How Does CO<sub>2</sub> Adsorption Alter Coal Wettability? Implications for CO<sub>2</sub> Geo‐Sequestration
Xiaoxiao Sun, Yanbin Yao, Dameng Liu, Derek Elsworth
Abstract
Abstract Successful sequestration of CO 2 into coalbeds relies on sufficient capacity and rates of uptake. Storage volumes are controlled by CO 2 adsorption which in turn is affected in a complex manner by the evolving wettability of the sorption surface. However, mechanisms and interrelations between CO 2 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 CO 2 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 H 2 O‐solid‐CO 2 surface to clarify the relationship between CO 2 adsorption and coal wettability – using measurements of adsorbed H 2 O, alone. This enables the first direct evaluation of coal wettability at in situ pressures of CO 2 . Constrained observations suggest that water wettability weakens significantly with increasing CO 2 pressure. Under low CO 2 pressure, changes in wettability are contributed directly by CO 2 adsorption and increases in CO 2 density ‐ when CO 2 adsorption reaches saturation at high gas pressure, then changes are determined primarily by changes in CO 2 density. We document a robust method and results for the accurate prediction of CO 2 storage capacity in coalbeds and concomitant enhanced methane recovery.