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Well completion strategy in sandstone reservoir with strong waterdrive
Background and geological information
This field produces from a series of stacked sandstone reservoirs situated in an angular unconformity trap consisting of an eroded 2 to 6° monocline and an oil/water contact common to all zones. The original oil column was 400 ft thick and contained a 43° API, highly undersaturated crude. Porosity averages 22%, and permeability averages 3 darcy.
The primary recovery mechanism is strong waterdrive. All wells are now gas lifted, although this was not necessary in early field life. Wells have typically been drilled through multiple sands, which have been produced sequentially and, in some cases, comingled. Initial development was on 300-acre spacing. Two subsequent infill-drilling programs have been carried out to drain thin zones and areas in the extreme updip portions of the reservoir. The platform is currently constrained by water-handling capacity and, when wells reach high water cut, they are typically cycled on and off so that total production matches facilities limits.
The estimated recovery factor is approximately 75% of original oil in place (OOIP). Good rock and fluid properties plus good lateral continuity have contributed to this high recovery through gravity-stable displacement of oil by the invading water. Estimated Sor in invaded sections of the reservoir is 10% of pore volume (PV).
Field surveillance and management
Cross section and 3D computer models have been used to study reservoir behavior and upgrade the depletion strategy. A sustained surveillance program includes logging to monitor displacement and regular field performance reviews by the geoscience and engineering team members.
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