| Problems of coal mine safety seriously restrict the sustainable development of coal industry and coal mine safety production,among which the casualties and economic losses caused by gas disasters are the most serious.At present,through the way of gas drainage,reducing the coal seam gas content is the main measure to control gas disasters in China.However,with the continuous increase of coal mining depth,the gas pressure and gas content are higher and higher,but the permeability of deep coal seam is poor and the permeability is low,which seriously affects the effect of gas drainage.Therefore,it is necessary to take some measures to increase the permeability of deep coal seam with high gas and low permeability and improve the extraction efficiency.In this paper,through theoretical analysis,numerical simulation and field experiments,the fracture initiation and propagation mechanism and pressure relief and permeability enhancement technology of hydraulic fracturing in Xuehu Coal Mine are studied.The fracture initiation mechanism and fracture propagation mechanism of coal seam hydraulic fracturing are studied.The fracture propagation process in the process of hydraulic fracturing is described,and the role of pressure relief and permeability enhancement of coal seam hydraulic fracturing is introduced.By simulating the fracture initiation and propagation rules in the process of hydraulic fracturing,the effects of fracturing time,ground stress difference and injection amount on fracture morphology are analyzed;The field hydraulic fracturing experiment was carried out in Xuehu Coal Mine,and the pumping effect after hydraulic fracturing was investigated,which provided theoretical basis and technical support for hydraulic fracturing technology in Xuehu Coal Mine.The main conclusions are as follows:1.Based on the analysis of the stress around the borehole before and after injecting high-pressure water,the fracture initiation condition of the fracture around the borehole is deduced,the calculation formula of the fracture initiation pressure and the judgment method of the fracture initiation position are given,and it is concluded that the fracture initiation pressure is mainly affected by the in-situ stress distribution and tensile strength.The greater the in-situ stress difference is,the greater the tensile strength is,the easier the coal seam is to crack The numerical value between the stress and the vertical stress determines the direction of the fracture.If the horizontal stress is larger,the horizontal fracture will be formed.If the vertical stress is larger,the vertical fracture will be formed.2.ABAQUS software is used to simulate the hydraulic fracturing process,and the effects of fracturing time,ground stress difference and injection amount on the fracturing effect are obtained.The specific conclusions are as follows:(1)the fracturing time has an important impact on the fracturing effect.If the fracturing time is too short,the coal seam around the borehole can not reach the initiation condition;however,due to the existence of leakage water pressure,after the initiation and extension of the internal fracture of the coal seam to a certain extent,continue to extend the fracturing time,most of the energy is consumed,and the fracturing efficiency is low.(2)In situ stress difference plays a key role in fracturing effect.At the same time,the distribution of in-situ stress also affects the direction of fracture propagation.For the mine with horizontal in-situ stress greater than vertical in-situ stress,the fractures formed by hydraulic fracturing are mostly horizontal fractures;on the contrary,if the vertical in-situ stress is greater,the fractures will eventually extend along the vertical direction.The fracture width near the borehole and the final fracture length are also related to the in-situ stress difference.With the increase of the in-situ stress difference,the fracture width near the borehole decreases and the final fracture length increases.(3)Injection volume also plays a certain role in hydraulic fracturing effect.The increase of injection volume will not have a great impact on the fracture initiation pressure,but it is conducive to the formation of wider and longer fracture,so that the fracturing effect is better;at the same time,increasing the injection volume will put forward higher requirements for the fracturing equipment and safety conditions,so it is of great significance to determine a reasonable injection volume and improve the hydraulic fracturing efficiency on the premise of efficient use of existing equipment and ensuring safety.3.The field experiment of hydraulic fracturing was carried out in Xuehu Coal Mine,and the gas drainage concentration,gas drainage flow and pure gas drainage flow were significantly improved.Among them,compared with the conventional drilling monitoring data,the gas drainage concentration of the fracturing hole increased from 17.575%to53.735%,increased to 3.07 times of the original;the gas drainage flow of the fracturing hole increased from 0.0125 m~3/min of the conventional hole to 0.0369 m~3/min,increased to 2.95 times of the original;the gas drainage pure flow of the fracturing hole increased from 0.0017 m~3/min of the conventional hole to 0.0199 m~3/min 7 times of the original.It can be found that hydraulic fracturing greatly increases the permeability of coal seam and effectively improves the gas drainage effect of boreholes in the fracturing area. |
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