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Production decline prediction is important to understand the performance and life span of oil and gas wells. The most common prediction method is decline curve fitting based on available production rate data. Such data are fit with different equations that extrapolate to future time. However, the parameters are commonly poorly constrained, especially when the production rate data are limited. In this study, we establish a novel gas isotope interpretation tool to better predict the resource quantity and life span of producing gas wells. This tool is based on the evolution of methane carbon isotope ratios (δ¹³C1) caused by different gas-releasing processes during production. It requires (1) real-time methane carbon isotope ratio data, (2) continuous gas production rate data for a certain period of time, and (3) basic geological and engineering conditions. We successfully applied the production decline prediction tool to …