Four-dimensional in-situ stress evolution law of shale reservoirs based on mechanical time-varying characteristics

Abstract

The influence of fractures on the mechanical properties of shale rocks is prominent. Accurately characterizing the changes in the mechanical properties of fractured shale is an essential basis for precisely depicting the four-dimensional in-situ stress field during the fracturing and production process of shale reservoirs. Existing studies on four-dimensional in-situ stress often attribute stress changes primarily to alterations in pore pressures， neglecting the impact of changes in the mechanical properties of rocks on four-dimensional in-situ stress. The relationship between the pore pressure and the volume of shale pores and fractures was clarified by utilizing online CT experiments. Theoretical relationships among fracture volume， Poisson’s ratio， and elastic modulus of shale rocks were derived based on strain energy theory. Furthermore， a time-varying theoretical model was established with pore pressure， Poisson’s ratio， and elastic modulus as variables. The results indicate that the opening of bedding fractures is the primary cause of the time-varying nature of the mechanical parameters of shale rocks. The number of fractures opening increases， and the fracture space exhibits exponential growth as pore pressure increases. With the increase in fracture volume， the Poisson’s ratio of shale rocks rapidly increases， while the Young’s modulus rapidly decreases， leading to a significant reduction in shale strength. An analysis of the shale in Jiyang Depression reveals that Young’s modulus decreases from 37.5 GPa to 15.06 GPa， and the Poisson’s ratio increases from 0.22 to 0.35 when the proportion of fracture volume increases to 1%. The influence of the time-varying nature of mechanical parameters of rocks on the evolution of four-dimensional in-situ stress is comparatively analyzed through numerical simulation methods. The four-dimensional in-situ stress simulation， considering the time-varying characteristics of rock mechanics， can better characterize the law of stress evolution and direction deflection during shale fracturing and production. It is more consistent with the evolution law of four-dimensional in-situ stress in the process of fracturing and production， and the simulation results are more reliable. The research results can provide theoretical support for the rational deployment， optimal design， and risk warning of three-dimensional well groups of shale oil.

Keywords

• shale oil
• four-dimensional in-situ stress
• mechanical properties of rock
• time-varying characteristics
• four-dimensional in-situ stress simulation