Comparing carbon sequestration in an oil reservoir to sequestration in a brine formation—field study

Abstract

Abstract Geologic sequestration of CO 2 in an oil reservoir is generally considered a different category than sequestration in formations that contain only brine. In this paper, the significance and validity of this conceptualization are examined by comparing the performance of CO 2 injected into a depleted oil and gas reservoir with the performance of similar injection into non-oil-bearing sandstones using a field test at Cranfield field, Mississippi, as a case study. Residual oil and gas in the reservoir under miscible conditions reduces CO 2 breakthrough time and rate of pressure buildup as compared to a reservoir containing only brine. Dense wells provide improved assessment of oil reservoir quality, leading to improved prediction, as well as verification of CO 2 movement in this reservoir as compared with the sparsely characterized brine leg. Assessment of the difference made by the presence of residual oil and gas requires a good understanding of reservoir properties to predict oil and gas distribution. Stratal slicing, attribute analysis, and petrographic analyses are used to define the reservoir architecture. Real-time pressure response at a dedicated observation well and episodic pressure mapping have been conducted in the reservoir under flood since mid2008; comparison measurements are planned for 2009 in downdip environments lacking hydrocarbons. Model results using a CMG-GEM compositional simulator compare well in general to measured reservoir response under CO 2 flood; imperfections in the model match of flood history document uncertainties. Time-lapse Reservoir Saturation Tool (RST) logging is under way to validate fluid composition and migration models. Monitoring performance of the wells during injection of CO 2 suggests at this site that the value of wells for characterization exceeds the risk of leakage.