Abstract
This study presents the effects of equations of state (EOSs) on methane adsorption capacity, sorption hysteresis and initial gas reserves of a medium volatile bituminous coal. The sorption experiments were performed, at temperatures of 25 °C and 40 °C and up to 7MPa pressure, using a high-pressure volumetric analyzer (HPVA-II). The measured isotherms were parameterized with the modified (three-parameter) Langmuir model. Gas compressibility factors were calculated using six popular equations of state and the results were compared with those obtained using gas compressibility factors from NIST-Refprop® (which implies McCarty and Arp’s EOS for Z-factor of helium and Setzmann and Wagner’s EOS for that of methane). Significant variations were observed in the resulting isotherms and associated model parameters with EOS. Negligible hysteresis was observed with NIST-refprop at both experimental temperatures, with the desorption isotherm being slightly lower than the adsorption isotherm at 25 °C. Compared to NIST-refprop, it was observed that equations of state that gave lower values of Z-factor for methane resulted in “positive hysteresis”, (one in which the desorption isotherm is above the corresponding adsorption curve) and the more negatively deviated the Z-factors are, the bigger the observed hysteresis loop. Conversely, equations of state that gave positively deviated Z-factors of methane relatively produced “negative hysteresis” loops where the desorption isotherms are lower than the corresponding adsorption isotherms. Adsorbed gas accounted for over 90% of the calculated original gas in place (OGIP) and the larger the Langmuir volume, the larger the proportion of OGIP that was adsorbed.
Highlights
Coal-seams are typically characterized by large proportions of the gas existing predominantly as adsorbed phase in the internal surface areas of the rock matrix while the natural cleats’ macropores may contain some water and free gas [1]
Compared to NIST-refprop, it was observed that equations of state that gave lower values of Z-factor for methane resulted in “positive hysteresis”, and the more negatively deviated the Z-factors are, the bigger the observed hysteresis loop
Adsorbed gas accounted for over 90% of the calculated original gas in place (OGIP) and the larger the Langmuir volume, the larger the proportion of OGIP that was adsorbed
Summary
Coal-seams are typically characterized by large proportions of the gas (mostly methane [1,2,3], with traces of impurities such as CO2 and N2 ) existing predominantly as adsorbed phase in the internal surface areas of the rock matrix while the natural cleats’ macropores may contain some water and free gas [1]. The amounts of gas adsorbed by a coal depend on different factors such as pressure, temperature and moisture contents [5]. 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]
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