Oil recovery from a CO2 injection in heterogeneous reservoirs: The influence of permeability heterogeneity, CO2-oil miscibility and injection pattern

This paper focuses on CO2 injections in heterogeneous reservoirs and provides an investigation of the effects of reservoir heterogeneity, CO2-oil miscibility, and injection patterns on oil recovery from an experimental perspective. The results show that CO2 consumption in heterogeneous cores is generally larger than that in homogeneous cores due to the existence of a low-oil-production stage under heterogeneous conditions. Oil recovery is very sensitive to heterogeneity in a permeability contrast (PC) range of 1.0-15.5, so even weak heterogeneity can lead to a large decrease of recovery for both immiscible and miscible flooding. As to the miscibility effects, oil recovery with multi-contact miscible (MCM) CO2 injections is higher than that of immiscible (IM) injections by 8.6%-14.1%, but it is difficult for MCM to reach a recovery as high as 90%, which is found in homogeneous cores. This phenomenon is different from some reported results from visual models, but in accordance with field tests characteristics. The injection pattern shows that the water alternating gas style (WAG) is more suitable for IM CO2 flooding than a soaking operation. For MCM injections, it is the opposite, and the soaking process leads to higher recovery than using WAG. However, the viscosity of the residual oil markedly rises after MCM soaking, which would increase the difficulty of future enhanced oil recovery (EOR) procedures.