Viscoelastic surfactants (VES) have been widely used in acidizing and acid fracturing. They are used as diversion agents during matrix acid treatments and leakoff control agents during acid fracturing. At high temperatures, viscoelastic surfactants hydrolyze, resulting in phase separation after a certain time. Their viscosities significantly decrease and it is much easier for them to flow back and this causes much less damage to the formation.
In this study, 4 to 8 wt% of a new VES-acid system were tested at temperature up to 250°F over hydrolysis times of 0 to 6 hours. Then the solutions were neutralized by calcium carbonate until pH reached 4.5. An HP/HT rheometer was used to measure the viscosity of the spent acids. Mass spectrometry (MS) was conducted to analyze the hydrolysis products of the VES. Coreflood tests were also conducted on Indiana limestone to determine the effect of the hydrolysis products on the permeability of these cores. Temperature was set at 250°F and flow rate at 2.5 cm3/s.
The viscosities of all VES-acid systems remained high at the beginning of hydrolysis, which was good for acid diversion. After that, the VES acid systems experienced significant viscosity reduction because of phase separation; it became much easier for the spent acid to flow back. Coreflood experiments showed little damage caused to the Indiana limestone. MS results indicated that hydrolysis was achieved through the peptide bond. Fatty acids formed the top oil layer and amine-based molecules formed the aqueous phase.
This paper will summarize and discuss the details of viscosity changes of the acid systems of this kind of viscoelastic surfactant, the damage caused by the hydrolysis products, and how this kind of viscoelastic surfactant can be used to improve the treatments.