Abstract:

The combination of CO2 enhanced oil recovery and CO2 storage is a promising technology to produce oil while simultaneously reducing net carbon emissions. Joint EOR and CO2 storage processes are often considered under conditions in which the oil phase and gas phase are miscible, leading to a near perfect displacement efficiency. Beneath a threshold minimum miscibility pressure, however, CO2-oil miscibility cannot develop. Immiscible cases are not well examined in literature but frequently occur in shallow reservoirs and heavy-oil reservoirs. Highly efficient miscible CO2, while ideal, would be unattainable in such reservoirs. This paper presents the results of reservoir simulations investigating the use of immiscible CO2 for combined EOR and carbon storage under gravity-assisted conditions. Immiscible CO2 is injected at a constant rate into the top of a water-flooded reservoir via horizontal wells. The downward motion of the expanding CO2 plume flows against gravity and is partially stabilized by buoyant forces against the heavier water phase, reducing viscous instability and improving sweep efficiency. Three-phase simulations were performed in a three dimensional, heterogeneous reservoir using ECLIPSE 300 with CO2SOL.

Several key variables – CO2 injection rate, injection strategy, oil viscosity, and oil zone thickness - were systematically modified across simulation runs to determine the effectiveness of gravity-assisted immiscible CO2 injection under varying degrees of plume instability. Lower CO2 injection rates were observed to more generate very high sweep efficiencies, producing more oil per quantity of CO2 injected than higher injection rates. The most successful injection rate by economic analysis was found to be 2 million standard m3/day of CO2, balancing the beneficial effects of slower injection with the financial returns of faster injection. Shutting off the oil production wells was a viable strategy under the appropriate economic conditions. A novel injection strategy was considered in which periods of CO2 injection were interchanged with periods of well inactivity; the inactive periods allowed the gas plume to stabilize under buoyant forces, leading to a greater efficiency of stored CO2 per unit CO2 injected than a strategy of constant CO2 injection. Higher frequency alternations were observed to sweep with CO2 even more efficiently; the highest frequency simulation was more successful than the best continuous injection scheme under a variety of economic conditions. Gravity-assisted drainage with immiscible CO2 was found to be effective in 40° API and 30° API oil, but far less so in 20° API oil. The process was also far more successful when applied to thicker oil banks. The results as a whole demonstrate that immiscible CO2 injection can be an effective solvent for enhanced oil recovery and CO2 storage under the proper conditions.

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Copyright 2017, Daniel C. Hatchell: Please note that the reports and theses are copyright to their original authors. Authors have given written permission for their work to be made available here. Readers who download reports from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the author, Daniel C. Hatchell.