Summary
Viscoelastic surfactant (VES) fluids have been used widely in the oil
industry as completion and stimulation fluids. The surfactants arrange
structurally to form rod-like micelles that increase VES-fluid viscosity for
regular fracturing and fracture-packing fluids. However, high fluid leakoff and
low viscosities at elevated temperatures have limited VES fluids to hydraulic
fracturing and fracture-packing applications.
This paper will introduce a nanotechnology application for maintaining
viscosity at high temperatures and controlling the loss of VES fluid without
generating formation damage. The nanoparticles studied are 35-nm inorganic
crystals that display unique surface morphology and surface reactivity. These
nanometer-scale particles associate with VES micelles through chemisorption and
surface-charge attraction to stabilize fluid viscosity at high temperatures and
to produce a pseudofilter cake of viscous VES fluid that reduces significantly
the rate of fluid loss and improves fluid efficiency. When internal breakers
are used to break the VES micelles, the fluid will lose its viscosity
dramatically and the pseudofilter cake will then break into brine and
nanoparticles. Because the particles are small enough to pass through the pore
throat of producing formations, they will be flowed back with the producing
fluids, and no damage will be generated. The results of rheology, leakoff, and
core-flow tests will be presented for the VES-fluid systems at temperatures 150
and 250°F.
Introduction
VES fluids have been used widely as gravel-packing, fracture-packing, and
fracturing fluids for more than a decade because the fluids exhibit excellent
rheological properties and maintain low-formation-damage characteristics
compared with crosslinked-polymer fluids. VES fluids are composed of
low-molecular-weight surfactants that form elongated micelle structures that
exhibit viscoelastic behavior to increase fluid viscosity (Nehmer 1988; Brown
et al. 1996; Samuel et al. 1999).
Traditionally, the industry depends on external breakers (or reservoir
conditions) to break VES fluids after treatment is completed. The two primary
external conditions have been (1) contact with reservoir hydrocarbons and (2)
contact and dilution with reservoir brine (Samuel et al. 1999). But, relying on
the 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 (Crews et al.
2006).
In a broad sense, internal breakers are hydrophilic compounds placed within
the VES elongated micelles during surface mixing that will go wherever the
fluid goes, ensure that the VES fluid breaks, and break the VES fluid so that
it cleans up easily, enabling oil and gas to flow to the wellbore to be
produced. Internal breakers generate VES-breaking compounds over time, which
penetrate and collapse the viscous, rod-like VES micelles into nonviscous,
more-spherical micelles, and the technology enables the VES breaker to
accompany the VES fluid during a fracture-pack or regular fracturing treatment
to enhance and ensure breaking and cleanup of the VES fluid from the reservoir
(Crews 2005; Crews and Huang 2007).
VES fluids are unlike polymer-based systems in that they are
nonwall-building and do not form filter cake on the formation face during
hydraulic fracturing and fracture-packing treatments. Without filter cake
development, the amount of VES fluid leaked off from the fracture into
formation during a fracturing treatment is primarily dependent on fluid
viscosity. Because of its nonwall-building property, VES fluid exhibits high
fluid leakoff from the fracture during a treatment and "screening out"
is a common problem. Because of poor fluid efficiency of VES fluid, (1) the
permeability of a reservoir is less than 400 md for most cases, (2) more total
fluid volume is required for a given treatment, and (3) a larger amount of
leaked off fluid within the reservoir matrix occurs, which needs to be removed
(cleaned up) after the treatment.
VES micelles are not stable at high temperatures and will rearrange
thermally into nonviscous structures. The stability at high temperatures and
fluid-loss property of VES fluids have limited their applications to fracturing
and fracture-packing treatments.
Nanoscience and nanotechnology have been used in many application areas,
such as biomedical, pharmaceutical, space, and information technology.
Nanotechnology represents the development and applications of materials,
methods, and devices in which critical length scale is on the order of 1 to 100
nm and in which critical functionality is not a direct manifestation of the
atomic or macroscale properties (Mokhatab et al. 2006). The laws that govern
materials at nanoscale are different from those that have been accepted widely
in larger scales. Some nanoparticles have been used in drilling fluids and have
exhibited extraordinary rheological properties. Those advanced drilling fluids
based on polymers that are physically or chemically associated with
nanoparticles, as well as with with amphiphilic surfactants or polymers have
been developed as stimuli-sensitive fluids. The fluid flow properties can be
altered in response to a change in stimuli, such as temperature, salinity, and
pH (Krishnamoorti 2006). The nanoparticles we used in this paper are less than
100 nm in size, with one select product with an average size of 35 nm. The
nanoparticles are inorganic crystals with no solubility in water, oil, or
solvent.
This paper presents a nanotechnology application for maintaining viscosity
at high temperatures and controlling the fluid loss of VES stimulation fluid
without generating formation damage.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
23 February 2007
- Meeting paper published:
30 May 2007
- Revised manuscript received:
11 March 2008
- Manuscript approved:
4 April 2008
- Version of record:
15 November 2008
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