This study examines challenges in CO2-EOR, like small-scale reservoir factors, viscous and interfacial forces, and dispersive effects due to concentration gradient, focusing on how these small-scale reservoir parameters impact oil recovery during CO2 injection.
A suite of simulation models representing various heterogeneity patterns was developed, and these models were subjected to differing pressures, both above and below the Minimum Miscibility Pressure (MMP). A simple 2D reservoir model was built as a compositional oil model using Eclipse 300. It consists of 100 cells in the x-direction and 20 cells in the z-direction, totaling 2000 cells. To simulate injection and production into the model, two vertical wells were placed at either end of the model, fully penetrating the z-direction. This allows the CO2 to sweep the oil from the injector to the producer.
After conducting 21 model simulations, the study revealed that below the Minimum Miscibility Pressure (MMP), gravitational effects were the primary driving force, causing gas to override oil, with permeability playing a minor role. Above the MMP, viscous forces induced fingering and early breakthrough. All models showed improved oil recovery due to miscibility, with the kv/kh ratio being a crucial factor. Coarsening of horizontal permeability, or both horizontal and vertical permeabilities, had a detrimental impact on recovery. Gas override was influenced by both horizontal and vertical permeability, leading to fingering under random permeability distributions. Horizontal permeability variations, regardless of vertical permeability consistency, had the most significant impact on oil recovery. Additionally, the study emphasized that the coefficient of variation (Cv) alone inadequately characterized heterogeneity, as models with the same Cv values exhibited varying recovery outcomes, ranging from 63% to 96.5%.
The novelty of this study is that it quantifies the impact of heterogeneity on CO2 EOR recovery. These findings underscore the intricate relationship between reservoir parameters and CO2 miscible flooding, offering essential insights for optimizing EOR strategies in heterogeneous reservoirs.
