The impact of geological and microbiological processes on oil composition and fluid property variations in heavy oil and bitumen reservoirs
Date
2008
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Abstract
Oil fluid property gradients at regional, field and reservoir scales are ubiquitous in
heavy oil and bitumen deposits in the Alberta Basin and develop via biodegradation at oilwater
transition zones, oil charge to the reservoir top and advective-diffusive mixing
controlled by oil composition and reservoir geometry. Orders of magnitude variations in
bitumen viscosity with reservoir depth adversely impact total recovery from SAGD and cold
production operations which require uniform fluid mobility.
Regional decreasing biodegradation trends to the west in northern Alberta result
from reservoir pasteurization near 80°C, west of Peace River oil sands (PROS), and
increasingly cooler time-integrated reservoir temperatures to the east, in the PROS and
Athabasca oil sands. Simple charge-degrade box models of these deposits show that early oil
charge around 100 to 90 Ma and limitation of the biodegradation rate due to reservoir
temperature, water and nutrient availability and oil composition are required to predict
observed westerly decreasing trends in oil biodegradation. Reservoir-scale concentration
gradients in PROS reservoirs are related to reservoir geometry, local shale barriers and
biodegradation rates controlled by water saturation, biodegradation susceptibility and
diffusivity of hydrocarbon substrates. 1D compositional diffusion models match observed
oil component concentration gradients by inclusion of geological barriers, variable thickness
oil-water biodegradation zones.
Numerical models of carbon isotope systematics identify the dominant reaction
pathway of subsurface hydrocarbon biodegradation as methanogenic alkane degradation
using carbon dioxide reduction, which produces isotopically light methane and heavy CO2 in
solution gas. Basin models show that regional biodegradation significantly impacts oil
quality, and fill, spill and leakage histories due to generation and migration of biogenic gas.
Full physics models of hydrocarbon biodegradation predict replacement of thermogenic gas
with biogenic gas, dissolution of biogenic CO2 into the water leg and development of gas
caps over geological time. None of these models adequately predict fluid properties required
for exploration and production operations, thus multivariate statistics models of
hydrocarbon composition were developed to predict PROS dead oil viscosity. Oil viscosity
assessed from core samples can increase dramatically during core storage and oil recovery
and a core storage time correction algorithm was developed to minimize "noise" in
calibration data sets.
Description
Bibliography: p. 544-549
Some pages are in colour.
Some pages are in colour.
Keywords
Biogeochemistry, Petroleum Geology, Geochemistry
Citation
Adams, J. J. (2008). The impact of geological and microbiological processes on oil composition and fluid property variations in heavy oil and bitumen reservoirs (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/2991