GEOMECHANICAL PROPERTIES OF THE LOWER COPPER RIDGE DOLOMITE (UPPER CAMBRIAN–LOWER ORDOVICIAN KNOX GROUP), A POTENTIAL CO2 SEQUESTRATION RESERVOIR IN THE APPALACHIAN BASIN, NORTHEASTERN KENTUCKY

The Kentucky Geological Survey 1 Hanson Aggregates stratigraphic research well was drilled in Carter County, Kentucky, in the southern Appalachian Basin, to determine the CO₂ storage capacity of the Upper Cambrian–Lower Ordovician Knox Group. The lower Copper Ridge Dolomite of the Knox has long served as a reservoir for the disposal of produced brine and hazardous liquids in the Appalachian Basin of Ohio and Kentucky. Thin porous and permeable intervals occur throughout the 260-m-thick lower Copper Ridge, but CO₂ storage reservoir-quality intervals of >7% porosity and >30 mD permeability are largely confined to the interval of 1143–1203 m in the wellbore. Overlying upper Copper Ridge is the primary confining caprock for fluids injected into the lower Copper Ridge. In any CO₂ storage project, fracturing the reservoir and loss of caprock integrity are risks that can be quantified from the geomechanical properties of the strata. Vertical stress gradient in the wellbore calculated from the formation density nuclear log, was 26.2 MPa/km. Open and drilling-induced fractures in the lower Copper Ridge section interpreted from the formation imaging log identified the principal horizontal stress orientation to be N32°E. Borehole breakouts followed the minimum stress axis orientation of N58°W. Geomechanical properties of the lower Copper Ridge were calculated from the formation density and compressive and shear acoustic velocities geophysical logs, and calibrated to values measured in core plugs to assess fracturing risk. Values of Young’s modulus and Poisson’s ratio suggested that the reservoir interval may be prone to fracturing. A step-rate test was conducted to determine lower Copper Ridge injectivity in a mechanically isolated interval of 1126–1202 m. The test interval fractured at 15.8 MPa, yielding a fracture gradient of 13.5 MPa/km. Additional pumping reached a final injection pressure of 16.3 MPa. Fracture closure pressure was 13.1 MPa. Average test interval permeability calculated from pressure falloff data was 44.8 mD and skin factor was −4.42, indicative of near-wellbore fracturing. Therefore, step-rate testing before injection of CO₂ and pressure monitoring during injection would be prudent for a CO₂ storage reservoir developed in the lower Copper Ridge.