Trapping of Oil Drops in a Noncircular Pore Throat and Mobilization Upon Contact With a Surfactant

Abstract

An experimental apparatus was developed to study trapping and mobilization of oil drops in a capillary of square cross section [100 microns × 100 microns (100 µm × 100 µm)] having a constriction also approximately square in shape. The throat of the constriction had dimensions of about 10 microns (10 µm) on a side. Experiments to investigate trapping consisted of injecting a drop of nonwetting phase liquid ("oil") into a flowing water stream (wetting phase). A nonane/water system was used for most of the experiments. Pre-equilibrated alcohol/water systems were used to study effects of interfacial tension (IFT). A drop was displaced toward the constriction by the flowing water. The behavior of the drop as it approached and Was trapped by the constriction or as it moved through the constriction was observed as a function of flow rate, drop length, and IFT between the water and nonwetting liquid. Mobilization by surfactants was investigated by conducting a series of displacement experiments in the capillary cell. In each experiment, an oil drop about 890 µm in length was trapped at the constriction by brine (10,100 ppm NaCl) flowing at an average pore velocity of 1.3 ft/D (0.4 m/d). A chemical slug containing surfactant and cosurfactant was injected into the capillary cell and displaced toward the trapped oil drop by the brine. Observations, measurements, photographs, and movies were made using a microscope. Provision also was made to monitor the pressure drop across the constriction.Two phenomena were observed in the trapping experiments. First, the position of a trapped, stable drop relative to the throat depended on IFT, displacing liquid velocity, and oil drop size. Second, by increasing the main stream velocity, increasing the drop size, or decreasing the IFT, the front interface of a stable drop could be caused to move further into the throat of the constriction until the drop became unstable and a "snap-off" of the drop started. In this snap-off process, small drops of oil broke away from the main drop and moved through the constriction. Eventually the oil drop achieved a size where the snap-off stopped. Data were correlated empirically to define conditions under which a drop was stable as well as those where snap-off occurred.Data from the mobilization experiments allowed two mechanisms to be identified. In Mechanism 1, minute quantities of the surfactant and/or cosurfactant were carried ahead of the main surfactant slug by brine that bypassed the slug. This reduced the IFT between the oil and brine. The oil drop moved further into the constriction and a snap-off process ensued. Mechanism 2 occurred when a sharp interface of the surfactant slug contacted the trailing edge of a nonane drop that was either trapped or in a snap-off process. A series of events that mobilized the drop occurred over a very short time period (<4.0 seconds). The events included rupture of the drop interface, rolling motions inside the drop with resulting emulsification, and passage of the emulsified drop through the pore constriction without snap-off. The emulsification reflected hydrodynamic instabilities —i.e., a Marangoni effect.