Study On Fracture Propagation Mechanism Of Shale Formation

As an important kind of unconventional resources,the shale oil and gas spread widely in China.However,the permeability of the shale reservoir is ultra low so that hydraulic fracturing technique is required to develop this kind of energy.In order to investigate the simultaneous propagation of multiple fractures during the multistage hydraulic fracturing,we established the two-dimension fully coupled seepage-stress-damage model based on Cohesive Zone Method(CZM).In this paper,we studied the effect of perforation cluster spacing,elastic modulus,injection rate and viscosity of fracturing fluid on the pattern of fractures.The results shows that:(1)perforation cluster spacing significantly affects the “stress shadow” area between the fractures,and smaller the clusters spacing is,the “stress shadow” will be more obvious,and the middle fracture will be limited in the propagation process.(2)the larger the elastic modulus is,the length of the fractures will be longer whereas the width of the fractures will be smaller,which increases the risks of screen-out.(3)With the increasing of the injection rate of the fracturing fluid,the length and width of fractures will both be increased,thus greater injection rate will finally maximize stimulated reservoir volume(SRV).(4)with the increasing of the fracturing fluid viscosity,fracture width will be increased and fracture length will be shorter,which reversely reduced SRV.Based on extended finite element method(XFEM),a fully coupled pore pressure-stress plane strain model is created.The model is used to investigated the influence from perforation phasing angle,elastic modulus,fracturing fluid viscosity,injection rate and in-situ stress difference on fracture re-orientation.The results shows that:(1)when increases from 0°to75°,the initiation pressure gradually increases with the radius of the re-orientation path grow slowly,and when is greater 60°,the radius of the re-orientation path grow rapidly.(2)with the increasing of elastic modulus,the initiation pressure decreases which means it is easier to create a fracture,and the radius of the re-orientation path grow step by step.(3)the injectionrate greatly impact the fracturing process,and with the increasing of the parameter,both initiation pressure and re-orientation radius increases obviously.(4)for formation with high stress difference,the perforation angle should be reduced to decrease the tortuosity of the fracture pattern,therefore ensuring the success of the operation.We investigated the simultaneous propagation of multiple fractures in multistage fracturing and the re-orientation of the fractures in vertical well.The research results is beneficial for optimizing the fracturing design in the oilfield.