Spontaneous imbibition and wettability characteristics of Powder River Basin coal

The wettability of coal is studied at scales that range from the microscopic to the core. While contact-angle measurements define wettability at microscopic (pore) and core scales, relative permeability curves define wettability and multiphase flow properties at core and reservoir scales. The microscopic wettability is evaluated based on the Frumkin–Derjaguin equation. Estimates for contact angles are made versus aqueous-phase pH. The calculations suggest a trend that goes through a maximum at a pH around 4. Contact angles become small at low and high pH suggesting an alteration of coal wettability with pH. Water imbibition studies indicate that the core-scale wetness has similar trends as those predicted for the pore scale. Contact angle goes through a maximum at a pH of around 7 and is low in highly acidic and basic solutions. X-ray computed tomography (CT) scanning based imbibition studies also provide a method of understanding multiphase flow in coal systems. Estimates for relative permeability of air–water flow in coal are obtained and reported. The results are encouraging as these are the first steps towards developing relative permeability curves for coal-methane and carbon-dioxide systems. These systems are important with regards to both CO2 sequestration as well as enhanced methane production.