Abstract
Excessive loss of high-density brines into the formation has always been a major concern during completion operations, since it is uneconomical and leads to formation damage and well control issues. A viscoelastic surfactant (VES) system was introduced as an alternative for polymers because of high-temperature instability, incompatibility problems, and the necessity of acid cleanup for the polymer-based system. However, past literature documented that VES systems were limited to only CaCl2/CaBr2 brines due to the negative effect of ZnBr2 solutions on the surfactant's ability to form gel which limits the applicable density for past fluid-loss pills. This paper introduces improved fluid technology that uses nanoparticles and new low molecular weight surfactants to effectively control fluid-losses for high-density brine that contains ZnBr2 at high temperature application.
Laboratory studies were conducted to examine the effect of the VES system on controlling leak-off rate using a HP/HT fluid-loss cell. The apparent viscosity of the VES solution was measured as a function of shear rate and temperature (up to 300°F).
Based on experimental results leak-off of surfactant based fluid is controlled by the elastic structure of the surfactant micelles. The strong elastic structure of gel will not deform to pass through the pores, it will plug this pores and eliminate any flow of the solution. Therefore, surfactant-based pill developed long-term stability as it tested for 7 days with a noticeable decrease in the leak-off rate with time. Optimum surfactant concentration does not depend on the brine composition or formation permeability but it was mainly dependent on temperature.
A breaker based on mineral oil caused a smooth viscosity decline while a breaker based on mutual solvent had a rapid viscosity decline. For mutual solvent, viscosity was instantaneously reduced by 50 to 90% depending on brine composition, surfactant concentration and temperature. However, temperature has most significant effect.
