How Does CO₂ Adsorption Alter Coal Wettability? Implications for CO₂ Geo-Sequestration

Successful sequestration of CO₂ into coalbeds relies on sufficient capacity and rates of uptake. Storage volumes are controlled by CO₂ adsorption which in turn is affected in a complex manner by the evolving wettability of the sorption surface. However, mechanisms and interrelations between CO₂ adsorption and coal wettability remain poorly constrained – especially under recreated in situ reservoir conditions where measurements are difficult. We circumvent this difficulty by combining direct measurements of adsorbed water and inferred wettability through Nuclear Magnetic Resonance spectroscopy with mechanisms recovered from molecular dynamic (MD) simulations. The MD simulations confirm that CO₂ gas molecules adsorb to the coal pore surface, partially displace the adsorbed water, and transform the coal surface into a heterogeneous surface comprising solid interspersed with gas pockets. We then use the Cassie-Baxter equation as a basis to characterize the wettability of this heterogeneous H₂O-solid-CO₂ surface to clarify the relationship between CO₂ adsorption and coal wettability – using measurements of adsorbed H₂O, alone. This enables the first direct evaluation of coal wettability at in situ pressures of CO₂. Constrained observations suggest that water wettability weakens significantly with increasing CO₂ pressure. Under low CO₂ pressure, changes in wettability are contributed directly by CO₂ adsorption and increases in CO₂ density - when CO₂ adsorption reaches saturation at high gas pressure, then changes are determined primarily by changes in CO₂ density. We document a robust method and results for the accurate prediction of CO₂ storage capacity in coalbeds and concomitant enhanced methane recovery.