CO2 recycling accounting and EOR operation scheduling to assist in storage capacity assessment at a U.S. gulf coast depleted reservoir

Abstract

From the start of CO2 injection on July 2008, through August 31, 2011, ∼3 million metric tons of CO2 have been stored in the lower Tuscaloosa Formation at Cranfield, Mississippi. We used the field to illustrate the importance of two operational choices on CO2 storage capacity: (1) reinjection of CH4 with the recycled CO2; and (2) injection rate in the presence of a spill point in the storage structure. The overall amount stored in the reservoir is approximately the amount purchased, corrected for the presence of impurities. We present a procedure to estimate how much CO2 is stored in an arbitrary volume smaller than that of the reservoir. The mass stored is corrected for CH4 and accounts for produced and recycled CO2. The procedure was applied to estimate the amount of CO2 stored in two Cranfield compartments, which are separated by a non-transmissive fault. We also investigated factors impacting storage capacity in EOR-then-storage and pure-storage contexts, with both being constrained by non-desirable migration of CO2 out of the reservoir through a spill-point. As expected, the longer the EOR operations continue, the larger the amount stored relative to the pure storage case. Although never large, the 14% gain in a hypothetical simplified model of the Cranfield reservoir, is strongly dependent on the injection/production strategy used.