Two-Phase Unsaturated Microscopic Seepage Simulation Study Based on Digital Core Technology

Abstract

ABSTRACT In recent years, digital core technology as an emerging numerical simulation method has been widely used in many fields. In this paper, the three-dimensional pore throat structure of sandstone was extracted by digital core computed tomography (CT) technique. And establish a core's realistic 3D pore network model by 3D visualization image processing technique for gas-liquid two-phase unsaturated microscopic seepage simulation. To realize two-phase unsaturated seepage simulation, using two-phase volume averaged momentum equation and continuity equation to construct the two-phase unsaturated seepage coupling equation. The two-phase saturation and relative permeability variations were described by the saturation relation and the VG-Mualem model. The results show that the saturation variation is uneven during unsaturated seepage due to the finger-in phenomenon. There is a dominant channel in the two-phase fluid seepage process, the flow lines are more intensive in the large pore throat, and the flow velocity varies widely. The relative permeability of two-phase at different parts is similar, the relative permeability curve is concave downward. The final relative permeability of the non-wetting phase is closed to 1. INTRODUCTION There are numerous seepage simulation methods, which can be mainly divided into macroscopic and microscopic from the seepage scale. Macroscale simulation methods are used to determine the fluid state in porous media by solving differential equations, including flow rate, saturation, etc. (Lei et al. 2018). The micro-scale simulation method is to simulate the seepage flow based on the pore network structure using finite element, lattice Boltzmann and other methods. And the boundary conditions are considered in the process (Liu et al. 2017). Due to the complexity of pore structure and fluid flow characteristics in porous media, conventional core images obtained by scanning methods are difficult to use for micro-scale seepage simulation. However, with the widespread application of CT scanning technology in the oil and gas industry, a three-dimensional high-precision digital core characterization technology based on CT scanning imaging has been formed, which is to extract rock structure data from digital rock, and process the data through a three-dimensional visualization image processing software to construct a pore network model. It has the advantages of high spatial continuity and high visualization, and the pore network model can better reflect the rock core pore structure characteristics and pore throat topological relationship than the random network defined only by porosity and permeability, which can greatly improve the accuracy of seepage simulation (Baldwin, 1996; F. Arzilli, 2016).