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Detailed geologic and mineralogic studies were conducted on the Hanna, Wyoming, and Hoe Creek, Wyoming, underground coal gasification sites. These studies demonstrate the importance geologic factors have on controlling overburden collapse into the reactor cavity during and after coal gasification and on subsequent environmental problems. Parameters that control the collapse of overburden material into the reactor cavity include: duration of the burn; maximum span of unsupported roof rock; lateral and vertical homogeneity, permeability and rock strength; and thickness of overburden materials. The duration of the burn, maximum span of unsupported roof rock, and total thickness of overburden rock are, in large part, determined by the technical engineering aspects of the burn The remaining parameters are determined by geological factors including original composition, depositional environment, diagenesis, and structural deformation.
The coals and overburden units at both sites are Eocene in age and are virtually flat lying. The three burn cavities studied at the Hoe Creek site are directly overlain by laterally discontinuous, poorly indurated, kaolinite-cemented sandstones, siltstones, and mudstones of channel and overbank origin. At the Hoe Creek I experiment, a small reactor cavity and a correspondingly short maximum span of unsupported roof rock consisting of fine-grained, low permeability overbank deposits resulted in minimal collapse. At the Hoe Creek II experiment, a significant amount of collapse occurred due to an increased span of unsupported roof rock comprised of poorly consolidated, more permeable channel sandstones and a limited amount of overburden mudstones and siltstones. Roof rock collapse extend d to the surface at the Hoe Creek III experiment where the maximum span of unsupported roof rock was largest and where the roof rock consisted of highly permeable, poorly consolidated channel sandstones.
The unit comprising the reactor cavity roof rock at the Hanna II experimental site is a laterally continuous lacustrine delta deposit, which primarily consists of sandstones with lesser amounts of interbedded siltstones and claystones. The overall strength, specifically the bridging capacity, of this unit is enhanced by abundant calcite cement. This cement reduced permeability and interstitial waters which probably kept spalling of the roof rock to a minimum. Consequently, roof rock collapse at the Hanna II experiment was much less extensive than at the Hoe Creek II and III experiments.
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