Changes that occur with increase in capillary number in the detailed structure of residual oil trapped in water-wet sandstone core samples have been investigated. The technique of using a nonwetting phase that can be solidified and separated from the porous medium has been applied with styrene monomer as the nonwetting phase and 2% CaCl2 brine as the wetting phase. The size distributions of residual oil blobs, obtained under various flow conditions, were measured by both image analysis and Coulter counter techniques. Specific features of blob shapes and dimensions were checked by optical and electron microscopy. The changes in size distribution and shapes of blobs provide insight into the mechanisms of trapping and mobilization of residual oil.
At the conclusion of waterflooding an oil-bearing reservoir, a significant fraction of the original oil still remains in the swept region as trapped residual oil. In water-wet reservoirs, this residual oil, S*or, may typically occupy 25 to 50% of the pore space and provides a main target for tertiary oil recovery. Trapped oil can provides a main target for tertiary oil recovery. Trapped oil can be recovered from a core sample at S*or, by immiscible displacement if the ratio of viscous to capillary forces, expressed in this work as the capillary number Nc = exceeds a critical value. Changes in microscopic distribution of oil within pore spaces can still occur at capillary numbers less than critical. Above the critical capillary number, Nc,(crit), oil is displaced from the core sample. In laboratory investigations, nondimensional relationships between capillary number and the ratio Sor/S*or (residual oil saturation, Sor, normalized with respect to S*or) have been found to be remarkably similar for a variety of sandstones. In addition to the amount of trapped oil. its microscopic distribution within the pore spaces of a reservoir rock is important to gain a better understanding of oil-recovery mechanisms. This knowledge may also be important to the design and implementation of tertiary recovery processes. For example, in modeling the recovery of residual oil, the viscous force required for mobilization of a residual oil blob trapped under water-wet conditions is expected to be inversely proportional to blob length. The technique of using a nonwetting phase, which after flooding to residual saturation can be solidified and then separated from the porous medium to study the microscopic structure of residual porous medium to study the microscopic structure of residual nonwetting phase, was probably first employed by Craze, who referred to the observed capillary structures as irregularly shaped blobs. Blob-size distributions have been measured in the past in sandpacks with styrene monomer as the oleic phase before solidification. The results of this study, although released, have not been made available through publication to the research community at large. A previous study in which styrene polymerization was used has also been cited but is not available. A technique for the study of residual oil structures that involved trapping of melted wax has been used by Morrow and Humphrey. Since Reed and Healy credit the method used by Humphrey to Taber's much earlier unpublished work, it is clear that blobs prepared by this technique have been examined by several investigators. Also, scanning electron micrographs (SEM's) of pore casts of blobs of residual nonwetting phase obtained through solidification of Wood's metal with hot toluene as the wetting phase have been presented by Swanson. Although considerable attention has been paid to the obviously important subject of residual oil structure, the amount of experimentally determined, quantitative information on blob structure and the statistics of blob populations is very limited. To obtain such information, satisfactory techniques for preparing statistically representative blob samples and measuring their size distributions must be devised. Once obtained, the experimentally determined blob-size distributions can be related to measured conditions for mobilization and compared with changes in size distribution predicted by theory.