Capillary Pressure and Wettability Behavior of CO2 Sequestration in Coal at Elevated Pressures

Enhanced coalbed-methane (ECBM) recovery combines recovery of methane (CH(4)) from coal seams with storage of carbon dioxide (CO(2)). The efficiency of ECBM recovery depends oil the CO(2) transfer rate between the macrocleats, via the microcleats to the coal matrix. Diffusive transport of CO(2) in the small cleats is enhanced when the coal is CO(2)-wet. Indeed, for water-wet conditions, the small fracture system is filled with water and the rate of CO(2) sorption and CH(4) desorption is affected by slow diffusion of CO(2). This work investigates the wetting behavior of coal using capillary pressures between CO(2) and water, measured continuously as a function of Water saturation at in-situ conditions. To facilitate the interpretation of the coal measurements, We also obtain capillary pressure curves for unconsolidated-sand samples. For medium- and high-rank coal, the primary drainage capillary pressure curves show a water-wet behavior. Secondary forced-imbibition experiments show that the medium-rank coal becomes CO(2)-wet as the CO(2) pressure increases. High-rank coal is CO(2)-wet during primary imbibition. The imbibition behavior is in agreement With contact-angle measurements. Hence. We conclude that imbibition tests provide the practically relevant data to evaluate the wetting properties of coal.