Experimental Study of Wettability Alteration to Preferential Gas-Wetting in Porous Media and Its Effects

In a recent theoretical study, Li and Firoozabadi showed that if the wettability of porous media can be altered from preferential liquid-wetting to preferential gas-wetting, then gas well deliverability in gas condensate reservoirs can be increased. In this paper, we present the results that the wettability of porous media may indeed be altered from preferential liquid-wetting to preferential gas-wetting. In the petroleum literature, it is often assumed that the contact angle through liquid phase θ is equal to 0 for gas-liquid systems in rocks. As this work will show, while θ is always small, it may not always be zero. In laboratory experiments, we altered the wettability of porous media to preferential gas-wetting by using two chemicals, FC754 and FC722. Results show that in the glass capillary tube θ can be altered from about 50 to 90 and from 0 to 60 by FC754 for water-air and normal decane-air systems, respectively. While untreated Berea saturated with air has a 60% imbibition of water, its imbibition of water after chemical treatment is almost zero and its imbibition of normal decane is substantially reduced. FC722 has a more pronounced effect on the wettability alteration to preferential gas-wetting. In a glass capillary tube θ is altered from 50 to 120 and from 0 to 60 for water-air and normal decane-air systems, respectively. Similarly, because of wettability alteration with FC722, there is no imbibition of either oil or water in both Berea and chalk samples with or without initial brine saturation. Entry capillary pressure measurements in Berea and chalk give a clear demonstration that the wettability of porous media can be permanently altered to preferential gas-wetting. Introduction In a theoretical work, we have modeled gas and liquid relative permeabilities for gas condensate systems in a simple network. The results imply that when one alters the wettability of porous media from strongly nongas-wetting to preferential gas-wetting or intermediate gas-wetting, there may be a substantial increase in gas well deliverability. The increase in gas well deliverability of gas condensate reservoirs is our main motivation for altering the wettability of porous media to preferential gas-wetting. Certain gas condensate reservoirs experience a sharp drop in gas well deliverability when the reservoir pressure drops below the dewpoint. Examples include many rich gas condensate reservoirs that have a permeability of less than 100md. In these reservoirs, it seems that the viscous forces alone cannot enhance gas well deliverability. One may suggest removing liquid around the wellbore via phase behavior effects through CO2 and propane injection. Both have been tried in the field with limited success; the effect of fluid injection around the wellbore for the removal of the condensate liquid is temporary. Wettability alteration can be a very important method for the enhancement of gas well