Abstract
Conventional hydraulic fracturing has several disadvantages, including a short effective extraction time and low fracture conductivity during long-term extraction. Aiming at overcoming these shortcomings, a similar simulation test of repeated hydraulic fracturing was conducted in this study, and the evolutionary rules regarding the injection water pressure and stress distribution of the coal seam roof during this repeated hydraulic fracturing were revealed. The research results show that after multiple hydraulic fracturing, the number of cracks in the coal seam and the range of fracturing influence have increased significantly. As the number of fracturing increases, the initial pressure required for cracking decreases. The highest water injection pressure of the first fracturing was 2.8 MPa, while the highest water injection pressures of the second and third fracturing were 2.7 MPa and 2.4 MPa, respectively. As the number of fracturing increases, the area of increased stress will continue to expand. After the first fracturing, the impact radius of fracturing is 100 cm. After the second fracturing, the radius of influence of fracturing expanded to 150 cm. When the third fracturing was over, the radius of influence of the fracturing expanded to approximately 250 cm. It can be seen that, compared with conventional hydraulic fracturing, repeated hydraulic fracturing shows better fracturing effect. The research results can be used as a basis for repeated hydraulic fracturing field tests to increase coal seam permeability.
Highlights
As coal mines of deep areas are mined, the characteristics of coal seams, i.e., their low permeability, great gas extraction difficulty, and low extraction efficiency, are becoming significant [1–5]
It is concluded that hydraulic fracturing can significantly improve the permeability of coal seams, which is an effective technology for controlling gas disasters [26, 27]
It can be seen from the figure that the entire fracturing process can be divided into the water pressure accumulation stage, crack initiation stage, and crack propagation stage in accordance with the water pressure change laws. e results of the specific analyses are presented as follows: Water pressure accumulation stage: after starting the water injection, the injected high-pressure water gradually wets the simulated coal seam, and the initial cracks of the coal seam are full of water
Summary
As coal mines of deep areas are mined, the characteristics of coal seams, i.e., their low permeability, great gas extraction difficulty, and low extraction efficiency, are becoming significant [1–5]. Underground hydraulic fracturing technology for coal mines, as an important measure for controlling gas disasters, can be used to open, propagate, and extend cracks in coal seams, which fully relieve the pressure on the coal mass and increase the permeability of the coal seams. This improves the gas extraction rate and effectively prevents dynamic disasters such as coal and gas outbursts. Erefore, based on a similar simulation test, this study compared and analyzed the water injection pressure, flow, crack initiation and propagation direction, and change law for the range of the roof stress of the coal seam during fracturing. It is expected to provide a basis for increasing the permeability of coal seams so as to achieve the purpose of extracting coal seam gas efficiently and at low cost
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