Simulated Recovery of Light, Nonaqueous Phase Liquid from Unconfined Heterogeneous Aquifers

A multiphase flow model (ARMOS) was used to evaluate the effects of subsurface heterogeneities on the recovery of light, nonaqueous phase liquids (LNAPLs or simply oil). Stochastic inputs for the model included the saturated hydraulic conductivity (K{sub sw}), van Genuchten {alpha} and n, the water saturation at field capacity (S{sub m}), and the maximum residual oil saturations in the saturated (S{sub or}) and the unsaturated zones (S{sub og}). The turning bands method was used to generate stochastic soil parameters representing three hypothetical sandy loam soils. Oil recovery in the three heterogeneous cases was compared to an equivalent homogeneous soil with effective parameters computed as the geometric means of the stochastic parameters. Distributions of the free oil plumes were described over time using statistical and spatial moment analyses. Due to the smoothing effect of the flow process, the predicted well oil thickness (H{sub o}) and free oil specific volume (V{sub of}) were less variable than the input stochastic parameter ln(K{sub sw}). However, H{sub o} and V{sub of} became more variable as free oil volume diminished and the oil distribution was controlled more by soil variability than gradients in the fluid levels. At the onset of oil recovery, the free oil areamore » was greater in all three heterogeneous soils than the homogeneous soil. Nevertheless, soil heterogeneities did not greatly affect oil recovery or trapping in the saturated and unsaturated zones. Heterogeneities may have had a greater influence on oil recovery if a smaller spill had been studied or if the vertical dimension had been simulated explicitly. The results suggest that the geometric mean soil properties provide a useful estimate of the potential for oil recovery from oil spills that span a large number of correlation scales.« less