Abstract

Highly saline reservoir brine and crude oil with ethoxylated carboxymethylates and cosolvents show a complicated phase behavior. Easily obtainable and well defined indicators are therefore required to characterize phase conditions, which prevail in displacement processes. The phase inversion temperature (PIT) has been investigated with respect to its ability to discern phase transformation in oil-brine-surfactant systems. PIT proved to be very sensitive to changes in salinity, ionic strength, water-oil ratio and surfactant concentration, which simultaneously are responsible for phase inversion of chemical systems in the porous medium. Core flood tests using an ethoxylated carboxymethylate as surfactant have shown that a rapid formation and production of a continuous oil phase in form of an oil bank can be achieved by a composition of the system in the boundary region of phase type III and II according to the terminology of NELSON and POPE. However these flood conditions may cause increased pressure gradients during the flood at the trailing end of the oil bank. Viscosity measurements on the emulsion in a low shear viscosimeter demonstrated that the increase of viscosity of a water-in-oil emulsion can be attributed to the increase of the WOR. In displacement processes this may be provoked by viscous fingering of the mobility buffer into the mobilized oil bank. The length and profile of the oil bank is controlled by phase conditions and viscous forces in the transition zone of the mobility buffer. By addition of a suitable alcohol as a cosolvent to the surfactant slug or to the mobility buffer the temperature range of the stable displacement process could be extended without loss of recovery efficiency and the pressure gradients observed during the flood were in the normal range.

Introduction

From the publications of WINSOR /1/, SHINODA /2/, NELSON and POPE /3/, HEALY et al. /4/, the significance of the phase behaviour of water and oil in the presence of surfactants and cosurfactants for the mobilization and banking of residual oil is well known. The forming of a stable microemulsion in the porous medium is viewed as a prerequisite for the partial miscibility of oil and water. HIRASAKI et al. /5/ have observed partial miscibility of oil and water. HIRASAKI et al. /5/ have observed the occurrence of a microemulsion after the displacement of the oil bank by a macroemulsion. If this microemulsion is water-external and of low viscosity, it can be expected that its mobility and adsorption will not lead to increased retention of surfactant in the porous medium. The microemulsion is in phase equilibrium with the aqueous and oil phases and exhibits a constant composition corresponding to the surfactant concentration; hence it must subsequently release the oil taken up through solubilization in the form of a continuous oil phase. A change in the surfactant concentration in the chemical slug, as a result of retention or dilution, affects both the volume of the microemulsion and its state.

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