Cleaning Up Spilled Gasoline With Steam: Compositional Simulations

Abstract

Abstract A finite-difference compositional simulator has been developed and tested at U.C. Berkeley to model the flow of mixtures of Nonaqueous Phase Liquids (NAPLS) through the air zone and into aquifers. The simulator has been successfully used to history-match a steam injection pilot at a "Clean Site" near the Lawrence Livermore National Laboratory in California — a test site for the Gasoline Spill Area (GSA) cleanup pilot planned for early '93. Because of its multicomponent capabilities, the simulator has been used to calculate (a) production rates of individual gasoline components in the GSA to size treatment facilities, (b) areal and vertical distribution of gasoline after the first cycle of steam injection, and (c) steam injection rate that limits growth of the steam zone beyond the cleanup area. It has been shown that gasoline present in the permeable sands and gravel layers can be successfully recovered by injecting steam into these layers in a 7-spot pattern. For the conditions assumed in the model, it will take less than 16 days to recover nearly all of the gasoline in the sands and gravel layers. By that time, the maximum aqueous concentrations of all hydrocarbon components in these layers will have dropped to less than 0.01 mg/l. The results show that vaporization, followed by bulk movement of the vapor to the production well is the dominant recovery mechanism. In terms of time required for cleanup, model results are most sensitive to permeability of the medium. Other parameters, such as the relative permeabilities and the number of components, also affect the outcome, but to a lesser extent. Introduction A compositional simulator M2NOTS (Multicomponent Non-isothermal Organics Transport Simulator) has been developed at U.C. Berkeley to model the flow of mixtures of Nonaqueous Phase Liquids (NAPLS) through the vadose zone and into aquifers. The NAPLs can have arbitrary densities, boiling temperatures, and viscosities, The M2NOTS simulator is a major extension of an existing two-phase (water and gas), two-component (H2O and air), and nonisothermal simulator TOUGH2, developed at the Lawrence Berkeley Laboratory for geothermal applications. The M2NOTS integrated finite-difference model is (a) three-dimensional; (b) fully implicit; (c) three-phase (aqueous, gaseous, and NAPL); (d) non-isothermal if needed; and (e) multicomponent (water, air, and any number of hydrocarbon components). The compositional and multiphase part of M2NOTS has the following features:each component of every phase in a grid block may partition into every other phase;each phase in a grid block may appear or disappear;the appearance or disappearance of a phase is established by a multicomponent, isothermal flash calculation performed at each iteration; andthe relative permeabilities are calculated from generalized power law equations for two-phase flow and the Stone II model for three-phase flow. The M2NOTS well model can handle multiple fluid or heat injection wells on pressure or rate constraints, and fluid producers with variable pump levels and on deliverability or pump pressure constraints. P. 261^