Measurements of Residual Gas Saturation under Ambient Conditions

ABSTRACT

The value of residual gas saturation to water influx (Srg) is a critical property when estimating recoverable reserves in gas reservoirs overlaying active aquifers, or natural gas storage reservoirs. However, it seems that very little work has actually been done to investigate this value. Early work in the 1950s and 1960s established that Srg was not going to be as low as 10-15% PV, as was commonly expected at the time. Further work at that time also demonstrated that there was a link between Srg and porosity or initial water saturation. However, in the last 25 years, not much has been published on this topic. Our group has conducted extensive experimental work to determine the value of Srg for a variety of reservoir types and conditions. Work was done under different scenarios including primary and spontaneous imbibition, secondary spontaneous imbibition and forced imbibition. The results of 27 core samples were interpreted using all existing published models. It was found that the value of residual gas saturation was sensitive to initial water saturation and to the permeability to porosity ratio. It was also found that this sensitivity does not always follow the same law. Some reservoirs followed Land’s model closely, while other reservoirs did not. The true residual gas saturation cannot be determined until after several weeks of constant imbibition. Regardless of this fact, the values of Srg after primary and spontaneous imbibition are very high. Under the current buoyant natural gas price scenario, such reservoirs are worth pursuing, but at lower gas prices many of these reservoirs may become marginal. Our findings are of particular importance in naturally fractured reservoirs that are plagued by early water breakthrough. Estimating the true residual gas saturation may lead to decisions on Enhanced Gas Recovery.

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