Gas content derivative data versus diffusion coefficient

Abstract

The study of the gas diffusion process has a main role in both Coalbed Methane (CBM) production and CO2 injection in geological sequestration projects. The accurate determination of gas diffusion coefficients in unconventional reservoirs such as coal seams, requires a consistent mathematical approach. The study of the gas diffusion process in coal seams was carried out using sorption isotherms. The Langmuir model for individual gases and the extended Langmuir model for multicomponent gas mixtures were applied to fit sorption isotherm data. “Gas content derivative data” and “gas content changes” emerged as crucial mathematical parameters to accurately study the gas diffusion process. The main goal of this paper is to define the degree of interaction between the gas content derivative data and the gas diffusion process. Experiments were performed on three samples selected from two different coals, which were submitted to three different gas compositions, viz 99.999% CH4; 99.999% CO2; and a gas mixture containing 74.99% CH4 + 19.99% CO2 + 5.02% N2, at 35ºC, and at pressures ranging from 0 up to 50 bar. Experimental results obtained from the three samples indicate that during adsorption/desorption processes, the diffusion coefficients increase and the gas content changes decrease when the pressure decreases, due to the sample saturation degrees and to the kinetic mechanisms increase. Additionally, the “gas content derivative data” scattering is slightly lower during the desorption process than during the adsorption process. These behaviors are clearly identified when using methane, but are even more evident when using CO2 and the gas mixture, due to the CO2 interaction with coal porous structure, which induces a considerable resistance to CO2 release. The results show that sample B (CH4 + CO2 + N2) displays higher diffusion coefficient values (this behaviour is mainly related to the presence of N2) than sample C (CH4) and than Sample A (CO2).