Research On The Influence Of Perforation On The Formation Of The Fracture Network Around The Shale Gas Horizontal Well

Shale gas as an unconventional oil gas resource has caused the concern of petroleum industry,perforated horizontal well staged fracturing is the key technique to the shale gas development.The effective of perforation mode on the fracture initiation and propagation is not comprehensive enough,which constrain the perofated parameters optimization and effective development of shale gas.This article taking Longmaxi formation shale of Sichuan basin for research objects,through the indoor experiments and elasticity theory,established the stress distribution model of shale gas horizontal well and analysed the fracture initiation laws.By using numerical simulation method,builded the finite element model of perforated horizontal well.Considering the influence of bedding and in-situ stress,studied the effective of perforation mode on the fracture initiation and propagation.The following results are obtained:(1)Based on the theory of elastic mechanics,established the stress distribution model of shale gas horizontal well.According to the maximum tensile stress failure criterion,analysed the effective of maximum and minimum horizontal main stress,vertical stress,degree of anisotropy,perforation mode on initiation pressure.(2)Perforation parameters plays a vital role to initiation pressure and initiation fracture face,especially the perforation azimuth angle has the biggest influence.When hole azimuth is less than 60 degrees,initiation pressure decrease with the perforation azimuth increase;When hole azimuth is greater than 60 degrees,initiation pressure increase with the perforation azimuth increase,and the initiation increase rapidly when hole azimuth range between 30 degrees to 60 degrees.(3)Without sonsideration of bedding,when the length of perforation tunnel is less than 1.3 times of borehole diameter,with the increase of tunnel length the initiation pressure decreases rapidly,when the tunnel length is 1.3 times larger than the borehole diameter,the fracturing pressure decreased slowly.lf the bedding considered initiation pressure almost does not change with the change of tunnel length.(4)The perforation phase has an effective on initiation pressure and fracture strike,perforation and staged fracturing is commonly used in shale gas reservoir,so horizontal section should along the direction of minimum horizontal stress,perforation phase choose 90 degree or 120 degree is reasonable.(5)For the strike slip type shale formation,when the inclination angle of bedding surface is low,initiation and extension of cracks are along the bedding plane direction,and the bedding surface is the main control factors.when the inclination angle of bedding surface is high,initiation and extension of cracks are effected by interaction of the bedding surface and the maximum principal stress,and the greater the angle of bedding,crack initiation direction is close to the direction of maximum principal stress.The higher the strength of the stratification plane is the greater impact of crack extension affected by the maximum principal stress is.The lower the strength of bedding surface is,the easier the crack initiate along the bedding plane.(6)Without the presence of bedding plane,when the perforation orientation is high,the lower the perforation azimuth is,the lower the steering distance is,the faster the crack back to the maximum principal stress direction.When the perforation orientation is low,the cracks along the hole extending in the direction of a distance and then gradually return to the direction of maximum principal stress.In the presence of bedding plane,the cracks can be hardly effected by changing of perforation azimuth,initiation and extension of cracks are along the bedding plane direction.The cracks develop along the bedding surface which near the hole tip.With the increasing of hole length,more bedding plane can be open,the fracure distribution will be more complex,the fracture network will be formed easier.