Impact of Acid Additives on the Rheological Properties of Viscoelastic Surfactants and Their Influence on Field Application

Abstract

Abstract This paper examines a new class of viscoelastic surfactants (amphoteric) that are used to enhance sweep efficiency during acid injection. Surfactant molecules align themselves and form rod-shape micelles once the acid is spent. These micelles cause the viscosity to significantly increase, and induce viscoelastic properties to the spent acid. The enhancement in these properties depends on the micelle shape and magnitude of entanglement. The effects of acid additives and contaminants (mainly iron (III)), on the rheological properties of these systems were examined over a wide range of parameters. Viscosity measurements were conducted on specially designed viscometers to handle very corrosive fluids. Measurements were made between 25 and 100°C, and 600 psi at various shear rates from 58 to 1,740 s−1. Acid additives included corrosion inhibitors, inhibitor aids, iron control agents, hydrogen sulfide scavengers, anti-sludge agents, and nonionic surfactants. Effects of mutual solvents and methanol on the apparent viscosity were also investigated. It is observed that temperature, pH, shear conditions and acid additives have a profound influence on the apparent viscosity of the surfactant-acid system. The viscosity and related properties are very different from what we have observed with both natural and synthetic polymers. These properties were characterized and correlated with the type and nature of the additive used. Optimum conditions for better fluid performance in the field were derived. Technical Contributions (1) Understanding the fundamentals of viscoelastic surfactants, and (2) Optimization of viscoelastic surfactant system for acid stimulation applications. Introduction Previous studies highlighted the need for proper diversion during matrix acidizing treatments of carbonate reservoirs.1 Various acid systems were introduced to enhance acid diversion by increasing the viscosity of the injected acid. Depending on the viscosifiying agent, these systems can be divided into two main categories: polymer-based acids and surfactant-based acids. Acid-soluble polymers have been used to increase the viscosity of HCl, and to improve its performance.2,3 As the viscosity of the acid increases, the rate of acid spending decreases and, as a result, deeper acid penetration into the formation can be achieved.4 Addition of uncross-linked polymers to HCl improved acid penetration, however, acid placement did not significantly improve.5 Cross-linked acids were introduced in the mid 70's as was cited by Metcalf et al.6 These acids have much higher viscosity than regular acids or acids containing uncross-linked polymers. Two types of cross-linked acids are available. The first type consists of a polymer, a cross-linker, and other acid additives.7 The acid in this case is cross-linked on the surface and reaches the formation already cross-linked. The second type of cross-linked acid consists of a polymer, a cross-linker, a buffer, a breaker, and other acid additives, e.g., corrosion inhibitors and surfactants. The acid in this case reaches the formation uncross-linked, and the cross-linking reaction occurs in the formation.5,8 The polymer used in the in-situ acid systems is a copolymer of acrylamide. This type of polymer is soluble in HCl acid over a wide range of acid concentration (1 to 28 wt% HCl). It is also soluble in calcium and magnesium chloride brines. The polymer is cross-linked using multivalent cations, e.g., Fe(III) and Zr(IV), via the carboxylate groups.4 A breaker is used to ensure a complete reversal of the cross-linking process. The breaker functions either by changing the valence of the multi-valent cation or by forming a more stable compound with the cross-linker. In either case, the breaker will not affect the integrity of the polymer molecules.