Finite element modeling of flow in a coal seam with underground coal gasification cavities

Abstract

A two-dimensional, finite element groundwater flow model was developed to study the movement of water in a coal seam in which large cavities were created by underground coal gasification (UCG) burns. The burns extract natural gas from the coal without any environmental disturbance at the ground level. On completion of the burn, groundwater mixes with the products of combustion in the cavity and the pollutants move into the coal seam. Mathematical modeling is used to study the movement and fate of these pollutants with time. Water quality modeling has to be preceded by flow modeling to determine velocities as a function of time. This paper studies the flow aspects of the problem. A new code was developed because of the unique requirements of the model. The model utilizes linear triangles to discretize the coal seam in plan and takes into account unsteady flow, anisotropic media, internal boundary conditions imposed by the cavities and a time-varying domain of flow in plan. The model was applied to a series of UCG burns in Hanna, Wyoming. Estimates of the time of filling of the five cavities were obtained. Comparisons of measured and computed potential head are presented at different points in the coal seam. Flow modeling can thus be used to predict the movement of water into and out of UCG cavities while the output of velocities is necessary for water quality modeling.