The role of methane in limiting CO₂ EOR : case study of offshore Gulf of Mexico oil reservoirs




Ogbuabuo, Prisca Chinwendu

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Data from the US Department of Interior - Bureau of Ocean and Energy Management - 2012 Offshore Gulf of Mexico Atlas were analyzed to: (i) compute reconnaissance-level estimates of CO₂ volumes for storage in sub-seabed offshore Gulf of Mexico (GoM) oil sands before and after carbon dioxide (CO₂) enhanced oil recovery (EOR), (ii) investigative technical and economic impacts of CO₂ injection in gas-rich offshore GoM hydrocarbon fields, and (iii) analyze legal issues and framework associated with offshore geologic sequestration or storage (GS). Part (i) of this study, Reconnaissance-level estimation of CO₂ sub-seabed GS potential in offshore GoM, builds on a similar study conducted by The University of Texas at Austin, Bureau of Economic Geology on potential onshore CO₂ GS in the GoM region, published in Nunez-Lopez et al. (2008). Part (ii) focuses on the use of two screening methodologies to investigate the impact of native methane (CH₄) in recycled CO₂. The impact of CH₄ on the effectiveness of CO₂ as a solvent for EOR is defined by: Calculating minimum miscibility pressure (MMP) of pure CO₂ for each oil sand (conventional oil reservoirs), Computing impure CO₂ MMP for each oil sand considering only native CH₄ as an impurity and neglecting other trace gas components in the oil reservoir. Five to 50 mole percent CH₄ impurity factor was computed as a function of the pseudocritical temperature (T[subscript pc]) of the CH₄-CO₂ mixture. Plotting miscibility against sub-seabed depth, total depth, play type, and API gravity. Part (iii) analyzes existing US outer continental shelf (OCS) regulations under the authority of the US Department of the Interior stated in Title 30 CFR Part 250 and Part 550 to determine their applicability to carbon capture, offshore GS, and CO₂ EOR. The study results show a potential storage capacity of approximately 3.5 billion metric tons of CO₂ after CO₂ EOR for the 3,598 offshore GoM individual oil sands assessed in Part (i). For Part (ii), results indicate that deeper reservoirs are most tolerant to miscible impure CO₂ EOR. Of the play types defined by the BOEM, fan and fold belt plays are most tolerant to impure CO₂ flooding. Further study on the impact of impure CO₂ on MMP resulted in a definition of 18 mole percent as the cutoff for economic and technically viable CO₂ flooding in offshore GoM oil fields. When a hypothetical CO₂ injection stream exceeded 18 mole percent CH₄ contamination, 72% of the case study oil reservoirs became immiscible. In Part (iii), policies that address offshore CO₂ GS, CO₂ EOR, and both price based and non-price based mechanisms in the OCS would accelerate a shift towards implementing GS and CO₂ EOR in offshore GoM.



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