A novel method to enhance methane hydrate exploitation efficiency via forming impermeable overlying CO2 hydrate cap

Highlights: • A new idea "reservoir reformation" is proposed to prevent sea water from invading NGH zone. • An artificial impermeable overlying CO₂ hydrate cap is constructed successfully. • The CO₂ hydrate cap improves production efficiency by decreasing water yield. • Optimal CO₂/H₂O volume ratio of the emulsion is suggested to form the desired CO₂ hydrate cap. • Large amounts of CO₂ is sequestrated in the depleted CH₄ hydrate zone. -- Abstract: To enhance the exploitation efficiency of natural gas hydrate by decreasing the yield of water, a novel "reservoir reformation" concept is proposed that involves the reformation of a natural gas hydrate reservoir by constructing an artificial impermeable overlying CO₂ hydrate cap. The feasibility of this concept has been demonstrated in this laboratory-scale experiment. After reformation by injecting CO₂ emulsion into the permeable overburden, a confined environment with an impermeable CO₂ hydrate cap is successfully constructed for depressurization operation. The cap can maintain mechanical stability until the end of production process. With the protection provided by the artificial CO₂ hydrate cap, the production efficiency was greatly improved to 83.3% and the water yield is remarkably decreased. Moreover, the optimal CO₂/H₂O volume ratio of the emulsion for forming the desired CO₂ hydrate cap was confirmed to be 1:1. The formation of CO₂ hydrate cap can also protect the geological stability of depleted methane hydrate zones and seal a large amount of CO₂, which is of both energetic and environmental significance; however, intensive and extensive research should be conducted in the future.