Numerical Analysis of Pore-Scale CO2-Oil Displacement in Heterogeneous Porous Media at Near-Miscible Flow Condition

Abstract

Abstract Gas flooding through the injection of CO2 is generally performed to achieve optimum oil recovery from underground hydrocarbon reservoirs, and CO2 capturing and storage. In fact, the second purpose is aimed at reducing the greenhouse effect in the atmosphere and achieving NetZero. Due to the laborious operational circumstances governing the gas injection process under miscibility conditions such as pressure maintenance, many reservoirs are situated in near-miscibility gas flooding operations. In this research, the displacement of oil through carbon dioxide injection under near-miscibility conditions is scrutinized exclusively at the pore scale. In this regard, based on the correlations and data available in the literature, first, the criteria of the near miscibility region are specified. Then, two separate numerical approach are implemented to examined the behavior of CO2-oil at lower-pressure limit of specified region. First, Phase-field coupled with Navier-Stokes equation is used to investigate the CO2-oil displacement by capturing the diffusive interface properties and hydrodynamic properties of fluids. Next, the effect of CO2 mass transfer into the oil phase is incorporated by coupling classical Fick’s law to the system of above equations to track the viscosity reduction of oil and the variation of CO2 diffusion coefficient using TDS module respectively. To better recognize the oil recovery mechanism in pore-scale, qualitative analysis indicates that interface is moved into the bypassed oil due to low interfacial tension in the near-miscible region. Moreover, behind the front ahead of the main flow stream, the CO2 phase can significantly displace almost all the bypassed oil in normal pores and effectively decrease the large amounts in small pores. This is because of mass transfer and capillary cross-flow mechanism caused by simultaneous CO2 and oil flow through the diffusive interface between the phases. The quantitative results also confirmed that taking mass transfer into account in pore-scale simulation and strengthening the diffusion term enhanced oil recovery from 92% to over 98%, approaching the output of miscible gas injection. The outcome of this research emphasizes the significance of applying the CO2-EOR process under near-miscible operating conditions.