Geoffrey S. Ellis Stephen C. Ruppel Kitty Milliken Mike Lewan Xun Sun Luis Baez Ken Beeney Steve Sonnenberg Tongwei Zhang 2013 <p>A series of CH₄ adsorption experiments on natural organic-rich shales, isolated kerogen, clay-rich rocks, and artificially matured Woodford Shale samples were conducted under dry conditions. Our results indicate that physisorption is a dominant process for CH₄ sorption, both on organic-rich shales and clay minerals. The Brunauer–Emmett–Teller (BET) surface area of the investigated samples is linearly correlated with the CH₄ sorption capacity in both organic-rich shales and clay-rich rocks. The presence of organic matter is a primary control on gas adsorption in shale-gas systems, and the gas-sorption capacity is determined by total organic carbon (TOC) content, organic-matter type, and thermal maturity. A large number of nanopores, in the 2–50 nm size range, were created during organic-matter thermal decomposition, and they significantly contributed to the surface area. Consequently, methane-sorption capacity increases with increasing thermal maturity due to the presence of nanopores produced during organic-matter decomposition. Furthermore, CH₄ sorption on clay minerals is mainly controlled by the type of clay mineral present. In terms of relative CH₄ sorption capacity: montmorillonite ≫ illite – smectite mixed layer > kaolinite > chlorite > illite.</p> <br/> <p>The effect of rock properties (organic matter content, type, maturity, and clay minerals) on CH₄ adsorption can be quantified with the heat of adsorption and the standard entropy, which are determined from adsorption isotherms at different temperatures. For clay-mineral rich rocks, the heat of adsorption (q) ranges from 9.4 to 16.6 kJ/mol. These values are considerably smaller than those for CH₄ adsorption on kerogen (21.9–28 kJ/mol) and organic-rich shales (15.1–18.4 kJ/mol). The standard entropy (Δs°) ranges from -64.8 to -79.5 J/mol/K for clay minerals, -68.1 to -111.3 J/mol/K for kerogen, and -76.0 to -84.6 J/mol/K for organic-rich shales. The affinity of CH₄ molecules for sorption on organic matter is stronger than for most common clay minerals. Thus, it is expected that CH₄ molecules may preferentially occupy surface sites on organic matter. However, active sites on clay mineral surfaces are easily blocked by water. As a consequence, organic-rich shales possess a larger CH₄-sorption capacity than clay-rich rocks lacking organic matter. The thermodynamic parameters obtained in this study can be incorporated into model predictions of the maximum Langmuir pressure and CH₄- sorption capacity of shales under reservoir temperature and pressure conditions.</p> application/pdf 10.1190/urtec2013-205 en Society of Exploration Geophysicists, American Association of Petroleum Geologists, Society of Petroleum Engineers Effect of organic matter properties, clay mineral type and thermal maturity on gas adsorption in organic-rich shale systems text