A Theoretical Study of the Effect of Porous Media on the Dew Point Pressure of a Gas Condensate

Abstract Some researchers studied the effect of porous media on the dew point, and they reached various conclusions. Even some influence trends are contrary. But up to now, there is no theory that can well explain these phenomena which look contradictory. In this paper, a theoretical model of calculating dew point is presented, which considers the effect of capillary pressure and adsorption in porous media. Calculation and analysis are performed for the two cases of laboratory and real formation. And the possibility of causing the difference of experimental results in experiment is described. The author also illustrates the influence trends of porous media on the dew point in reservoir. Introduction By now, in oil and gas reservoir engineering, phase behavior testing of formation fluid and modeling calculation both hold that porous media make no influence on phase behavior, but whether porous media affect phase behavior or not, researchers haven't attained identical views yet. The results of several reported experimental studies indicate that the presence of a porous medium has a significant influence on the equilibrium behavior of hydrocarbon mixtures. Trebin and Zadora contend that the initial condensation pressures(dew points) of gas condensate mixtures in porous media can be 10 to 15 percent higher than those observed in conventional PVT cells. Tindy and Raynal reported that saturation pressures of crude oil in porous media were several percent higher than those measured in conventional test cells. On the other hand, earlier results reported by Weinaug and Ordell indicated that vapor-liquid equilibrium relationships of the system methane-n-butane and ethane-n-pentane were not affected by the presence of dry sand. Oxford and Huntington studied the revaporization of n-hexane by nitrogen and found that withdrawal rate and the presence of brine in the porous medium had little effect on the revaporization process. In a study of the effects of wettability change, Smith and Yarborough concluded that the detailed form of the capillary structures of retrograde liquid held in a porous medium had no effect on the revaporization process. Clark studied the adsorption and desorption of light paraffinic hydrocarbons in clay and partially water-saturated sand and sand-clay packs to determine their effect on equilibrium behavior. Compressibility factors for propane at 100 F in the presence of dry sand-clay packs were lowered by 13 percent. However, in sand-clay mixtures containing water, the compressibilities differed by less than 1 percent from those obtained in the absence of the porous media. Clark also studied the effect of a dry sand-clay media on the PVT properties of mixtures of methane and propane. P.M. Sigmund thought the reasons causing the contradiction of the previous investigated conclusions were partly that the fluid was not recirculated through the porous media, so he measured the effect of porous media on phase behavior, dew point, and bubble point by recirculating the fluid through the media; He also calculated the influence of interfacial tension on phase behavior by using the method in paper, and concluded that the effect could be neglected except at very high curvatures (curvature radius<0.00001 cm). And he pointed out such high curvatures were thought unlikely to exist if water-wetting rocks contained connate water. P. 617