Rate Effects in Tertiary Micellar Flooding of Bradford Crude Oil

Micellar flooding of the tight Pennsylvania oil reservoirs invariably is accompanied by low flood advance rates -on the order of a fraction of 1 ft/D. This investigation, therefore, was devoted to the effect of rate on tertiary oil recovery. The experiments were conducted in 2- and 4-ft-long Berea sandstone cores at rates as low as 0.10 ft/D. All runs were tertiary in nature. The micellar solutions being used in the field tests in Pennsylvania also were used for this research. The majority of the runs were carried out in horizontally positioned cores (although a few runs used vertical cores) to determine if gravity was a factor in the observed effect. It was found that oil recovery decreased with a decrease in the flood advance rate up to a point. Thereafter, it showed a small increase with further decrease in rate. This effect has not been reported by other investigators. The rate effect is discussed and analyzed in terms of the system phase behavior, sulfonate adsorption, dispersion, diffusion, and mobility control. The role of the rate effect on the formation of the stabilized bank also was developed for the experimental conditions involved. The implications of very low rates are discussed in the light of field results. Introduction During recent years, much effort has been devoted to investigations of oil displacement by micellar solutions. This process was proposed and described in detail by Gogarty and Tosch1 and Davis and Jones.2 An analysis of the mechanism of the process is given by Bleakley.3 The micellar solutions are composed primarily of a hydrocarbon, water, a surfactant, and a cosurfactant. Micellar flooding involves sequential injection of a micellar slug, a mobility buffer, and drive water. Since one of the significant characteristics of Pennsylvania oil reservoirs is the low formation permeability, water flood rates in many of these reservoirs are much less than 1 ft/D. Currently, tertiary micellar floods are being conducted in these reservoirs. Thus, it is important to investigate the recovery behavior of micellar displacement at comparable flow rates. Taber et al.4 observed that at both higher and lower rates the displacement of oil and water by an alcohol was efficient. This was confirmed later by Taber and Meyer.5 An increase in micro emulsion displacement efficiency at high rates was observed by Healy et al.6 In our previous work,7 the effect of flood advance rate in micellar/polymer displacement process under a wide variety of conditions was investigated to determine (1) what effect do flow rates have on oil recovery and (2) whether flow rate itself or some other factor, such as gravity segregation, contributes to the observed behavior. Oil recovery was found to be rate dependent under these conditions. One objective of this work is to analyze the effect of flood advance rates in terms of system phase behavior, sulfonate adsorption, mixing mechanism, and mobility control. Another purpose is to understand the effect of rate on the formation of the stabilized oil/water bank and discuss the implications of very low rates.