Residual Gas Saturation Revisited

Abstract The determination of the true residual gas saturation in gas reservoirs is addressed in this paper. It is customarily assumed that when a gas reservoir is overlaying an aquifer, water will imbibe into the gas-saturated zone with the onset of gas production. The process of gas displacement by water will be forced imbibition in areas of high drawdown and spontaneous imbibition in the areas of low drawdown. It is further assumed that in the bulk of the reservoir spontaneous imbibition will prevail and that the reservoir is water-wet. A final assumption is that the gas behaves as an incompressible fluid. All these assumptions are challenged in this paper. The topics of imbibition and wetting phase film flow are reviewed along with a brief summary of the work on the determination of residual gas saturation to date. Furthermore, a series of experiments are presented whereby it is demonstrated that the residual gas saturation obtained by a short imbibition test is not necessarily the correct residual gas saturation. Imbibition tests by different methods yield very different results, while saturation history and core cleaning also seem to have a strong effect on the determination of residual gas saturation. In some cases, it was found that the residual gas by spontaneous imbibition was unreasonably high. This was attributed to weak wetting conditions of the core (no water "pull" by imbibition). It is expected that this work will shed some new light in an old but not so well understood topic.