Research On Hydraulic Fracturing And Permeability Enhancement Technology For High Gas And Low Permeability Coal Seam In Zhongxing Coal Mine

As a kind of clean energy,if gas is effectively exploited,it will not only ease the energy shortage in China,but also have great significance for gas disaster prevention and environmental protection.It is the key development direction of high-gas coal mines in the future and conforms to the national energy development"13th Five-Year Plan"to build a modern coal industry system that is economical,safe,efficient and green.Coal seams in China are generally characterized by low permeability,high gas and high pressure,which results in low coal gas drainage efficiency,which seriously affects coal mine safety production.At present,high-gas coal mines generally use conventional dense hole layout to extract gas.This method not only consumes a lot of manpower and material resources to a certain extent,but also restricts coal mine production capacity.This paper studies the gas extraction technology of high-fracture and low-permeability coal seam in Zhongxing coal mine,and researches on the technology of enhanced gas drainage by hydraulic fracturing and permeability enhancement in low-permeability coal seam.The mechanism of hydraulic fracturing and permeability enhancement in permeable coal seams was simulated to analyze the effect of in-situ stress difference and hole spacing on the hydraulic fracturing crack propagation process and permeability enhancement effect.The effect of the on-site hydraulic fracturing process in Zhongxing coal mine was investigated.Low permeability coal seam gas drainage provides technical support and theoretical basis.The main research results are as follows:?1?The factors affecting gas adsorption and permeability,the failure deformation and stress distribution of surrounding rock after hydraulic fracturing were analyzed.The fracture propagation process and influencing factors of hydraulic fracturing were studied.The research showed that:coal seam gas pressure and temperature were the main factors affecting coal seam gas adsorption capacity and permeability.The higher the pressure,the greater the adsorption capacity and the lower the permeability.The higher the temperature,the smaller the adsorption capacity.By analyzing the stress distribution around the borehole after hydraulic fracturing and the law of fracture propagation,it is revealed that the fracture zone,plastic zone and elastic zone were formed around the borehole after coal fracture.Crack propagation was mainly affected by the magnitude of in-situ stress and the compressive strength of coal.?2?The RFPA^2D numerical simulation method was used to analyze the influence of the in-situ stress difference on the hydraulic fracturing crack propagation process and permeability enhancement effect.The simulation results showed that for every increase of 2 MPa in the in-situ stress difference,the initiation pressure was reduced by at least 1 MPa.It showed that under the same stress level,the greater the in-situ stress difference,the more obvious the coal body failure,the lower the initiation pressure,the more obvious the crack development in the vertical minimum principal stress direction and the better the permeability.The closer the vertical in-situ stress to the horizontal in-situ stress,the higher the crack the complexity of the network,the more balanced the development.?3?The RFPA^2D numerical simulation method was used to analyze the effect of hole spacing on the process of hydraulic fracturing crack propagation and permeability enhancement.The simulation results showed that the larger the pore spacing of the fracturing pores,the weaker the superposition effect of the coal body stress between the two pores,and the lower the penetration of the fractures.Simulated pore spacing of 5m,6m,7m,8m and 9m coal seam fracturing permeability,the maximum effective fracturing radius of the double-hole fracturing coal seam was 8m.When the borehole spacing was 8m,the coal seam reaches the maximum fracturing effect range of 0¹6m.The maximum seepage flow in the x-direction was 340 m³/?d·m?,and the maximum seepage flow in the y-direction was 671 m³/?d·m?.?4?An on-site hydraulic fracturing process effect survey was conducted in Zhongxing coal mine.Field tests show that after hydraulic fracturing,pressure relief and permeability enhancement,the permeability of coal seam increases significantly.It was concluded that the extracted gas concentration and the extracted gas flow are increased by 3.6 times and 2.7 times respectively.The gas emission from the gas control roadway of the overlying north wing in the hydraulic fracturing borehole section was reduced by 26.98%.The content of raw coal gas which extracted by fracturing was reduced more obviously.Compared with the same period of the previous year,the raw coal gas content of the conventional borehole was reduced by18.5%year-on-year.Under the condition that the coal seam gas effective extraction radius was 3m,the hydraulic fracturing borehole extraction time was shortened by 17days.The application of hydraulic fracturing and permeability enhancement technology improved the efficiency of coal mine gas extraction,and had guiding significance for coal mine safety production.