Stabilizing Viscoelastic Surfactants in High-Density Brines

Abstract

Summary Viscoelastic-surfactant (VES) systems are the preferred gelling and viscosity-generating agents for fluids used in the production zone because of their nondamaging effects on the reservoir. Polymer gels have relatively higher rock-retention values and can often damage the invaded zone, in most cases requiring acid treatment to remove. In contrast, VES systems behave as "equilibrium" or "living" polymers, and their viscosity can be reduced by contact with the produced hydrocarbons or with an internal breaker. This can eliminate the need for remedial treatments, greatly reducing operating cost/time and damage to the formation. Cationic, anionic, and cationic/anionic VES systems have historically demonstrated limitations in high-density brines. Electrostatic screening generally reduces the viscosity or causes phase separation. Many surfactants generally have low salt tolerance and minimal to zero tolerance of divalent brines. Most surfactants exhibit a dramatic decrease in viscosity after a certain concentration of salt is reached and even phase separation in some cases. A novel method has been developed to stabilize different VES packages in high-density completion brines. By controlling the curvature of surfactant aggregates using low-molecular-weight surfactant polymers, nanometer-scale manipulation of the phase behavior is achieved. We have viscosified mono-and divalent high-density brines using cationic, anionic, and catanionic mixtures that were previously considered ineffective. This unlocks a vast range of surfactants for use in high-density completion brines, fracturing/stimulation/acidizing fluids, and sand-control operations. This paper details the laboratory work performed to develop VES packages for fluids ranging from fresh water to completion brines. Extensive rheology experiments have been run on several fluids, showing the viscoelasticity of these systems. The selfbreaking nature of VES systems when in contact with hydrocarbons or exposed to a built-in breaker has also been demonstrated by laboratory experiment.