Study On Crack Propagation Law Of Hydraulic Fracturing Of Bedding Coal

Hydraulic fracturing technology is the core of the development and utilization of unconventional oil and gas resources.By injecting fracturing fluid,fracturing cracks are generated in the reservoir,the cracks expand and communicate with the natural fracture structure in the reservoir,form a complex crack network,and increase the permeability of the reservoir to achieve the purpose of increasing production.The formation process and propagation law of hydraulic fracturing cracks in coal reservoir are of great significance to the efficient exploitation and utilization of coal measure gas reservoir resources.Therefore,based on the previous research on coal hydraulic fracturing,this paper makes a systematic study on the crack propagation law of layered coal hydraulic fracturing.In order to study the control effect of coal bedding on water pressure crack propagation,the law of coal water pressure crack propagation under different bedding dip angles was studied by means of laboratory test,numerical simulation and theoretical analysis.The mechanical difference characteristics of loaded coal under different bedding conditions are studied,and the failure crack morphology of loaded coal sample is analyzed.The hydraulic fracturing test of true triaxial coal is carried out,and the injection pressure and crack propagation pattern are comprehensively analyzed.A method of combining cohesive force elements and pore pressure nodes to simulate the hydraulic fracturing of coal rock fracture network is proposed.The effects of bedding inclination,stress field and fracturing fluid injection displacement on the propagation of hydraulic fracturing crack in coal are studied.The theoretical criterion for the bifurcation of water pressure crack when encountering bedding structural plane is established.The basic mode of water pressure crack propagation when encountering bedding structural plane is analyzed.Research indicates:(1)The elastic modulus,compressive strength,splitting modulus and tensile strength of coal samples with bedding dip angle of 0° are the largest.With the continuous increase of bedding dip angle(within the range of 0°-90°),the mechanical property parameters of coal samples first decrease and then increase.The loading fracture of coal samples with bedding dip angles of 0°,30° and 60° is mainly shear failure,and the coal samples with bedding dip angles of 90 ° are mainly tensile splitting,accompanied by shear failure.Its fracture form is to form approximately parallel tensile splitting surfaces along multiple weak bedding surfaces.(2)Under the condition of bedding dip angle of 0°,both bedding and maximum principal stress control the propagation of hydraulic crack.Under the condition of bedding dip angle of 60°,the control effect of bedding on hydraulic crack propagation is stronger than the maximum principal stress.When the bedding dip angle is 90°,the water pressure crack only propagates along the bedding direction.Small displacement is conducive to the opening of primary natural fractures,while large displacement injection is also conducive to the formation of branch fractures.Reasonable setting of fracturing fluid displacement is conducive to the formation of complex hydraulic fracture network structure in coal.(3)The effectiveness of the hydraulic fracturing numerical model of bedding coal is verified by classical theoretical model and laboratory test.Different variables such as bedding and stress are studied.It is found that the main fracture is formed along the main stress direction and the secondary branch fracture is formed along the bedding direction under the condition of 30 ° bedding dip angle.It shows that the comprehensive action of the stress condition and bedding can form a more complex hydraulic fracture network structure.(4)The influence characteristics of displacement on the water pressure crack of bedding coal are studied.The greater the injection displacement is,the greater the pressure of coal initial failure is,the stronger the disturbance effect on the coal near the injection point is,and the wider the crack at the injection point is;Under the condition of fluctuating displacement injection,the propagation range of water pressure crack in the bedding structural plane is wider.Fluctuating displacement injection is more likely to cause fatigue damage and fracture of coal,further increase the micro cracks in coal and promote the initiation and propagation of water pressure crack.