Effect of Equation of States on High-Pressure Volumetric Measurements of Methane–Coal Sorption Isotherms—Part 1: Volumes of Free Space and Methane Adsorption Isotherms

Abstract

High-pressure volumetric analysis (HPVA) experiments were carried out using Particulate System's HPVA-II 200 at temperatures of 25 and 40 °C and pressures up to 7 MPa to quantify methane adsorption capacities of a coal sample. Six well-known equations of state (EOSs), namely, Peng−Robinson (PR), Soave−Redlich−Kwonk (SRK) and their volume translated forms (PR-Peneloux and SRK-Peneloux), Soave modified Benedict−Webb−Rubin (SBWR), and Lee−Kesler (LK) were used for data interpretation, and the results were compared with those calculated by the equipment using Z-factors from NIST-refprop software implementation of McCarty and Arp EOS for helium and Setzmann and Wagner EOS for methane. Because of variations in the Z-factors of helium and supercritical methane obtained from the different EOSs, large variations were observed in calculated isotherms and Langmuir parameters with the PR-Peneloux and PR-EOS family showing the largest relative deviations, while SRK-EOS and SRK-Peneloux gave more negative adsorption for each temperature. All the EOSs gave significantly lower density for the adsorbed phase at 40 °C compared to the value at 25 °C. This is consistent with the negative effect of temperature on adsorption.

Numerical Simulation of Gas Production from Gas Shale Reservoirs—Influence of Gas Sorption Hysteresis

“…It is equipped with a Microsoft Excel ® -based software that automatically computes the isotherms using fluid data obtained from refprop ® software. Details of the formulae used in calculating the isotherms and the dependence of the isotherms on equation of state have been discussed in our previous studies [18,24]. Thus, based on the findings from those studies, the equation of state by Soave-Benedict-Webb-Rubin (SBWR-EOS) [25] was adopted to calculate the gas compressibility factors for both helium (for free-space volume calculation) and methane in this study.…”

Volumetric Measurements of Methane-Coal Adsorption and Desorption Isotherms—Effects of Equations of State and Implication for Initial Gas Reserves

“…Methane adsorption and desorption isotherms were measured for the sample using Particulate System's HPVA-II ® 200 unit described in [10,13,30,31]. This equipment uses volumetric technique for gas adsorption and desorption measurements.…”

“…This equipment uses volumetric technique for gas adsorption and desorption measurements. The volume of free-space was measured using helium expansion method [10,13,30,31]. In addition to the isotherms computed by the equipment (reference data), six other isotherms were calculated, at each temperature, from the raw experimental data, using Z-factors obtained from the EOSs discussed in Ekundayo and Rezaee [10].…”

“…The dependencies of adsorption capacity and gas reserves estimates on gas compressibility factors have been illustrated by different researchers [7][8][9][10][11]. Despite this knowledge, indiscriminate uses of EOSs for Z-factors needed for generating sorption isotherms is very common [10,[12][13][14].…”

“…The dependencies of adsorption capacity and gas reserves estimates on gas compressibility factors have been illustrated by different researchers [7][8][9][10][11]. Despite this knowledge, indiscriminate uses of EOSs for Z-factors needed for generating sorption isotherms is very common [10,[12][13][14]. Redlich-Kwong EOS [12], 32-parameter modified Benedict-Webb-Rubin (MBWR) EOS [13] and a combination of McCarty and Arp and Setzmann and Wagner EOSs [12] are some examples found in the literature.…”