CO2 EOR and Associated Storage in Residual Oil Zones: Modelling the Evolution and Significance of Oil Saturation in Residual Oil Zones

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Residual oil zones (ROZs) are reservoirs in which oil is largely at levels of residual saturation. Such reservoirs cannot be produced by conventional production techniques. ROZs in carbonate reservoirs in the Permian Basin of West Texas were initially interpreted (from wireline logs), as being productive oil zones. If these zones were completed for production, they produced largely water. Over the last decade it has been demonstrated that CO2 injection can make these zones economic to produce. As a result, ROZs in the Permian basin and elsewhere have become attractive targets for CO2-EOR (CO2 enhanced oil recovery) and have a large potential for CO2 sequestration consequential to the EOR activity. The viability of CO2 EOR in ROZs is currently being demonstrated by the results of CO2 injection into the ROZs at the Seminole, Wasson Denver Unit, and Goldsmith oil field. The recoverable oil from ROZs in both the San Andres and Canyon Reef formations of Permian Basin, have been estimated as 12 billion barrels. A key question is the capacity of ROZs to sequester CO2. The current project is the first study of ROZs based on extensive studies of cores, wireline logs, and production data from several ROZs in the San Andreas Formation. Understanding the magnitude of oil saturation and how it varies within ROZs is important to modelling both EOR and sequestration. The commonly accepted model for the formation of ROZ is based on the hydrodynamic effects of tectonically-controlled increased water flows in aquifer at the base of oil fields. In this work, the nature of this process was modelled using a commercial reservoir simulator. These flow simulations were designed to understand how the effects of strength of aquifer flow, flow direction, and capillary pressure on the nature and distribution of oil saturations in ROZs. A special emphasis was on understanding the impact of reservoir heterogeneity on the variation of capillary pressures throughout ROZs. Heterogeneities in capillary pressures appear to dominate the distribution of oil saturation within the ROZ and will also strongly influence the performance for both oil production from CO2 injection, as well as associated CO2 storage. Finally, we discuss the implications of our results to the understanding of both CO2 EOR and storage in ROZs.