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

Abstract

AbstractSuccessful 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.