New insights on water-gas flow and hydrate decomposition behaviors in natural gas hydrates deposits with various saturations

The efficient and safe exploitation of natural gas hydrates (NGHs) has been a worldwide hot topic. Water migration is a fundamental process during NGHs production due to large amount of water produced from hydrate decomposition and in-situ seawater in seabed. Water flow erosion, a continuous water flow process to decompose hydrates, is a novel production strategy proposed to enhance hydrate decomposition by introducing chemical potential difference. In order to understand the water-gas flow characteristics in hydrate-bearing sediment and evaluate the influence of water flow erosion on hydrate decomposition, we employed different fluid flows (single water phase and water-gas two-phase flow) and magnetic resonance imaging (MRI) to visualize the water-gas migration process and methane hydrate decomposition. Methane hydrate sediment samples were formed with various saturations and the saturation-permeability relation was matched with the grain-coating/pore-filling models. The results revealed that samples with lower hydrate saturation could benefit more from water flow erosion. The average hydrate decomposition rate for a lower-saturation sample (22.68% saturation) was around four times higher than that of a higher-saturation sample (38.27% saturation). The water phase flow in hydrate-bearing sediment was studied using heavy water (D2O) and found to be a continuous dilution process. In addition, the water-gas two-phase flow showed a two-stage evolution: separated two-phase flow followed by dispersed two-phase flow. Finally, the interaction mechanism between gas-water seepage process and hydrate decomposition was proposed. Overall, the water flow erosion strategy showed a great potential to be synergistically combined with typical production methods to enhance methane hydrate decomposition.