Potential for CO2 Sequestration in Coal and Shale

Unconventional hydrocarbon reservoirs viz. coals and shales, beyond their importance as a vast and cleaner energy option, provides the dual benefit of sequestrating CO2, owing to their gas storage properties within the microporous structures. Organic matter type and its thermal maturity primarily controls gas storage capacity of coals and shales, however, additional factors viz. mineral matter type, moisture content, temperature etc., can critically impact the overall gas holding capacity of the reservoir. Results clearly demonstrates that organic matter in coals and shales have several times higher affinity towards CO2, than for CH4, allowing competitive adsorption between the two gases and when CO2 is injected into producing wells at appropriate stages, it is preferentially adsorbed by the organic matter, releasing additional CH4, and permanently sequestrating the injected CO2. In this chapter we detail the mechanisms of gas storage in coals and shales. CO2, when injected in deep reservoirs, beyond a critical temperature and pressure, behaves as a supercritical phase (SC-CO2). SC-CO2 can significantly impact the geomechanics, wettability, and petrophysics of these reservoirs, which can have certain implications towards subsurface storage of injected CO2. In the later part of the chapter, we discuss the above-mentioned points in great details, in addition to the usage of SC-CO2 as a drilling and fracturing fluid.