EVALUATION OF CO2 STORAGE CAPACITY IN AN UPPER KNOX GROUP SANDSTONE (CAMBRIAN-ORDOVICIAN KNOX GROUP), SOUTHERN ILLINOIS BASIN, KENTUCKY

The Kentucky Geological Survey drilled a test well to a total depth of 2,477 m in Hancock County, Kentucky, southern Illinois Basin, to test CO₂ injection and storage in deep saline reservoirs in the Knox Group. Testing was completed in two phases: phase 1 injected supercritical CO₂ in the entire Knox, whereas phase 2 focused on a sandstone in the upper Knox (previously termed Gunter Sandstone). A 49-m section of interbedded sandstone and dolomite was mechanically-isolated by plugging the wellbore with cement at 1,606 m, then cementing 14-cm (5½-in) casing from 1,470–1,536 m in the overlying dolomite section to ensure a seal for an inflatable injection packer run on tubing. Before a step-rate test to verify injection rates and pressures was conducted, static reservoir temperature and pressure of 40.9°C and 15.9 Mpa, respectively, were measured at 1,593 m pressure-gauge depth. Injection of 333 tonnes of supercritical CO₂ was followed by a second step-rate test that determined the fracture gradient in the reservoir to be 15.2 kPa/m (0.672 psi/ft), after which shut-in pressure fall-off was monitored. Pressure transient analysis modeled a two-porosity system in the reservoir, with average 9.7% porosity and 12.5 mD permeability. Near-wellbore permeability was likely enhanced by the previous CO₂ injection test. A net isopach map of the sandstone in the 221.8 km² evaluation area was constructed at the industry-standard 7% porosity cutoff. Average reservoir thickness at the cutoff was 23 m and sandstone/dolomite facies-weighted porosity was 10.2%. Supercritical CO₂ storage capacity was calculated using industry-standard methodology, with CO₂ density under reservoir conditions of 784 kg/m³ and using facies-weighted storage efficiencies. Supercritical CO₂ storage capacity ranged from P₁₀ capacity of 1.1 Mt/km² to P₉₀ capacity of 9.8 Mt/km², with a P₅₀ capacity of 3.8 Mt/km². P₅₀ storage capacity in the evaluation area is 8.4 Gt, or an average surface area of 26.2 ha required to achieve 1 Mt of supercritical CO₂ storage capacity. Due to the low fracture gradient and permeability of the Gunter, multiple wells would be required for an industrial-scale injection project. It is more likely, however, that the sandstone would contribute storage capacity to a stacked deep saline reservoir project developed in the larger Knox section.