The Midwest Geological Sequestration Consortium (MGSC) carried out a small-scale carbon dioxide (CO2) injection test in a sandstone within the Clore Formation (Mississippian System, Chesterian Series) in order to gauge the large-scale CO2 storage that might be realized from enhanced oil recovery (EOR) of mature Illinois Basin oil fields via miscible liquid CO2 flooding.As part of the MGSC-�s Validation Phase (Phase II) studies, the small injection pilot test was conducted at the Bald Unit site within the Mumford Hills Field in Posey County, southwestern Indiana, which was chosen for the project on the basis of site infrastructure and reservoir conditions. Geologic data on the target formation were extensive. Core analyses, porosity and permeability data, and geophysical logs from 40 wells were used to construct cross sections and structure contour and isopach maps in order to characterize and define the reservoir architecture of the target formation. A geocellular model of the reservoir was constructed to improve understanding of CO2 behavior in the subsurface. At the time of site selection, the Field was under secondary recovery through edge-water injection, but the wells selected for the pilot in the Bald Unit had been temporarily shut-in for several years. The most recently shut-in production well, which was surrounded by four nearby shut-in production wells in a five-spot pattern, was converted to CO2 injection for this pilot. Two additional wells outside the immediate fivespot pattern, one of which was an active producer, were instrumented to measure surface temperature and pressure. The CO2 injection period lasted from September 3, 2009, through December 14, 2010, with one three-month interruption caused by cessation of CO2 deliveries due to winter weather. Water was injected into the CO2 injection well during this period. A total of 6,300 tonnes (6,950 tons) of CO2 were injected into the reservoir at rates that generally ranged from 18 to 32 tonnes (20 to 35 tons) per day. The CO2 injection bottomhole pressure generally remained at 8.3 to 9.0 MPag (1,200 to 1,300 psig). The CO2 injection was followed by continued monitoring for nine months during post-CO2 water injection. A monitoring, verification, and accounting (MVA) program was designed to determine the fate of injected CO2. Extensive periodic sampling and analysis of brine, groundwater, and produced gases began before CO2 injection and continued through the monitored waterflood periods. Samples were gathered from production wells and three newly installed groundwater monitoring wells. Samples underwent geochemical and isotopic analyses to reveal any CO2-related changes. Groundwater and kinetic modeling and mineralogical analysis were also employed to better understand the long-term dynamics of CO2 in the reservoir. No CO2 leakage into groundwater was detected, and analysis of brine and gas chemistry made it possible to track the path of plume migration and infer geochemical reactions and trapping of CO2. Cased-hole logging did not detect any CO2 in the near-wellbore region.An increase in CO2 concentration was first detected in February 2010 from the gas present in the carboy during brine sampling; however, there was no appreciable gas volume associated with the detection of CO2 . The first indication of elevated gas rates from the commingled gas of the pilot-�s production wells occurred in July 2010 and reached a maximum of 0.36 tonnes/day (0.41 tons/day) in September 2010. An estimated 27 tonnes (30 tons) of CO2 were produced at the surface from the gas separator at the tank battery from September 3, 2009, through September 11, 2011, representing 0.5\% of the injected CO2. Consequently, 99.5\% of the injected CO2 was stored at the Bald Unit Field after nine months of post-CO2 injection monitoring. Project improved oil recovery (IOR) was estimated at 412 m3 (2,590 bbl) and CO2 EOR as 325 m3 (2,045 bbl), although estimation of an EOR baseline was difficult because recovery was also increased by preproject well work. These figures would have been higher if not for variations in oil production rate due to winter weather. Oil production rates did not return to pre-shut-in level after the lengthy winter injection hiatuses, but remained elevated relative to production rates immediately before the pilot. The pilot was designed to measure and record data that could be used to calibrate a reservoir simulation model of the Clore sandstone to project the EOR potential of a larger-scale project at the Bald Unit. A model calibrated to field data (including geologic data and oil and water production) was used to assess the full-field EOR potential of the Field. Projections based on these models indicated that full-field CO2injection for 20 years could have 12\% oil recovery or 27,000 scm (170,000 stb) with a CO2 net utilization of 4,900 scm/scm (31,000 scf/stb). The potential CO2 storage is estimated to be 193,600 to 277,450 tonnes (213,000 to 305,200 tons).