Twelve practicing petroleum engineers participated in a study comparingdeterministic with probabilistic reserve estimates. Production data for fivewells were provided sequentially in four phases covering the first 5 years ofproduction history. A deterministic and probabilistic reserve estimate was madeby each evaluator for each phase.
The deterministic reserve estimates were based on Securities & ExchangeCommission (SEC) definitions. For the probabilistic reserve estimates, theevaluators were asked to supply three values: P90, P50, and P10. P90 is thevalue where there is a 90% probability that the actual reserves are greaterthan this value; P50 is the value where there is a 50% probability that theactual reserves are greater than this value; and P10 is the value where thereis a 10% probability that the actual reserves are greater than this value. Mostevaluators used their deterministic estimate as the basis for the probabilisticestimates. In these cases, subjective estimates were made rather than using astatistical approach.
The evaluators have more difficulty in estimating the P90 value when thedecline exponent b is low. The evaluators have the same difficulty inestimating the P10 value when the decline exponent b is high. In comparing theindividual evaluators, there is considerable variability in deterministic, P90,P50, and P10 values. The most variability is in the P10 values. Also, it wasobserved that over time, as more production history is recorded, P90 values donot approach truth from the low side.
This study was designed to compare the currently used deterministicdefinition of proved reserves (where the estimator is "reasonablycertain") with a probabilistic definition with three levels of certainty.The first level is defined as "proved" or P90. This is the amount ofreserves where there is a 90% probability that the actual reserves will begreater. The second level is "proved plus probable" or P50. This isdefined as the amount of reserves where there is a 50% probability that theactual reserves will be greater. The third level is "proved plus probableplus possible" or P10. This is defined as the amount of reserves wherethere is a 10% probability that the actual reserves will be greater.
Includes associated paper 36358, "Discussion of a Comparison ofProbabilistic and Deterministic Reserve Estimates: A Case Study," byChapman Cronquist