Capillary Wicking in Gas Wells

Abstract

Summary Gas expansion near the wellbore during production causes the evaporation of connate water. When the reservoir permeability is low, capillarity is controlling, causing liquid movement to the near-wellbore region, where drying rates are higher. In tight-gas sands or in shale gas formations, where capillarity is high, the gas production itself can cause depletion of the water saturation below residual values because of such evaporation. In this work, we present a study of the fundamental processes involved during the flow of a gas in a liquid-saturated porous medium. We have modeled evaporation by accounting for the capillary driven film flow, or "wicking," of saline liquid to the wellbore or the near-fracture region and the effect of gas expansion. It is shown that, for gas reservoirs with connate water saturation, large pressure drawdowns lead to a drying front that develops at the formation face and propagates into the reservoir. When pressure drops are lower, water rapidly redistributes because of capillarity-induced movement of liquid from high- to low-saturation regions. This phase redistribution causes higher drying rates near the wellbore. The results show, for the first time, the effect of both capillarity-induced film flow and gas compressibility on the rate of drying in gas wells. The model can be used to help maximize gas production under conditions such as water blocking by optimizing the operating conditions. Additionally, it can be used to obtain a better understanding of the impact of capillarity on evaporation and consequent processes, such as salt precipitation.