Isotope fractionation of methane during natural gas flow with coupled diffusion and adsorption/desorption

Fractionation of carbon isotopes of methane in natural gas due to mass transport is evaluated based on statistical thermodynamics, microkinetics and fluid dynamics analysis. A continuum flow model with coupled adsorption/desorption and diffusion is defined and calibrated to describe isotope fractionation of natural gas due to flux in low permeability rocks. Isotopic fractionations due to post-generation mass transport are evaluated with analytical and numerical solutions under different conditions. Calculations strongly suggest that isotopic fractionation in adsorption/desorption systems are mostly dominated by mass transport, rather than the fractionation between adsorbate phase and gas phase. At steady state, the fractionation is determined by the drop of partial pressure and the diffusivity difference of the isotopologues. At non-steady state, the coupling of adsorption/desorption may bring about non-linear deviation of isotope fractionation. Stronger adsorption affinity and larger adsorption capacity result in a remarkably negative isotopic composition of early gas, but this is not the case of the weak adsorption of methane on organic matter. Large isotopic fractionation due to gas transport may happen under laboratory conditions with sufficient degassing, but the fractionation is limited under geological conditions. (C) 2011 Elsevier Ltd. All rights reserved.