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.
Fabrication of injectable hydrogels based on poly(l-glutamic acid) and chitosan