Multiscale random pore network modeling of oil-water two-phase slip flow in shale matrix

The large-scale hydraulic fracturing technology is commonly employed to significantly improve oil recovery in shale. An exceptional phenomenon from many field tests is worthy of attention that the majority of fracturing fluid (mainly water) does not return to the surface during hydraulic fracturing, but flows into shale matrix. As a consequence of this phenomenon, oil-water two-phase flow would happen. However, the fluid transportation in nano-pores is different from that of the conventional sandstone reservoirs. To understand oil-water two-phase flow in shale, an integrated pore network model (PNM) framework including network construction and flow simulation is developed. In the network construction, combined with a pore-size distribution (PSD) curve of shale system, a shale multiscale random pore network model is proposed, where organic pores and inorganic pores can be distinguished appropriately. In the flow simulation, considering that the traditional Hagen-Poiseuille equation may not be applicable in shale system, new bulk fluid and corner film flow models with slip length in various geometric pores (circular, square, equilateral triangle) are developed using the computational fluid dynamic (CFD) method. Through the quasi-static simulation technique, we estimate the flow properties in shale including capillary pressure and relative permeability. Simulation results confirm the slip effect would significantly affect oil-water two-phase flow in shale. And then, influences of shale pore structure on two-phase flow are further investigated. Through coupling network construction and two-phase flow simulation, we believe our work could shed light on the further research for multi-phase transportation in shale.