Occurrence characteristics and formation mechanism of water invaded residual gas in sandstone gas reservoir
Tapping the potential of water sealed residual gas is one of the key measures to improve the recovery factor of water invaded unconsolidated sandstone gas reservoirs.At present,however,there is a lack of systematic cognition on the formation process and occurrence form of water sealed residual gas under different energies.To clarify the occurrence characteristics and formation mechanisms of residual gas in conventional sandstone gas reservoirs under different water invasion energies,this paper tracks and depicts the dynamic evolution of the gas-water interface during the water invasion process and reveals the trapping and retention mechanisms of different types of residual gas by using the glass etching model,the micro-visualization experimental setup and the pixel extraction method.And the following research results are obtained.First,the strength of water invasion energy is a key factor controlling the occurrence form of residual gas.When the water invasion energy is weak,the water phase can hardly overcome the deformation resistance of the gas-liquid interface,and capillary force is dominant.As a result,residual gas is formed gradually mostly in the dynamic process of"water film interception and division-water film shearing and stretching-water film thickening interception and compression",exhibiting features of water locking and residual gas entrapment.Second,when the water invasion energy is strong,displacement force is dominant,and the phenomenon of heterogeneous fingering is evident.The retention process of residual gas involves two stages:water phase squeezing interception and water film thickening compression,with edge residual gas band being predominant.Third,Overall,the residual gas saturation under high water invasion energy is lower than that under low water invasion energy,but the sweep uniformity under strong water invasion energy is significantly weaker than that under low water invasion energy.In conclusion,based on the two types of water drive energy(or water drive rate),the formation mechanisms of water sealed gas in water invaded zones are investigated,and five types of residual gas are identified.The research results can provide a scientific basis for the formulation of potential tapping countermeasures for water sealed gas in water invaded zones and the design of EOR schemes for water-invaded gas reservoirs.
Sandstone gas reservoirTypes of residual gasWater invasion energyMicroscopic visualizationOccurrence formFormation mechanism
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