Study on the Anisotropy Characteristics of Deep Shale in the Southern Sichuan Basin and Their Impacts on Fracturing Pressure

Deep shale formations in Southern Sichuan Basin show significant stratigraphic anisotropy and strong in-situ stress heterogeneity,which result in serious fluid loss during drilling and difficulty in fracturing pressure prediction.On the basis of microstructure observations and mechanical experiments on deep shale,the anisotropy characteristics of deep shale were systematically studied and a circumferential stress model was built for a wellbore.A deep shale formation fracturing pressure prediction model was established in combination with the tensile failure of deep shale rock body,shear slip failure and tensile failure of fracture to analyze the influence of anisotropy characteristics on the borehole fracturing pressure.The analysis results showed that stronger shale anisotropy and higher in-situ stress difference would lead to easier fracture slippage in highly deviated wells;higher cohesion would effectively restrain the dislocation of fracture weak planes,and fracture inclination would dominate the tensile leakage of fracture.The prediction model was adopted to predict the fracturing pressure of shale wells in the Pengshui Block of the Southern Sichuan Basin.Results showed that the impact of in-situ stress difference is more significant than that of elastic difference,and the cohesion of rock is the main factor in inducing shear slip failure,while fracture inclination is the main factor in leading to the tensile failure of the fracture.Fracture developed shale formations are dominated by fracture damage,and fracturing pressure is significantly affected by rock cohesion,fracture inclination and in-situ stresses.In practical fracturing pressure prediction,three types of fracture modes should be used integrally to determine the fracture mode and fracturing pressure.