Compositional simulation of three-phase flow in mixed-wet shale oil reservoir

Abstract

Three-phase flow is often involved during the production of shale oil reservoir. Relative permeability, which is often used to characterize multiphase flow, is found a strong function of wettability. Meanwhile, the mixed wettability in shale reservoir has been well characterized and may have impacts on fluid flow. In this work, to describe the mix-wet condition in shale, matrix is divided into organic and inorganic matrix which are hydrophobic and hydrophilic respectively. Relative permeabilities under different condition of wettability are applied in each kind of matrix. To construct permeability for three-phase flow, water-oil and oil-gas systems are evaluated separately with the assumption that water/gas permeabilities are only functions of their own saturations. Specifically, water-oil relative permeability is obtained by simulating multiphase flow directly on a digital rock sample using the lattice Boltzmann method. Oil-gas relative permeability is calculated from capillary pressure obtained from confined vapor liquid equilibria (VLE) coupled with Young-Laplace equation. Compositional simulation shows that the rich hydrocarbon in organic matrix is difficult to be produced due to the relatively lower permeability and poorer oil mobility under oil-wet condition. Since organic matter is a dispersed phase inside inorganic matter, fluids in organic matrix must flow through inorganic matrix before reaching the fractures. The dispersed nature of organic matter makes the relative permeability in inorganic matrix a dominant factor that controls the overall production in shale oil reservoir. The effect of relative permeability in organic matrix however has limited effect.