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.
© 2008. Society of Petroleum Engineers
View full textPDF
(
2,079 KB
)
History
- Original manuscript received:
19 January 2004
- Meeting paper published:
17 April 2004
- Revised manuscript received:
6 August 2007
- Manuscript approved:
1 September 2007
- Version of record:
20 March 2008
View Cart
