Summary
Carboxybetaine viscoelastic surfactants have been applied in acid diversion
and fracturing treatments in which high temperatures and low pH are usually
involved. These surfactants are subjected to hydrolysis under such conditions
because of the existence of a peptide group (-CO-NH-) in their molecules,
leading to changes in the rheological properties of the acid. The objective of
this paper is to study the impact of hydrolysis at high temperatures on the
apparent viscosity of carboxybetaine viscoelastic surfactant-based acids, and
propose the mechanism of viscosity changes by molecular dynamics (MD)
simulations.
Surfactant-acid solutions with different compositions (surfactant
concentration varied from 4 to 8 wt%) were incubated at 190°F for 1 to 6 hours.
Solutions were then partially spent by CaCO3 until the sample pH was
4.5, and the apparent viscosity was measured using a
high-temperature/high-pressure (HT/HP) viscometer. To understand the mechanism
for viscosity changes on the molecular level, MD simulations were carried out
on spent surfactant-acid aqueous systems using the Materials Studio 5.0
Package.
It was found that short-time hydrolysis at high temperatures (for example, 1
to 2 hours at 190°F) led to a significant increase in surfactant-acid
viscosity. However, after incubation for 3 hours, phase separation occurred and
the acid lost its viscosity. Simulation results showed that viscosity changes
of amido-carboxybetaine surfactant acid by hydrolysis at high temperatures may
be caused by different micellar structures formed by carboxybetaine and fatty
acid soap, its hydrolysis product. The optimum molar ratio of
amido-carboxybetaine and fatty acid soap to form worm-like micelles was found
to be nearly 3:1 from our simulations.
Our results indicate that hydrolysis at high temperatures has a great impact
on surfactant-acid rheological properties. Short time viscosity build-up and
effective gel breakdown can be achieved if surfactant-acid treatments are
carefully designed; otherwise, unexpected viscosity reduction and phase
separation may occur, which will affect the outcome of acid treatments.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
1 April 2011
- Meeting paper published:
27 March 2011
- Revised manuscript received:
2 March 2012
- Manuscript approved:
7 March 2012
- Published online:
27 November 2012
- Version of record:
6 December 2012
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