Abstract

One of the most severe limitations of surfactant flooding is the instability of the commonly employed surfactants in the usual ionic environments of oil reservoirs. Of crucial concern is their salt tolerance and, especially, their sensitivity to the presence of divalent and monovalent ions. In this study, such interactions were investigated by implementing experiments using etched glass micromodels. Micromodels are produced with the objective of directly observing fluid flow through porous media and contain an etched flow pattern that can be viewed with a microscope. In this study glass was used to construct one-quarter five-spot etched-glass micromodels toprobe into the effects of salinity on macroscopic behaviour of surfactant flooding. Numerous runs were performed in order to investigate the effect of salinity on the performance of surfactant solution at a given surfactant, cosurfactant and polymer concentration. Salts used in this set of floods were sodium chloride, magnesium chloride and calcium chloride. It was implicated that four different salinity (in terms of CaCl2 concentration) ranges exist. Each of these ranges render a unique behaviour regarding the ultimate recovery trends resulted from image analysis techniques. There exists a certain range of salinity in which ultimate recovery does not change with salinity increase. The second salinity range is beyond the salt tolerance (first salinity range) of the surfactant solution which results in decrease in ultimate recovery. In the third range of salinity ultimate recovery was surprisingly enhanced due to plugging of high permeable pores. In the fourth salinity range precipitation increases in such a way that most of pore throats, even those with intermediate permeability are plugged and ultimate recovery commences to decrease as salinity increases.

Introduction

Surfactant flooding is renowned as a method to produce trapped residual oil after waterflooding. The micro-emulsions used to lower the interfacial tension (IFT) contain surfactant, hydrocarbon and water. Cosurfactant or an electrolyte may be added. One of the most severe limitations of surfactant flooding is the surfactants sensitivity to the reservoir environment. Of particular concern is their salt tolerance and, especially, their sensitivity to the presence of divalent and monovalent ions. Such interactions have been investigated by Bansal and Shah [1]. In particular, they reported an increase in IFT with increasing CaCl2 (or MgCl2) concentration in connate water. Bansal and Shah [2] also noted that the optimum salinity (in terms of NaCl concentration) dropped as they increased the concentration of divalent ions, this effect being more dramatic in the case of calcium. Hirasaki and Lawson [3] studied the association of surfactant micelles with sodium and calcium ions using lectrostatic and equilibrium models. Glover et al.[4] reported that divalent ions form sulfonate species which can strongly influence the microemulsion phase behavior. This effect has been studied in detail by Celik et al.[5]. They reported that the interaction between divalent ions and petroleum sulfonates initially involved precipitation, followed by redissolution at higher surfactant concentrations. Work carried out has shown that the IFT of dilute petroleum sulfonate solutions against oil increased dramatically upon additions of calcium or magnesium salts (Kumar et al.[6]).

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