Estimate of Equation of State Uncertainty for Manometric Sorption Experiments: Case Study With Helium and Carbon Dioxide

The sorption of gses on coal rs 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 particulartly sensitive to experimental errors, because commonn 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 determination 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 is most accurate at the experimental conditions However there is generally little information available on the accuracy of an EoS for the condition 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 a EoS using a manometric sorption apparatus. This is demostrated by usin He and CO2 at 31811 k and pressures between 0 1 and 17 MPa, whch covers the range of interest for deep coalbeds The accuracies of the following EoSs have been estimated Peng-Robinson with Stryject-Vera modification for CO2. Pen-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 of an EoS This estimate can be used as a first approximation of the EoS accuracy in the priori erro analysis of manometric sorption experiments