ADSORPTION OF ANIONIC AND AMPHOTERIC FOAM-FORMING SURFACTANTS ON DIFFERENT ROCK TYPES

Surfactant adsorption studies carried out with surfactants suitable for mobility control by foams in gas-flooding enhanced oil recovery are described. The extent of adsorption of three surfactants (an anionic surfactant, a betaine, and a sulfobetaine) is measured on four types of rock representing reservoir materials (Berea sandstone, Indiana limestone, Baker dolomite and quartz) from three brines (a sodium chloride solution and two synthetic reservoir brines with different concentrations of total dissolved solids). This allows the assessment of the effects of surfactant type, rock type, brine salinity and the presence of divalent cations on surfactant adsorption. In order to probe into adsorption mechanisms, an attempt is made to correlate adsorption levels with rock surface charges. Adsorption of the anionic surfactant on sandstone and dolomite is significantly lower than that of both amphoterics, while adsorption of the amphoterics on limestone is similar to or lower than adsorption of the anionic surfactant. The presence of divalent cations increases the adsorption of the anionic surfactant and the betaine on sandstone and limestone, but has very little effect on the adsorption of the sulfobetaine on all rocks, or on the adsorption of all three surfactants on dolomite. The effect of the total dissolved solids concentration in the brine on adsorption is different for different surfactant/rock combinations. A correlation of surfactant adsorption with rock surface charge shows that adsorption of the anionic surfactant is consistent with an electrostatic mechanism of adsorption. Adsorption of both amphoteric surfactants occurs through a complex interplay of mechanisms, some of which can be suggested from the observed trends.