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Abstract

The gravity drainage mechanism in the Prudhoe Bay Field has been evaluated by means of repeat Compensated Neutron Log (CNL) data, centrifugal gas/oil displacements in small preserved cores, and by an analytical stochastic approach.

From the repeat CNL data, in-situ displacement efficiencies as observed above the advancing gas-oil contact in 50 gravity drainage wells have been estimated. A Monte Carlo-type error analysis is presented to give the range of uncertainty associated presented to give the range of uncertainty associated with the results.

Gas/oil centrifuge displacements in small preserved cores were performed in order to validate the CNL results, and to investigate the time dependency of recovery in gassed out zones which is otherwise beyond CNL resolution. Conventional scaling theory was subsequently used to find homologous times for the reservoir and laboratory models, thus making comparisons (between CNL and centrifuge data) possible. In addition, relative permeability to oil (to be possible. In addition, relative permeability to oil (to be used in the analytical study) was computed from the centrifuge data.

Finally, an approximate analytical description of the gravity drainage process was formulated and a Monte Carlo analysis performed to cover the wide range of values the input parameters in the equations may take on. This analysis yields a statistical distribution in displacement efficiencies at each point in time. The "average" behavior may be compared to the CNL and centrifuge data. The results demonstrate that the gravity drainage mechanism in Prudhoe Bay is very time-effective and yields high quantifiable displacement efficiencies. The consistency obtained among the various evaluation techniques gives confidence in the results.

Introduction

The Prudhoe Bay Field is the largest, most prolific and also one of the most complex combination drive reservoirs in the U.S. (1-6) The main objective for development is to maximize economic recovery while maintaining a high and stable rate of oil production. To achieve this, reservoir engineers have had to consider all classical drive mechanisms. gas and water injection programs, and finally EOR processes to be implemented programs, and finally EOR processes to be implemented at a later stage.

Efficient reservoir management is dependent upon accurate knowledge of residual oil saturations, displacement efficiencies, and the time dependency of recovery associated with the various drives.

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