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We present a simple, fast, analytical expression which can be used to estimate the reservoir pressure as a function of gas saturation, for gas saturations between zero and the critical gas saturation. Use of the relation also makes it extremely easy to assess the local recovery factor as a function of pressure from the bubble point down to the pressure at which the critical gas saturation is reached. We hope that the application of this method will reduce the likelihood that operators will let the average pressures of their oil reservoirs drop to the point where significant quantities of free gas flow.
Few things devastate the ultimate recoveries of oil reservoirs more than allowing gas saturations to increase until free gas flows. The threshold saturation where free gas begins to flow is called the critical gas saturation. Unfortunately, estimating the pressure where the reservoir reaches the critical gas saturation has involved long, tedious, material balance calculations. The material balance method is complicated enough to discourage many operators from attempting to make the estimate.
We have developed and present herein a very simple method, one explicit equation, for detennining the pressure at the critical gas saturation, or any lesser gas saturation. Charts are included which facilitate the estimation of required parameters. We hope that the material presented here will facilitate wider application of better reservoir management techniques by oil field operators.
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