An Emerging CO2 Storage Option: CO2 Storage and By-Product Oil Recovery from Shale Oil Formations

Recent studies, sponsored by the United States Energy Association (USEA) and prepared by Advanced Resources International, have identified an emerging CO2 storage option – injecting and storing CO2 in shale oil formations. Because this CO2 storage option also provides by-product oil recovery, it offers a source of revenues and thus a potential “business case” for pursuing CO2 capture and storage.<br><br>Shale oil (also called tight oil) has become the dominant source of U.S. oil production, increasing from less than 1 million barrels per day (MMB/D) a decade ago to about 8 MMB/D at the end of 2019. In addition, large shale oil basins and resources exist in many parts of the world, including Argentina, Australia, Canada, China, North and South Africa, and other settings, as evaluated by our company for the U.S. Energy Information Administration. (World Shale Gas Resources: An Initial Assessment of 14 Regions Outside the United States, April 2011) Because of ultra-low permeability, oil recovery efficiency from shale oil formations using primary (pressure depletion) practices is low, on the order of 5 to 10% of the original oil in-place (OOIP). This leaves behind a large resource target for application of more advanced shale oil recovery technologies.<br><br>Use of advanced recovery practices involving the injection of CO2 into shale oil formations (Shale EOR) provides one such technology and thus an additional pathway for economically viable storage of CO2. While a series of CO2 injection pilots are underway in U.S. shale oil basins, industry has held the results of these pilots “close to the vest.” <br><br>To overcome this lack of information, the USEA sponsored a series of studies, involving the use of geologic characterization and reservoir engineering, to examine the potential of Shale EOR in four major U.S. shale oil basins - the Bakken Shale, the Eagle Ford Shale, and the Wolfcamp Shale in the Midland and Delaware Basins of the Permian Basin. This work showed that cyclic injection of CO2 can lead to additional recovery of oil from these shale oil formations while trapping and storing significant volumes of CO2. The study involved building a geologic model for a representative portion of each shale oil formation, history matching reservoir properties with well completion practices against actual well performance, and using a compositional reservoir simulator (GEM) to calculate oil recovery and CO2 storage from Shale EOR.<br><br>Shale reservoir characterization and modeling for a “type area” of the Wolfcamp Shale in the Midland Basin illustrates the methodology and results of the Shale EOR study. With 12 cycles of CO2 injection, soak and production (over 8.5 years), Shale EOR stored 102,000 metric tons of CO2 in a representative 180-acre well pattern of the Wolfcamp Shale, while producing 223,000 barrels of shale oil. The 223,000 barrels of incremental oil, in addition to 355,000 barrels from primary oil recovery, provided an uplift of 63% in oil recovery. Using an additional CO2 injection cycle followed by well shut-in, Shale EOR stored 155,000 metric tons of CO2 in this 180-acre Midland Basin’s Wolfcamp Shale “type pattern”, equal to 0.7 metric tons of CO2 stored for every barrel of incremental shale oil produced. <br><br>Similar evaluations of cyclic CO2 injection in other shale basins also showed that with Shale EOR more CO2 could be stored than the CO2 content in the oil produced. The Bakken Shale had a CO2 stored to oil produced ratio of 0.62 metric tons per barrel, the Eagle Ford Shale had a ratio of 0.39, and the Wolfcamp Shale of the Delaware Basin had a ratio of 0.69. As such, given that the combusted portion of a barrel of oil creates 0.38 metric tons of CO2, Shale EOR enables more CO2 to be stored than the CO2 content in a barrel of oil when combusted. (Approximately 12% of an oil barrel is used for petrochemicals and other non-combustion sources.)<br><br>The study noted that special geologic conditions and development practices are important for optimum application of the cyclic CO2 injection and storage process. Future pathways being evaluated for this emerging CO2 storage option and a potential new “business case” for CCUS include: (1) examining continuous, rather than cyclic, injection of CO2 and (2) using conformance and other CO2 storage enhancing technologies. These technologies would appreciably increase the volumes of CO2 able to be stored as well as improve the oil recovery efficiency of Shale EOR.