RIASSUNTO
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.
Introduction
Numerous papers have been written on the subject of residual oil saturation from imbibition, but fewer on the subject of residual gas saturation of imbibition. The common conception is that many of the principles that cover oil and gas reservoirs are the same.
Agarwal1 addressed the relationship between initial and final gas saturation from an empirical perspective. He worked with 320 imbibition experiments and segmented the database to develop curve fits for common rock classifications.
Land2 noted that available data seemed to fit very well to an empirical functional form given as:
Equation
In this model, the only free parameter is the maximum observable trapped non-wetting phase saturation corresponding to Sgr (Sgi=1). This expression does not predict residual phase saturation, only how residual saturation scales with initial saturation.
Zhou et al.3 studied the effect of wettability, initial water saturation and aging time on oil recovery by spontaneous imbibition and waterflooding. A correlation between water wetness and oil recovery by waterflooding and spontaneous imbibition was observed.
Geffen et al.4 investigated some factors, which effect the residual gas saturation, such as flooding rate, static pressure, temperature, sample size and saturation conditions before flooding. They found that water imbibition on dry plug experiments was different from water flooding experiments with connate water. They concluded that the residual gas saturation was essentially the same from the two types of experiments.
Keelan and Pugh5 concluded that trapped gas saturation exists after gas displacement by wetting phase imbibition in carbonate reservoirs. Their experiments showed that the trapped gas varied with initial gas in place and it was a function of rock type.