An investigation of factors affecting the interaction of CO2 and CH4 on shale in Appalachian Basin

Depleted unconventional gas reservoirs have been proposed reservoirs for long-term storage of anthropogenic CO₂. The injection of CO₂ in such reservoirs may benefit from, (1) the presence of existing infrastructure and right-of-way to reduce sequestration costs, (2) the presence of an existing network of fractures to increase reservoir contact efficiency, and (3) potential to enhanced gas recovery using CO₂. However, there remain significant technical challenges and uncertainties about the behavior of these reservoirs, and how they might respond to CO₂ flooding. Toward addressing those uncertainties, the present study considers results of select experiments intended to improve understanding of the fundamental characteristics of shale matrix and shale interactions with methane and carbon dioxide. Outcrop samples from the low permeability sedimentary Marcellus formations in the Appalachian Basin of the eastern United States were characterized using various analytical techniques, including FTIR, XRD, ICP-OES, TOC analyzer, surface analysis, and pycnometry. FTIR confirmed CO₂ adsorption by appearance of an absorption band near 2349 cm⁻¹, however, CH₄ absorption band at 1303 cm⁻¹ was comparatively weak. Total organic carbon (TOC) exhibits significant statistical correlation with Cu, K, and Ni, while several other metals (As, Ba, Ca, Cd, Co, Cr, Fe, Mg, Mn, Na, Sr, and Ti) correlated with total inorganic carbon (TIC). Shale adsorption capacities of both CO₂ and CH₄ showed linear relationships to the organic matter content with CO₂ exhibiting consistently higher adsorption capacities than CH₄. At organic matter content greater than 2 wt%, the ratios of adsorption capacity of CO₂ over CH₄ were in a range between 1.3 and 1.9, which is similar to the ratios of critical temperatures between CO₂ and CH₄. This study evaluates the role of various physical and chemical parameters on CO₂/shale and CH₄/shale interaction, and considers implications for sequestration of CO₂ in depleted shale reservoirs.