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Computer Modeling of Regional Groundwater Flow and BTEX Migration in Sedimentary Basins of the Colorado Plateau Region
By
Ye Zhang1, Michael Szpakiewic2, and Mark Person1
1Indiana University, Department of Geological Sciences, Bloomington, IN
2Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID
Aromatic hydrocarbons such as benzene, toluene, and xylene (BTEX) can be found in high concentrations in basinal brines (up to 18.6 mg/l) adjacent to petroleum reservoirs. BTEX has been used as geochemical indicators in exploration. However, BTEX compounds dissolved in groundwater attenuate in the subsurface due to retardation and biodegradation. To date, the mechanism of aqueous BTEX transport from oil reservoir to the land surface is poorly understood. In this study, the long term transport and fate of BTEX components within the oil-prolific Uinta Basin has been assessed. BTEX concentrations were analyzed from both oil-field brines and springs located hydro-dynamically down gradient from relevant oil field. The springs sampled were situated within regional discharge areas of the Uinta Basin. BTEX concentrations measured in oil field brines have a concentration range of between 0.1 to 9.5 mg/l but were undetectable in most springs.
Mathematical
models were constructed along a northwest-east cross
-
section
across the Uinta
Basin through the Altmount-Bluebell field following the regional groundwater
flow direction. The models represent advective-dispersive BTEX transport by
regional groundwater and BTEX diffusion within the oil reservoir. Rates of
biodegradation and retardation estimated from shallow aquifer systems were also
represented in the model. Model results indicated that BTEX concentrations in
the basinal brines ranged from 0.0 to 17.0 mg/l. Significant concentrations
(above detection: 0.01mg/l) were not detected in the surface discharge areas.
Models which represent biodegradation predicted BTEX concentrations in oil field
brines and springs that are consistent with field measurements. Models without
biodegradation produced concentrations two to three orders of magnitude too
high.
A
suite of generic models were also constructed using an idealized basin-scale
cross
section
(60km) to evaluate the influence of aquifer and geochemical
parameters on BTEX transport. This
cross
section
has three lithological units
– aquifer in the middle, an overlying confining unit and an underlying
confining unit. A single oil reservoir is located in a stratigraphic trap within
the aquifer. Results indicated that hydraulic conductivity of the cap rock
(overlying confining unit) greatly influenced the direction of transport and
location of BTEX discharge. Interestingly, BTEX transport was not sensitive to
the mass-transfer coefficients and saturation profiles assigned to the oil-water
interface. Model results also demonstrated that biodegradation controlled the
fate of BTEX transport and magnitude of their concentrations in groundwater. In
simulations without biodegradation, BTEX concentrations in the basin dropped to
below 0.01 mg/l at a distance up to 47km down gradient from the oil reservoir.
With biodegradation, BTEX concentrations dropped below detection limit at 15km (l*=0.0001)
and 3km (l=0.001) and
concentrations in springs were mostly undetectable. We conclude that BTEX
concentrations detected in exploration wells may provide a useful indicator of a
nearby, hydrodynamically, up-gradient oil reservoir. However, surface
geochemical sampling of soluble BTEX compounds in springs is probably not a
useful exploration tool due to the effects of biodegradation.
*l is the biodegradation rate assigned to the model.