Summary
Viscoelastic surfactant systems are used in the industry for several
applications. Initially, the application was focused on low-friction and
solids-suspension (fracturing and CT-cleanout) characteristics of the fluid. In
the last 4 years, the application of viscoelastic surfactants was extended to
acid-based systems for carbonate stimulation. These surfactants have the
ability to significantly increase the apparent viscosity and elastic properties
of the treating fluids. This is because of the ability of surfactant monomers
to associate and form rod-shaped micellar structures under certain
conditions.
Viscoelastic surfactant-based acid systems have been used in Saudi Arabian
fields in matrix acid stimulation, and in leakoff control acids during
acid-fracturing treatments. These surfactants were used to provide diversion
during acidizing of vertical, long horizontal, and multilateral wells. They
were used in sour environments where hydrogen sulfide levels reached nearly 10
mol%. They were also utilized in gas wells to reduce acid leakoff, and create
deep fractures in dolomitic carbonate reservoirs (250 to 275°F). In addition,
they were successfully employed to stimulate seawater injectors and disposal
wells where the bottomhole temperature was in the range of 100 to 150°F.
More than 250 wells (oil, gas, water injectors, and disposal wells) were
treated with viscoelastic surfactant-based acid systems. The acid was placed
either by bullheading, by using coiled tubing with or without a tractor. In
some cases, these treatments included stages of emulsified or regular
acids. All these wells responded positively to the treatment. There were
no operational problems encountered during pumping these acids even when
low-permeability reservoirs were treated. Because these acid systems do not
contain polymers, there was no need to flow back water injectors. The spent
acid in oil and gas wells was lifted from the treated wells in a very short
period of time. Finally, wells treated with surfactant-based acid systems
showed sustained performance for longer times than those treated with other
acid systems.
Introduction
Matrix acidizing and fracturing treatments have been used to enhance the
performance of oil, gas, and water wells for several decades. Water-soluble and
acid-soluble polymers have been used in these treatments to increase the
viscosity of the treatment fluids and hence enhance diversion during matrix
acidizing treatments. High-viscosity fluids are needed during acid-fracturing
treatments to reduce leakoff rate during acid injection into the fracture.
Various chemicals were developed 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 and
surfactant-based.
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 can be achieved (Deysarkar et al. 1984).
The addition of uncross-linked polymers to HCl improved acid penetration;
however, acid placement did not significantly improve (Yeager and Shuchart
1997). Crosslinked acids were introduced in the mid 70’s as was cited by
Metcalf et al. (2000). These acids have much higher viscosity than regular
acids or acids containing uncrosslinked polymers. Two types of crosslinked
acids are available. The first type consists of a polymer, a crosslinker, and
other acid additives (Saxon et al. 2000). 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 (e.g., corrosion inhibitors and surfactants). The acid
in this case reaches the formation uncrosslinked, and the crosslinking reaction
occurs in the formation (Taylor and Nasr-El-Din 2003).
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
6 January 2005
- Meeting paper published:
26 September 2004
- Revised manuscript received:
24 July 2006
- Manuscript approved:
24 July 2006
- Version of record:
20 February 2007
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