Geologic Carbon Storage Through Enhanced Oil Recovery

Abstract

The advancement of carbon capture technology combined with carbon dioxide (CO2) enhanced oil recovery (EOR) holds the promise of reducing the carbon footprint of coal-fired power plants and other industrial sources, while at the same time boosting production of oil. CO2 injection in deep formations has a long track record. Tertiary EOR with CO2 has its origins in West Texas in the 1970's, when CO2 was first used at large scale at the SACROC field to produce stranded oil following primary and secondary production (water flooding). Because CO2 mixes with oil and changes oil properties, CO2 floods are effective at producing additional oil following water flooding. Carbon dioxide is a valuable commodity both because of its ability to stimulate oil production from depleted reservoirs, and because of the limited volumes of naturally-sourced CO2 in the U.S. Therefore, during large-scale commercial floods, CO2 that is produced with oil during EOR is separated, compressed and re-injected and recycled numerous times. Venting to the atmosphere is a rare event, quantifiable, and constitutes an insignificant fraction of the injected CO2. The CO2 purchased mass, net any venting during EOR activity is sequestered in the reservoir by a combination of capillary, solution and physical trapping mechanisms. Approximately 600 million metric tonnes of purchased CO2 have been utilized in the southwest U.S. Permian Basin (PB) alone, the rough equivalent of 30 years worth of CO2 from a half dozen medium-sized coal-fired power plants.Although CO2 EOR technology is mature in the U.S., many reservoir targets have not been flooded because of limited CO2 supply. Moreover, very large newly discovered EOR resources, known as “residual oil zones” (ROZs) occur in naturally water-flooded intervals below the oil-water contact in reservoirs that possess pore space containing immobile oil. ROZs are also now being documented in geologic settings without overlying conventional oil and gas accumulations. ROZ exploration and production using CO2 promises the supplemental capacity to accept very large volumes of CO2 in order to access and produce the remaining immobilized oil.Many existing EOR sites may be ideal for sequestration because they: 1) provide known traps that have held hydrocarbons over geologic time, 2) provide existing CO2 transportation and injection infrastructure, 3) occur in areas where the general public widely accepts injection projects, 4) provide CO2 commoditization capability for capturing companies, 5) facilitate management of underground CO2 plumes, 6) have proven reservoir injectivity, 7) may offer additional stacked storage potential, and, 8) are advantageous for monitoring because of available well infrastructure, experienced service company presence, and dense pre-injection data.Despite these advantages, in order to assure long-term containment of CO2 for atmospheric purposes and related CO2 reduction credits, the following best practices will ensure credit for captured and sequestered CO2: 1) demonstrate the appropriateness of the reservoir and existing wells for long term CO2 storage (integrity of the reservoir and seal, and identifying/remediating existing penetrations that are historically documented as the highest risk for unexpected pathways for CO2 to the surface), 2) evaluate well construction practices to ensure they are compatible with long-term exposure to low pH fluids (carbonic acid), 3) account for the net CO2 volumes stored- separately from the volumes purchased and recycled, and 4) demonstrate the long-term “permanence” of the CO2 plume in the subsurface through flood surveillance, monitoring and careful site closure.EOR provides a readily available pathway to large volume storage though oil production offsetting major capital costs of capture facility and pipeline construction, boosting public acceptance through experience and community benefits. Moreover, after completion of EOR operations, sequestration activities can be continued via maximizing CO2 storage in the depleted field, and by injection into qualified and associated brine formations.