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.

Viscoelastic surfactants (VESs) are used as diverting agents in carbonate matrix acidizing. However, these surfactants can adversely affect the wettability around the wellbore.Receding contact angles were measured with the conventional method to study the effect of spent-acid solutions with an amphoteric amine-oxide VES and the mutual solvent ethylene glycol monobutyl ether (EGMBE) on the wettability of Austin cream chalk rocks. The conventional method involves the injection of oil droplets into the surfactant solution. However, contact angles could not be measured when oil droplets were injected into spentacid solutions with VES and EGMBE, because low oil/acid interfacial tensions (IFTs) cause them to spread on the rock surface.A new procedure was used for these contact-angle measurements. Rocks were centrifuged in spent-acid solutions with VES and EGMBE, so that the surfactant changed the interfacial properties of the rock. Then, contact angles were measured in spent acid with hydrochloric acid (HCl) only, to prevent VES and EGMBE from reducing the oil/acid IFT. The effect of the surfactants in the spent acid on the acid/rock and acid/oil IFT, which is the wettability, is shown by the difference in contact angles before and after centrifuging.With the new procedure, a spent-acid solution with HCl, 1 vol% of VES, and 10 vol% of EGMBE made an oil-wet rock water-wet and a water-wet rock strongly water-wet at 25 C. This suggests that an EGMBE post-flush enhances the relative permeability to oil, under the parameters investigated.Contact angles are a function of the oil/rock, acid/rock, and oil/acid IFTs. However, the wettability of the rock is a function of the oil/rock and acid/rock IFT only. The new procedure measures contact angles in such a way that the surface-active agents change the oil/rock and acid/rock IFT only so that "spreading," an artifact caused by oil/acid IFT reduction, may not occur.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractViscoelastic 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 rockoil/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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