Three-Phase Flow in Porous Media: Wettability Effect on Residual Saturations During Gravity Drainage and Tertiary Waterflood

Abstract

Abstract Three-phase flow configurations occur in many situations in petroleum engineering, gravity assisted gas displacement, tertiary waterflood, water alternating gas process (W.A.G.). Therefore residual saturations (oil and gas) knowledge during three-phase flow is a crucial point for reservoir engineers. In this paper we present experimental results dealing with the influence of wettability on two and three-phase flow in porous media in short (0.25m) and long (1m) cores. Forced displacement of gas by water and oil have been performed on the short cores while gas/oil gravity drainage in presence of irreducible water (Swi) followed by a tertiary waterflood have been performed on the long cores. For all the experiments the saturation profiles are measured using a dual-energy gamma-ray attenuation technique. Originality of the experiments lies on the fact that gas/oil gravity drainage and tertiary waterflood are performed on the same core during the same experiment. Two phase flow is obtained at the bottom of the core (in the capillary head region) while three-phase flow is observed in the upper part of the core. Local measurement of saturation profiles show that residual oil saturation at the end of gravity drainage depends on wettability. However we do not observe a wettability effect on three-phase residual gas saturation. Behaviour of three-phase residual oil saturation is much more complicated because its value and saturation profile depends strongly on wettability. Introduction Recovery techniques involving gas injection, water-alternating-gas (W.A.G.) process or gravity drainage in oil reservoirs in presence of connate water are widely used to improve oil recovery. Development of these techniques, among others, illustrates the growing interest devoted three-phase studies. In this paper we focus our attention on the effect of wettability and flow type (two- and three-phase flow) on gas and oil residual saturations. Literature results dealing with that topic can be can be summarized, in a non exhaustive way as follows. Kortekaas et al.1 showed during a waterflood following pressure depletion on water-wet core, residual saturation in three-phase configuration (Sor3?) was lower than residual oil saturation obtained by waterflooding (Sorw). Ma et al.2 confirmed this behaviour for W.A.G. experiments. In the same way, but using different wettability cores Skauge et al.3 showed that residual oil saturation in three-phase flow condition (Sor3?) is significantly lower than residual oil obtained after water (Sorw) or gas (Sorg) flooding. Moreover, for water-wet system they compared the residual saturations with the trapped gas saturation and found that Sorw= Sor3?+Sgt4. Nevertheless data are in contradiction with the results from Fayers5, who provided empirical relationships dealing with Sor3? for different wettability, Sor3?=Sorw-0.5.Sgt for water-wet core and Sor3?=Sorw for intermediate-wettability which is in good agreement with the results of Kyte et al.6 and Holmgren et al.7. Dealing with three-phase residual gas saturation (Sgr3?) different results are published in the literature. Kralik et al.8 and Skauge et al.9 showed experimentally that three-phase residual gas saturation is lower than two-phase residual gas saturation (Sgr3?< Sgr2?). On the other hand Maloney et al.10 and Jerauld11, presenting different imbibition experiments for two and three-phase flow showed that Sgr3?= Sgr2?. Our experimental study deals with the influence of wettability on two and three-phase flow on short core (0.25m) and long (1m) cores of the same origin. Short cores were used for forced displacement of gas by water and oil while long cores were used to perform gas-oil gravity drainage in presence of irreducible water (Swi) followed by tertiary waterflood. Final and transient saturation profiles are measured using dual energy attention gamma ray attenuation technique. We focused our attention on residual (gas and oil) saturations deduced from the final profiles.