Supercritical-CO Adsorption Quantification and Modeling for a Deep Coalbed Methane Reservoir in the Southern Qinshui Basin, China.

Accurate depiction of the adsorption capacity of supercritical CO (ScCO) by existing adsorption models is an important focus for deep coal seams in CO-enhanced coalbed methane (CO-ECBM) recovery. To investigate the applicability of different adsorption models for the adsorption isotherms of ScCO, the validities of 10 different adsorption models were analyzed, based on analyses of the adsorption characteristics of ScCO from deep coal seams of the Southern Qinshui Basin, China. These models include the Langmuir (L) model, two-parameter Langmuir (TL) model, Toth (T) model, Langmuir-Freundlich (LF) model, extended Langmuir (EL) model, double parameter Brunauer-Emmett Teller model, three-parameter BET (TBET) model, Dubinin-Radushkevich (D-R) model, Dubinin-Astakhov (D-A) model, and Ono-Kondo lattice (OK) model. These models were tested for both the excess and absolute adsorption capacities of ScCO under various temperatures and pressures. The simulation accuracy of the different adsorption models was analyzed. The optimal models for the adsorption of ScCO in deep coal seams were selected based on a comprehensive analysis of the simulation parameters, standard error, and residual sum of squares. There were obvious differences in the validity of the different adsorption models in terms of the excess adsorption capacity and absolute adsorption capacity of ScCO. The D-A and D-R models are the optimal adsorption models for the adsorption isotherms of the excess adsorption of ScCO for the whole tested pressure range. The T, TL, and D-R models are the optimal adsorption models in simulation of the excess adsorption capacity of ScCO for the selected adsorption models when the equilibrium pressure is divided into two sections at the point of 8.13 MPa. In simulation of the absolute adsorption capacity of ScCO, the TBET and LF models are the optimal adsorption models among the selected models when the equilibrium pressure is less than or equal to 8.13 MPa. The linear, exponential, logarithmic, power function, and polynomial adsorption simulation all have good precision in the simulation of the absolute adsorption capacity of ScCO when the pressure is beyond 8.13 MPa.