SUITABILITY OF THE ST. FRANCOIS CONFINING UNIT AS A CAPROCK ABOVE CO2 INJECTION ZONES IN MISSOURI

We measured the hydraulic conductivity of the St. Francois confining unit (Derby-Doerun and Davis Formations) in southwest Missouri as part of the City Utilities Shallow Carbon Sequestration Project. These strata generally are thought to be a poor confining unit due to (purported) low shale contents and similar TDS values of pore fluids above and below these formations. Nevertheless, they form the caprock above a proposed CO₂ injection zone within the underlying St. Francois aquifer (Lammotte Sandstone), so a high degree of confinement is necessary for permitting and trial injections.

At the City Utilities Southwest Power Plant (Greene County, Missouri) we cored the St. Francois confining unit and completed shut-in packer tests in 6.5-m intervals spanning that unit. Visual and geophysical logging show that these strata are ~ 50% shale, and that interbedded carbonates (both limestone and dolomite) generally consist of dense, discrete clasts encased within shale. These characteristics are visually identical to those in mineral-exploration cores throughout southwest Missouri. The shut-in tests give (radial) hydraulic conductivities generally ranging from approximately 10^-12 - 10^-14 m/sec, and provide a (calculated) vertical hydraulic conductivity of ~10^-13 m/sec at the local scale. Thus, any significant leakage across this confining layer over short time frames would have to be along discrete avenues such as fault/shear zones.

This material is based on work sponsored by the Department of Energy National Energy National Energy Technology Laboratory under Award Number DE-NT0006642 to City Utilities of Springfield, MO and Award Number DEFG2610FE0001790 to Missouri State University.