Capillary Pressure and Wettability Behavior of the Coal-Water-Carbon Dioxide System at High Pressures

Enhanced Coal Bed Methane (ECBM) combines enhanced recovery of CH4 from coal seams with simultaneous storage of CO2. The efficiency of ECBM depends on the transfer rate between cleats and coal matrix. Diffusive transport of CO2 in the small cleats is enhanced when the coal is CO2-wet. Some coals only become CO2-wet at higher pressures. Indeed for water-wet conditions the small fracture system is filled with water and the rate of CO2 adsorption and CH4 desorption is affected by slow diffusion 
of CO2.

This paper investigates the wetting behavior of coal, using capillary pressure measurements at in situ conditions. To facilitate the interpretation of these measurements we also obtain capillary pressure curves for an unconsolidated sand sample. We measure both the drainage and imbibition capillary pressure in the coal-water-CO2 system. For medium and high rank coal the primary drainage capillary pressure curves show a water-wet behavior. Secondary imbibition experiments show that the medium rank coal becomes more CO2-wet as the CO2 pressure increases. High rank coal is CO2-wet during primary imbibition. The imbibition behavior is in agreement with contact angle measurements. Hence we conclude that imbibition tests provide the most practically relevant data to evaluate the wetting properties of coal.