Summary
For a number of years, viscoelastic surfactant (VES) fluids have been used
for a variety of stimulation treatment applications, including hydraulic
fracturing, acid diverting, and gravelpacking. VES fluid systems typically
offer higher-retained permeability and conductivity of the formation sand and
proppant pack than polymeric systems. However, preliminary cost, a 200°F
temperature limit, excessive leakoff, and no internal breaker mechanism for dry
gas applications have limited VES use.
New VES fluid technology has been developed that substantially improves
product performance and cost effectiveness. The temperature range has been
extended to 300°F by using newly developed VES stabilizer technology. The
system works with high-density brines up to 14.4 ppg. Internal breakers have
been developed that permit a controlled viscosity break from ambient to 300°F.
Laboratory tests have determined that an internally broken fluid rapidly
achieves >90% returned permeability and conductivity of the formation sand
and proppant pack without the presence or need for contacting hydrocarbons.
Fluid loss-control technology has been developed that reduces VES fluid leakoff
similar to wall-building fluids, but without filtercake damage.
This paper discusses the development of the new VES system chemistry and its
properties. The paper also addresses the merits of a viscous fluid that can
work in a variety of base fluids for high-pressure applications, such as
managing surface-treating pressure or for gas-hydrate inhibition in deep gas or
deepwater environments. Breaker technology discussion addresses the ability to
ensure and enhance VES fluid-viscosity breaking. Fluid loss-control technology
effective to at least 2,000 millidarcies (mD) is presented. This paper also
presents rheological, return permeability and conductivity, fluid loss control,
treating pressure, and financial results.
Introduction
VES fluid systems have been used for gravelpack completions since the mid
1980s and for frac-packs since the mid 1990s (Nehmer 1988; Brown et al. 1996).
These fluid systems have been mainly applied to completions requiring treatment
fluids that are relatively low-damaging to the reservoir. The viscosity
developed by VES is created by the unique arrangement of the surfactants into
elongated, or worm-like, micelle structures (Samuels et al. 1997). A select
type and amount of counterion in the mix water is typically required for
elongated micelle structures to form and retain their stability. These
structures are typically sensitive to counterion concentration and fluid
temperature. Type and amount of common counterions used are 4% potassium
chloride, 3% ammonium chloride, 1.5% magnesium chloride, and 30 to 70 pptg
sodium salicylate. The temperature stability for VES fluids used for frac-packs
has increased to approximately 200°F. Because the viscosity of VES fluids is
caused by the arrangement of low-molecular weight surfactants and not by
high-molecular weight polymers like guar and hydroxypropyl guar, VES fluids are
nonwall-building fluids (i.e., they do not form a filtercake on the formation
face). Therefore, they readily leak into the reservoir matrix. The amount of
leakoff is much higher than polymeric systems and is fluid-viscosity dependent.
In times past, no internal breaker has been used within VES fluids (i.e., no
breaker is added to the VES fluid to go wherever the VES fluid goes). Instead,
the VES fluid has been considered to break by reservoir conditions. The two
primary external conditions have been: 1) contact with reservoir hydrocarbons;
and 2) contact and dilution with reservoir brine. However, relying on external
or reservoir conditions to break down the leaked-off VES fluid to achieve quick
and complete treatment fluid flowback has been a point of contention and is
questionable, especially for dry gas reservoirs. Internal breaker technology
for VES fluids has not existed until recently (Crews 2005). Additionally,
conventional counterions that work within a low- and narrow-concentration range
for VES viscosity yield and temperature stability have limited the range in
salinity and density of VES fluids to only light brines.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
22 June 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
15 May 2007
- Manuscript approved:
17 August 2007
- Version of record:
20 March 2008
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