Summary
The sorption of gases on coal is of great importance for the use of gas
injection to enhance coalbed methane production. Furthermore, when injecting
carbon dioxide (CO2), the project is eligible for carbon credits as
the CO2 is sequestrated.
Accurate measurements of sorption at in-situ conditions have proven to be a
complicated matter. Especially, the sorption of CO2, pure or in a
mixture, are particularly sensitive to experimental errors, because common
in-situ conditions of deep coalbeds are near the critical point of
CO2. One source of uncertainty in sorption measurements is the
accuracy with which gas densities have been determined. Accurate determinations
of gas densities at in-situ conditions are complex and require the inclusion of
specialized equipment. Therefore, gas densities are usually calculated from
pressure and temperature measurements using an Equation of State (EoS). The
accuracy of this EoS then influences the uncertainty in the sorption
measurement. Therefore, it is essential that an EoS is used that is most
accurate at the experimental conditions. However, there is generally little
information available on the accuracy of an EoS for the conditions of interest.
This is particularly true for the calculation of densities of mixtures with an
EoS.
This paper describes an experimental method to estimate the accuracy of an
EoS using a manometric sorption apparatus. This is demonstrated by using He and
CO2 at 318.11 K and pressures between 0.1 and 17 MPa, which covers
the range of interest for deep coalbeds.
The accuracies of the following EoSs have been estimated: Peng-Robinson with
Stryjek-Vera modification for CO2; Peng-Robinson with an alpha
parameter proposed by Twu in 1995 for He; multiparameter crossover equation of
state by Sun, Kiselev, and Ely for CO2; modified Benedict-Webb-Rubin
EoS by McCarty and Arp for He; and the reference EoS for CO2 by Span
and Wagner.
The experimental method provides a lower limit estimate of the accuracy for
an EoS. This estimate can be used as a first approximation of the EoS accuracy
in the a priori error analysis of manometric sorption experiments.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
2 August 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
17 February 2009
- Manuscript approved:
17 February 2009
- Published online:
16 July 2009
- Version of record:
12 March 2010
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