Impact of Hydrolysis at High Temperatures on the Apparent Viscosity of Carboxybetaine Viscoelastic Surfactant-Based Acid: Experimental and Molecular Dynamics Simulation Studies
- Meng Yu (Texas A&M University) | Yan Mu (South China University of Technology) | Hisham A. Nasr-El-Din (Texas A&M University)
- Document ID
- Society of Petroleum Engineers
- SPE Production and Operations Symposium, 27-29 March, Oklahoma City, Oklahoma, USA
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 1.2.3 Rock properties, 1.8 Formation Damage, 5.8.7 Carbonate Reservoir, 4.1.5 Processing Equipment, 4.3.4 Scale, 5.4.10 Microbial Methods, 4.1.2 Separation and Treating, 2.5.2 Fracturing Materials (Fluids, Proppant), 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 3.2.4 Acidising
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Carboxybetaine visco-elastic surfactants have been applied in acid diversion, matrix acidizing and fracturing treatments, in which high temperatures and low pH are usually involved. Amido-carboxybetaine surfactants are subject to hydrolysis under such conditions due to the existence of a peptide bond (-CO-NH-) in their molecules, leading to alteration of 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 visco-elastic surfactant-based acids, and determine the mechanism of viscosity alterations 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 HT/HP viscometer. To determine the mechanism for viscosity alteration on 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 long time incubation, phase separation occurred and the acid lost its viscosity. Simulation results showed that the viscosity alteration of amido-carboxybetaine surfactant-acid by hydrolysis at high temperatures may be due to different micellar structures formed by carboxybetaine and fatty acid soap, its hydrolysis product. The optimum molar ratio of amido-carboxybetaine and fatty acid soap was found to be nearly 3:1 from our simulations.
Our results indicate that hydrolysis at high temperatures has great impact on surfactant-acid rheological properties. Short time viscosity build-up and effective gel break-down 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.
Viscoelastic surfactants have been successfully applied in the oilfield as fracturing fluids (Al-Muhareb et al. 2003; Artola et al. 2004; Bustos et al. 2007; Fontana et al. 2007; Bulat et al. 2008), fluid loss pill (Samuel et al. 2003) and matrix acidizing fluids (Nasr-El-Din et al. 2003; Al-Mutawa et al. 2005; Zeiler et al. 2006; Liu et al. 2009; Nasr-El-Din et al. 2009a,b). As the pH increases above 2, viscoelastic surfactants result in rapid viscosity buildup by forming long worm-like micelles, which entangle to exhibit viscoelastic behavior (Samuel et al. 1997). After the treatment, highly viscous gel is broken down by contacting either the formation hydrocarbons or pre/post flush fluids (Chang et al. 2001; Taylor et al. 2003).
Worm-like micelles can be formed by individual surfactants with certain molecular structures (Yang 2002). One kind of amphoteric surfactants, amido-carboxybetaine, has been used for matrix acidizing treatments (Nasr-El-Din et al. 2006a; Nasr-El-Din and Samuel 2007). However, Fu et al. (2002) observed that this type of surfactant-acid fluid experienced viscosity reduction when subjected to high temperatures. For 4 wt% of surfactant-acid fluid incubated at 88°C for 90 min, viscosity was significantly decreased. Phase separation occurred in those samples with longer incubation time. These observations indicate that, on one hand, high temperature may cause fluid viscosity reduction for surfactant-acids, and cause fluid phase separation. On the other hand, it helps breaking down the gel. In this case, no additional breaker is needed.
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