Experimental and numerical simulation of dynamic fractionation for methane carbon isotope during shale gas depletion development

• An independently designed physical simulation experiment on isotope fractionation during shale gas depletion development was conducted. • Three-stage isotopic fractionation (stable–decrease–increase) during shale gas depletion was observed, and its mechanism was analyzed. • A novel CIF model for 12 CH 4 and 13 CH 4 during mass transport in the dual-porosity medium was established for the first time. • The multi-solution of traditional mass transport numerical models was effectively minimized by the CIF model. • The gas production/recovery ratio for free and adsorbed gas at each fractionation stage can be quantitatively evaluated using the CIF model. Evaluating the adsorbed gas ratio (AGR) in the cumulative/daily production of shale gas is essential for optimizing development strategies, but no widely accepted method exists in the industry. The isotope fractionation method has emerged as a potential tool to achieve this goal. This study conducted isotope fractionation experiments on full-diameter shale under overburden pressure conditions during the shale gas depletion development. The results demonstrate that carbon and hydrogen isotopes during shale gas production exhibit a three-stage fractionation behavior, characterized by “stable (Ⅰ)–decreasing (Ⅱ)–increasing (Ⅲ)”. On this basis, an innovative carbon isotope fractionation model for 12 CH 4 and 13 CH 4 during mass transport in the dual-porosity medium was established, which effectively reduced the multi-solution of traditional numerical models by simultaneously matching the experimental data of production, pressure, and δ 13 C 1 value. Quantitative calculations indicate that in Stage I of the isotope fractionation, fracture gas is dominant. In Stage II, the produced gas is contributed by both fracture gas and matrix gas, with free gas dominating the matrix gas and adsorbed gas being secondary (the turning point of Stage II and Stage III (P 2 ) corresponds to an AGR of 48.30% in daily gas production), and a large amount of adsorbed gas still not effectively utilized (P 2 corresponds to an adsorbed gas recovery ratio of 18.84%). Starting from Stage III, adsorbed gas gradually replaces free gas and assumes a dominant role. This study is crucial for understanding the dynamic fractionation mechanism during isotopic molecule transport in complex pore-fracture systems and provides technical support for the optimization of development plans.