The United States has large reserves of methane in shale-gas and tight-gas reservoirs that is usually extracted via the hydrofracking, a controversial technology that has been linked to adverse environmental consequences. A molecular-scale understanding of how methane and carbon dioxide at the hydrostatic conditions of a typical reservoir interact with the organic and inorganic components of shale, whose inorganic porosity is generally dominated by smectite, is crucial to design and implementation of cleaner, more effective extraction methods such as methane recovery via injection of supercritical carbon dioxide (scCO2). Here we compare the results of novel in situ spectroscopic and diffraction investigations of supercritical methane and carbon dioxide interactions with a proxy for the smectitic layers (the smectite clay hectorite) at 90 bar and 323 K (equivalent to ~1 km depth). The data show that carbon dioxide can actively intercalate and expand the smectite phase when the charge-balancing cation has a large radius and low affinity for H2O or when a small amount of water is present to prop the interlayer slightly open. In contrast, methane is intercalated via a passive space-filling mechanism only once the interlayer has expanded via adsorption of H2O such that it is large enough to fit the methane molecule (~11.5 ± 0.3 Å). Intercalated CO2 is a maximum when the basal spacing is equivalent to that of a monolayer hydrate (~12.5 Å) and decreases with increasing relative humidity of the scCO2 phase for all cations. However, we find evidence of some residual CO2 in the interlayer at all RH with the highest adsorption always associated with charge-balancing cations with large radii and low hydration energy. Intercalated methane is at a maximum in systems with large interlayer void volume and at a basal spacing of 11.5-11.8 Å, but it is rapidly displaced from both interlayer and external surface adsorption sites by H2O with increasing fluid relative humidity regardless of the interlayer cation. The dynamics of adsorbed CO2 are significantly affected by the smectite, with adsorbed CO2 experiencing restricted dynamic motion while the dynamics of methane do not appear to be affected as strongly. Implications for developing supercritical CO2-based shale-gas extraction will also be discussed.

Understanding alternative shale gas extraction technologies: Comparing and contrasting supercritical methane and carbon dioxide interactions with smectite clay on the molecular scale using in situ spectroscopy and diffraction

255th National Meeting (2018)

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