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Sensitivity Analysis Applied to Unsaturated Flow Modeling of a Retorted Oil Shale Pile¹

ABSTRACT

Commercial recovery of oil from oil shale will require disposal of large quantities of retorted or spent shale. Because no commercial disposal piles have been constructed and monitored, uncertainties exist in predicting water movement through large spent-shale disposal piles. To better understand these uncertainties, we investigated water movement through spent shale by numerical modeling. A first-order sensitivity analysis was used as a part of the numerical investigation of water movement through a hypothetical pile of spent oil shale. The sensitivity analysis was useful in identifying the most important components of the flow model. Results indicate that the flow model is sensitive to parameters that directly supply water to or control extraction of water from the profile, namely the initial condition of spent shale, precipitation, and estimates of potential evapotranspiration. The flow model is less sensitive to parameters that control water movement, specifically hydraulic conductivity of the spent shale and soil cover. The sensitivity analysis demonstrates that the flow model is also sensitive to parameters describing the sink term, namely rooting depth and rooting density. To improve predictions of the flow model, the input parameters contributing the most to model sensitivity should be measured carefully; parameters that produce less model sensitivity can be estimated without as much effort.

¹ Prepared for the U.S. Department of Energy's Office of Health & Environmental Research by Pacific Northwest Laboratory Under Contract DE-AC06-76RLO 1830.

² Research Engineer, Research Scientist, and Staff Scientist, respectively, Pacific Northwest Laboratory, Richland, Washington.