SIMULATION OF FLUID DISTRIBUTIONS OBSERVED AT A CRUDE-OIL SPILL SITE INCORPORATING HYSTERESIS, OIL ENTRAPMENT, AND SPATIAL VARIABILITY OF HYDRAULIC-PROPERTIES

Subsurface oil, water, and air saturation distributions were determine d using 146 samples collected from seven boreholes along a 120-m trans ect at a crude oil spill site near Bemidji, Minnesota. The field data, collected 10 years after the spill, show a clearly defined oil body t hat has an oil saturation distribution that appears to be influenced b y sediment heterogeneities and water table fluctuations. The center of the oil body has depressed the water-saturated zone boundary and the oil appears to have migrated laterally within the capillary fringe. A multiphase cross-sectional flow model was developed and used to simula te the movement of oil and water at the spill site. Comparisons betwee n observed and simulated oil saturation distributions serve as an indi cator of the appropriateness of using such models to predict the actua l spread of organic immiscible liquids at spill sites. Sediment hydrau lic properties used in the model were estimated from particle size dat a. The general large-scale features of the observed oil body were repr oduced only when hysteresis with oil entrapment and representations of observed spatial variability of hydraulic properties were incorporate d into the model. The small-scale details of the observed subsurface o il distribution were not reproduced in the simulations. The discrepanc y between observed and simulated oil distributions reflects the consid erable uncertainty in model parameter estimates and boundary condition s, three-phase capillary pressure-saturation-relative permeability fun ctions, representations of spatial variability of hydraulic properties , and hydrodynamics of the groundwater flow system at the study site.