Mechanistic Processes Controlling Gas Sorption in Shale Reservoirs

Abstract:

Utilization of CO₂ to stimulate natural gas production in previously fractured shale-dominated reservoirs where CO₂ remains in place for long-term storage may be an attractive new strategy for reducing the cost of managing anthropogenic CO₂. A preliminary analysis of capacities and potential revenues in US shale plays suggests nearly 390 tcf in additional gas recovery may be possible via CO₂ driven enhanced gas recovery. However, reservoir transmissivity properties, optimum gas recovery rates, and ultimate fate of CO₂ vary among reservoirs, potentially increasing operational costs and environmental risks. In this paper, we identify key mechanisms controlling the sorption of CH₄ and CO₂ onto phyllosilicates and processes occurring in mixed gas systems that have the potential of impacting fluid transfer and CO₂ storage in shale dominated formations. Through a unique set of in situ experimental techniques coupled with molecular-level simulations, we identify structural transformations occurring to clay minerals, optimal CO₂/CH₄ gas exchange conditions, and distinguish between adsorbed and intercalated gases in a mixed gas system. For example, based on in situ measurements with magic angle spinning NMR, intercalation of CO₂ within the montmorillonite structure occurs in CH₄/CO₂ gas mixtures containing low concentrations (<5 mol%) of CO₂. A stable montmorillonite structure dominates during exposure to pure CH₄ (90 bar), but expands upon titration of small fractions (1-3 mol%) of CO₂. Density functional theory was used to quantify the difference in sorption behavior between CO₂ and CH₄ and indicates complex interactions occurring between hydrated cations, CH₄, and CO₂. The authors will discuss potential impacts of these experimental results on CO₂-based hydrocarbon recovery processes.