ANALYSIS OF THE DEVONIAN BLACK SHALE IN KENTUCKY FOR POTENTIAL CARBON DIOXIDE SEQUESTRATION AND ENHANCED NATURAL GAS PRODUCTION

Shales are often regarded as the reservoir seal in many geologic sequestration scenarios. Injecting CO₂ into organic-rich gas shales is an untested carbon management strategy. Devonian black shales underlie approximately two-thirds of Kentucky. These shales are the source and trap for large quantities of natural gas. Most of this natural gas is adsorbed on clay and kerogen surfaces, analogous to methane storage in coal beds. In coals, CO₂ is preferentially adsorbed, displacing methane. Black shales may similarly desorb methane in the presence of CO₂.

Current research is investigating the black, organic-rich Devonian shales as a potentially significant geologic sink for CO₂. Drill cuttings from the Kentucky Geological Survey Well Sample and Core Library are being sampled. CO₂ adsorption isotherms are being collected to determine the gas-storage potential of the shale and to identify optimum shale facies. Sidewall core samples have been acquired to investigate specific black-shale facies, their potential CO₂ uptake, and the resulting displacement of methane. Advanced logging techniques (ECS) are being investigated for possible correlations between adsorption capacity and geophysical log measurements.

Average random vitrinite reflectance (R_0random) data range from 0.78 to 1.59 in the upper oil to wet gas and condensate hydrocarbon maturity range. Total organic content determined from acid-washed samples ranges from 0.69 to 4.62 percent. CO₂ adsorption capacities at 400 psi range from a low of 19 standard cubic feet per ton of shale (scf/ton) in the less organic-rich zones to more than 86 scf/ton in the Lower Huron Member of the shale.

Initial estimates based on these data indicate a sequestration capacity of 5.3 billion tons of CO₂ in the Lower Huron Member of the Ohio Shale of eastern Kentucky and as much as 28 billion tons total in the deeper and thicker parts of the Devonian shales in Kentucky. Should the black shales of Kentucky prove to be a viable geologic sink for CO₂, their extensive occurrence in Paleozoic basins across North America would make them an attractive regional target for economic CO₂ storage and enhanced natural gas production.