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The CO2 replacement technique is considered as a promising approach for both gas hydrate recovery and CO2 sequestration. Based on this technique, the continuous CO2/H2 injection-production mode had been studied in our previous work. However, there are limitations on this continuous mode, such as lower CH4 recovery and CO2 sequestration ratios, lower CH4 concentration in produced gas and higher injection-production ratio caused by the fast breakthrough of injected gas. Here we proposed a so called semi-continuous injection-production mode, in which continuous injection-production process is interrupted periodically by stopping injection and production operations for letting injected gas diffuse sufficiently, delaying its breakthrough and increasing its effective sweep region. A series of experimental simulations were performed with respect to this mode in a three-dimensional simulator with a volume of 10.6 L. The results indicated that for the injected gas with low CO2 concentration, the CH4 recovery and concentration in the produced gas could be enhanced dramatically by the semi-continuous gas injection method. Additionally, the fast decomposition stage of CH4 hydrate could be retained separately in the replacement process, thereby effectively improving production efficiency. However, the corresponding CH4 concentration in the produced gas decreased and the injection production ratio increased. Notably, the process combining the CH4 steam reforming with cyclic injection-production was first simulated. The results showed an excellent CO2 storage capability and CH4 hydrate recovery ratio, which was mainly controlled by the gas composition of the re-injected gas. Through systematically comparing and analyzing the CH4 concentration, recovery ratio, production efficiency and injection-production ratio, it is demonstrated that the semi-continuous injection-production mode is superior to the continuous one. The results obtained in this work are of significance for guiding future NGHs exploitation.

Nowadays, the combination of natural gas hydrates exploitation and CO2 storage is considered of excellent application. In this work, we explored reservoir reformation technology to enhance gas recovery from the unsealed water-saturated hydrate-bearing deposits in a large-scale tridimensional simulator. Firstly, recurrent CO2 injection into the overburden was to gradually form CO2 hydrate and reduce its permeability. Then traditional depressurization operations were conducted to compare the influences of overburden thickness and additional reservoir reformation on the evolution of pressure distribution and the gas/water production. The results showed that reservoir reformation could effectively improve the mining efficiency, reaching ∼2–4 times of gas production and ∼3–5 times of gas–water ratio compared with the direct depressurization. After a long-term reformation, CH4 recovery ratio was increased from 24.5% to 84.9%. Additionally, we further demonstrated the complete spatiotemporal evolution mechanism of the initial CH4 hydrate and artificial overlying CO2 hydrate during depressurization. A large amount of CO2 was ultimately stored in the depleted reservoir in the form of CO2 hydrate and the CO2 storage ratio was up to 79.5%. Therefore, this reservoir reformation approach is greatly promising for high-efficient and safe exploitation of marine hydrate and CO2 storage.

The exploitation of unsealed marine hydrate deposits is of greater commercial value, but also more challenging because of the risks of the invading of huge body of sea water and subsidence of the seabed. In this work, we proposed a new method to recover natural gas from the usealed hydrate deposits saturated with water by pressure-retaining gas (CO2 + H2) injection. A series of experimental simulations was performed to prove its feasibility through establishing a highly simulated unsealed marine hydrate-bearing sediments system. The results demonstrated that overlying water invading was almost completely inhibited for the pressure-retaining approach while unimpeded for the simple depressrization process. Further, we designed a temporary depressurization process before gas injection to establish a flow field for gas in the sediments. This combined operation effectively enhanced the migration and spread of the injected gas, eventually increasing gas recovery ratio from 25% to 60% while the produced water/gas ratio remains as low as 12.6 ST kg/m3. Additionally, it is proved that the balance between CO2 sequestered and CH4 recovered could be achieved, which was an excellent support for reservoir stability. The combination method realized a higher CH4 concentration in the produced gas and a lower injection-production ratio, which greatly reduced the costs of gas injection and subsequent treatment of produced gas. For comparison, similar simulation work was also performed with respect to sealed gas-rich hydrate-bearing sediments. Overall, this temporary depressurization aided pressure-retaining gas injection approach is very promising for high efficient and safe exploitation of marine hydrtates.