AN INITIAL SET OF WORKING HYPOTHESES CONCERNING SOME CHEMICAL AND PHYSICAL EVENTS WHEN CO2 IS INJECTED INTO A COALBED

An initial set of working hypotheses concerning the effects of supercritical CO2 injection on both the organic and mineral matter in coal will be presented. This will include a discussion of the displacement of CH4, the adsorption and imbibition of CO2 into the coal, coal swelling, extraction of small organic molecules trapped in the coal macromolecular network, the migration of the extracted molecules through the coal and their effect on permeability, the concept of threshold pressure, as well as the breaking and formation of weak intermolecular interactions between adjacent macromolecules and between the macromolecules and CO2. The presence of high pressure CO2 and water can also have a profound effect on the mineral matter present in coal. Many minerals present in coal are soluble in acidic aqueous solutions. The solubility of CO2 in water at various temperatures as a function of pressure and the reaction of CO2 with water to form carbonic acid will be described. As pressure increases, the solubility of CO2 increases, and in general, as the temperature decreases, CO2 solubility increases. As pressure increases, pH decreases. As temperature increases, pH increases. Many of the minerals present in coal are soluble in acidic solutions. The Alkaline Earth metals are removed from coal by treating coal with acidic aqueous solutions. We expect calcite, dolomite, and other carbonate minerals to be removed from coal when it is in contact with acidic aqueous solutions of carbonic acid. Other minerals and clays may also be removed. The solubility trends of a number of minerals common to coal will be described. Lastly, there have been a number of studies of the chemical and physical effects of CO2 injection on reservoir rocks. These previous studies will be briefly reviewed in an attempt to glean useful information concerning the possible effects of injection of supercritical CO2 into coal.