Characteristics of Oil Displacements by Aqueous Surfactant Solutions

Abstract The aim of-this laboratory work was a detailed study of oil recovery ability of aqueous surfactant solutions. The first part included determination of relevant physical chemical properties of additive solutions, physical chemical properties of additive solutions, i.e. interfacial and rheological properties, phase diagram exploration as well as an evaluation of the specific adsorption-retention properties of surfactant polymer solutions in porous media used for oil polymer solutions in porous media used for oil recovery tests. In the second part, the results of the series of oil displacements were analysed - to give a precise description of the main features of displacements by aqueous surfactant solutions, - to assess their oil recovery efficiency, shown to depend on surfactant concentration in slug injected, consecutive or simultaneous injection mode of polymer and surfactant and additive retention properties in the porous media chosen, - to compare the oil recovery performances of surfactant solutions and microemulsions taken from the same Winsor III type diagram. Finally, application of a Buckley-Leverett type approach was considered to aid in the interpretation of the successive displacement steps observed during flooding by surfactant solutions at low or moderate concentrations. Introduction Oil recovery processes by means of surfactant micellar solutions or microemulsions include the injection of slugs of varied compositions. For instance, the concentrations of additives (sulfonates and alcohols) used in pilot tests were respectively 2.7, 5.5, 8 and 13.4 weight % on Benton, Big Muddy, North Burbank and Wilmington Fields. Also injected surfactant formulations can contain crude or refined oil (as in the case of recent pilots of Wilmington or Bell Creek Field). Despite a lot of published work dealing with micellar processes, one of the published work dealing with micellar processes, one of the still pending questions concerns the definition of pertinent criteria for selection of the optimal pertinent criteria for selection of the optimal composition and type of micellar solution for a given field. The aim of this laboratory work was a precise assessment of the features, performances and limitations of aqueous surfactant solutions which were studied in the concentration range from 0.2 to 10 weight % of additives. Such solutions can yield, even at low concentrations, to partial oil miscibility, depending on the specific characteristics of the corresponding phase diagram involving all the components present within the swept pore space. The initial compositions of sulfonate solutions, then, are represented along the water-additive side of phase diagrams; they can thus be considered as limit cases of so-called water-rich microemulsions. The following study was divided in two parts - extensive determination of the physical chemical properties of micellar solutions, involved in oil properties of micellar solutions, involved in oil displacement mechanisms inside porous media, - analysis and interpretation of oil displacements by means of aqueous surfactant solutions. DETERMINATION OF PHYSICAL-CHEMICAL PROPERTIES OF SYSTEMS Interfacial properties and phase diagram : Aqueous surfactant solutions, used in subsequent oil recovery experiments, were prepared with two distinct petroleum sulfonates, Petrostep 420 (60 % active material-cosurfactant hexanol) and TRS 10-80 (average M.W. 420 - 73 % a.m.-co-surfactant t-C5 alcohol), dissolved in a brine of constant salinity: 15 g/l sodium chloride. 5 % Petrostep solutions (exact composition given in Table III) in contact with Petrol D (an odorless oil-cut marketed by Esso-Chimie), give two phase systems with interfacial tensions of about 2.10-3 mN/m.