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 CO 2 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% CH 4 ; 99.999% CO 2 ; and a gas mixture containing 74.99% CH 4 + 19.99% CO 2 + 5.02% N 2 , at 35 C, 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 behaviours are clearly identified when using methane, but are even more evident when using CO 2 and the gas mixture, due to the CO 2 interaction with coal porous structure, which induces a considerable resistance to CO 2 release. The results show that sample B (CH 4 + CO 2 + N 2 ) displays higher diffusion coefficient values (this behaviour is mainly related to the presence of N 2 ) than sample C (CH 4 ) and than sample A (CO 2 ).

Diffusion and Migration Law of Gaseous Methane in Coals of Different Metamorphic Degrees

“…The former provides the main channel for gas migration, while the latter bridges up pores and fractures. The coalbed gas is mostly stored and thus mainly diffused in the pores [4][5][6][7]. Much research has been done on the micropore structures of coal, yielding fruitful results.…”