High-Temperature Viscoelastic Surfactant Fluids with Low Scaling Tendency and Based on Seawater, Produced Water, or Brines

摘要

The viscoelastic surfactant (VES) fluids have been widely used in oilfield operations due to their advantages over polymer-based linear or crosslinked fluids that include low formation damage, good proppant suspending and carrying ability, strong tolerance to low-quality water, to name a few. Most of the conventional VES fluid systems, however, have upper temperature limits at around 250°F, beyond which the fluid performance and stability quickly deteriorate. In this paper, a new type of VES fluids formulated with seawater, produced water, or brines showed excellent performances at 350°F or higher, while at the same time had low scaling tendency upon mixing with high-TDS formation water downhole. To reduce the chemical cost of the new VES fluids, a number of novel and low-cost additives were successfully identified that significantly increased the fluid viscosity at elevated temperatures, resulting in similar or better fluid performances with reduced dosages of the VES molecules. The selected VES-enhancing additives strengthened the VES fluids formulated with various sources of water including low-quality water like seawater or produced water at high temperatures up to 350°F or more. In one example, when additive-I was added at the dosage of about 0.8% by weight, the VES fluid viscosity was enhanced by about 35% on average between 300 and 350°F. At 350°F, the viscosity enhancement was about 50% with the same loading of additive-I. The overall cost of the additive-I applied was below 3% that of the baseline VES fluid. In another case, compared with the baseline VES fluid, additive-II at the dosage of about 0.8% by weight raised the fluid viscosity at 350°F to over four times. The cost of the additive-II used in the case was also trivial when compared with that of the baseline fluid. The fluid viscosity could also be enhanced when a number of other selected additives were added to the VES fluids at appropriate dosages. The additive-enhanced VES fluids, in the meantime, caused only minimal damage to the formation, as the regained permeability was about 90%. The novel VES-enhancing additives might have, through van der Waals forces, simultaneously attached to multiple VES micelles in the fluids, thus strengthening the three-dimensional network of the VES micelles. This way, the overall fluid viscosity could be increased. Further discussions about the enhancing behaviors and mechanisms of the selected additives in the VES fluids formulated with seawater, produced water, or brines, and the laboratory test results will be presented in detail.