A coupled model for methane carbon isotope fractionation during gas-water two-phase flow in low-permeability reservoirs

Le résumé fourni par la source

Carbon isotope fractionation of methane during gas-water two-phase flow in low-permeability reservoirs is crucial for interpreting natural-gas geochemical signatures and evaluating well production status; however, existing models are mainly developed for a single gas phase and cannot account for the influence of water phase on gas migration and isotope fractionation. To fill this gap, this study, for the first time, developed a carbon isotope fractionation model for gas-water two-phase flow (GWF-CIF model) by establishing a coupled system of partial differential equations that integrates gas-water two-phase flow dynamics, effective-stress evolution, real gas effect, and multi-mechanism gas transport, while systematically accounting for the differential diffusion and adsorption/desorption behavior of 12 CH 4 and 13 CH 4 . This model enables simultaneous prediction of gas-water two-phase production behavior and dynamic isotope fractionation, providing a novel quantitative framework for reserve evaluation and development optimization of low-permeability gas reservoirs. The model reliability was validated using field test data from two deep coalbed methane (CBM) wells in the Daji area of the Ordos Basin, China. History matching indicates that the model effectively reproduces the evolution of cumulative gas production, cumulative water production, and methane carbon isotopic composition. Simulation results demonstrate the characteristic “water production preceding gas production” behavior and a three-stage methane isotope evolution characterized by an initial stable (Stage I), a middle decreasing (Stage II), and a late increasing (Stage III). Initial water saturation significantly affects isotope fractionation; higher water saturation reduces the fractionation magnitude and delays the timing of turning points for various stages. The 20-year forecast indicates total gas productions of 1229.5×10 4 m 3 and 992.8×10 4 m 3 for the two wells, with free gas contributing 28.4% and 34%, respectively. During the first two stages of isotope fractionation, produced gas is dominated by free gas; after entering the third stage, the adsorbed-gas contribution gradually exceeds that of free gas. The turning point of carbon isotope fractionation can serve as a key indicator for identifying transitions in gas production mechanisms and provides a scientific basis for timing production-enhancement measures. This study provides a theoretical basis for interpreting gas-water flow behavior and isotopic geochemical characteristics in low-permeability gas reservoirs, with significant implications for optimization strategies including CO 2 injection, casing-pressure reduction, and secondary refracturing.

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