A Pore-Level Investigation of Relative Permeability Hysteresis in Water-Wet Systems

Abstract

Summary Oil-recovery efficiency is known to depend crucially upon the surface chemistry of the particular crude oil/brine/rock system under investigation. Mineralogy, brine chemistry/pH, crude-oil composition, and ageing conditions all serve to modify the effective contact angles operating within a mixed-wet, porous medium. Recent network-modeling studies have helped to clarify the important role played by wettability during multiphase flow in nonuniformly wet systems. The consequences of modified-contact angles during waterflooding have been described in terms of various regimes. These have, subsequently, been used to reconcile a wide range of apparently contradictory wettability experiments. The purpose of this paper is to apply these ideas to the study of hysteresis phenomena in both strongly and weakly water-wet, drainage/imbibition, relative permeability curves. Such hysteresis effects have long been recognized, but the precise direction of hysteresis seems to vary from study to study. Indeed, in water-wet media, even the degree of sample consolidation appears to play an important role. A three-dimensional (3D) network model is reported, which takes into account a great deal of the underlying pore-scale physics. Initial water saturation, wettability alterations, film flow, phase trapping, and realistic variations in advancing and receding contact angles are all incorporated into the simulations. More specifically, this study examines relative permeability hysteresis in systems from Regime IA, which corresponds to strongly and weakly water-wet, porous media (i.e., media where some degree of modification in the distribution of effective contact angles has occurred). The consequences of different combinations of pore-scale-displacement mechanisms (viz., snap-off and piston-like displacement) for relative permeability hysteresis has been calculated by means of the pore-scale simulator. The effects of combining these mixed mechanisms with other network properties, such as coordination number (z) and certain volume and conductivity exponents (ν and λ), are also reported. The nature of hysteresis is found to change quite dramatically as these parameters are varied, and all experimentally observed trends in relative permeability hysteresis are reproduced under suitable conditions. This may be the first time that a pore-scale model has been used to provide a clear interpretation of all experimentally observed hysteresis trends during primary drainage (PD), secondary imbibition (SI), and secondary drainage (SD) displacements in both strongly and weakly water-wet systems.