A new method is presented for predicting the performance of depletion drive oilreservoirs that takes into consideration the effect of pressure and saturationradients. The pressure at the well is related to the average reservoir pressureby an equation derived from analytical solution of the diffusivity equation. Another equation is used to relate the oil saturation at the well to theaverage oil saturation in the reservoir. The pressure and saturation at thewell are used to evaluate the producing gas-oil ratio which is substituted intothe material balance equation to compute the fractional oil recovery at theaverage reservoir pressure.

The modified method shows the effect of such parameters as production rate, well spacing and well damage on reservoir performance. Such effect is absent inclassical prediction methods that ignore pressure and saturation gradients inthe reservoir.

A systematic iterative procedure for the performance calculations is outlined. Results of the prediction by the proposed method are presented and comparedwith those obtained using the classical prediction method.


Predicting the future performance of an oil reservoir is one of the mostimportanttasks in reservoir engineering. Arelatively fast and simple predictionmethod is based on the material balance equation together with the gas-oilratio equation and the saturation equation. This procedurewas used by Tarner(1), Muskat(2), Tracy(3) and Pirson(4) using different forms of the material balance equation. For a depletion drive oil reservoir these methods require only the knowledge of PVT data and relative permeability data. Their accuracy is limited by theassumption of uniform pressure and fluid saturation throughout the reservoir. On the other hand elaborate and sophisticated reservoir simulation methods canbe used for performance prediction provided an accessto a computer withrelatively large storage LS available. Furthermore reservoir simulation methodsrequire detailed description of the reservoir parameters such as physicalproperties of rocks and fluids r well locations, production rates, formationthickness at different points in the reservoir.

The accuracy of the results will depend on the reliability of this supplieddata. At early stages of the reservoir life where such information is notavailable the resultsof reservoir simulation may not justify the extra cost andeffort involved.

The use of prediction methods based on the material balance equation could beenhancedlf the assumption of uniform pressure and saturation could beeliminated. Average pressure is used to estimate the PVT data needed in thematerial balance equation while average pressure and average saturation areused to estimate the instantaneous gas-oil ratio. Since a pressure gradient isestablished between the reservoir boundaries and the production wells, a partof the reservoir is at a pressure higher than average reservoir pressure whilethe other part is at a lower pressure. It is therefore reasonable to useaverage values of the oil and gas formation volume factors to convert reservoirvolumes of oil and gas to surface volumes and to use an average value of gassolubility to find the volume of gas in solution.

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