Effect of capillary force on performance of shale rock fracturing

Compared to conventional reservoirs, shale gas and oil reservoirs have super-low matrix permeability. Hydraulic fracturing is an effective technology to develop shale gas and oil resources. Water has been used as fracturing fluid, which is immiscible with formation fluid (i.e., oil and gas). When immiscible multiphase flow occurs in shale rock, the capillary force has large effect. Conventional fracturing simulation models for shale gas reservoirs assume that flow field is dominated only by viscous force. This assumption is unreasonable for low permeable rock because seepage effects influenced by both capillary force and viscous force. This study develops a new numerical model to simulate multiphase flow in low permeable rocks and to investigate the effects of capillary force on fracturing performance. The numerical model couples flow model and solid model. The flow model consists two parts: matrix flow model and fracture flow model. The matrix flow model based on Pore-scale Network Method (PNM) reproduced two-phase flow in matrix considering viscous forces and capillary forces. The fracture flow model based on unsteady flow model simulated unsteady flow in opened fractures. The solid model uses the Finite Element Method (FEM) with cohesive zone element. The validation was carried by comparison with analytical solution and experiment results. Then, fracturing performance of different capillary force were analyzed. The simulation results show that capillary force has significant impact on fracture geometry.