Effect of the organic matter characteristics on methane adsorption in shale.

A series of methane adsorption isotherm experiments on bulk organic shale from the Longmaxi Formation in Sichuan Basin were conducted at 35, 50 and 65 °C and the CH4 pressure up to 15 MPa under dry conditions. Comparing these experimental results to previously published methane adsorption data of different organic types and thermal maturity, the effect of the characteristics of organic matter on methane adsorption in shale were discussed. The results show that the Longmaxi shale follows Type II kerogen trend line in the In K-1/T diagram of different kerogen type, and the Langmuir maximum sorption capacity per ton rock is obviously less than that of three types of kerogen which were isolated from the shales in USA. The thermodynamic parameters of adsorption on organic rich shale of Longmaxi Formation were determined based on the experimental CH4 isotherms. The heat of adsorption (q) and the standard entropy (Δs0) ranges from 19.8 to 25.4 kJ/mol and from -94. 0 to -111.1 J/(mol ⋅ K), respectively, which are close to the values of type II or III kerogen and greater than that of clay minerals. This indicates that the organic matter is the important adsorbent in shales. Note that the Langmuir maximum CH4 adsorption capacity per gram TOC of the Longmaxi shale is much greater than that of three types of kerogen in USA, which may indicate that this phenomenon is related to the pore structure micropore development in the shales of high thermal maturity, which needs to be investigated in future research. [ABSTRACT FROM AUTHOR]