Summary
Growing demand for natural gas in North America is driving the exploration
and production industry to look for new resources in previously unexplored
areas, and the deep Gulf of Mexico (GOM) continental shelf is currently
attracting substantial attention. Several current deep-shelf
high-pressure/high-temperature (HP/HT) wells have anticipated bottomhole
temperatures that significantly exceed the operating limits of existing
measuring-while-drilling and logging-while-drilling (MWD/LWD) tools; therefore,
downhole annular-pressure measurements will not be available for pressure
management. This leaves temperature and hydraulic models as the best, if not
the only, source of downhole-pressure information for these wells. These models
depend on accurate surface inputs and laboratory-measured fluid properties
under downhole conditions. Unfortunately, these anticipated temperatures and
pressures also exceed the operating limits of conventional HP/HT viscometers.
This lack of measured fluid properties under these extreme conditions will
severely limit the ability of hydraulic models to predict downhole
pressures.
A new extreme-HP/HT (XHP/HT) concentric-cylinder viscometer was designed and
built to fill this important technology gap for GOM deep-shelf HP/HT wells. The
instrument is capable of measuring typical drilling-fluid viscosities up to
600°F (316°C) and 40,000 psig (276.0 MPa) and is capable of accurate property
measurements for drilling fluids containing ferromagnetic materials. Subsequent
verification and validation proved that the new viscometer compares favorably
to commercially available field viscometers and more-sophisticated laboratory
rheometers and therefore lends itself to widespread industry use. This paper
reviews the development of the instrument and associated automated control
system and explores health, safety, and environment (HSE) issues related to
testing drilling fluids at these extreme conditions. The paper also presents
results of verification and validation testing on invert-emulsion drilling
fluids.
Introduction
Developing deep-shelf gas requires overcoming some formidable drilling and
drilling-fluid challenges. Rigs capable of drilling to these depths are larger,
more robust, and more expensive than ordinary rigs. Penetration rates tend to
be low, extending time on location and adding to drilling costs. The extreme
pressures, temperatures, and acid-gas levels limit downhole tool, material, and
fluid selection.
During the planning stage for several potential record-depth deep-gas wells,
a technology gap was recognized for the measurement of fluid viscosity at the
expected downhole temperatures and pressures. HP/HT-viscometer technology at
the time was limited to measurements at ≤500°F (260°C) and ≤20,000 psig (138.0
MPa). Some of the deep-shelf HP/HT wells had anticipated bottomhole conditions
approaching 600°F (316°C) and 40,000 psig (276.0 MPa). Mathematical
extrapolations of fluid properties could result in significant inaccuracies in
hydraulic models because fluid behavior has never been evaluated under these
extreme conditions. Because current MWD/LWD tools are unusable under these
extreme conditions, measurement of valid fluid properties for input into
hydraulic models is critical for determination of the best available predicted
values of downhole annular pressures. Because of these limitations, it was
apparent that a new HP/HT viscometer would have to be developed for the
industry.
Oilfield Couette Viscometers
Specialized concentric-cylinder, or Couette, viscometers are used throughout
the oilfield industry to determine the rheological properties of drilling
fluids, cement slurries, and fracturing fluids. International Standards
Organization (ISO)/American Petroleum Institute (API) standards [ISO
10414-2:2002, ISO 10414-1:2002, API RP 13B-2 (2005)] exist that define and
recommend test conditions, methods, bob and rotor geometries, and shear rates
for determining fluid characteristics. From the results of these tests, the
apparent viscosity of the sample is calculated at each shear rate and test
condition. The data modeling methods differ with the fluid being tested, as
most of these fluids do not exhibit Newtonian behavior.
The term “Couette flow” originated from Maurice Frédéric Alfred Couette,
professor of physics at the University of Angers in France during the 19th
century (Couette 1890). He described laminar flow of a liquid in the space
between coaxial cylinders, now known as “Couette flow” in his honor. Equations
used to calculate values for shear stress, shear rate, and viscosity for
Couette flow are included in Appendix A.
A coaxial-cylinder, or Couette, viscometer consists of an outer cylinder
that rotates around a stationary inner cylinder. The outer component is known
as the “rotor,” and the inner cylinder is known as the “bob.” A shear gap
exists in the annular space between the bob and the rotor. In the interest of
industry standardization, the diameters and lengths of the bob and rotor are
defined by applicable ISO/API recommended practices [ISO 10414-2:2002, ISO
10414-1:2002, API RP 13B-2 (2005)].
The bob and rotor are immersed in the target fluid. As the rotor turns at
standard speeds ranging from 1 to 600 RPM, creating a specific fluid shear rate
in the annular gap at each speed, the torque induced on the stationary bob by
the fluid is measured accurately. The torque transducer connected to the bob is
calibrated to indicate shear stress using known viscosities of Newtonian oils
over the desired range of shear rates. Viscosity at a given shear rate is
determined as the ratio of shear stress to shear rate.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
20 December 2005
- Revised manuscript received:
26 December 2006
- Manuscript approved:
2 January 2007
- Version of record:
20 June 2007
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