Properties of Capillary Fluids at the Microscopic Level

Abstract

Summary: Results are presented of experimental measurements of the properties of model reservoir fluids (water and hydrocarbons) at the microscopic and submicroscopic level—i.e., when they are confined in narrow spaces comparable to the pore sizes of reservoir rocks. Measurements of capillary forces (or pressures) were carried out for curved oil/water interfaces (menisci) of radii as small as 20 A [2 nm]. The results show that the capillary pressure is accurately given by the Laplace equation for even the smallest pore radii of interest in waterflooding. The Kelvin equation describing the equilibrium radius of an oil/water interface has also been verified down to very small radii, but the effect of ions leaching out from the silicate surfaces can greatly enhance the equilibrium meniscus (Kelvin) radius and must not be ignored in such processes. The phenomenon of the spontaneous condensation of water bridges between two surfaces has been observed, highlighting an additional important mechanism for trapping residual oil. The first measurements of the microviscosity of liquids in narrow channels show that the bulk liquid viscosity pertains down to very thin films in the case of both aqueous salt solutions and hydrocarbons. Results of the effects of salinity and pH on clay-swelling pressures are also described. These show that in addition to the expected long-range repulsive electrostatic force between charged surfaces in salt solutions, there is also a short-range (<20-Å [2-nm]), monotonically repulsive "hydration" force, which is an oscillatory function of distance below 15 Å [1.5 nm], with minima at discrete surface separations corresponding to the thickness of layers of water molecules: e.g., at 2.8, 5.6, 7.8 Å, etc. [0.28, 0.56, 0.78 nm]. The oscillatory force is believed to be responsible for the crystalline swelling of clays.