Residual gas saturation is a crucial number to estimate the gas recovery in gas reservoirs with active aquifers. Water influx in gas reservoirs has long been recognized as an important cause of gas trapping in water-wet reservoirs. In this study, the residual gas saturation to water influx was investigated in 47 core plugs, including 29 sandstone plugs from different areas, 2 Berea sandstone plugs and 16 carbonate plugs. Over 100 different experiments were performed, including spontaneous water imbibition and forced water imbibition tests, primary imbibition and secondary imbibition tests, counter current and co-current imbibition tests. Measurements indicated that the value of residual gas saturation depends on many factors, including reservoir properties, the capillary number, experimental procedures, fluid properties and also very strongly depends on the gas solubility and compressibility. The residual gas saturation value from primary and secondary imbibition tests was also used to compare against literature models. Modified models were developed in order to fit the experimental data better.
The residual gas saturation results show that gas recovery should be high under spontaneous imbibition and extremely high under the forced imbibition. However, trapped gas in reservoirs with active aquifers remains as high as 90%. Hopefully this paper can provide some insight for enhance gas recovery in gas reservoirs with active aquifers.
Recovery of natural gas from reservoirs with a naturally occurring underlying aquifer and aquifer gas storage are common projects in gas reservoir engineering. In both types of projects large volumes of gas become trapped and cannot be recovered. Once gas becomes trapped, conventional wisdom dictates that it is very difficult to remobilize. It is very important to calculate the optimum gas recovery and if residual gas saturation values are high, to further develop a strategy of enhancing gas recovery. Experimental research work was presented in Kantzas et al.1, in which experiments were performed in both sandstone and carbonates reservoirs and residual gas saturation was evaluated. Some factors such as wettability, imbibition rate and experimental procedures, which affect the residual gas saturation, were discussed. Also different residual gas saturation predictive models from the literature were applied in their work. Continuation of this research work was presented in Ding and Kantzas 2–4, in which the residual gas saturation evaluation from different reservoirs and at different conditions was addressed.
The efficiency of gas recovery by water imbibition was described in Crowell et al5. The different factors, which affected residual gas saturation, were addressed. It was shown that gas recovery is a strong function of the initial gas saturation and that the maximum recovery is obtained at zero initial water saturation. Different sandstone plugs were used in the experiments. It was shown that similar residual gas saturation values were obtained by both free imbibition and imbibition at a constant flow rate. A slight increase in gas recovery with a reduction of interfacial tension was observed in Berea slabs.
Suzanne et al.6, 7 evaluated the residual gas saturation through 60 relationships between initial gas saturation (Sgi) and residual gas saturation (Sgr), which covered a large set of sandstone plugs.