Underground Coal Gasification Cavern Subsidence

GasTech, Inc. (GasTech) is in the process of permitting a series of underground coal gasification (UCG) wells as part of a pilot project in the deep coals of the Powder River Basin (PRB), Wyoming.  GasTech commissioned Agapito Associates, Inc. (AAI) to analyze the long-term behavior of UCG caverns after completion of the pilot project.  The analysis provided estimates of surface and aquifer displacements, and stability assessments of the pillar between rows of caverns.

Numerical analysis was performed to account for extraction-induced stress changes and the non-elastic behavior of the weaker coal seams.  Five types of input were considered in the analysis: cavern geometry, stratigraphy, rock properties, in-situ stress, and groundwater pressure.  The main coal-producing seams were represented in the model, including the target Upper Wyodak coal bed which is about 60 feet thick at a depth of about 1,000 feet below ground.  In-situ stresses were based on nearby hydrofracture measurements.  Modeling was performed using FLAC3D.

Geotechnical issues addressed include estimated height of roof collapse above the UCG caverns, estimated disturbance of aquifers, and the potential for surface subsidence.  Recommendations were developed for well spacing to optimize resource recovery given limitations on subsidence.

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