Liquid plugging in in situ coal gasification processes

The presence of liquids can severely alter the spacial propagation characteristics of the flame front in an in situ coal gasification process. In a cocurrent burn, the liquids, water, and coal tars will be baked or pyrolyzed out of the coal in the hot zone near the flame front and will condense on cooler coal further downstream from the flame front. In the region where condensation is taking place, the liquids can plug the formation and thus alter or stop the gas flow pattern necessary for maintaining a spacially controlled flame front. Liquid plugging effects and their relationship to the permeability and the absolute crack sizes in the formation are discussed in a semiquantitative manner. The calculations presented are a rough guide to the requirements for preparing the coal seam with hydraulic or explosive fracturing, when such fracturing is needed.