Study On The Mechanism And Simulation Of The Hydraulic Fracture Propagation In The Shaly Loose Sandstone

In terms of shale content,the value of the shaly loose sandstone is between 15%~50%.Traditionally,it is considered that when shale content is beyond 15%,the sandstone has a poor sorting performance,and the porosity and permeability of sandstone are significantly worse,so it is difficult to be an industrial oil and gas reservoir.However,with the full use of high-quality oil and gas reservoir,people have to turn to the development of poor oil and gas reservoirs which contains the shaly loose sandstone.Fracturing practice in shaly loose sandstone layer shows that due to high clay content,rock plasticity is strong,fracture toughness is high,crack initiation is difficult and the morphology of cracks is complex.Therefore,to carry out the study of fracture propagation mechanism in shaly unconsolidated sandstone,is to lay the theoretical foundation of effective development of shaly loose sandstone reservoir,and is of important significance to ensure Chinese ' oil and gas security strategy.Based on the analysis of the physical property of the reservoir,the uniaxial experiment,the corresponding mechanical parameters and the formula are obtained.The experimental results showed that the content of shale has a great influence on the shape of hydraulic fracture.With the increase of clay content,hydraulic fracture will be fishbone seams which are in twists and turns with a seepage zone.The discrete element software PFC is used for further study on the mechanism of propagation of hydraulic fracture in shaly unconsolidated sandstone.Based on the comparison with the physical experiment,the shale content had a significant effect on hydraulic fracture morphology of shaly unconsolidated sandstone.With the increase of shale content,the rock is more and more plastic,crack opening is getting smaller and smaller,cracks are more and more curved presenting fishbone shaped branch fractures and the seepage fractured zone near cracks is increasingly obvious,the fracture pressure is more and more small.The natural fractures under hydraulic pressure will be communicated by fracturing fluid,forming branch fractures.But the communication does not affect the hydraulic fractures perpendicular to the minimum principal stress.Stress fields have a influence on the form of hydraulic fracture in shaly unconsolidated sandstone.The fracture zone is twisted and disordered when the stress difference is small;the greater the stress difference,hydraulic fracture is more straight.Stress chain analysis displayed by PFC software,found that the stress chain direction is very obvious in the shaly loose sandstone,but the non-homogeneous phenomenon is very serious.This phenomenon causes crack appeared in the process of shear stress,the hydraulic fractures are more easily to branch with broken belt.Comparing hydraulic fracturing fracture curve with PFC micro parameter,the stiffness and shear stiffness of sandstone is relatively large,shale is relatively small.To achieve the same stress,shale requires relatively larger displacement and the pressure holding time is longer,with a smaller slope fracture curve.From the analysis of microscopic parameters,Poisson's ratio and plastic have a certain connection with shear failure of shaly loose sandstone.