Experimental Study On Fracture Initiation Mode And Propagation Law Of Natural Shale During Hydraulic Fracturing

Due to the low porosity and low permeability physical properties of shale gas reservoirs restricting the flow of internal gas,the use of hydraulic fracturing technology to form fracture networks within the shale reservoir is the key to the successful development of shale gas.Because the geological structure conditions of organic-rich shale reservoirs in China are complex,the physical properties of shale in different blocks are quite different,and the mechanical properties show obvious anisotropic characteristics,the hydraulic fracturing fracture propagation mechanism of conventional shale reservoirs can not be fully applied.Therefore,it is of great practical significance for the successful development of shale gas to carry out experimental research on anisotropic characteristics of shale matrix and fracture propagation mechanism of hydraulic fracturing,analyze its mechanical properties and failure characteristics,and grasp its hydraulic fracturing fracture initiation conditions,propagation rules and influencing factors.This paper takes the outcrop shale of Longmaxi Formation in Sichuan Basin as the research object,and uses the indoor test method to explore the anisotropy characteristics of shale,and then uses the self-developed large-scale true triaxial hydraulic fracturing simulation test device,the fracture initiation mode,propagation law and influencing factors in the process of shale hydraulic fracturing are studied.The results show that:(1)The microstructural difference at the bedding is the essential cause of the anisotropy of the shale.The micro-porosity of bedding is 13.63% and that of non-bedding is only 8.93%~9.57%.(2)Shale anisotropy is manifested in velocity and modulus of elasticity of ultrasonic wave gradually decrease with the increase of bedding angle;he compressive strength and Poisson's ratio change with the bedding angle as U-shaped,with higher values at 0° and 90° and lowest values at 30°;The tensile strength increases with the increase of bedding splitting angle;The failure mode changes from vertical splitting tension failure to splitting shear failure with the increase of bedding angle.(3)Hydraulic cracks are easy to initiate and propagate along the direction of maximum principal stress,and have the characteristics of hysteresis and high speed.When they propagate,the influence areas of displacement on both sides are semi-elliptical and asymmetrical,and the regions of high tension strain,tension strain and compression strain are in turn along the center of the crack.(4)With the injection of fracturing fluid,the pump pressure curve can be divided into four stages: continuous pressurization,wellbore initiation,fracture propagation and fracture penetration,and these four stages correspond to the four processes of fracture development respectively: crack initiation process,crack formation process,mesh formation process and mesh balance process.in addition,there are six different modes of propagation of hydraulic fracture interacting with weak bedding surface.(5)The fracturing effect has anisotropy with the change of the bedding angle.As the bedding angle increases,the pump pressure fluctuates more and the fracture pressure increases gradually.The shape of the new hydraulic fracture after fracturing is more complicated.(6)The anisotropy of the fracturing effect corresponds to the anisotropy of the shale mechanical parameters,and the shale with large elastic modulus and small tensile strength is more likely to crack during the hydraulic fracturing process.The paper has 61 figures,6 tables and 66 references.