Study on the influence of stress difference coefficient on hydraulic fractures in deep shale reservoir and the formation method of complex fracture network

Abstract

Abstract With the progress of underground energy exploration and development, the research on deep shale was becoming increasingly important. Laboratory tests were designed and combined with theories to study the fracture characteristics of deep shale true triaxial hydraulic fracturing under different stress difference coefficients. Combined with the results of laboratory triaxial test and the evaluation method of rock brittleness based on energy change, it was shown that with the increase of confining pressure, the brittleness of shale decreased, indicating that it was difficult for deep shale to form complex fracture networks. In high-stress hydraulic fracturing tests with different stress difference coefficients, combining fractal theory and energy release rate principle, it was found that hydraulic fractures were easy to communicate with shale bedding planes under low horizontal stress difference coefficients, which can improve the complexity of hydraulic fractures ignoring the overall increase of the triaxial stress. Although the low horizontal stress difference coefficient in deep shale reservoir makes it easy for hydraulic fractures to communicate with shale strata and thus promote the formation of complex shale fracture network, deep shale reservoir fracture expansion is difficult and reservoir reconstruction scope is limited. Therefore, hydraulic fracturing tests under different viscosity and displacement were designed and multiple linear regression method was adopted to obtain the relationship between the coupling effect of fracturing fluid viscosity and displacement on the fractal dimension, which can not only improve the fracture complexity to a large extent, but also control the fracture propagation direction, provide reference for field fracturing work.