CONTROLS ON CO2 STORAGE IN ORGANIC-RICH SEDIMENTARY ROCKS: A COMPARISON OF COAL AND SHALE

Organic-rich sedimentary rocks have significant capacity for the geologic storage of CO₂, and injection of CO₂ has significant potential as a technology to enhance the recovery of hydrocarbons from unconventional coal and shale reservoirs. These strata have gas storage and permeability characteristics that differ significantly from those in conventional hydrocarbon reservoirs and saline formations because large volumes of gas can be adsorbed on organic matter and because fluid movement within rock matrix is influenced by non-Darcy processes, such as diffusion and slippage.

Adsorption isotherms indicate that thermal maturity, maceral composition, and mineral matter are dominant controls on storage capacity in coal. Hydrolysis is an important mechanism of CO₂ storage in lignite and subbituminous coal, which have high inherent moisture. By contrast, organic carbon content is the dominant control on the adsorption capacity of shale, and high water saturation over a great range of thermal maturity makes hydrolysis a critical consideration for geologic CO₂ storage, even in the dry gas window. Free gas storage as governed by ideal gas law is as important as adsorption in shale, and so understanding the relationship between free and adsorbed gas is critical for accurate assessment of gas mobility.

Permeability in coal is highly stress-sensitive and tends to decrease exponentially from 1,000 to < 1 mD as overburden stress increases. Permeability also is sensitive to changes of matrix volume driven by the adsorption and desorption of gases and may change by more than an order of magnitude over the life of a well. Permeability in shale is typically lower than 0.1 µD and correlates strongly with organic matter content. Shale is substantially less stress-sensitive than coal; however, significant stress-sensitivity of gas-slippage in shale has been demonstrated, indicating that it is a significant consideration for the application of carbon storage and enhanced recovery technology.