Abstract: Natural gas hydrate is widely recognized as one of the most promising clean energy alternatives with the potential to replace coal, petroleum and conventional natural gas in the 21st century. However, it still remains a major challenge to accurately analyze multi-physical field evolutions in the entire exploitation process. In this study, based on the time-updating approach, a semi-analytical model is developed within a multi-physical coupling framework. This model comprehensively considers the entire exploitation process, including both the drilling and support phases. Specifically, the proposed model is capable of analyzing the interactions between hydraulic flow, heat transfer, mechanical properties and hydrate saturations. Meanwhile, more actual mechanical properties of hydrate are incorporated in this model, such as shear expansion and strain-softening post-failure behaviours. As a verification step, a good agreement is observed between the results obtained by numerical predictions and those derived from the developed semi-analytical model. In parametric analyses, it is found that appropriately reducing the depressurization rate is beneficial for increasing gas production, while appropriately increasing the depressurization rate can enhance extraction safety. Interestingly, compared to the classical elastic-perfectly-plastic or elasto-brittle-plastic models, the developed strain-softening model can significantly improve the predicting ability to assess the safety performance during the extraction process.

Key words: methane hydrate, semi-analytical model, multi-physics coupling, strain-softening, depressurization