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The evaluation of vertical sweep has been a long standing problem when determining waterflood efficiency in massive 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 3-0 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 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 fiat slumped out of the thief zone, an estimate of vertical permeability was determined and our understanding of vertical sweep efficiency greatly enhanced.
The Prudhoe Bay Oil Field on the Alaskan North Slope is the largest producing oil field in North America with over 21 Billion STB oil originally in place. The main reservoir interval is the Triassic Ivishak sandstone of the Sadlerochit group. This main oil-bearing formation is at a depth of about 9000 fl. The rock quality is generally high with permeabilities in the range from 10 md to 5 darcies. Two permeabilities in the range from 10 md to 5 darcies. Two recovery mechanisms operate in the field. Gravity drainage is the principle process under the original gas cap. Water is currently being injected to promote oil production in peripheral parts of the reservoir. The waterflooding is being peripheral parts of the reservoir. The waterflooding is being performed using variations of inverted nine spot patterns with performed using variations of inverted nine spot patterns with interwell distances of about 1500 ft.
The occurrence and extent of water and oil gravity segregation i.e. slumping in Prudhoe Bay waterfloods has long been a issue of high uncertainty and conjecture. Field surveillance data has been misleading and at best speculative in establishing a understanding of the vertical sweep mechanism.
Historically waterflood injection has been dominated in Prudhoe Bay by thief zones as was anticipated by original Prudhoe Bay by thief zones as was anticipated by original design studies. To maximized oil recovery it is necessary to produce as much of the waterflooded interval as possible. As a produce as much of the waterflooded interval as possible. As a result, it is inevitable that larger and larger quantities of water will be produced. Consequently, any knowledge of the factors controlling waterflood sweep efficiency will allow these reserves to be produced while handling the lowest volume of water. Recently a replacement water injection well has provided much needed insights into the vertical sweep efficiency in the reservoir.
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