Shallow light oil accumulations in reservoirs within the permafrost are rarely recognized and, when identified, are often considered uneconomic because of their shallow depths and low reservoir energy. Horizontal drilling technology will improve the economics of these shallow accumulations, but a better understanding of the reservoir and fluid behavior under these low temperature and pressure conditions will improve recovery and lower development risk. However, little data is available on how a rock/ice/light oil system behaves at low pressures. This information and a robust reservoir model are needed to evaluate the effectiveness of different production methods in this type of accumulation.

The Umiat field of northern Alaska was discovered in the 1940's and is a remote, shallow, light oil accumulation consisting of multiple deltaic and marginal marine Cretaceous sandstones deformed by a thrust-related anticline. While the accumulation may be significant (1.2 billion barrels OOIP), it is shallow and partly in the permafrost zone.

New sedimentologic and structural studies indicate the reservoir has a complex permeability structure that will both impact both the placement of horizontal wells and subsequent reservoir performance. Reservoir sands generally consist of prograding wave-dominated shoreface deposits with good vertical permeability overlain by river-dominated deltaic deposits with poorer vertical permeability. Two different sets of natural fractures may also impart a strong permeability anisotropy to the reservoir.

The temperature profile of the reservoir will also impact reservoir performance and will have to be incorporated into the reservoir simulation. Fluid flow experiments on samples of the Umiat reservoir at sub-freezing temperatures show a reduction in gas and oil relative permeability in the presence of interstitial ice, with the greatest reductions at lower temperatures. A representative Umiat oil sample has been recreated and used to calibrate an Equation of State (EOS) model that can then be used to predict the properties of a representative Umiat fluid in the simulation.

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