Capillary Pressure and Wettability Behavior of Coal - Water - Carbon Dioxide System

Enhanced Coal Bed Methane (ECBM) recovery techniques, based on carbon dioxide injection, have been proposed for the recovery of larger fraction of methane in place. Laboratory studies and recent pilot field tests demonstrate that CO₂ injection has the potential to enhance coal bed methane recovery through displacement desorption mechanism with an added advantage that this technology can be employed to sequester large volumes of CO₂ in deep coal seem reservoirs, thereby reducing emissions of industrial CO₂ as a greenhouse gas.

It can be expected that carbon dioxide enhanced ECBM is detrimentally slow unless operated at high enough pressures such that the coal becomes carbon dioxide wet. Indeed in water wet coal the small fracture system remains almost entirely filled with water and kinetics of CO₂ adsorption and CH₄ desorption is determined by the slow diffusion in the relatively large aggregates of water filled fractures and coal. Capillary diffusion is of the order of hundred times faster and fills the small fractures with carbon dioxide proportionally faster provided that conditions are such that the coal is CO₂ wet.

For this reason we undertook to measure the capillary pressure of grounded coal between water and carbon dioxide. The grounded coal precisely exposes the coal surface of relevance here. A computer code is written, which simulates the capillary pressure experiment. The code makes it possible to get useful results before complete capillary pressure equilibrium in our laboratory cell have been attained.