The Evaluation of Prudhoe Bay Waterflood Conformance and Reservoir Characterization by Thermal Modeling

Abstract

ABSTRACT The evaluation of vertical sweep has been a long standing problem when determining waterflood efficiency in massive sandstones. As a result of reservoir heterogeneities and gravitational effects the vertical distribution of water near an injection well is typically different than its distribution in the reservoir. A thermal modeling approach has been successfully applied to this problem in the Sadlerochit sands at the Prudhoe Bay field to attain a greater understanding of the vertical sweep efficiency in a portion of this reservoir. A thermal simulator was used to construct a 3-D model of one half of a 320 acre inverted 9-spot waterflood pattern in the Flow Station 2 waterflood area of Prudhoe Bay. The model was history matched to a temperature profile taken in a replacement injector drilled 720 feet (220m) away from its original location. The amount and location of reservoir cooling observed is a function of the water volume that has passed through it at a given location and its associated conductive and convective cooling. The replacement well modeled was drilled 4.75 years after injection had been initiated in the original well. The analysis thus provides an understanding of the vertical water distribution in the reservoir 720 feet (220m) from an injector, 4.75 years after the initiation of secondary recovery. The producing sand modeled is approximately 170 feet thick and is characterized by a high permeability (0.1-5.0 darcy) thief zone at its top and 0.5 to 1.0 darcy sands below. In this waterflood pattern approximately 62% of the injectant had entered this thief zone. Thermal modeling indicated that 60% of the water had slumped from this thief interval into the sands below, 720 feet (220 m) from its injection point. This result indicates that although the thief zone has taken a high percentage of the water injected into the pattern, the vertical distribution of that water in the reservoir is far different from its distribution at the sand face as measured in the wellbore. By quantifying the volume of water that slumped out of the thief zone, an estimate of vertical permeability was determined and our understanding of vertical sweep efficiency greatly enhanced.