Summary
Steam injection is often not a good alternative for oil recovery from shallow bitumen reservoirs. For instance, the thin caprock creates the danger of steam breakthrough. For deeper reservoirs, the heat losses from injection wells may be prohibitive. A technology that may be better suited is oil recovery aided by low-frequency electrical heating of the reservoir. This technology, well known for environmental remedial applications, has been field tried recently, yielding promising results. The process uses electric conductivity of connate water to propagate an alternating current between electrodes, inducing the Joule heating of the reservoir. An associated problem is the appearance of hot spots around the electrodes that may be relieved by water circulation. However, the water circulation may have a significant effect on the heating process because the electric conductivity of the circulated water depends on its salt content.
To find out the influence of salt concentration on process efficiency, we have studied the process of salt-water recirculation around an electrode using numerical simulation. The physical properties and operational data for Athabasca bitumen have been collected from the literature. The model built with Computer Modelling Group's STARS simulator and tested first with available analytical solutions has been validated, and the proper choice of the underlying grid and numerical tuning parameters has been verified. The process was also simulated at field scale for a common pattern of electrodes and production wells. The salt penetrated into the reservoir, far beyond the major water-circulation zone around the electrodes. This process increases the electric conductivity in a large region between electrodes, which improves the heating of the reservoir. The single-electrode simulation studies using different tools yielded similar results for a simple problem. More-complex (and more-realistic) field-scale simulations show that adding salt enhances the oil production. In practice, an upper concentration limit may be given by corrosion problems at the electrodes.
The reservoir simulation of bitumen recovery assisted by low-frequency heating is a challenging multiphysics problem. The understanding of the influence of salt concentration on the circulated water provided by this work is an important key in process-design considerations.
Accuracy of salt concentration measurement using test paper.
