Capillary Retention of Petroleum in Unconsolidated Sands

Abstract

Petroleum retained in oil-reservoir rocks after ordinary methods of recoveryhave ceased to be effective is held by capillarity and adhesion, both of whichresist its displacement. Capillarity retains minute filaments of oil in thesmaller pore spaces and acute interfacial angles between the sand grains.Adhesion results in the retention of a film of oil that completely encases eachindividual sand grain. In fine-grained sands and with the more viscous oils, high percentages of the original oil content of the reservoir rock may thus beretained by the operation of these two forces, after the expelling force due toexpanding natural gas has been dissipated. Improved processes designed torecover a larger percentage of the oil content of the reservoir sand must bepredicated on a proper understanding of the magnitude and mode of operation ofthese two forces. This paper records the results of a theoretical and experimental study ofthe retentive effects of capillarity on crude petroleum in unconsolidatedsands. The physical factors responsible for the capillary attraction of sandfor oil are first reviewed. The results of a series of laboratory experiments, designed to disclose the magnitude of the capillary force operative undervarying sand conditions and with different types of oils, are then presented.The experimental data are next used in the development and evaluation of amathematical formula which theoretically provides a means of computing themagnitude of the capillary force under any prescribed conditions. Finally, several practical applications for such a formula are discussed. Physical and Mathematical Concepts of Capillarity Capillarity may be defined as a natural phenomenon by virtue of which aliquid is drawn into and held by minute cavities in a solid substance. It is aforce which is developed by the combined action of the surface tension andcohesion of the liquid and the molecular attractions existing between theliquid and the solid substance composing the walls of the capillarycavities. It is convenient to explain capillary force with reference to the rise of aliquid in a tube of capillary size.