Regional Assessment of Suitability of Organic-Rich Gas Shales for Carbon Sequestration

Abstract

In gas shales, natural gas occurs both as free gas in intergranular and fracture porosity and as an adsorbed phase onto the surfaces of clays and organic matter, analogous to natural gas storage in coalbeds. The adsorption capacity of shales from Kentucky, Indiana, and West Virginia was estimated using drill cuttings and sidewall cores to determine both CO 2and CH 4adsorption isotherms. Elemental capture spectroscopy logs were analyzed to investigate possible correlations between adsorption capacity and mineralogy.

The maturity of the shale was characterized using average random vitrinite reflectance data yielding values ranging from 0.78 to 1.59 (upper oil to wet gas and condensate hydrocarbon maturity values). Total organic carbon (TOC) content ranges from 0.69 to 14%. Calculated CO 2adsorption capacities at 2.75 MPa range from a low of 0.4 m 3/t (14.1 ft 3/t) to more than 4.2 m 3/t (148.3 ft 3/t). A direct linear correlation between measured TOC and the adsorption capacity of the shale has been determined; CO 2adsorption capacity increases with increasing TOC. Data also suggest that CO 2is preferentially adsorbed (5.3:1) and would displace CH 4, leading to a potential method for enhancing natural gas recovery in gas shales.

Initial estimates of the volume of CO 2sequesterable in the shale based on these data indicate a capacity of as much as 25 billion t in the deeper and thicker parts of the Devonian shales across Kentucky. Discounting the uncertainties in reservoir volume and injection efficiency, these results indicate that gas shales could provide a potentially large geologic sink for CO 2. Moreover, the extensive occurrence of gas shales in Paleozoic and Mesozoic basins across North America makes them an attractive regional target for economic CO 2storage and enhanced natural gas production.