Estimation of Free Hydrocarbon Volume from Fluid Levels in Monitoring Wells

Abstract

ABSTRACTUnder the assumption of local vertical equilibrium, fluid pressure distributions specified from well fluid levels in monitoring wells may be used to predict water and hydrocarbon saturation profiles given expressions for air‐water‐hydrocarbon saturation‐pressure relations. Vertical integration of the oil‐saturation profile yields the actual oil volume in porous media per unit area adjacent to the well. Three‐phase fluid distributions are predicted using a scaling procedure which requires knowledge of two‐phase air‐water saturation‐pressure relations, hydrocarbon density, and hydrocarbon surface tension. Air‐water saturation‐pressure relations are parameterized by either the Brooks‐Corey or van Genuchten expressions. Parameters in the models are estimated from grain‐size distribution data for two hypothetical soils.Results reveal that whereas the distance above an oil‐water table at which oil saturations become zero may be independent of soil type, estimated light nonaqueous phase liquid (LNAPL) volumes per unit area may differ substantially. Hence, estimates of LNAPL volume cannot be inferred directly from soil LNAPL thickness or well LNAPL thickness data without consideration of effects of soil properties. Furthermore, it is demonstrated that no simple linear conversion scheme can be employed to relate the height of LNAPL in a monitoring well to the LNAPL volume in porous media. Effects of grain‐size distribution and well LNAPL thickness on the ratio of actual LNAPL thickness in the aquifer to well LNAPL thickness are shown.