International Journal Of Scientific & Engineering Research, Volume 7, Issue 7, July-2016 1173
ISSN 2229-5518
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http://www.ijser.org
Natural Gas Compressibility Factor
Measurement and Evaluation for High
Pressure High Temperature Gas Reservoirs
I.I. Azubuike, S. S.Ikiensikimama,O.D.Orodu
Abstract— The Natural gas compressibility factor is an important reservoir fluid property used in reservoir engineering computations either
directly or indirectly in material balance calculations, well test analysis, gas reserve estimates, gas flow in lines and in numerical reservoir
simulations. Existing gas compressibility factor correlations were derived using measured data at low to moderate pressures(less than 8,
000 psia) and temperatures (less than 212
o
F), and an extrapolation to High Pressure High temperature (HPHT) is doubtful. The need to
understand and predict gas compressibility factor at HPHT has become increasingly important as exploration and production has moved to
ever deeper formations where HPHT conditions are to be encountered. This paper presents laboratory measurement of gas compressibility
factors at HP HT natural gas systems and the ev aluation of s ome selected gas compressibility factors correlations. Samples of gas
mixtures were collected from the high pressure gas reservoirs from the Niger Delta region of Nigeria. Vinci PVT Cell was used to measure
the gas compressibility factors for a pr essures ranging from 6,000 to 14,000 ps ia and tem peratures at 270
o
F and 370
o
F. T he new
laboratory data was compared to some of the gas compressibility factor correlations/ models used in the petr oleum industry. Results
showed that majority of the correlations studied overestimated the gas compressibility factor at HPHT. Mean relative and absolute error
analysis were done based on the temperature difference; it was found that the total mean relative and absolute errors for the 370
o
F cases
are higher than those for 270
o
F. Among all the correlations assessed, Hall and Yarborough equation performed better than other existing
correlations with a mean absolute error of 3.545 and relative error of -2.668 at 270
o
F. At 370
o
F, Beggs and Brills correlation predicted better
than other correlations studied with a mean relative error of -4.77 and absolute error of 7.187.
Index Terms—Correlation, Evaluation, High Pressure, High Temperature, Gas Compressibility Factor, Gas Reservoir, Natural gas
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1. INTRODUCTION
In dealing with gases at low pressures, the ideal gas
relationship is a convenient and generally satisfactory tool. At
higher pressures, the use of the ideal gas equation-of-state
may lead to errors as great as 500%, as compared to errors of
2–3% at atmospheric pressure [1].
• I . I. Azubuike is currently pursuing doctoral degree program in Petroleum
Engineering Department, world Bank Center of Excellence for Oil Field Chemical
Research, University of Port-Harcourt, Nigeria.
• Dr. S. S. Ikiensikimama is currently Associate Professor in the Department of
Petroleum & Gas Engineering University of Port-Harcourt, Rivers State,
Nigeria.
• Dr. O. D. Orodo is currently Associate Professor in the Covenant University,
Ota, Ogun State, Nigeria.
Numerous equations-of-state have been developed in the
attempt to correlate the pressure-volume-temperature (PVT)
variables for real gases with experimental data. In order to
express a more exact relationship between the variables P, V,
and T, a correction factor called the gascompressibilityfactor,
gas deviation factor, or simply the z-factor, must be
introduced into the Ideal gas law. Its value reflects how much
the real gas deviates from the ideal gas behaviour at a given
pressure and temperature.
Compressibility factor is defined and expressed as:
=
(1)
Introducing the z-factor to ideal gas law results to real gas
equation (Equ. 2),
= (2)
where n is the number of moles of gas, P is the pressure, V is
the volume and T is the absolute temperature. Gas
compressibility factor can be determined on the basis of
measurements in PVT laboratories. At low temperature and
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