Experimental Investigation of Critical Condensate Saturation and Its Dependence on Interstitial Water Saturation in Water-Wet Rocks

Abstract

Summary An experimental investigation of retrograde condensation in water-wet poreswas conducted in micromodels and long cores to determine the criticalcondensate saturation and to evaluate the significance of interstitial water oncondensate mobility. Although the interstitial water lowered the criticalcondensate saturation, it also could reduce the initial recovery rate byrestricting condensate drainage Introduction When gas-condensate reservoirs are developed by pressure depletion, retrograde condensation occurs at pressures below the dewpoint of the reservoirgas. The condensate liquid dropout is commonly considered to remain largelyimmobile in reservoir pores, leading to a loss of valuable hydrocarbon liquids. This pores, leading to a loss of valuable hydrocarbon liquids. This assumptionarose mainly from relative permeability studies conducted on gas/oil systemsand a limited number of gas-condensate studies that indicated that a minimumliquid saturation (critical saturation) of 30 to 50% is required for the liquidto become mobile. Yet, field experiences indicate condensate flow at much lowersaturations (around 10%), particularly in the presence of interstitial water. Saeidi and Handy studied the retrograde condensation of methane/propanemixtures in a horizontal sandstone core by depleting it to a maximum volumetriccondensate dropout of 18% and did not observe condensate flow even with a 30%interstitial water saturation. Asar and Handy determined gas and liquidrelative permeabilities of a methane/propane mixture in a 0. 190- m-permeabilities of a methane/propane mixture in a 0. 190- m- permeability coreat various pressures. They observed that the liquid permeability core atvarious pressures. They observed that the liquid could flow even at saturationsas low as 10% and postulated that the saturation approaches zero as the fluidsystem approaches the critical condition and the interfacial tension (IFT)between the phases vanishes. They concluded that the liquid could flow at lowphases vanishes. They concluded that the liquid could flow at low liquidsaturations in condensate reservoirs. Gravier et al deposited condensate inhorizontal cores with an interstitial water saturation from 19.5 to 30% bycooling the cores, which were presaturated with a gas mixture that was aboveits dewpoint. The critical condensate saturation, determined by injectinggascondensate into these cores, ranged from 24.5 to 50.5%, giving a totalliquid saturation of more than 70%. They could find no correlation between thecritical condensate saturation and the petrophysical properties of the rocks. Recently, further insights into the formation and flow of condensate in poreswas obtained by conducting depletion tests on horizontally and verticallymounted cores and micromodels. It was shown that the initial formation ofcondensate in reservoir pores below the dewpoint is a film process, withhydraulic continuity being maintained throughout the pores. The occurrence offilm flow and low values of gascondensate IFT were considered to enhancegreatly the effect of gravity on gas-condensate flow behavior in reservoirs. Itwas also shown that interstitial water significantly influences the growth andflow of condensate in pores. Knopp compared gasflooding results on four coreswith water saturations from 19.3 to 26.8% with those from water-free tests. Gas/oil saturations were not affected by the presence of water. Delclaud et al. studied gas-flooding in high-permeability sandstone cores (k 1 to 1.5 m) andreported that the oil recovery and relative permeabilities were nearlyidentical with or without interstitial water saturations of 16 to 25 %. Theresidual liquid saturations were found to be very low at 3.5 to 6% PV and didnot depend on the interstitial water saturation. Dumore and Schols studied thedrainage of oil in Bentheim sandstones (k=2 m) with and without interstitialwater. The residual oil saturation decreased from 17 to 3% when a watersaturation of 24% was restored in the core. Pavone et al. reported oil recoveryresults in a free-fall gravity-drainage experiment on a Fontainebleau sandstonewith a permeability of 1.5 m and concluded that a small amount of interstitialwater dramatically reduces the oil relative permeability. The interstitialwater was found to shift the oil relative permeability curve toward thatapplicable at higher total liquid saturations and, in some cases, to a valuecomparable with that observed with as much as four times the amount ofinterstitial water. The cited reports envision the effect of inter-stitialwater on liquid hydrocarbon recovery completely differently: one sees anegative effect, one sees no effect, two see a drastic improvement. Theunderstanding of interstitial water effects on the liquid recovery clearly islargely unresolved, particularly for gascondensate systems. particularly forgascondensate systems. An experimental investigation of retrograde condensationin water-wet pores was conducted to determine the critical condensatesaturation and to evaluate the effect of interstitial water saturation oncondensate mobility. Depletion tests at simulated reservoir conditions usedhigh-pressure micromodels and long cores. The interstitial water saturation wasvaried in each experiment to establish its effect on condensate flow. Micromodels were used to observe the condensate formation in the retrograderegion and its subsequent flow at the pore level as the liquid dropoutincreased during pressure depletion. These microscopic observationsdemonstrated the dominance of gravitational and shear forces on the flow andgave insight into the significance of the interstitial water saturation. Theexperiments provided the opportunity to relate the observed phenomena to thosethat occurred in a series of core tests that provided, in addition to themicromodel work, quantitative information. Low values of critical condensatesaturation were observed in these experiments, and the importance of the watersaturation was noted. It was clear that the two parameters wereinterrelated.