Ground-Water Effects Of Underground Coal Gasification Experiments In Northeastern Wyoming

Abstract

Abstract The Lawrence Livermore Laboratory (LLL) carried out two underground coal gasification (UCG) experiments at its Hoe Creek site in Northeastern Wyoming. We are conducting environmental studies in conjunction with these UCG experiments, including an investigation of changes in local ground-water quality. Such changes are a consequence of the residual reaction products coal ash, char, some of the coal tars, and roughly 10% of the product gases - which remain underground after gasification. When ground water returns to the gasification zone, leaching and dissolution processes lead to the formation of a plume of contaminated ground water, which begins to move through the coal seam. In addition, fissuring and roof collapse may destroy the integrity of the underground "reaction vessel" permitting contaminants to escape to the surface permitting contaminants to escape to the surface or into overlying aquifers. Our field investigations include ground-water quality monitoring near the UCG experiments, geotechnical measurements of ground deformations, and Post-burn coring. Limited water quality analysis is performed in the field, and more extensive analysis, including GC-MS, is carried out at LLL and U.S. Geological Survey laboratories. The field program is augmented by laboratory measurements and program is augmented by laboratory measurements and modeling studies. We have monitored ground-water quality in wells near the first LLL experiment for a period of 2 years following gasification. A large number of contaminants were found in wells located up to 100 ft (30 m) from the burn zone, but concentrations have decreased substantially as a result of sorption by the coal. During the second gasification experiment (Hoe Creek II), roof collapse and the inadvertent gasification of an overlying coal seam caused the interconnection of three aquifers, including the two gasified coal seams. We have measured the resulting changes in local hydrology and in the dispersal of reaction-product contaminants. Introduction The conversion of coal into combustible gases, as a source for synthetic fuels, promises to become an important method of coal utilization. If this conversion is carried out with the coal in place underground - in situ gasification, or UCG (Underground Coal Gasification) - additional economic and environmental advantages may be realized. For example, UCG may make the recovery of deep coals economically attractive, and it can be accomplished without the need to expose workers to the hazards of underground mining operations. Furthermore, the U. S. coal resource that is suitable for UCG, and not economical to strip or deep mine, has been estimated at nearly 2 trillion tons. The exploration and assessment of this prospective technology must include a critical investigation of potential environmental problems, together with potential environmental problems, together with efforts to identify effective environmental controls. We are attempting to explore some of the environmental implications of UCG - particularly, ground-water effects - in parallel with UCG process investigations, so that significant concerns process investigations, so that significant concerns can be addressed in a timely manner. Although underground coal gasification generally involves a complex series of chemical reactions, it can be simply characterized as the heating of coal in the presence of gasifying agents such as oxygen and steam. Some of the coal is burned to provide heat to drive the gasification reactions. In the simplest form of UCG, two or more process wells drilled into the coal seam are used, after the coal is ignited, to inject air or other gasifying agents and to withdraw the resulting combustible gas mixture (Figure 1). In most cases, the coal's permeability must be enhanced, before gasification, along a path connecting the process wells. (The need to achieve this preliminary connection reliably and economically represents an important current challenge in the development of a practicable UCG technology.)