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

The falling body viscometer is selected to measure gas
viscosity for a pressure range of 3,000 to 24,500 psia
and temperature range of 100 to 415°F. Nitrogen was
used to calibrate the instrument and to account for
the fact that the concentrations of non-hydrocarbons
are observed to increase dramatically in HPHT
reservoirs. Then methane viscosity is measured to
reflect the fact that, at HPHT conditions, the reservoir
fluids will be very lean gases, typically methane with
some degree of impurity. The experiments showed
that while the correlation of Lee et al. accurately
estimates gas viscosity at low to moderate pressure
and temperature, it does not provide a good match
to gas viscosity at HPHT conditions.
higher than the values provided by the NIST and
by previous investigators. The difference increases
as temperature decreases, and it increases as
pressure increases. These 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 estimation. To this aim, further tests are
ongoing at Texas A&M University.
Accomplishments
Comparing our result with NIST values and data at
low to moderate pressure and temperature from
previous investigators showed that:
» Nitrogen viscosity—The lab data matched the
NIST values as well as those reported by other
investigators at low to moderate pressures, while
they are lower at high pressure. The difference
between measured data and NIST values increases
as temperature decreases; this difference also
increases as pressure increases.
» Methane viscosity—New lab data matched the
NIST values at low to moderate pressure, but the
new experimental viscosities are higher at high
pressure. The mismatch decreases as temperature
increases, and increases as pressure increases.
Significance
Gas viscosity correlations derived from data obtained
at low to moderate pressures and temperatures cannot
be confidently extrapolated to HPHT conditions. The
gas viscosity correlations that are currently available
to the petroleum industry were derived from data
obtained with gases with limited impurities, and so
their accuracy for use with gases containing large
quantities of impurities is unknown.
The laboratory investigations performed at TAMU
show that, at high pressure, the experimental
nitrogen viscosities are lower than the values
provided by the NIST and by previous investigators.
The observed mismatch increases as temperature
decreases, and it increases as pressure increases.
For methane, the TAMU investigations show that,
at high pressure, the experimental viscosities are
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Crisman Annual Report 2009