Kinetics of Breaking of a Sulfobetaine Viscoelastic Surfactant Gel in the Presence of Model Oils

Viscoelastic surfactant (VES) gels are used for well stimulation. Breakers such as ethylene glycol mono butyl ether, are compounds that reduce viscosity of VES gels, and are required after the stimulation job to prevent formation damage. The focus here was to delineate the steps involved in the breaking of a VES gel in the presence of model oils. It also aimed to determine the effect of increasing aliphatic chain length on the gel breaking rate. A sulfobetaine VES (40 g/L) was mixed with calcium chloride (600 mM) and three model oils (280 mM) in water. The model oils were n-decane, n-dodecane, and n-hexadecane. The complex viscosity (frequency-dependent viscosity) with time was measured for at most 24 hours at 10 rad.s-1 and strain of 2 % using a rheometer. The rheological experiments were conducted at 50 °C. The viscosity of the VES/oil mixtures increased with time and reached a maximum. The magnitude of the maximum viscosity was dependent on the oil. In the presence of n-decane and n-dodecane, the maximum viscosity steadily dropped for a few minutes before a sharp drop occurred. The drop continued until the viscosity was close to 1 cp. Meanwhile, n-hexadecane increased the viscosity and steadily dropped for 24 hours. The complex viscosity after 24 hours was > 150 cp. The time it took for the gel to break depended on the type of oil. The breakage time increased in the following order: n-decane