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

Abstract

Summary This paper examines a new class of viscoelastic surfactants (amphoteric) that are used to enhance sweep efficiency during matrix acid treatments. It appears that surfactant molecules align themselves and form rod-shaped micelles once the acid is spent. These micelles might 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 using specially designed viscometers to handle very corrosive fluids. Measurements were made between 25 and 100°C, and at 300 psi at various shear rates from 58 to 1,740 s-1. Acid additives included corrosion inhibitors, inhibitor aids, an iron control agent, a hydrogen sulfide scavenger, an anti-sludge agent, and a nonionic surfactant. 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 were observed with both natural and synthetic polymers. The differences in these properties were characterized and correlated with the type and nature of the additives used. Optimum conditions for better fluid performance in the field were derived. Introduction Previous studies (Thomas et al. 1998) highlighted the need for proper diversion during matrix acidizing treatments of carbonate reservoirs. Various systems were introduced to enhance 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 (Pabley et al. 1982; Crowe et al. 1989). 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 (Deysarkar et al. 1984). Addition of suitable synthetic or natural polymers to HCl improved acid penetration; however, acid placement did not significantly improve (Yeager and Shuchart 1997). Crosslinked acids were introduced in the mid-70s, as cited by Metcalf et al. (2000). These acids have much higher viscosity than regular acids or acids containing uncross-linked polymers. Two types of crosslinked acids are available The first type consists of a polymer, a crosslinker, and other acid additives [e.g., corrosion inhibitors and iron control agents (Johnson et al. 1988)]. The acid in this case is crosslinked on the surface and reaches the formation already crosslinked. The second type of crosslinked acid consists of a polymer, a crosslinker, a buffer, a breaker, and other acid additives. The acid in this case reaches the formation uncrosslinked, and the crosslinking reaction occurs in the formation (Yeager and Shuchart 1997; Saxon et al. 2000). In-situ gelled acids were the subject of several lab and field studies. In general, lab and field results were positive; however, there were several concerns raised about these acids. Taylor and Nasr-El-Din (2002, 2003) noted that in-situ gelled acids caused loss of core permeability in tight carbonate cores. Permeability loss was attributed to polymer retention in the core and on the injection face of the core. A similar observation was noted by Chang et al. (2001). Lynn and Nasr-El-Din (2001) noted precipitation of the crosslinker (iron) when in-situ gelled acids were used to enhance the permeability of tight cores at high temperatures. Nasr-El-Din et al. (2002) showed that the crosslinker (Fe(III)) may precipitate in sour environments. Mohamed et al. (1999) reported poor field results when large volumes of polymer-based acids were used to stimulate seawater injectors with tight carbonate zones.

Development of a New Multifunctional Cationic Surfactant System with Corrosion Inhibiting Ability

“…Compared with 2e without a propargyl group, corrosion rate in 2a-2d solutions commonly decreased one to two orders of magnitude (entry (12) versus entries (3), (4), (6), and (11)). It is claimed that alkynyl groups can polymerize on the metal surface, form a protective film on the metal surface, and thus retard corrosion rate [9].…”

“…However, VES systems frequently encounter considerable difficulties in corrosion inhibitors (CIs) selection. In many cases, corrosion inhibitors would break these VES gels, and this is because inhibitors usually have oil-soluble constituents (e.g., acetylenic alcohol) or contain mutual solvents (e.g., ethylene gylcol monobutyl ether) [6]. Such a conflict encourages us to look for new corrosion chemicals which will have a very good compatibility with VES systems.…”

Quantitative Analysis of Reaction-Rate Retardation in Surfactant-Based Acids

“…In addition, 20-wt% HCl acid was used to prepare all acid solutions examined. This concentration was selected because this is the acid concentration typically used in the field, and this acid has the lowest apparent viscosity (Nasr-El-Din et al 2008a). The latter will minimize problems associated with mixing this acid in the laboratory.…”

“…The viscosity of the gel is a function of many parameters, including pH and Ca 2+ concentration (Nasr-El-Din et al 2008a). The bulk solution contains high acid concentration, and its pH is less than zero.…”

The Mechanism of Leakoff Reduction of Clean Self-Diversion Acid in Acid Fracturing

“…After the treatment, surfactant gel will be broken by mixing with hydrocarbon or diluted with water, so it is considered a clean stimulation fluid. The mechanism of viscosifying of VES acid was studied by many researchers (Chang et al, 2001(Chang et al, , 2003Taylor et al, 2003;Al-Mohammad et al, 2006;Zhao, 2006;Nasr-El-Din et al, 2008;Li et al, 2009) under experiment conditions. How the viscosity changes with different factors was measured in experiment.…”