Capillary Wicking in Gas Wells

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. Introduction Problems involving gas flow past trapped liquids in porous media are encountered in a variety of contexts, such as water block removal in gas wells, evaporation of volatile oils, and recovery of residual oil. In the case of a binary system, such as gas and water, the thermodynamic phase equilibrium can be represented by a simple linear law and gas injection that reduces to a drying problem in which the remaining liquid is evaporated by the flowing gas. Drying of wetting liquids in porous media has been studied by several authors. These studies mainly focused on pass-over drying, in which gas is passed over a porous medium saturated with the wetting liquid. This form of drying is controlled by the gas flow rate. However, when the liquid recedes into the porous medium, drying is controlled by the rate of diffusion of the components in the liquid phase in the pore spaces. Early in 1949, Allerton et al. studied through-drying of packed beds of crushed quartz and other porous materials by convection of dry gas. The study, however, did not consider the effect of gas compressibility or capillarity. Whitaker developed a diffusion theory of drying using volume averaging methods with constant pressure in the gas phase. This eliminated the effect of compressibility of gas on the drying rates and therefore is useful only in a pass-over drying context. Experimental and simulation studies of gas injection (Dullien et al. 1989; Holditch 1979; Kamath and Laroche 2003) showed that trapped water is first removed by a viscous displacement followed by a long period of evaporation. These studies showed that higher pressure drop, permeability, and temperatures caused greater rates of evaporation and faster progression of saturation drying fronts in both fractured and unfractured wells.