Modeling commercial-scale CO₂ injection in Mt. Simon Sandstone near Decatur, IL storage sites

Advancing carbon capture and storage (CCS) to commercialization necessitates exploiting CO2 storage resources efficiently, which likely will involve multiple injection sites (or entire projects) accessing the same regional storage complex. This situation is presently occurring in central Illinois, where a series of three U.S. Department of Energy-sponsored storage projects are utilizing the Cambrian Mt. Simon Sandstone storage complex near Decatur, Illinois. From 2011 to 2014, the Illinois Basin – Decatur Project (IBDP) injected CO2 at a rate of 0.33 million tonnes per year (Mt/yr), and in 2017, the Illinois Industrial CCS Project (IL-ICCS) began injecting approximately 1 Mt/yr (planned rate) in a well 1.6 km apart. Recently, the CarbonSAFE Illinois – Macon County Project began investigating whether more than 50 Mt of CO2 can be injected into the Mt. Simon Sandstone over several decades (a target rate of 2 Mt/yr) located at a site about 5 km northwest of the other injection locations. As evaluated for this study, the Macon County well would be within the Forsyth Oil Field and drilled to access the same interval of the Mt. Simon storage complex as used by the other injection projects, namely the lower Mt Simon Sandstone. The intent of this paper is to assess the feasibility of the Forsyth site for commercial CO2 storage to further develop the storage resource by evaluating whether the target rate can be achieved, designing an injector to meet the target rate, and describing the potential contact of subsurface plumes resulting from these separate but proximal central Illinois projects. This entails modelling the behaviour of the injected CO2 and evaluating the well perforated intervals, plume sizes, pressure responses, and contact with existing CO2 plumes at the Decatur sites. To understand the interaction of three subsurface plumes, a static geologic model was generated to encompass the proposed Forsyth site (CarbonSAFE project) and the existing Decatur sites (IBDP and IL-ICCS projects), an area 22.5 × 22.5 km² and a vertical section of 664 m. The static model was developed to represent the geological architecture and heterogeneity of the Mt. Simon storage complex at the Forsyth and Decatur sites through regional correlations based on geophysical data from wells at the Decatur sites. A dynamic model with integrated fluid and rock–fluid properties was constructed from the static model. The primary CO2 storage mechanisms modeled were structural and stratigraphic trapping, residual gas trapping, and solubility trapping. Mineral trapping resulting from geochemical reactions among CO2, rock, and formation water were considered negligible because the target formation is quartz sandstone. To reflect current plans at each of the three sites, the following CO2 injection schedule was modeled: CO2 was injected via two existing injectors, CCS#1 and CCS#2, at Decatur sites, followed by a planned injection, CCS#3, at Forsyth site. The CO2 injection began via CCS#1 at 0.33 Mt/yr for 3 yrs (from 2011 to 2014), then CCS#2 at 1 Mt/yr for 5 yr (from 2017 to 2023), and followed by CCS#3 at 2 Mt/yr for 25 yr (from 2025 to 2050) to meet the projected commercial injection target of 50 Mt. A number of cases with a perforated interval ranging from 5 to 91 m were simulated to determine the perforated interval for the planned well required to achieve the target injection rate, minimize the build-up of well bottomhole pressure, and minimize the plume size. For a selected perforated interval, the CO2 plume size and injection zone pressure (entire gridded model pressure) response were evaluated at the end of injection and 20 yr afterward (the time required for injection zone pressure to reach the pre-injection level). Simulation results showed that no plume contact occurred between CO2 injected at the Forsyth and Decatur sites, and that the average injection zone pressure (entire gridded model pressure) increased less than 4%. This relatively small pressure increase suggests that commercial stable injection of CO2 (ca 50 Mt) via the planned well is highly feasible. A minimum perforated interval of 30 ft (9 m) in the lower Mt. Simon is needed to achieve the target rate of 2 Mt/yr. However, the short perforation interval resulted in the most increase (27%) in well bottoomhole pressure. Generally, injection pressure decreases with increasing perforated interval and permeability thickness. A perforated interval ranging from 100 to 200 ft (30.5 to 61 m) was recommended considering both minimizing the pressure buildup and plume size and lowering the well completion costs. For a 200 ft (61 m) perforated interval, 50 Mt of CO2 injection at Forsyth resulted in a CO2 plume with an equivalent radius of 1782 m and a height of 372 m. This plume at Forsyth site did not contact with the plumes at Decatur sites, which had an equivalent radius of 635 m and a height of 364 m. The estimated area of review (AoR) at Forsyth, based on a 91 × 91 × (4.6–12.2) m³ cell pressure increase of 100 psi (689 kPa), was 192 km² (a radius of 7.8 km) at the end of 50 Mt of CO2 injection. Simulation results indicated that at 20 yr post-injection, the CO2 plumes at Forsyth and Decatur sites would extend laterally by less than 20% of the plume at the end of CO2 injection and that the injection zone pressure would return to its pre-injection level. Results of this study suggest multiple CO2 projects can be designed to exploit a regional storage complex. For commercial-scale injection of 50 Mt of CO2 in the Mt. Simon storage complex in central Illinois, no plume contact is expected for projects located 5 km apart. The characterization and design of sites are paramount for the co-usage of storage complexes, which presents a unique opportunity to commercialize CO2 storage.