The Brazeau River Nisku "A" pool is located approximately 150 km southwest of Edmonton, Alberta, Canada. It has produced 45 °API light oil since 1978 from the pinnacle reefs of Later Devonian Winterburn group. The stock tank oil initially in-place (STOIIP) was estimated from the 2D seismic data to be 33 million bbl. The primary oil production of the under-saturated reservoir pool only lasted two and half years before a dry-gas cycling pressure maintenance scheme was initiated to enhance the pool’s recovery. An infill horizontal well was added in 1992 to the existing two vertical producers, with one-injector to re-inject the produced lean gas back to the reservoir pool from the top of the reef structure, enhancing the pool recovery from a typical gravity drainage mechanism. After 1995, the dry gas injection stopped and the reservoir began pressure blowdown and resumed depletion drive production. By 2010, the pool’s pressure was near a complete depletion. However, the Nisku "A" pool’s performance history data have suggested that 7 bcf more natural gas was produced than expected, raising questions about the long established STOIIP estimate. Numerical simulation studies also failed to reconcile the oil volume estimate conflict between the production performance data and the seismically well-defined volumetric evaluation.

This paper demonstrated, by carefully examining the reservoir appraisal data that the Nisku "A" pool most probable STOIIP is 40 million bbl, instead of 33 million. The 7 million barrels of oil in-place estimate gap could have been easily identified in the first two and half years of production by correctly performing simplistic material balance analysis of the pool’s pressure versus oil and gas rate history and the reservoir compressibility values, which are often taken into granted by engineers in resource and reserve evaluations.

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