Experimental and theoretical studies of the effects of buoyancy forces on liquid/liquid displacement processes in porous media.

When a higher-viscosity fluid is displaced by a lower-viscosity fluid in a porous medium, the displacing fluid tends to channel through the paths of lesser hydrodynamic resistance, thereby forming pronounced "fingers". The effects of buoyancy (gravity) forces on the stability of such displacement processes are of considerable importance in many practical situations, especially during the recovery of oil from underground reservoirs, or during the clean-up of subterranean toxic chemical spills, by means of aqueous fluid injection. The present work is confined to the former (i.e., oil recovery) situation. In this work, the effects of buoyancy forces on immiscible water/oil displacement processes and miscible oil/oil displacement processes were studied for linear flows occurring in a two-dimensional, consolidated, transparent porous medium aligned in either the vertical or horizontal plane. Experiments were performed using three different flow modes, namely horizontal, vertical-upwards and vertical-downwards. A wide range of injection flow rates were employed in order to elucidate the relative effects of buoyancy forces, viscous forces and capillary forces. For each displacement, the breakthrough condition was measured, and photographs of the evolving fingering patterns were taken. A mathematical analysis of the observed displacement phenomena produced an analytical relationship expressing the macroscopic water saturation profile as a function of dimensionless distance. The horizontal flow mode was employed as the reference condition for the prediction of the oil recovery efficiency in the two vertical flow modes, since buoyancy forces can be effectively neglected in horizontal flow. A reliable technique for measuring the saturation profile from the displacement photographs was developed in order to interpret the experimental data. Good agreement between the theoretical and experimental results was obtained, thereby permitting a reliable quantitative prediction of the effects of buoyancy forces on oil recovery during immiscible water/oil displacement processes. The extent to which the viscosity ratio, interfacial tension, and flow rate influenced the relative effects of the buoyancy forces will be discussed in relation to measured breakthrough times and oil recoveries. Also, the instability theory of the displacement will be discussed. It is evident from the results obtained that buoyancy forces are capable of exerting very significant effects on the stability of liquid-liquid displacement processes occurring in porous media, on the formation of fingers, and on the ultimate recovery efficiency during practical oil recovery processes involving aqueous fluid injection