Fracture Diversion Mechanism And Parameter Optimization Study By Temporary Plugging Hydraulic Fracturing On Shale Reservoir

The Long Maxi shale reservoir in Sichuan-Chongqing area is characterized by large horizontal stress difference,great variation of mechanical parameters such as shale strength and brittleness,etc.During the operation of temporary plugging fracturing,it is affected by the bedding and natural fractures,and the laws of fracture initiation and propagation are unclear.Therefore,it is urgent to master the basis of optimization design of temporary plugging fracturing construction parameters through laboratory physical simulation experiments.Therefore,this paper carried out true triaxial hydraulic fracturing physical simulation experiment and numerical simulation experiment to study under the influence of natural factors(horizontal in-situ stress difference and natural fractures)and construction factors(pump rate and temporary plugging agent ratio)how the temporary plugging fracturing hydraulic fracture initiation mechanism and expansion law in shale reservoir behaves.The research results of this paper are as follows:(1)True triaxial physical simulation experimental study on fracture initiation and propagation behavior of shale reservoir.Based on the real in-situ stress characteristics of shale in Long Maxi area,the application of true triaxial physical simulation experiment shows that the horizontal in-situ stress difference of 12 MPa to 20 MPa is more suitable for temporary plugging fracturing construction;for the selection of temporary plugging agent,the mode of mixed addition of proppant and tackifier should be chosen,and compared with the use of a 10% concentration of tackifier,when using a 5% concentration of tackifier,temporary plugging fracturing fractures are more developed;for the choice of pump rate,high pump rate should be selected when construction.After the hydraulic fracturing experiment of temporary plugging,the fractures mainly formed four types of fracture networks: 1.Temporary plugging,cracks diversion;2.Blocking old fracture,initiating new fracture;3.Blocking old fracture,communicating bedding;4.Blocking old fracture,initiating new seams,and communicating bedding.(2)Numerical simulation study on the fracture propagation law of temporary plugging hydraulic fracturing with preset artificial fracture.The extended finite element method is used to conduct a numerical simulation study on the propagation path of temporary plugging fracturing fractures.When the preset fracture and the old fracture are at an angle of 90 °,the fracture first extends for a long distance perpendicular to the old fracture,then the angle between the fracture propagation direction and the horizontal maximum principal stress direction gradually decreases,and the final fracture extension direction is and the horizontal maximum principal stress direction is parallel;when the preset fracture and the old fracture are at an angle of 60 ° and 30 °,the preset fracture turns quickly after initiation and extends along the horizontal maximum principal stress direction.When the temporary plugging agent achieves effective temporary plugging,the natural fractures will crack and extend,while the old fracture will not crack,and the new fracture will crack along the natural fracture and eventually turn to expand along the direction of the maximum principal stress.(3)Optimization of temporary plugging hydraulic fracturing construction parameters for shale reservoir.Reasonable optimization of temporary plugging hydraulic fracturing construction parameters can reduce shale fracture pressure,increase fracture complexity,and achieve effective reservoir reconstruction.When the temporary plugging fracturing construction is started,high pump rate is used to form initial fractures;when the temporary plugging agent is pumped,the pump rate should be reduced,and the area to be broken is slowly moved and disturbed,and the adjacent bedding and natural cracks are opened step by step to form a stepped crack.