Abstract

This work aims at evaluating a new surfactant based viscoelastic fluid for Chemical EOR applications. The surfactant based fluid exhibits a viscous phase at low concentration and high temperature, useful for improving oil recovery.

The viscoelastic fluid is induced by wormlike micelles formed by self-assembled surfactants. The phase diagram of the surfactant in pure water was established using a pervaporation-based microfluidic device (Leng et al., PRL, 96, 2006). Isotropic wormlike micelles have been observed up to 12 % w/w. In a second step, Particle Tracking Microrheology (PTM) was used to investigate the rheological properties of the fluid for surfactant concentrations below 2% w/w in water. Viscosity at low surfactant concentrations (0.1% to 0.3 % w/w), T= 80°C, in synthetic sea water (3.9 % w/w TDS) and in sodium chloride (2 % w/w TDS) has been recorded. Data shows that the viscosity is weakly dependent on brine concentration and evolves between 3 and 15 mPa.s (g=10 s-1), for surfactant concentrations between respectively 0.1% to 0.3 % w/w.

The second series of tests consisted of core-flood experiments at 80°C in Clashach sandstone with brine solution (NaCl 2% w/w) containing surfactant concentrations between 0.1 %w/w and 0.3 %w/w. The surfactant is shown to adsorb moderately on the sandstone (50 mg/g) and displace a great fraction of residual oil (from Sor=0.49 to Sor=0.20).

These preliminary results show a strong potential for this new surfactant based viscoelastic fluid in chemical EOR. Compared to other viscoelastic fluids this product shows the following advantages:

Superior viscosity, at low surfactant concentration, in hard brine and at high temperature

Better displacement of residual oil in core-flood with moderate adsorption

1. Introduction

For a majority of oil reservoirs, large amount of oil are still left unrecovered after extensive water flooding, (today average worldwide recovery factor is 32%). Chemical EOR technology is the most promising tertiary recovery technique to both improve sweep and displacement efficiency. The well known process to improve reservoir sweep efficiency consists in injecting low concentration of polymer that viscosifies water. This process known as polymer flood has been extensively used at large scale especially at the Daqing field in China. More complex chemical enhanced oil recovery processes use both surfactant to reduce oil-water interfacial tension and polymer to improve sweep efficiency (SP). The addition of alkali to surfactant flooding reduces the amount of surfactant required and form the process known as alkaline surfactant polymer flooding (ASP). Low interfacial tensions are requested in these processes and formulations are thus more complex and sensitive to reservoir conditions. Here we describe a new surfactant-based viscoelastic fluid extending the potential of chemical EOR.

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