The Effect of Viscoelastic Surfactants and a Mutual Solvent on the Wettability of a Carbonate Rock

Abstract

Abstract Proper acid placement/diversion is required to make matrix acid treatments successful. Viscoelastic surfactants (VES) are used as diverting agents in carbonate matrix acidizing. However, these surfactants can adversely affect wettability around the wellbore. Spreading droplets may not reflect wettability, if they result from low oil-acid IFT's. Therefore, a procedure was proposed for contact angle measurements when surfactant solutions, such as spent acid with VES and EGMBE,reduce interfacial tensions (IFT's) and cause oil droplets to spread (Adejare et al. 2012). The effect of two amphoteric amine-oxide VES', designated as "A" and "B", and an EGMBE preflush and postflush on the wettability of Austin cream chalk was studied using the proposed procedure.In addition, the two-phase titration experiment was used to measure VES adsorption. A treating schedule sequence typical of carbonate matrix acidizing was used. Rocks were centrifuged in fluids representing the preflush, main acid stage, diverting stage, and postflush.The difference in contact angles before and after centrifuging shows the effect of surfactants in the spent acid on wettability.Contact angles were measured in spent acid with HCl only to prevent VES and EGMBE from reducing IFT's. VES "A" and "B" adsorb on the rock surface at 25 and 80°C.Experiments with acid treatments with 4 vol% VES "A" and "B" diversion stages and a 10 vol% EGMBE preflush and postflush made initially oil-wet rocks water-wet at 25°C, 80°C, and 110°C. Acid treatments with a 4 vol% VES "A" diversion stage only made rocks water-wet at 25°C and 80°C. For the parameters investigated, our results suggest that diversion with VES "A" only, andan EGMBE flush following diversion with VES "A" and "B",can alter wettability to water-wet and increasethe relative permeability to oil.