EFFECTS OF BUOYANCY FORCES ON MISCIBLE LIQUID-LIQUID DISPLACEMENT PROCESSES IN A POROUS-MEDIUM

The effects of buoyancy forces on the displacement of one liquid by an other in a consolidated porous medium have been investigated experimen tally for the specific case in which the two liquids are completely mi scible. In order to obtain a clear understanding of the favorable and unfavorable effects of buoyancy forces, experiments were carried out i n three different flow modes, namely horizontal, vertical-downward and vertical-upward. As the effects of buoyancy forces are negligible for two-dimensional porous media in the horizontal flow mode, the results obtained in this mode were used as a reference for comparison with th ose obtained in the two vertical modes. The miscible system employed c onsisted of an aqueous glycerol solution and distilled water (dyed wit h methylene blue) as the displaced and displacing fluids respectively. Displacements using five different density ratios were studied, with the density ratio being varied by changing the concentration of the gl ycerol solution. The breakthrough time and fractional recovery of the displaced fluid were measured in each case and photographs of the unst able fingering patterns were taken for a wide range of injection flow rates. The results obtained indicate that the effects of buoyancy forc es can be very significant, especially at low flow rates and/or at hig h density ratios. When the density of the displacing fluid is less tha n that of the displaced fluid (which was the case in the present work) , buoyancy forces play a deleterious role in vertical-upward displacem ents since they tend to promote fingering and reduce breakthrough time s, thereby producing low recoveries of the displaced phase. In contras t, under the same density conditions, buoyancy forces play a positive role in vertical-downward displacements since they then tend to stabil ize the displacement process (i.e. reduce fingering), thereby producin g increased breakthrough times and correspondingly high recoveries of the displaced phase. However, at high flow rates, the observed fingeri ng patterns and recoveries are very similar for horizontal, vertical-d ownward, and vertical-upward displacements, since viscous forces are d ominant under such conditions and buoyancy forces are insignificant.