Review of field reserves from different parts of the world reveals that reliability of reserves estimates is rather poor. Many fields show wide fluctuations in reserves estimates through time, both at pre- and post-production stage, with a tendency toward underestimation. Accompanying this trend, uncertainty attached to the "best" reserve estimate does not shrink with additional data, defying intuition. Reasons for such phenomena are varied, but one fundamental reason is the approach the industry generally takes toward reserves estimation. Reserves variations, whether in upward or downward direction, adversely affect project economics.

Factors that affect reserves evaluation include reservoir aspects, development scheme, operations and technology, economic and regulatory aspects, and "intangibles" that relate directly to human input and conduct. In the last-named category, proclivity to ignore statistical rules leads to distortion in reserves and associated uncertainty estimates.

The vision toward improving reserves reliability involves a multi-prong approach that tackles all these root causes. An optimal appraisal strategy that addresses the key subsurface uncertainty, due regard to statistical rules, and use of volumetric methods that consider different scenarios and put the evaluator in contact with "real" data, are viewed, among others, as providing tangible improvements.

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