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Energies 2021, 14, 387 for the mixed gases lie between these two limits. The figure shows that at a fixed temper- ature, the equilibrium conditions move to lower pressure and, at a fixed pressure, they move to a higher temperature when the CO2 mole fraction in the mixture increases. The experiments above are conducted in the bulk phase; however, the dissociation of methane hydrate and its replacement via carbon dioxide occur in sediment reservoirs. Therefore, this process needs to be studied in porous media. Figure 13 compares the hy- drates equilibrium conditions in porous media with those in the bulk phase [54]. Figure 13. Equilibrium condition for methane and carbon dioxide hydrate in porous media and in bulk phase reprinted from [54] with permission from Elsevier. Gambelli et al. have tested the CO2 replacement into NGH deposit via a combination of two strategies. Once the CH4 hydrate formation is carried out, the replacement phase is performed via depressurization and CO2 injection. The quantity of CO2 stored depends on the sum of two contributions: the methane replacement and an ex-novo CO2 hydrate formation. The CO2-CH4 exchange in methane hydrates is favorable; however, the new CO2 hydrate formation around and the superficial replacement process limit the transition of the CO2 into the deeper layer inside the deposits and, therefore, the overall replacement process. As a result, the storage efficiency (i.e., the ratio between the moles of CO2 permanently stored into the NGH and the total amount of CO2 injected inside the deposit) is limited at 36% with a value of sand pore saturation degree equal to 7.6% [56]. ηstorage = nCO2 stored (9) nCO2 injected An experimental apparatus able to promote methane production via a combined CO2 replacement and depressurization was tested by Zhao et al. [57]. In this manuscript, they run two tests conducted with and without depressurization combined replacement method. This experiment shows the great benefit of the combined method, increasing methane replacement from 7% without depressurization to 25% with a mixed method [57]. Japan and China have carried out methane production testing in oceanic NGH reser- voir, showing the possibility and feasibility of methane production by depressurization. Four tests are conducted, three in Nankai Trough (Japan), and one in Shenhu Area, China. The first Japanese test was done in March 2013, but the methane production was inter- rupted only after six days due to the abrupt sand production. The other two tests were performed in the same location in 2017 but also, in this case, the trials lasted 12 and 24 days, respectively. China has extracted from an oceanic NGH reservoir via a single vertical well by depressurisation method in the same period. China’s test was performed in the South China Sea for 60 days. The average daily gas production rate falls between 2.9 × 103–8.3 × 103 in both test sites, with a peak of 2 × 104 m3/day [58]. 5.2. Carbon Dioxide Replacement Cost and Conventional Storage Cost The recovery of methane from NGH and its replacement with CO2 molecule remains very challenging, and no previous work on the open literature have reported its cost. 21 of 32PDF Image | Energies 14
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