Entrapment and Mobilization of Residual Oil in Bead Packs

Abstract

Summary Phase behavior, interfacial tension (IFT), viscosity, and density data were determined for the system 2% CaCl2 brine/isopropyl alcohol (IPA)/isooctane. Liquid pairs from this system were used in a test of capillary number as a correlating function for mobilization of residual oil in geometrically similar porous media as provided by bead packs. Close correlation of results was obtained for a more than five-fold variation in permeability and a more than six-fold variation in IFT. Extensive permeability and a more than six-fold variation in IFT. Extensive investigation was also made of the change in trapped oil saturation given by vertical upward flooding; the ratio of gravity to capillary forces varied more than 100-fold. A correlation between trapped oil saturation and Bond number was obtained that was in good agreement with previous results obtained for gas entrapment. However, capillary numbers for entrapment of a given reduced residual oil saturation (ROS) were found to be slightly higher than those for entrapment of gas. Relative permeabilities were independent of whether the trapped phase was oil or gas and were determined mainly by the magnitude of the trapped nonwetting-phase saturation. Capillary numbers for mobilization of residual oil from bead packs were much higher than typical values for sandstones. For bead packs that had been consolidated by sintering, capillary numbers for prevention of entrapment increased and those for mobilization decreased. The net result was that differences in capillary numbers for mobilization and entrapment were greatly reduced and results became more akin to relationships observed for consolidated sandstones. Introduction Secondary recovery by waterflooding leads to entrapment of oil as a result of capillary action. The oil remaining in the swept zone will be referred to as normal waterflood ROS, S*,. Enhanced recovery of oil over that produced by secondary recovery can be achieved under immiscible conditions either by reducing the amount of oil entrapped or by mobilization of some of the trapped oil. For strongly wetting conditions, which are assumed to apply through-out the present work, trapped oil is held as discrete blobs. The processes of mobilization and entrapment are associated respectively with displacement of discontinuous and continuous oil. Minimization of entrapment is particularly important to maintain the integrity of banks of recovery agents and developed banks of continuous oil. Reductions in ROS with an increase in the ratio of viscous to capillary forces have been demonstrated previously. This ratio is often expressed as the dimensionless group vu/o, where o is the IFT, v is the superficial velocity, and tt is the viscosity of the displacing (wetting) phase. Relationships between capillary number and oil recovery by mobilization have been correlated fairly satisfactorily for consolidated sandstones having a wide range of permeabilities. Capillary numbers for mobilization from selected carbonate cores were much lower than for sandstones, however, showing that the correlation determined for sandstones is by no means general for consolidated rocks. One approach to more detailed delineation of the role of pore geometry in mobilization and trapping, which also provides a more meaningful testing of capillary number as a correlating function, is to investigate geometrically similar systems. In the laboratory, porous media are commonly prepared from glass beads or unconsolidated sands. With due attention to the method of packing, close-sized particles provide media that, in a statistical sense, are geometrically similar. provide media that, in a statistical sense, are geometrically similar. For such media, porosity is constant and permeability varies as r2, where r is the particle radius. Ability to scale porous media ge-ometrically is of particular value with respect to making a directest of correlations between capillary number and ROS. Furthermore, theoretical estimates of capillary numbers for oil recovery need to be tested further against experimental results. In the present work, experimental results are reported for mobilization and entrapment in unconsolidated and consolidated bead packs.