Recent displacement tests have indicated that under certain conditions the stabilized oil bank formed during dilute surfactant flooding may be composed of as many as three distinct oil banks. These secondary oil banks are characterized by the arrival of high oil cuts, high sulfonate concentrations, and low interfacial tensions in the effluent stream. The mechanisms governing the formation of these three banks, although different for each bank, are related to the interactions between the oil, rock, surfactant solution, and mobility buffer.
To determine the mechanisms governing the formation of these banks, a series of core floods was designed to isolate and study each bank separately. The experimental results, including the production histories and effluent analysis of these tests are presented and discussed. presented and discussed. The results of these tests indicate that two of the banks are formed by the chromatographic separation of high and low equivalent weight sulfonates, while the third is formed by the desorption of sulfonate during the injection of low saline fluids (mobility buffers). Also discussed are the means to alter the production profiles of displacement tests by manipulation of these mechanisms.
Previous investigators have attempted to describe the mechanisms governing the development and propagation of the stabilized oil bank formed during surfactant/polymer flooding. Davis and Jones presented experimental results which indicated a region of high oil cut at oil breakthrough. The oil cuts decreased and eventually stabilized later in the flood. This behavior was attributed to the mobile water bank advancing on and beyond the stabilized oil bank. The oil cut was observed to decrease as the water bank invaded the oil bank.
Gladfelter and Gupta reported a similar trend in their experimental work. Using a microwave scanning apparatus, they observed a "hump" region in the oil saturation profile of linear sandstone cores at the leading edge of the stabilized oil bank. This region of high oil saturation was followed by a steady state region of lower oil saturation. They determined that the "hump" was formed through the hysteresis in the relative permeability curves and the related hysteresis in the fractional flow curve. The growth of the "hump" was found to be solely dependent on the extent of hysteresis and the fraction of oil flowing in the lower steady state region.
Wasson et al. described the development and production of two oil banks formed during low production of two oil banks formed during low concentration surfactant flooding. The formation of these two oil banks was found to be related to the mobilities of the injected fluids. The first bank was formed by the fingering of the low viscosity slug through the more permeable channels of the core. The surfactant solution in these channels came into contact with enough oil to form an oil bank. The second bank was formed during the injection of the mobility buffer. It was believed that the plug flow associated with high viscosity fluids was responsible for the formation of this bank.
Production histories of early displacement tests in this study indicated that three distinct oil banks were being produced. Examples of these banks are shown in Figures 1-3. The arrivals of the first two banks were observed after 50% PV and 100% PV of fluid production. The third bank, similar to that observed by Wasson, was produced at the end of the buffer injection. The purpose of this paper, therefore, is to give a detailed description of the stabilized oil bank formed by the low concentration slugs studied and attempt to describe the mechanisms governing the formation of these secondary oil banks.