Pore-Scale Mechanisms of Enhanced Oil Recovery by CO2 Injection in Low-Permeability Heterogeneous Reservoir

Since recently, the oil reserves of low-permeability reservoirs comprise a larger and larger percentage of newfound oil reserves in China, the efficient development of these reserves becomes a matter of urgent concern. CO2 flooding is a promising method to solve this problem. In low-permeability heterogeneous reservoirs, the oil displacement efficiency by CO2 flooding is affected by a number of interactions between different fluids, such as CO2 dissolution, viscosity reduction, oil swelling, and extraction of light hydrocarbon components of crude oil, but also affected by interactions between fluids and rocks due to heterogeneity and low permeability. The dynamic interplay between these various mechanisms is complicated and cannot be captured by existing models and simulations. In this paper, we present the highlights of the results of a series of visualization experiments performed using a low-permeability heterogeneous glass micromodel. CO2 injection was carried out at 323 K and different pressures to investigate the interactions between CO2 and oil in different size of pores. In particular, we find that strong mass transfer has happened when experimental pressure is close to but less than minimum miscible pressure, and CO2 injection promotes oil recovery significantly. Our results also reveal that in a heterogeneous glass micromodel, strong mass transfer did not happen simultaneously at different size of pores, and happened at higher pressure in smaller size of pores than larger size of pores.