Abstract
Carbon-neutral oil production is one way to improve the sustainability of petroleum resources. The emissions from produced hydrocarbons can be offset by injecting capture CO_{2} from a nearby point source into a saline aquifer for storage or a producing oil reservoir. The latter is referred to as enhanced oil recovery (EOR) and would enhance the economic viability of CO_{2} sequestration. The injected CO_{2} will interact with the oil and cause it to flow more freely within the reservoir. Consequently, the overall recovery of oil from the reservoir will increase. This enhanced oil recovery (EOR) technique is perceived as the most cost-effective method for disposing captured CO_{2} emissions and has been performed for many decades with the focus on oil recovery. The interaction between existing oil and injected CO_{2} needs to be fully understood to effectively manage CO_{2} migration and storage efficiency. When CO_{2} and oil mix in a fully miscible setting, the density can change non-linearly and cause density instabilities. These instabilities involve complex convective-diffusive processes, which are hard to model and simulate. The interactions occur at the sub-centimeter scale, and it is important to understand its implications for the field scale migration of CO_{2} and oil. In this work, we simulate gravity effects, namely gravity override and convective mixing, during miscible displacement of CO_{2} and oil. The flow behavior due to the competition between viscous and gravity effects is complex, and can only be accurately simulated with a very fine grid. We demonstrate that convection occurs rapidly, and has a strong effect on breakthrough of CO_{2} at the outlet. This work for the first time quantifies these effects for a simple system under realistic conditions.
Highlights
Carbon dioxide (CO2 ) injection in oil reservoirs to increase recovery and simultaneously store CO2 is perceived as a highly cost-effective method for disposing captured CO2 emissions (Heidug et al 2015). This enhanced oil recovery (EOR) technique has been performed for many decades with the focus being on hydrocarbon recovery and not long-term CO2 storage (Dooley et al 2010)
Various techniques can be implemented to increase CO2 stored during oil extraction and reduce the carbon footprint of oil production, socalled CO2-EOR+ technologies (Heidug et al 2015)
Relative permeabilities affect the mobility contrast for a given viscosity ratio and phase behavior can change local fluid properties, which can enhance or mitigate viscous and gravitational instabilities. This is subject of active research, where convective mixing will be exploited in a multiphase compositional framework within Open Porous Media (OPM)
Summary
Carbon dioxide (CO2 ) injection in oil reservoirs to increase recovery and simultaneously store CO2 is perceived as a highly cost-effective method for disposing captured CO2 emissions (Heidug et al 2015). Density inversion can trigger gravity fingers, resulting in density-driven convection in the miscible zone This phenomenon can be observed in examples of field-scale injection of CO2 into oil using both commercial (Ahmed et al 2012; Both et al 2015) and high-resolution simulations (Shahraeeni et al 2015). The known effects of gravity fingering in CO2-brine systems indicate that convection has the potential to improve sweep and reduce breakthrough in oil systems Some evidence of this is demonstrated in the simulations by (Ahmed et al 2012; Shahraeeni et al 2015). No similar analysis has been carried out for convective mixing under flowing conditions, and a detailed analysis is required to quantify the balance between viscous and gravity forces in a miscible CO2 flood This work addresses this challenge through simulation of fine-scale coupled viscous and gravitational processes during miscible displacement. We determine the necessary grid resolution to obtain a physical solution where we observe fingers
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