Mobilization of Waterflood Residual Oil by Gas Injection for Water-Wet Conditions

Abstract

Summary Mechanisms by which waterflood residual oil is mobilized and recoveredduring tertiary gasflooding at quasistatic rates and strongly water-wetconditions were investigated with 2D glass micromodels. Two three-phaseoil/water/gas systems were used in the displacement experiments. One system hada positive spreading coefficient, the other a negative coefficient. Results forthe two systems were compared to determine the differences in displacementmechanisms and oil recovery efficiency. Displacement in both systems proceedsby a double-drainage mechanism where a gas/oil displacement is alwaysassociated with an oil/water displacement. The oil/water displacement leads tocoalescence and reconnection of oil blobs. Oil recovery was significantlyhigher for the positive spreading system. The higher displacement efficiencyresulted from f low through thin but continuous oil films that always separatedthe oil and water phases in the positive spreading system. The absence of oilfilms and the possibility of direct gas/water displacements reduced oilrecovery for the negative spreading system. Introduction The mobilization and subsequent recovery of waterflood residual oil bymiscible hydrocarbon-gas flooding depends on the development of favorablemass-transfer (phase-behavior) effects, which in turn require direct contactbetween the injected gas and residual oil phases. Although the phase-behaviorprocess has been the subject of intensive investigation and is now wellunderstood, few previous studies looked at how injected gas contacts waterfloodresidual previous studies looked at how injected gas contacts waterfloodresidual oil and little is understood about the factors that govern theefficiency of this process. Jones and Holm suggested that capillary pressure plays an important role indetermining the accessibility of residual oil to gas. This was confirmed by Kantzas et al. who studied the mechanisms of residual oil recovery bygravity-assisted, inert-gas injection. Moreover, Kantzas et al. showed thatgravity forces, flow through thin films, and gas/water and gas/oil interfacialtensions (IFT's) are important factors in the development of oil banks ingravity-stabilized immiscible gasfloods of waterflood residual oil. Well-defined oil banks, however, are not observed in miscible floodingexperiments where, typically, solvent breakthrough quickly follows thecommencement of oil production. This paper reports the results from an experimental study of the pore-scalemechanisms responsible for the mobilization of pore-scale mechanismsresponsible for the mobilization of water-flood residual oil by immisciblegasflooding in horizontal glass micromodels for strongly water-wet conditions. The fluids used were a refined oil, water, and air. In some of the experiments, a small quantity of isobutanol was added to the water to reduce the water/gas IFT. IFT's for the water/gas, oil/gas, and oil/water systems were measured tocalculate the spreading coefficient, Sow = wg - og - ow,............................ (1) for a displacement experiment. Previous studies of displacements inmicromodels are characterized by very low recoveries and rapid solventbreakthrough as a result of severe fingering of gas. This problem has beenovercome in this study by incorporating a problem has been overcome in thisstudy by incorporating a capillary barrier at the outlet of the model. Thisproduces micromodel displacements that are more typical of displacements inconsolidated porous media where higher capillary pressures can begenerated. The experimental results show that IFT and flow through thin but continuousoil films play major roles in determining oil recovery for strongly water-wetconditions at quasistatic displacement rates. Recoveries for positive spreadingsystems are much higher than for negative spreading systems where there is noflow through oil films. Water recoveries for both systems are high and verysimilar because water is always the wetting phase and thus is able to flowthrough continuous wetting films. Waterflood residual oil is mobilized and reconnected by a double drainagemechanism for both the positive and negative spreading systems. Gas firstdisplaces oil and then oil displaces water. The two drainage events usuallyoccur in close proximity because this minimizes the resistance to flow ofdisplaced oil from the first to the second drainage site. The results show that reconnection of oil is not sufficient to ensure highoil recovery because reconnected oil is trapped easily by the formation oflarge gas loops that cut off direct oil and water paths to the outlet. Forpositive spreading systems, continued oil paths to the outlet. For positivespreading systems, continued oil and water production are possible by flowthrough thin films. For negative spreading systems, there are no oil films andonly water can flow. This is the major reason for the higher observed oilrecoveries for positive spreading systems. Experimental Micromodel. The displacement experiments were carried out in a 2D glassmicromodel containing a regular square network of approximately 4,600intersecting capillaries on a nominal 500-40m spacing. The micromodel wasfabricated with chemical-etching techniques. Although the photographic patternfor the network was regular, the transfer of the pattern to the glass surfaceand the subsequent etching and fusing processes introduced small randomvariations in the dimensions of the actual capillaries and in thresultingpore-body and -throat dimensions. pore-body and -throat dimensions. When viewedthrough a microscope, the capillaries appeared approximately rectangular incross section, with a depth of about 1540m. Pore-body widths varied between 300and 400 40m and Pore-body widths varied between 300 and 400 40m and pore-throatwidths ranged from 100 to 300 40m. Pore-throat lengths ranged pore-throatwidths ranged from 100 to 300 40m. Pore-throat lengths ranged between 300 and500 40m. The inlet side of the micromodel contained a wide channel along theentire edge to distribute injected fluid uniformly across the width of thenetwork. The outlet, on the opposite side, was connected to the network byshort capillaries having effective diameters considerably smaller than those ofthe pore throats in the network. These acted as a capillary barrier to porethroats in the network. These acted as a capillary barrier to the nonwettingphase, delaying gas breakthrough and snowing a greater range of capillarypressures to be generated within the network. Both edges along the length ofthe micromodel contained pores that were smaller than those in the body of themodel. Fig. pores that were smaller than those in the body of the model. Fig. 1shows major features of the experimental micromodel. Fluids. The fluids used in the displacements were air, Soltrol-130 (refinedoil), and distilled water. Table 1 gives the IFT's measured by the dynamic Wilhelmy method with clean glass slides. SPEFE P. 70