Impact of Hydrolysis at High Temperatures on the Apparent Viscosity of Carboxybetaine Viscoelastic Surfactant-Based Acid: Experimental and Molecular Dynamics Simulation Studies

Abstract

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.

Impact of Surfactant on the Retention of CO2 and Methane in Carbonate Reservoirs

“…This endothermic adsorption behavior is attributed to the decomposition of surfactant at high temperatures, which decreases the surfactant viscosity and correspondingly weakens the attachment to the rock surface. Furthermore, the detached surfactant conglomerates, and aggregations can provide more entrapment pores that act as additional conduits to hold more gas. , This also indicates that previously VES treated carbonate reservoirs with medium to high temperatures have great potential for CO 2 and CH 4 adsorption, making them excellent candidates for CO 2 sequestration. However, using CO 2 surfactant based foams in gas reservoirs allows for more CH 4 to be adsorbed and hence reduces the natural gas recovery.…”

“…This is because of the strong attachment of surfactant to the rock surface and covering a huge percentage of the adsorption sites previously available on nontreated sample. 16,42 At 100 and 150 °C, surfactant treated samples exhibited a significant increase in adsorption uptake as compared to untreated samples (Figure 6, Figure 7, Figure 8, and Figure 9). At 44 bar and 100 °C, CH 4 uptake increased from 21.15 mg/g on untreated samples to 34.18 mg/g.…”

“…The CO 2 sequestration operation usually commences at later stages of development. Hence, the long incubation time at high temperatures induces phase separation of brine and surfactant resulting in high viscosity reduction because of the break of micelles surfactant groups. , While these studies provide an excellent insight of how rocks and surfactants behave at reservoir conditions, still the effect of these residual chemicals such as VES on gas–rock interactions and EGR process are yet to be addressed.…”

A Novel Formula of Diverting Acid System and Application

“…In 2001, the United States Schlumberger developed a viscous-elastic surfactant (VES) diverting acid fluid system. Then BJ Company had developed this product [3] . In 2009(Ghawar oilfield in Saudi Arabia), Abdulazi Al-Harbi added AP polymers in diverting acid as a new type of diverting agent and applied successfully in many gas wells, average stimulation ratio was up to 10 [4] .…”