U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings
of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993,
Water-Resources Investigations Report 94-4015
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Multiphase-Flow Modeling of the Bemidji, Minnesota Crude-Oil-Spill
Site with Geostatistical Simulation of Hydraulic Properties
by
L.A. Dillard (Department of Applied Earth Sciences, Stanford
University, Stanford, CA, 94305-2225 and U.S. Geological Survey, 435 Middlefield
Rd., Menlo Park, CA, 94025), H.I. Essaid (U.S. Geological Survey, 435 Middlefield
Rd., Menlo Park, CA, 94025), and W.N. Herkelrath (U.S. Geological Survey,
435 Middlefield Rd., Menlo Park, CA, 94025)
Abstract
This paper describes a study aimed at improving the understanding of
multiphase flow in a heterogeneous aquifer by using data collected at the
north pool sub-region of a crude-oil-spill site near Bemidji, Minnesota.
Data collected include grain-size distributions, oil and water saturations
(fluid saturation refers to the volume of the fluid phase relative to the
volume of pores in the sample), and porosities of samples collected at the
north pool site. These data are used to estimate permeability (k), retention
curves, and the geometric mean and standard deviation of the sampled grain-size
distributions. The k estimates are statistically distributed in a bimodal
lognormal fashion, whereby the two k population distributions correspond
to the two predominant aquifer lithologies at the north pool: a coarse glacial-outwash
deposit and interbedded fine-silt lenses. A dual geostatistical approach
involving sequential indicator and Gaussian simulation is used to characterize
the bimodal aquifer heterogeneity present at the north pool by creating
one realization of a three-dimensional grid of log(k) values conditioned
on the original data.
Field oil-saturation data indicate that the spatial distribution of oil
in the subsurface has been affected by the bimodal aquifer heterogeneity
present at the north pool. The shape of the subsurface oil-saturation distribution
is highly irregular; more specifically, the center of the oil lens is actually
depressed below the water table in some locations, and oil saturations almost
as high as 0.3 occur in the unsaturated zone. A cross-sectional numerical
model is used to simulate the flow of oil and water at the north pool, assuming
the air phase is at atmospheric pressure. A slice of the three-dimensional
grid of log(k) values produced through geostatistical simulation is used
as input in the multiphase model to investigate the effect of aquifer heterogeneity
on multiphase flow. Flow simulations using the bimodal representation of
heterogeneity produce an oil-saturation distribution that is similar to
that found in the field. Other factors affecting multiphase flow, such as
hysteresis and three-phase oil relative permeability, also are investigated.
The effects of hysteresis are not as significant when heterogeneity is adequately
characterized.
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