistant packer 5 is 25Mpa, and the maximum pressure resistance value of the second high-pressure resistant packer 6 is 30MPa; then the high-pressure fracturing fluid is injected into the vertical drilling 2, and the injection pressure of the fracturing fluid is continuously increased until it penetrates the first high-pressure resistant packer 5, and the pressure value is maintained so that the high-pressure fracturing fluid is injected into the first horizontal well 3 to fractur the thin coal seam 1. Since the first horizontal well 3 has perforations 7, the cracks generated after the first horizontal well 3 is pressurized will extend and develop along each perforation 7 toward the thin coal seam 1.

[0033] F. After completing the fracturing of the first horizontal well 3, the injection pressure of the fracturing fluid is increased until it penetrates the second high-pressure packer 6, and the well is shut down for a period of time. The fracturing fluid is an alkali solution. Using alkali solution as the fracturing fluid can not only fracture the thin coal seam during high-pressure fracturing, but also, during the well shut-down, the high-pressure alkali solution can flow into the thin coal seam 1 through the cracks generated by fracturing and chemically dissolve the mineral components inside it, further improving the roughness and spatial volume of the fracturing cracks, and further improving the permeability enhancement effect on the thin coal seam 1. After completion, the high-pressure water in the first horizontal well 3 and the second horizontal well 4 is drained, and finally the vertical drilling well 2 is connected to the gas drainage system to drain gas from the thin coal seam 1.

[0034] As an improvement of the present invention, the first horizontal well 3 and the second horizontal well 4 are both parallel to the direction of the thin coal seam 1; the distance between the first horizontal well 3 and the top plate of the thin coal seam 1 is 0.5-1m, and the distance between the second horizontal well 4 and the bottom plate of the thin coal seam 1 is 0.5-1m. The thickness of the thin coal seam 1 is 1.5-2m, and the firmness coefficient f value is greater than 0.8.

[0035] 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 directional long drilling pressure relief and efficient gas extraction in thin coal seams, characterized in that: The specific steps are: A. First, geological exploration is conducted on the thin coal seams deep underground, and the coal rock cores of the thin coal seams and their top and bottom rock layers are obtained by core drilling, and the geological parameters of the coal rock cores are measured; B. Determine the construction parameters of the horizontal well according to the effective stress principle and the coal rock geological parameters of the coal rock core, wherein a ground drilling rig is used to construct a vertical well in a thin coal seam. When approaching the top rock layer of the thin coal seam, the drilling rig turns to construct a first horizontal well in the top rock layer of the thin coal seam, and withdraws the drill after the construction is completed; then continue to drill downward along the vertical drilling well to pass through the thin coal seam, and turn to construct a second horizontal well in the bottom rock layer of the thin coal seam. After the construction is completed, withdraw the drill, and keep the second horizontal well parallel to the first horizontal well; C. After the first horizontal well and the second horizontal well are washed, perforating bullets are used to vertically perforate the thin coal seam at different positions in the first horizontal well and at different positions in the second horizontal well, and the perforations in the first horizontal well and the second horizontal well are arranged crosswise; D. After the perforation of the first and second horizontal wells is completed, the first high-pressure packer is sent into the first horizontal well to plug the entire first horizontal well; then the high-pressure fracturing fluid is injected into the second horizontal well along the vertical drilling to fractur the thin coal seam. Since there are perforations in the second horizontal well, the cracks generated by the second horizontal well after being pressurized will extend and develop along each perforation toward the thin coal seam; E. After the second horizontal well is fracturing, the second high-pressure resistant packer is sent into the coal-rock interface between the thin coal seam and the roof rock layer along the vertical drilling to seal the vertical drilling entering the thin coal seam and the entire second horizontal well; the maximum pressure resistance value of the second high-pressure resistant packer is greater than the maximum pressure resistance value of the first high-pressure resistant packer; then, the high-pressure fracturing fluid is injected into the vertical drilling, and the injection pressure of the fracturing fluid is continuously increased until the first high-pressure resistant packer is penetrated, and the pressure value is maintained to inject the high-pressure fracturing fluid into the first horizontal well to fractur e the thin coal seam. Since the first horizontal well has perforations, the cracks generated after the first horizontal well is pressurized will extend and develop toward the thin coal seam along each perforation; F. After completing the fracturing of the first horizontal well, increase the injection pressure of the fracturing fluid until it penetrates the second high-pressure packer and keep the well shut down for a period of time. After completion, drain the high-pressure water in the first and second horizontal wells. Finally, connect the vertical well to the gas drainage system to drain gas from the thin coal seam.

2. The method for decompression and efficient gas extraction in thin coal seams according to claim 1 is characterized in that: The geological parameters of the coal rock core in step A include mechanical parameters, gas content and pressure, and ground stress environment information.

3. The method for decompression and efficient gas extraction in thin coal seams according to claim 1 is characterized in that: The fracturing fluid is alkaline solution.

4. The method for decompression and efficient gas extraction by directional long drilling in thin coal seams according to claim 1 is characterized in that: The strikes of the first horizontal well and the second horizontal well are both parallel to the strike of the thin coal seam; the distance between the first horizontal well and the top plate of the thin coal seam is 0.5-1 m, and the distance between the second horizontal well and the bottom plate of the thin coal seam is 0.5-1 m.

5. The method for decompression and efficient gas extraction by directional long drilling in thin coal seams according to claim 1 is characterized in that: The thickness of the thin coal seam is 1.5 to 2 meters, and the firmness coefficient f value is greater than 0.

8.

6. The method for decompression and efficient gas extraction by directional long drilling in thin coal seams according to claim 1 is characterized in that: The perforating bullet adopts a large-aperture bullet, the penetration depth of the perforating bullet is 600 mm, the perforation diameter is 20 mm, and the perforation can penetrate into the thin coal seam to 0.2 to 0.5 m.

7. The method for decompression and efficient gas extraction by directional long drilling in thin coal seams according to claim 1 is characterized in that: The maximum pressure resistance value of the second high-pressure resistant packer is 25 MPa, and the maximum pressure resistance value of the second high-pressure resistant packer is 30 MPa.

8. The method for decompression and efficient gas extraction by directional long drilling in thin coal seams according to claim 1 is characterized in that: The distance between adjacent perforations in the first horizontal well and the second horizontal well is 10 m.

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