UNDERSTANDING GEOCHEMICAL INTERACTIONS RESULTING FROM SEQUESTRATION OF CO2 IN A DEPLETED OIL RESERVOIR

Sequestration or disposal of Carbon-dioxide (CO2) in deep, geologic formations is one of the options currently being explored to control the amount of CO2 introduced in the atmosphere and its impact on global environment. The target geologic formations in this category include, depleted oil and gas reservoirs, deep saline aquifers and un-mineable coal beds. Even though improved oil recovery operations have resulted in large-scale injection of CO2 in depleted oil reservoirs over past 4-5 decades, a number of unknowns still exist regarding the interactions between the host reservoir and injected CO2. Further detailed understanding of these interactions is necessary before this option can become a safe, economical and long-term feasible sequestration option. This paper describes results of the laboratory experiments and numerical simulations work performed as part of a DOE sponsored project on CO2 sequestration in a depleted oil reservoir. The main thrust of this project is characterization of CO2 migration through a field injection experiment. A depleted oil reservoir near Hobbs, NM was used for the field demonstration. Laboratory experiments were performed to understand the geochemical interactions that might result due to the introduction of CO2 in the reservoir. Rock samples from a core from the field were exposed to CO2 in the presence of brine from the reservoir. The experiments were carried out over a period of 19 months at 700 psi and 40 deg C. Pre- and post-experiment rock and fluid samples were characterized to understand mineralogic alterations. Rapid dissolution of carbonate minerals and later dissolution of plagioclase feldspars was observed. Trace amounts of clay growth accompanied with feldspar dissolution was also observed. Numerical simulations are currently being performed to develop a computational model for the geochemical interactions observed in the laboratory experiments. Coupled fluid-flow and geochemical reaction models will be developed for the field, based on the laboratory experiments and numerical simulations related to the experiments. These coupled models will ultimately be used to understand long-term impact of CO2 storage on this reservoir.