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

Summary In a recent theoretical study, Li and Firoozabadi [Li, K. and Firoozabadi, A.: "Phenomenological Modeling of Critical-Condensate Saturation and Relative Permeabilities in Gas-Condensate Systems," paper SPE 56014 available from SPE, Richardson, Texas (2000)] 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 article, 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 ? is equal to 0° for gas-liquid systems in rocks. As this work will show, while ? 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 ? 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 ? 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,1 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 non-gas-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.2–4 Examples include many rich gas-condensate reservoirs that have a permeability of less than 100 md. 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 deliverability. If one can alter the wettability of the wellbore region to intermediate gas-wetting, gas may flow efficiently in porous media. As early as 1941, Buckley and Leverett5 recognized the importance of wettability on water flooding performance. Later, many authors studied the effect of wettability on capillary pressure, relative permeability, initial water saturation, residual oil saturation, oil recovery, electrical properties of reservoir rocks, reserves, and well stimulation.6–16 reported that it might be possible to improve oil displacement efficiency by wettability adjustment during water flooding. In 1967, Froning and Leach8 reported a field test in Clearfork and Gallup reservoirs for improving oil recovery by wettability alteration. Kamath9 then reviewed wettability detergent flooding. He noted that it was difficult to draw a definite conclusion regarding the success of detergent floods from the data available in the literature. Penny et al.12 presented a technique to improve well stimulation by changing the wettability for gas-water-rock systems. They added a surfactant in the fracturing fluid. This yielded impressive results; the production following cleanup after fracturing in gas wells generally was 2 to 3 times greater than field averages or offset wells treated with conventional techniques. Penny et al.12 believed that increased production was due to wettability alteration. However, they did not demonstrate that wettability had been altered. Recently, Wardlaw and McKellar17 reported that only 11% pore volume (PV) water imbibed into the Devonian dolomite samples with bitumen. The water imbibition test was conducted vertically in a dry core (saturated with air). Based on the imbibition experiments, they pointed out that many gas reservoirs in the western Alberta foothills of the Rocky Mountains were partially dehydrated and their wettability altered to a weakly water-wet or strongly oil-wet condition due to bitumen deposits on the pores. The water imbibition results of Wardlaw and McKellar17 demonstrated that the inappropriate hypothesis for wetting properties of gas reservoirs might lead to underestimation of hydrocarbon reserves.