Increasing energy demands and tighter controls governing CO2 emissions is spurring development of unconventional fuel resources, primarily focused on shale gas and other low permeable reservoirs. Injection of CO2 into shales, both to drive additional gas recovery and to permanently isolate the CO2 from the atmosphere, could provide an economic incentive as well as a permanent storage option for CO2. Experimental research into mechanisms controlling gas (CH4 and CO2) adsorption and desorption onto important shale minerals (i.e., illite, smectite, kaolinite, and chlorite) reveal a dynamic system that is not well understood. In this paper, we demonstrate, through a unique set of in situ experimental techniques coupled with atomistic simulations, distinctive interactions occurring between clay minerals and gases at reservoir conditions. For example, surface-sorbed concentrations of CO2 increase linearly as a function of pressure, with a slight rise in the rate of sorption under supercritical conditions. After a maximum surface concentration is researched, desorption dominates, and the sorbed surface concentrations decrease. The peculiarly shaped isotherm is explained by density function theory (DFT) simulations, which reveal a significant change in surface binding energies for CO2. This type of sorption-desorption behavior is not observed in the CH4 system and thus could play an important role in reservoir management during CO2 enhanced gas recovery (EGR) operations. Other notable findings include structural changes occurring to montmorillonites when the clays are exposed to scCO2. High-pressure infrared spectroscopy experiments identified maximum sorbed concentrations of CO2 on a Ca-saturated montmorillonite as a function of adsorbed water content, and these concentrations correlate with structural changes observed by in situ x-ray diffraction. Based on DFT, the developing intermolecular CO2 interactions under supercritical conditions as well as the cation-CO2 interactions are responsible for CO2 expanding clay structures. The authors will discuss potential impacts of these experimental results on CO2-based hydrocarbon recovery processes.

Dominant interactions between clay minerals and sorbing gases relevant to shale gas reservoirs

248th National Meeting (2014)

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