Abstract
Abstract Viscoelastic surfactants (VES) are used as diverting agents in carbonate matrix acidizing. However, these surfactants can adversely affect wettability around the wellbore. Receding contact angles were measured to study the effect of spent acid solutions with an amphoteric amine-oxide VES, and the mutual solvent EGMBE on the wettability of Austin cream chalk rocks. However, when oil droplets are injected into spent acid solutions with VES and EGMBE, low oil-acid interfacial tensions (IFT) cause them to spread on the rock surface. Contact angles cannot be measured when droplets spread. In addition, spreading may result from low oil-acid IFT's, rather than indicating strong oil-wetness. A procedure is proposed for contact angle experiments for surfactant solutions that cause oil droplets to spread. Rocks were centrifuged in spent acid solutions with VES and EGMBE. Then, contact angles were measured in spent acid with HCl only, to prevent VES and EGMBE from reducing the oil-acid IFT. The effect of the surfactants in the spent acid on the rock-oil/acid IFT, which is the wettability, is shown by the difference in contact angles before and after centrifuging. Using the proposed procedure, a spent acid solution with HCl, 1 vol% VES and 10 vol% EGMBE made an oil-wet rock water-wet, and a water-wet rock strongly water-wet at 25°C. This suggests that an EGMBE postflush enhances the relative permeability to oil, under the parameters investigated. Contact angles are a function of the rock-oil/acid and oil-acid IFT's. The wettability of the rock is determined by the rock-oil/acid interface. The proposed procedure is effective because, in contrast with the conventional procedure, the oil-acid IFT is kept high and constant, so that changes in the rock-oil/acid interface can be observed. The proposed procedure will be used in future studies of the effect of spent acid solutions with VES and EGMBE on wettability.
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