Polymer/Surfactant Transport in Micellar Flooding

Abstract

Abstract For the surfactant formulations used (particular surfactant concentration, surfactant type, cosolvent type, cosolvent concentration, etc.), the results show that surfactant systems containing polymer as a mobility control agent may exhibit adverse polymer transport behavior during flow through porous media. Polymer generally lagged behind the surfactant even though the two species were injected simultaneously in the surfactant slug. This poor polymer transport definitely could have a detrimental effect on the efficiency of a micellar flooding process in the field. Phase studies show that when some surfactant systems containing xanthan gum are mixed with crude oil at various salinities, a polymer-rich, gel-like phase forms. The polymer lag phenomenon in core tests can be related to phase separation due to divalent cations generated in situ as a result of ion exchange with the clays and the surfactant. Introduction and Background Proper design for mobility control is important in micellar or surfactant flooding to maintain stable displacement and prevent or reduce viscous fingering. Generally, effective mobility control is obtained by having the mobility of the polymer drive less than the mobility of the surfactant slug and the mobility of the surfactant slug less than the mobility of the oil/water bank. Oil-external and aqueous surfactant systems are designed to attain mobility control by two distinct approaches. In the oil-external case, increases in viscosity (compared with water) are largely a result of the presence of oil and the relatively high concentration of surfactant in the slug. The viscosity of aqueous surfactant systems frequently is increased by the addition of a polymer, usually either xanthan gum or polyacrylamide. There are drawbacks to both of these approaches. Use of a hydrocarbon such as crude oil (in the surfactant slug) to increase viscosity may result in injectivity problems due to wax and/or asphaltenes in the crude oil.Trushenski investigated the effects of sulfonate, cosurfacant, water, and salt concentrations on sulfonate/polymer incompatibility (multiple phases may form when polymer solution dilutes with a sulfonate containing micellar fluid). In dynamic core tests, this phase separation may result in one phase being trapped in the porous media. For the sulfonate/cosurfactant system (Mahogany AA and isopropyl alcohol) used, the results of static phase studies correlated with the incidence of phase trapping in dynamic core tests. Trushenski concluded that increasing the concentration of cosurfactant or cosolvent in the surfactant and polymer slugs could eliminate or reduce sulfonate/polymer incompatibility. In his dynamic core tests, the absolute brine permeability of the Berea cores was characteristically about 500 md. Polymer type (polyacrylamide or polysaccharide) did not appear to affect phase behavior appreciably. He identified sulfonate/polymer incompatibility as a previously unreported source of sulfonate loss. Trushenski also concluded that invasion of a surfactant slug by a water-soluble polymer can be eliminated or minimized if the surfactant slug contains low concentrations of crude oil.Pope et al. studied the phase behavior of surfactant/brine/alcohol systems both with and without polymer. Some of their various combinations also were equilibrated with a synthetic oil consisting of n-octane or an n-octane/benzene mixture. Both anionic and nonionic polymers were used. At a sufficiently high sodium chloride concentration, the oil-free (no added oil) mixtures showed phase separation into an aqueous polymer-rich phase and an aqueous surfactant-rich phase. They termed the salinity at which phase separation occurred the critical electrolyte concentration. SPEJ P. 603^