Estimation of Reserves and Water Drive From Pressure and Production History

A study has been made of the material balance-fluid flow method ofestimating reserves and degree of water drive from pressure and productionhistory data. By considering the effect of random pressure errors it is shownthat in a particular example a standard deviation of three and one-half poundsin each of ten pressure surveys permits the determination of the reserves witha standard deviation of 8 per cent and the water drive with a standarddeviation of 15 per cent, assuming that certain basic geologic data arecorrect. It is believed that this method of estimating reserves and water driveis useful and reliable in a number of cases. The method is particularlyvaluable when reservoir pressure data are accurate within a very few pounds, but may also be applied with less accurate pressure data if a relatively largereservoir pressure decline occurs early in the life of the field, as forexample in an undersaturated oil field. Introduction A knowledge of the magnitude of reserves and degree of water drive presentin any newly discovered petroleum reservoir is necessary to early applicationof proper production practices. A number of investigators have contributed tomethods of relating reserves, degree of water drive, and production andpressure history. Three types of problems of increasing complexity may bementioned. If a reservoir is known to have no water drive, and if the ratio ofthe volume of the reservoir occupied by gas to the volume of the reservoiroccupied by oil (which ratio permits fixing the overall compressibility of thereservoir) is known, then only one further extensive reservoir property remainsto be determined, namely the magnitude of the reserves. A straightforwardapplication of material balance considerations will permit this determination.The problem becomes very much more difficult if we wish to determine not onlythe magnitude of the reserves but also the magnitude of water drive, if any, which is present. In principle, a combination of material balance and fluidflow considerations will permit this evaluation. Finally, if neither themagnitude of reserves, the degree of water drive, nor the ratio of oil to gaspresent in the reservoir is known and it is desired to determine all three ofthese variables, the problem could in principle be solved by a fluidflow-material balance analysis which determines the overall compressibility ofthe reservoir at various points in its history. The change in compressibilitywith pressure would provide a means of determining the ratio of gas to liquidpresent, since the compressibilities of gas and liquid vary differently withpressure variation. However, in practice this problem is probably so difficultas to defy solution in terms of basic data precision apt to be available. T.P. 2589