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Electro-osmotically generated two-phase flow in porous media

Final Activity Report Summary - EOTIP (Electro-osmotically generated two-phase flow in porous media)

When fluid flows through porous rock, an electrical potential is often generated, due to the charges on the surface of the rock and the convection of counter-ions within the pore fluid. This can be used to indicate the direction and magnitude of fluid flow within the rock. The phenomenon is well understood for single-phase flow. However, little is known about the effect of 2-phase flow. Such flow might occur in partially saturated soil, where the fluids would usually be air and water, or in a petroleum reservoir, where the fluids might be oil and water, or gas and water. Changes in streaming potential measured in an oil well might indicate that water is about to arrive at the well unless pumping is reduced. An understanding of how streaming potentials are modified may lead to an early warning of water arrival that allows the total amount of oil recovered from the reservoir to be increased.

The project studied the ideal case of a single, non-conducting droplet flowing along the centreline of a straight, uniformly charged capillary. The first step was to compute the flow around the drop, and the drop deformation. This study investigated greater drop deformations (i.e. higher flow rates) than have been studied before. The numerical work was supplemented by analysis that predicted the drop behaviour at high flow rates. It has always been known that the ratio of the viscosity of the drop to that of the surrounding fluid plays an important role. However, we have shown, for the first time that this role is fundamentally different depending on whether the ratio is greater than or less than 1/2. Instability eventually appears at the trailing end of a low viscosity drop at high velocities, and the numerical predictions agree well with previously published experiments. Capillary waves are created on the surface of high viscosity drops at high velocities. Such waves have been predicted and observed on infinitely long cylinders, but not on finite drops: the waves grow slowly and observation would require an experiment in an exceedingly long capillary.

The project then studied the effect of the drop on the streaming potential generated by the flow. The effect of the drop depended on its size (relative to the capillary), its viscosity (relative to that of the surrounding fluid), and the drop deformation. All these effects were investigated. Analytic theories were developed for very small drops and for long, highly deformed drops; these theories allowed the accuracy of the numerical code to be checked. The presence of the drop sometimes increased the streaming potential and sometimes decreased it, depending on the drop viscosity. However, it was shown that a combination of the change in streaming potential and change in pressure always increased when a drop was added. The theoretical work accomplished in this project now requires experimental confirmation, and the proposed combination of potential and pressure should be straightforward to measure, and should allow experimental results to be interpreted in a consistent fashion.