Net-zero produced water strategy on gas production in marine gas hydrate bearing sediment

Gas hydrates, commonly found in deep marine sediments, have been considered a potential long-term energy source. Recent field trials demonstrated that well depressurisation, converting solid gas hydrates into fluid in situ, is the most efficient method for gas production. However, this process involves continuously pumping a large volume of water from the ocean that cannot be simply disposed of due to environmental concerns. The produced water, with temperature, salinity and residues different from those of shallower seawater, could impact marine species, habitats and ecosystems if directly discharged into the sea. Storing or transporting this water for treatment is economically impractical. Furthermore, as hydrate dissociation increases sediment permeability, water production rates rise over time. Therefore, re-injecting produced water into gas hydrate reservoirs emerges as a promising net-zero produced water strategy. This study investigates the effects of produced water re-injection on gas productivity and sediment deformation through coupled thermo-hydro-chemo-mechanical numerical analysis. The results indicate that re-injecting the water could improve gas productivity but may cause heave and differential displacement near the injection well. This highlights the importance of considering thermo-hydro-chemo-mechanical processes in comprehensively evaluating energy production strategies and suggests that further studies are necessary for future commercial gas production from gas hydrate reservoirs.