The reservoir performance and impact from using large-volume, intermittent, anthropogenic CO₂ for enhanced oil recovery
Coleman, Stuart Hedrick
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Anthropogenic CO₂ captured from a coal-fired power plant can be used for an enhanced oil recovery (EOR) operation while mitigating the atmospheric impact of CO₂ emissions. Concern about climate change caused by CO₂ emissions has increased the motivation to develop carbon capture and sequestration (CCS) projects to reduce the atmospheric impact of coal and other fossil fuel combustion. Enhanced oil recovery operations are typically constrained by the supply of CO₂, so there is interest from oil producers to use large-volume anthropogenic (LVA) CO₂ for tertiary oil production. The intermittency of LVA CO2 emissions creates an area of concern for both oil producers and electric utilities that may enter into a CO₂ supply contract for EOR. An oil producer wants to know if intermittency from a non-standard source of CO₂ will impact oil production from the large volume being captured. Since the electric utility must supply electricity on an as-needed basis, the CO₂ emissions are inherently intermittent on a daily and seasonal basis. The electric utility needs to know if the intermittent supply of CO₂ would reduce its value compared to CO₂ delivered to the oil field at a constant rate. This research creates an experimental test scenario where one coal-fired power plant captures 90% of its CO₂ emissions which is then delivered through a pipeline to an EOR operation. Using real emissions data from a coal-fired power plant and simplified data from an actual EOR reservoir, a series of reservoir simulations were done to address and analyze potential operational interference for an EOR operator injecting large-volume, intermittent CO₂ characteristic of emissions from a coal-fired power plant. The test case simulations in this study show no significant impact to oil production from CO₂ intermittency. Oil recovery, in terms of CO₂ injection, is observed to be a function of the total pore volumes injected. The more CO₂ that is injected, the more oil that is produced and the frequency or rate at which a given volume is injected does not impact net oil production. Anthropogenic CO₂ sources can eliminate CO₂ supply issues that constrain an EOR operation. By implementing this nearly unlimited supply of CO₂, oil production should increase compared to smaller-volume or water-alternating-gas (WAG) injection strategies used today. Mobility ratio and reservoir heterogeneity have a considerable impact on oil recovery. Prediction of CO₂ breakthrough at the production wells seems to be more accurate when derived from the mobility ratio between CO₂ and reservoir oil. The degree of heterogeneity within the reservoir has a more direct impact on oil recovery and sweep efficiency over time. The volume of CO₂ being injected can eventually invade lower permeability regions, reducing the impact of reservoir heterogeneity on oil recovery. This concept should mobilize a larger volume of oil than a conventional volume-limited or WAG injection strategy that may bypass or block these lower permeability regions. Besides oil recovery, a reservoir's performance in this study is defined by its CO₂ injectivity over time. Elevated injection pressures associated with the large-volume CO₂ source can substantially impact the ability for an oil reservoir to store LVA CO₂. As CO₂, a less viscous fluid, replaces produced oil and water, the average reservoir pressure slowly declines which improves injectivity. This gradual improvement in injectivity is mostly occupied by the increasing volume of recycled CO₂. Sweep efficiency is critical towards minimizing the impact of CO₂ recycling on reservoir storage potential. Deep, large, and permeable oil reservoirs are more capable of accepting LVA CO₂, with less risk of fracturing the reservoir or overlying confining unit. The depth of the reservoir will directly dictate the injection pressure threshold in the oil reservoir as the fracture pressure increases with depth. If EOR operations are designed to sequester all the CO₂ delivered to the field, additional injection capacity and design strategies are needed.