Page 154 - Environmental Control in Petroleum Engineering

Environmental Transport of Petroleum Wastes 141

highly dependent on soil type and composition, particularly the amount
of clays present. Factors that affect adsorption are the structural
characteristics of the chemical, the organic content of the soil, the pH
of the fluid medium, the soil grain size, the ion exchange capacity of
the soil (clay content), and the temperature. Migration of heavy metals
away from drill sites generally does not occur.
A number of numerical models having different levels of capabilities
are available (American Petroleum Institute, 1986, and American
Petroleum Institute, 1988). Unfortunately, most models neglect capil-
lary trapping of the oil and air and hysteresis of relative permeability.
Monte Carlo models allowing multiple realizations of possible con-
taminant transport have also been developed (Parker et al., 1993),
Another important pathway for the transport of petroleum wastes
is improperly plugged and abandoned wells. These wells allow fluids
from geologic formations having high hydrocarbon, salt, and/or heavy
metals concentrations to flow into formations containing fresh water.
Wells that are properly plugged and abandoned do not provide a
permeable flow channel for fluids. Fluid flow, however, is not possible
between layers if they are in hydrostatic pressure equilibrium, regard-
less of whether channels exist between the layers.
Numerical modeling of fluid flow in improperly abandoned wells
can indicate the likelihood of freshwater contamination at a particular
site (Warner and McConnell, 1993). The relative contamination poten-
tial of abandoned wells ranges from highly likely to impossible,
depending on the age of the well, the depth of the well, the type of
well, how the well was constructed, how it was plugged, the history
of well activity, and the hydrogeologic conditions at the site,

4.2.2 Releases on Water

Transport of petroleum wastes below the surface of water depends
primarily on the currents in the water and the topography of the floor
of the water body. Produced waters typically have a greater salinity
than fresh water or seawater, making them more dense. Discharged
produced waters then sink until they either reach a density equilibrium
with the seawater or reach the sea floor. Numerical models have been
developed to model the transport of discharged drilling muds and
produced water (Arscott, 1989). Two such models are the EPA's
CORMIX1 and the Offshore Operators Committee models.