Geochemical modeling of carbon isotope fractionation during methane transport in tight sedimentary rocks

Abstract

Methane transportation in tight sedimentary rocks results in significant and complex isotope fractionation. Existing models and mechanistic studies of isotope fractionation cannot fully explain the actual observations in gas transport processes that occur in natural reservoirs and in the laboratory. Thus, geological applications based on transport-related isotope fractionation have been lacking in substantive progress. Here, we established a multi-scale model in which seepage in fractures, diffusion and adsorption/desorption in matrix pores, and concentration diffusion in kerogen structural pores are coupled. The results show that both diffusion and adsorption/desorption lead to significant isotope fractionation, while the contribution of pressure-driven seepage, is limited. The diffusion of kerogen-dissolved gas is of considerable significance to isotope fractionation in the later stage of transport, despite its weak contribution to gas production. The sequence of controlling factors of isotope fractionation during pure diffusion is as follows: diffusion coefficient ratio (D⁎/D) > initial pressure (P0) > others, where the value of D⁎/D mainly depends on the average pore size of rocks. The isotope fractionation caused by adsorption/desorption is closely related to the Langmuir parameters of sedimentary rock. It is essentially a dynamic non-equilibrium fractionation during adsorbed gas transport, rather than thermodynamic equilibrium fractionation between adsorbed gas and free gas. The model developed herein determines the contribution of each single effect to the apparent isotope fractionation and provides a novel method for obtaining the Langmuir parameters of rocks under in-situ geological conditions. Under variable boundary conditions, this model can be used to evaluate the gas resource potential and recoverable reserves in unconventional reservoirs and demonstrates the great potential for monitoring the production status of shale gas wells.