Calculating immobile saturation

JOUI

Journal of Petroleum Science and Engineering 15 (1996) 33-43

Calculating immobile gas saturations
Eric S. Carlson, Philip W. Johnson
SPE, The Uniuersiry of Alabama, Tuscaloosa, AL 35487-0207, USA

Received 11 October 1994; accepted 19 September 1995

Abstract
We present a simple, fast, analytical expression which can be used to estimate the reservoir pressure as a function ofgas 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.

1. Introduction The threshold saturation where free gas begins to flow (during solution gas drive production forexample), is called the critical gas saturation. Unfortunately, estimating the pressure at which the reservoir reaches this critical gas saturation has previously involved long, tedious, material balance calculations. Collecting and correlating the data for the material balance method is enough to discourage many operators from attempting to make the estimate. We have developed a method consistingof one explicit equation for determining the pressure at the critical gas saturation, or any lesser gas saturation. Charts are included which facilitate estimation of the required parameters. We hope that the material presented here will promote better reservoir management techniques by oil field operators. 2. Background For an undersaturated oil reservoir (the pressure is above the bubble point),the examination of recovery
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versus pressure indicates that incrementally more oil can be produced per unit pressure drop at pressures immediately below the bubble point than at pressures above the bubble point. This change in reservoir efficiency is due to the tremendous amount of energy supplied byfree gas evolving from the oil. The apparent improvement of reservoir efficiency, however, is short-lived. As soon as the critical gas saturation is reached, free gas begins to flow, and the pressure declines rapidly. Free gas formation caused by pressure depletion usually has detrimental effects on ultimate oil recovery, although it may briefly improve production. An increase in gas saturationcauses the oil permeability to drop, while the loss of gas (from the oil) makes the oil shrink and causes the oil viscosity to increase. These changes in the oil and formation properties make it progressively harder for the oil to flow through the reservoir. Under the best of circumstances, these changes reduce oil mobility and consequently the efficiency of any secondary recovery project, even ifthe project is implemented prior to free gas flow.

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P. W. Johnson / Journul

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Science und Enginrerirrg

15 (19961 33-43

In solution gas drive and gas cap drive fields it is common for the pressure to drop below the bubble point. As long as little free gas flows, the change in ultimate recovery may be negligible (for instance, if the extra productiondata leads to a significant improvement in the reservoir characterization, the secondary recovery design can be improved). It is wise to limit free gas production. When free gas flows, the most important source of natural pressure maintenance is lost. The effects of the decrease of oil phase viscosity, increase in oil shrinkage, and reduction of oil permeability become very pronounced. After theinevitable drop in reservoir pressure, high saturations of free gas will remain in the reservoir at low pressure. For all practical purposes, a volume of liquid equal to the volume occupied by the free gas must be injected before a strong response to a secondary recovery program occurs. Therefore, a secondary recovery project will not only be less efficient because of lower oil mobility, but also...