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

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.

Investigation of the Attenuation and Release of Cu2+ Ions by Polymer-Treated Tailings

“…In the case of a Type III isotherm, there is no Point B (the PT data in Figure 5C) and consequently no detectable monolayer formation and the adsorbed molecules are clustered around the most favourable sites on the surface of a macro-porous solid [55]. The hysteresis loop on the N 2 gas sorption-desorption isotherm occurred in the range of 0.5 to 1.0 P/P 0 which is greater for the polymer-treated sample, implying the presence of more mesopores [56,57].…”

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.…”

“…Moreover, significant hysteresis can be seen between the adsorption and desorption isotherms for each sample. It is believed that the primary reason for the observed large hysteresis between the sorption isotherms for each sample is the equation of state used in calculating volume changes during the sorption processes [18]. It has been reported that both the size and type of hysteresis between methane adsorption and desorption isotherms are dependent on the applied equation of state [18].…”

“…Different hysteresis behavior have been reported, at conditions beyond which capillary condensation can occur, mostly through experimental studies, in methane-coal, carbon dioxide-coal and nitrogen-coal systems [7,[15][16][17]. However, very few literatures exist on shale gas desorption isotherms and by implication, limited resources exist on shale gas sorption hysteresis in spite of the large volume of published work on shale gas adsorption isotherms [7,16,[18][19][20]. While it is not exactly clear why shale gas desorption is not so widely studied like adsorption, despite that it is clearly understood that desorption is akin to shale gas production, it can be speculated that the assumption that the Langmuir isotherm is valid for modeling shale gas sorption processes invariably makes researchers assume that the desorption isotherm should be exactly the same as the adsorption isotherm.…”

Physical Experiment and Numerical Simulation of the Depressurization Rate for Coalbed Methane Production

“…Traditionally, the volumetric method depends on the gas equation of state, and the shrinkage effect of the coal matrix leads to an increase in free space. Meanwhile, the change in gas compressibility factor makes the calculation and correction more complicated. , In recent years, low-field nuclear magnetic resonance (LF-NMR) technology has been extensively used in reservoir fluid analysis, adsorption characteristics, and stress sensitivity. The dynamic changes of the coal reservoir during gas exploitation can be quantitatively characterized by the hydrogen-bearing fluid (e.g., CH 4 and H 2 O) relaxation time. − …”