Study On Induced Stress Field Model Considering Natural Fracture

Shale gas has high recoverable reserves and broad prospects for development in China.At present,most of these reservoirs are developed by fracturing technology in horizontal Wells.However,some shale gas wells are less productive after fracturing.One of the important reasons is that the expansion of hydraulic fracture is limited by the interference of induced stress field during fracturing process,which can't effectively communicate reservoirs and release productivity.And the induced stress field model is an important part of the hydraulic fracture propagation model.Therefore,it is necessary to calculate the induced stress field of shale reservoir,and provide a stress field model for fracture propagation simulation,and complete the relevant research contents of the 13th Five-Year National Science and Technology Major Project.Based on the induced stress field calculation model of reservoir and hydraulic fracture physical model,this paper established hydraulic fracture induced stress field model by using two-dimensional displacement discontinuity theory and introducing fracture height correction coefficient,and solved it by using Gauss-Jordan elimination method and column principal element elimination method.The accuracy of this model is verified by comparing with previous research results.On this basis,the stress of natural fractures under induced stress and in-situ stress is analyzed.Combined with the failure criterion of natural fractures,the failure state of natural fractures is judged,and the model of induced stress field considering natural fractures is established.Then,based on the model,the influence rules of reservoir parameters,hydraulic fractures and natural fractures are analyzed,and the influence of induced stress field considering natural fractures on the interaction between hydraulic fractures and natural fractures is studied.The following conclusions and recognitions have been obtained:(1)When natural fractures are not destroyed,the induced stress difference at the tip of hydraulic fractures is positive.When natural fractures are destroyed,in terms of the magnitude of induced stress difference at the crack tip,it is considered that the open type natural fracture is larger than the closed type natural fracture and larger than the hydraulic fracture,indicating that the destruction of natural fractures increases the stress difference at the tip and makes it difficult for the hydraulic fracture to turn,which is not conducive to the formation of fracture network;(2)For different reservoir induced stress field models,with the increase of net pressure,the induced stress difference at the tip of hydraulic fracture also increases,but the increase of induced stress difference is slightly less than the increase of net pressure,indicating that the effect of increasing the net pressure in the fracture to make the fracture turn to increase the reservoir volume is not obvious.With the increase of hydraulic fracture length or the decrease of natural fracture length,the induced stress difference at the tip of hydraulic fracture increases,which is not conducive to the formation of complex fracture network.When the hydraulic fracture deflection Angle less than 900 or natural fracture deflection Angle is not more than 45°,the induced stress difference at the crack tip is small and negative in some areas,It has less hindrance to hydraulic fracture steering and is beneficial to the formation of complex fracture networks;The elastic modulus and Poisson's ratio have no influence on the calculation results of the model;(3)The shear failure range of open natural fractures is greater than that of closed natural fractures which is larger than the tensile failure of nature fractures.With the hydraulic fracture tip as the center,the longest distance for shear failure of open natural fractures is 18 m;the longest distance for shear failure of closed natural fractures is 12 m;and the longest distance for tensile failure of natural fractures is 8 m;(4)Reservoir induced stress field considering natural fracture has no effect on the form of tensile failure and shear failure of natural fracture intersecting with hydraulic fracture.However,it has different effects on the tendency of tensile or shear failure of natural fractures.As far as tensile failure is concerned,it will increase the tendency of tensile failure of the natural fracture units on the right side of intersecting points,and weaken the tendency of tensile failure of units on the left side of intersecting points.As far as shear failure is concerned,it will weaken the tendency of shear failure of natural fracture units at intersecting points and increase the tendency of shear failure of other units.