Williston basin associated co2 storage field laboratory

The Williston Basin Associated CO2 Storage Field Laboratory (WBCFL), led by the Energy & Environmental Research Center (EERC) in partnership with Denbury (now ExxonMobil), was established to advance understanding of CO2 enhanced oil recovery (EOR) and associated geologic CO2 storage in the Williston Basin. The specific objective of the WBCFL was to conduct field and laboratory activities to inform decisionmakers regarding the 1) efficacy of associated storage in stacked geologic reservoirs during EOR operations; 2) CO2 injectivity, oil production, and storage in residual oil zones (ROZs); 3) use of a novel CO2 storage monitoring, verification, and accounting (MVA) technology (active seismoelectric [ASE] monitoring), and 4) life cycle and techno-economic factors for EOR development scenarios targeting ROZs. The project location was the Pennel Field in southeast Montana, which is part of the Cedar Creek Anticline (CCA) geologic structure within the Williston Basin. The field was selected because it was being developed by Denbury for tertiary-phase CO2 EOR targeting the Red River Formation, and the availability of CO2, which was being supplied by the Greencore Pipeline, offered opportunities to evaluate the EOR potential of the other oil-bearing formations within the structure. The focus of this project was a vertically stacked reservoir system, including the Interlake Formation, a complex carbonate interval with both main pay zones (MPZs) and underlying ROZs. ROZs are oil-bearing zones that have been naturally waterflooded and, while they often contain moderate oil saturations, they cannot be economically produced using conventional primary or secondary recovery. However, CO2 EOR of ROZs in the Permian Basin have been successful, as the injected CO2 can mobilize oil trapped in the reservoir by swelling the oil, reducing oil viscosity, and achieving miscibility under reservoir conditions. The initial project activities focused on data collection and preparation of the field demonstration within the Pennel Field. Denbury drilled, logged, and cored a new well, 23-18B, as a CO2 injector paired with a nearby production well, 13-18B. Samples were collected from the field location, including rock, brine, and oil samples, for subsequent analysis at the EERC and to support the laboratory testing, geomodelling, and reservoir simulation components of the project. Comprehensive laboratory analyses were performed to characterize produced oil and brine, evaluate CO2–oil miscibility, and measure interfacial tension and contact angles at reservoir temperature and pressure. Results showed that Interlake oils achieve miscibility with CO2 at approximately 2600–2800 psi, indicating potential suitability for CO2 EOR. Brine and carbonatedbrine testing highlighted modest but measurable changes in interfacial properties, while contact angle measurements indicated predominantly water-wet rock—an important condition that can strongly influence CO2 storage and oil displacement behavior. These laboratory results were integrated into a geologic model of the Pennel Field, constructed from well logs, core data, structural horizons, and petrophysical properties. The Interlake Formation model was subdivided into six depositional cycles that capture heterogeneity between the MPZ and ROZ intervals. A 1-sq-mi dynamic reservoir simulation model was vii developed from this geologic framework, incorporating five producing wells and the drilled injection well. Historical well performance, including more than five decades of production, was used to history-match reservoir behavior and calibrate the simulation model. Using the calibrated simulation model, the EERC evaluated three CO2 injection strategies: continuous CO2 injection, a 1:1 water alternating gas (WAG) scheme, and a tapered WAG schedule designed to reduce early gas breakthrough. Across all strategies, CO2 injection increased oil production relative to primary depletion, including meaningful contributions from the ROZ. The tapered WAG strategy produced the highest incremental oil recovery and the most efficient CO2 utilization, aided by improved reservoir pressure support and reduced early gas migration toward producers. Notably, Well 22-18B, located farther from the injector and historically more productive, significantly outperformed the closer 13-18B, both in projected incremental oil and in CO2 storage amount. Simulations suggested that developing CO2 EOR in the 1-sq-mi area of interest in the Interlake MPZ/ROZ system could yield 34,000–54,000 barrels of incremental oil at 13-18B and 181,000–217,000 barrels at 22-18B over the 5- and 6-year modeled injection periods. Associated CO2 storage ranged from 80,000 to over 190,000 tonnes, depending on the case, with WAG strategies storing a larger fraction of the injected CO2. These high storage rates reflect the strong retention characteristics of carbonate ROZs and their compatibility with miscible CO2 flooding. Dimensionless curve analysis was used to extend these simulation results to 15 years, enabling long-term projections for economic and life cycle analysis (LCA). A screening-level LCA and techno-economic analysis evaluated the environmental and economic impacts of each EOR scenario, integrating simulation-based forecasts of oil production, recycled CO2, and long-term storage. Results emphasized that CO2 EOR—particularly tapered WAG—can provide substantial net storage while increasing oil recovery, producing favorable CO₂ storage per barrel, and strengthening alignment with federal incentives such as the Section 45Q tax credit. The stacked-storage development concept, targeting both MPZ and ROZ horizons together, offers a pathway for maximizing combined resource recovery and geologic CO2 containment at legacy oil fields. To explore the feasibility of cost-effective monitoring strategies, the project deployed a baseline active seismoelectric survey—a method that detects electrical signals generated by seismic waves interacting with fluid-filled rock layers. Although deep detection (~8000 ft) proved challenging due to electromagnetic noise and signal attenuation, the survey established an important baseline and highlighted potential for shallower reservoir applications using improved acquisition geometry, denser receiver arrays, and refined filtering approaches. The work contributes valuable insights to the growing body of research on novel monitoring tools for CO2 EOR or permanent geologic storage. The final component of the project was for Denbury to perform the field test to validate the results of the modeling, simulation, and economic analysis; however, after Denbury was acquired by ExxonMobil in late 2023, the project experienced multiple delays in execution of the field pilot. In early 2025, the EERC was informed that the pilot project no longer aligned with Denbury/ExxonMobil’s priorities and that they were unable to continue the collaboration with the viii EERC and DOE. As a result, the field-based activities, including CO2 injection and subsequent monitoring and fluid sampling, were not completed. Despite the lack of data from a field test, the geologic, fluid composition, and production data collected from the field allowed for significant work to be performed, the results of which are relevant to future CO2 EOR projects across the county. In total, WBCFL provides a robust, transferable technical foundation for future CO2 EOR and storage developments in similar carbonate reservoirs that contain ROZs. Even without the execution of the planned field pilot, the project’s fluid-behavior studies, geologic modeling, dynamic simulation, monitoring evaluation, and integrated LCA/techno-economic results collectively demonstrate that ROZ-targeted CO2 EOR holds potential to produce incremental oil while achieving meaningful and secure CO2 storage. These findings support the continued investigation and testing of tapered WAG schedules, the optimization of producer/injector spacing, and the further development of cost-efficient monitoring technologies to advance the commercial viability of stacked CO2 storage systems across the Williston Basin and beyond.