Crisman Annual Report 2009 - Harold Vance Department of ...

Measurement and Correlation of Gas Viscosities at High Pressures and High
Temperatures
Introduction
High-pressure and high-temperature (HPHT) gas
reservoirs are defined as having pressures greater
than 10,000 psia and temperatures over 300°F.
Modeling the performance of these reservoirs
requires the understanding of gas behavior at
elevated pressure and temperature. An important
fluid property is gas viscosity, as it is used to model
the gas mobility in the reservoir and can have a
significant impact on reserves estimation during field
development planning. Accurate measurements of
gas viscosity at HPHT conditions are both extremely
difficult and expensive, thus this fluid property is
typically estimated from published correlations
based on laboratory data. Unfortunately, the
correlations available today do not have a sufficiently
broad range of applicability in terms of pressure and
temperature, so their accuracy may be doubtful for
the prediction of gas viscosity at HPHT conditions.
Objectives
This project will review the databases of hydrocarbon
gas viscosity that are available in the public domain,
and discuss the validity of published gas viscosity
correlations based on their applicability range.
Approach
A falling body viscometer was used to measure the
HPHT gas viscosity in the laboratory. This system is
very common for the measurement of liquid viscosity
and, in some specific circumstances (lubrication or
small percentage of liquid phase), can also measure
low viscosities. The decision to use such a viscometer
was based on the consideration that it is the only
device built to withstand extreme high pressure at
an acceptable cost. The instrument was calibrated
with nitrogen and then, to represent reservoir gas
behavior more faithfully, pure methane was used.
The subsequently measured data, recorded over a
wide range of pressure and temperature, was then
used to evaluate the reliability of the most commonly
used correlations in the petroleum industry. The
results of the comparison suggest that at pressures
higher than 8000 psia, the laboratory measurements
drift from the National Institute of Standards and
Technology (NIST) values by up to 7.48%.
Finally, a sensitivity analysis was performed to
assess the effect of gas viscosity estimation errors
on the overall gas recovery from a synthetic HPHT
reservoir, using numerical reservoir simulations. The
result shows that a -10% error in gas viscosity can
produce an 8.22% error in estimated cumulative
gas production, and a +10% error in gas viscosity
can lead to a 5.5% error in cumulative production.
Significance
The preliminary results indicate that the accuracy
of gas viscosity estimation can have a significant
impact on reserves evaluation.
Future Work
This project has led to the following conclusions:
» Accurate measurements of natural gas viscosity
under HPHT conditions are yet to be obtained,
» Gas viscosity correlations derived from data
obtained at low to moderate pressures and
temperatures cannot be confidently extrapolated
to HPHT conditions,
» Gas viscosity correlations currently available to
the petroleum industry were derived from data
obtained with limited impurities, and so their
accuracy for use with gases containing large
quantities of impurities is unknown,
» Laboratory investigations performed using
nitrogen showed a consistently negative error
when compared to the NIST reported values.
Preliminary results stress the importance of
obtaining an exhaustive range of measurements of
the viscosity of natural gases under HPHT conditions
in order to ensure better reserves estimations. To
this aim, further tests are ongoing.
Project Information
3.2.4 Measurement and Correlation of Gas Viscosities at
High Pressures and High Temperatures
Contacts
Gioia Falcone
979.847.8912
gioia.falcone@pe.tamu.edu
Catalin Teodoriu
catalin.teodoriu@pe.tamu.edu
Ehsan Davani
CRISMAN INSTITUTE
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Crisman Annual Report 2009