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A New Methodology for Assessing the Impacts of Small-Scale Solvent Supply Variations with the Prudhoe Bay Miscible Flood

WEAVER, JASON, ARCO Alaska Inc., Anchorage, AK

As the Prudhoe Bay Miscible Gas Project (PBMGP) approaches its fourteenth year of operation, the water alternating gas (WAG) flood is facing new challenges. These challenges typically involve projects that lead to small solvent supply variations. To accurately assess these projects, a new methodology was developed for forecasting enhanced oil recovery (EOR) responses. This methodology, and the generated analytical equation, provides increased ease of use and greater reliability in assessing small solvent supply variations when compared with direct application of large-scale models.Full-field modeling of miscible injectant (MI) allocation and enhanced oil recovery for the Prudhoe Bay WAG flood is currently performed with large scale-up models. While these models are excellent for evaluating large, full-field decisions related to miscible injection at Prudhoe Bay, they can yield ambiguous results when used to evaluate projects that cause small solvent supply perturbations. The difficulty in resolving the impacts of small incremental projects is a common weakness of large-scale reservoir models.Through analysis of numerous Prudhoe Bay full-field simulations of large variations in the MI supply, it was found that a two-component analytical equation was excellent for fitting the character of the cumulative EOR impact. The components of this equation include a logarithmic decline function and a variation of the Weibull function. The logarithmic function is ideal for representing the ultimate EOR impact of a MI supply variation, whereas the Weibull function represents the EOR oil rate acceleration (or deceleration) component. The optimum coefficients for the composite analytical equation were found through regression of large-scale simulation results.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California