Summary
Problems with existing procedures used to estimate gas
pressure/volume/temperature (PVT) properties are identified. The situation is
reviewed, and methods are proposed to alleviate these problems. Natural gases
are derived from two basic sources: associated gas, which is liberated from
oil, and gas condensates, where hydrocarbon liquid, if present, is vaporized in
the gas phase. The two gases are fundamentally different in that a high-gravity
associated gas is typically rich in ethane through pentane, while gas
condensates are rich in heptanes-plus. Additionally, either type of gas may
contain nonhydrocarbon impurities such as hydrogen sulfide, carbon dioxide, and
nitrogen. Failure to distinguish properly between the two types of gases can
result in calculation errors in excess of those allowable for technical work.
Sutton (1985) investigated high-gravity gas/condensate gases and developed
methods for estimating pseudocritical properties that resulted in more-accurate
Z factors. The method is suitable for all light natural gases and the
heavier gas/condensate gases. It should not be used for high-gravity
hydrocarbon gases that do not contain a significant heptanes-plus component.
The original Sutton database of gas/condensate PVT properties has been expanded
to 2,264 gas compositions with more than 10,000 gas-compressibility-factor
measurements. A database of associated-gas compositions containing more than
3,200 compositions has been created to evaluate suitable methods for estimating
PVT properties for this category of gas. Pure-component data for methane
(CH4), methane-propane, methane-n-butane,
methane-n-decane, and methane-propane-n-decane have been compiled
to determine the suitability of the derived methods. The Wichert (1970)
database of sour-gas-compressibility factors has been supplemented with
additional field and pure-component data to investigate suitable adjustments to
pseudocritical properties that ensure accurate estimates of compressibility
factors. Mathematical representations of compressibility-factor charts commonly
used by the engineering community and methods used by the geophysics community
are investigated. Generally, these representations/methods are robust and have
been found suitable for ranges beyond those recommended originally. Natural-gas
viscosity, typically estimated through correlation, has been found to be
inadequate for high-gravity gas condensates, requiring revised procedures for
accurate calculations.
Introduction
Since its publication, the Standing and Katz (1942) (SK) gas Z-factor
chart has become a standard in the industry. Several very accurate methods have
been developed to represent the chart digitally. The engineering community
typically uses methods published by Hall and Yarborough (1973, 1974) (HY),
Dranchuk et al. (1974) (DPR), and Dranchuk and Abou-Kassem (1975) (DAK). These
methods all use some form of an equation of state that has been fitted
specifically to selected digital Z-factor-chart data published by
Poettmann and Carpenter (1952). The geophysics community typically uses a
method developed by Batzle and Wang (1992) (BW). Recently, Londono et al.
(2002) (LAB) refitted the chart with an expanded data set, resulting in a
modified DAK method. They provided two equations: one fit to an expanded data
set from the SK Z-factor chart and another that included pure-component
data.
A general gas Z-factor chart, such as the one developed by Standing
and Katz (1942), is based on the principle of corresponding states (Katz et al.
1959). This principle states that two substances at the same conditions
referenced to critical pressure and critical temperature will have similar
properties. These conditions are referred to as reduced pressure and reduced
temperature. Therefore, if two substances are compared at the same reduced
conditions, the substances will have similar properties. In the context of this
paper, the property of interest is the gas Z factor. Mathematically, the
SK chart relates Z factor to reduced pressure and reduced
temperature.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
14 July 2005
- Revised manuscript received:
18 December 2006
- Manuscript approved:
26 December 2006
- Version of record:
20 June 2007
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