Study on characteristics of CO2 seepage within natural gas hydrate reservoirs

Carbon dioxide (CO₂) storage in natural gas hydrate (NGH) reservoirs through hydrate-based methods has emerged as a promising carbon sequestration technology. The dynamic permeability evolution of the reservoirs plays a crucial role in determining both the storage capacity and stability of CO₂ sequestration, particularly due to hydrate phase transitions during the injection process. However, the evolution of dynamic permeability and its influence mechanism are yet to be elucidated. In this paper, we investigated the dynamic permeability characteristics of NGH reservoirs with different hydrate saturations during CO₂ injection. The results show that the reservoir permeability can be influenced significantly by the water-CO₂ flow rate ratio (r). When r is 5:1 (mL/min), the relative permeability (K_r) of the water phase demonstrates instability, with values consistently exceeding 0.4, significantly reducing the likelihood of flow blockage formation. Moreover, r directly affects the fluid seepage dynamics. The variation in r (from 5:1 to 2:1 mL/min) promotes the blockage formation in mid-to-rear sections of the reservoir, thereby facilitating the expansion of the storage extent. Meanwhile, a method was developed for detecting blockage locations within reservoirs under non-visualized conditions, suitable for applications in CO₂ sub-seabed storage. Notably, a threshold effect of initial NGH saturation (S_h0) on ultimate storage capacity was observed. At S_h0 below 21 %, the storage capacity increases with S_h0.