136-4 Dynamic fractionation of methane isotopologues during mass transport in coal with bidisperse pore structures

Session: Micro-Nano Scale Pore-Fracture Architecture and Fluid Dynamics in Shale and Coal Reservoirs

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Abstract:

Gas-in-place (GIP) content and the in-situ free/adsorbed gas ratio are crucial factors for evaluating resource potential and optimizing production strategies of deep coalbed methane (CBM). While the prevailing notion that adsorbed gas predominates is widely accepted in shallow CBM, this idea has been contested in deep CBM. Various numerical models for calculating GIP content based on gas volume data from canister degassing experiments commonly suffer from low accuracy, strong multiplicity, and inability to evaluate critical parameters of free/adsorbed gas ratio. Isotope fractionation is an important tool for studying the occurrence state and transport mechanism of natural gas, and numerical modeling that considers both cumulative degassing content and carbon isotope data has proven effective in determining the GIP content and adsorbed gas ratio in shale. However, the applicability of the existing isotope fractionation models in deep CBM has yet to be demonstrated experimentally and theoretically. In this study, four coal samples from the Shanxi and Taiyuan formations in Ordos Basin were subjected to canister degassing, gas metering, and methane isotopic composition testing. The results revealed significant multi-stage carbon isotope fractionation characteristics throughout deep CBM degassing, reflecting the strong heterogeneous pore structure characteristics within the coal. Existing carbon isotope fractionation (CIF) models are unable to effectively characterize this complex phenomenon. Based on these findings, we innovatively developed a CIF model that considers bidisperse pore structures and multiple transport mechanisms. The model can better characterize the complex isotope fractionation behavior and reveal the mechanisms of carbon isotope transport in series structures of multi-scale pores. Using the model, we calculated the GIP content of four experimental samples from the Shanxi and Taiyuan formations on the southeastern edge of the Ordos Basin, ranging from 27.78 m³/t to 39.13 m³/t, with an average of 31.68 m³/t, and the in-suit free gas ratio ranges from 13.30% to 32.89%, with an average of 19.45%. These research findings provided new insights for assessing the resource potential of deep CBM, screening sweet spots, and predicting gas well performance.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7505

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