Characterizing gas-water transport behavior in tight shale and itsapplication on the well productivity

This study provides a systematic method to study the gas/water transport behavior and water retentionbehavior in gas shale reservoir using analytical and numerical modelling approaches. The critical watersaturation determined from the relative permeability curve is regarded as the turning point between twogas production stages: Stage I and Stage II. At Stage I, the gas /water two phase flow dominates shalegas production period, while at Stage II the single gas flow and the flow of water in adsorbed phase aredominative. The improved permeability evolution model incorporates both the gas and water sorption-induced swelling strains. The permeability ratio is a time- and location-dependent parameter and its changecan be divided into three stage, namely, the increase at early time, the decrease at mid-term time, and theincrease at later production time. With gas depletion, gas pressure drawdown significantly dominates theevolution of permeability ratio. Continuously, the effect of sorption induced matrix shrinkage strain willbecome dominative with gas pressure decreases to low values. Due to the gas source supplement inducedby gas desorption at Stage I, the gas production rate will temporarily increase but then will continuouslydecrease till to the lowest level at critical water saturation. At Stage II, the gas production rate is significantlyinfluenced by residual water content in shale controlled by the flow of water in adsorbed phase. In addition,the effects of elastic properties (i.e. Young's modulus and Poisson's ratio), initial permeability and diffusiontime on permeability ratio and gas/water production profiles were discussed. These results provide a firstrational method for analyzing the gas/water transport behavior and water retention behavior with potentialapplication of shale gas recovery.