Investigation of Water Leakoff Considering the Component Variation and Gas Entrapment in Shale During Hydraulic-Fracturing Stimulation

Abstract

Summary The water leakoff into the shale matrix during the hydraulic-fracture treatment has been a critical issue in determining fracture geometry. Furthermore, water leakoff also affects mechanical properties of the surrounding rock matrix which, in turn, affects fracture propagation. Conventional approaches for the prediction of leakoff were inadequate because several important phenomena are ignored. In this paper, several effects on water leakoff into shale matrix during shale-gas reservoir stimulation are considered. A simplified structure is used to depict the complex pore network in shale. Different interactive forces involved in water displacement considering the osmotic and capillary effects are taken into account in the mathematical formulation of the model. The proposed numerical model is used to study the water leakoff and the consequent pressure increase caused by gas entrapment. The potential influence of the increase in pore pressure on the generation of microfractures is also discussed. The simulation results show reasonable agreement with the previous studies, and indicate that the water leakoff greatly depends on composition and structure of shale matrix. Clay minerals, for example, are naturally prone to water invasion, and draw water faster than hydrophilic minerals and organic matter because of the osmotic effect. Furthermore, the invaded water significantly increases the pore pressure within the shale matrix because of gas entrapment, which leads to a strong nonlinear relationship between leakoff and the square root of time. An increase in pore pressure also results in a decrease in effective stress that leads to the generation of tension-induced microfractures in shale matrix. This study emphasizes the significance of osmotic and capillary effects as well as gas entrapment on hydraulic-fracturing treatment of shale-gas reservoirs. Moreover, the new leakoff model can be applied to assist the investigation of fracture-propagation behavior in a shale-gas reservoir.