A numerical model for areal migration of water and light hydrocarbon in unconfined aquifers
A finite element model has been developed to simulate simultaneous flow of water and light hydrocarbon in an areal flow region of an unconfined aquifer for analyses of hydrocarbon spreading from subsurface leaks or spills and for use in design of free product recovery systems. Vertically integrated governing equations for water and oil flow are employed which assume local vertical equilibrium and negligible gas pressure gradients. Multiple water and free product recovery wells are handled as internal type-I boundary conditions by stipulating air-oil table elevation and free product height with corrections to convert grid averaged nodal heads to actual well bore fluid levels. An automatic updating scheme for well bore correction factors is introduced which ensures consistency of well flux calculations with the global mass balance. Areal model predictions are compared with two dimensional vertical cartesian and radial simulations with multiphase seepage faces for hypothetical trench and well free product recovery systems, respectively. The results indicate that the assumption of vertical equilibrium and lack of explicit treatment of seepage faces in the areal model produce minor loss in accuracy while conferring major reductions in computational effort. Simulations of various spill spreading and free product recovery scenarios with multiple pumping wells are investigated to demonstrate the model capabilities.