CO2 replacement-based natural gas hydrate production: Insights from the semi-analytical model to optimised exploitation strategy

Natural gas hydrate, a promising clean energy source, encounters the issue of declining mechanical properties in the hydrate-bearing reservoirs during its exploitation. As an alternative, CO 2 replacement represents a potential solution, enabling the simultaneous production of methane gas while sequestering carbon dioxide underground. However, this method remains limited by the challenge of low gas production. In this study, a semi-analytical model is developed that (1) incorporates a comprehensive phase equilibrium framework capturing complex multi-phase interactions, (2) accounts for the coupled effects of gas, water, and heat production on multiphysical field evolution during exploitation, and (3) identifies key parameters influencing efficiency and supports optimisation of exploitation strategy. As a verification step, the developed semi-analytical model is compared with numerical predictions and with previous solutions. Combining the developed model, this study introduces a novel three-stage extraction scheme for the first time, offering a new approach to hydrate recovery. This scheme not only provides important guidance for optimizing hydrate exploitation strategies but also demonstrates its potential in real-world oil and gas field development, significantly enhancing natural gas recovery efficiency. Furthermore, by orthogonal experimental design, the study identifies and quantifies the key parameters influencing the efficiency of simple CO 2 replacement and the combined depressurisation and CO 2 replacement method.