Natural gas hydrates of Bulgaria and Romania in the Black Sea have been subject to studies by several European research projects. The current understanding of the hydrate distribution, and the total amounts of hydrate in the region, makes it interesting to evaluate in terms of commercial potential. In this study, we have evaluated some well-known hydrate production methods. Thermal stimulation and adding chemicals are considered as not economically feasible. Pressure reduction may not be efficient due to the endothermic dissociation of hydrates and long-term cooling of the sediments. Chemical work due to pressure reduction is an additional mechanism but is too slow to be commercially feasible. Adding CO2/N2, however, has a dual value. In the future, CO2 can be stored at a price proportional to a CO2 tax. This is deducted from the value of the released natural gas. The maximum addition of N2 is around 30 mol% of the CO2/N2 mixture. A minor addition (in the order of 1 mol%) of CH4 increases the stability of the hydrate created from the injection gas. The maximum N2 amount is dictated by the demand for the creation of a new hydrate from injection gas but also the need for sufficient heat release from this hydrate formation to dissociate the in situ CH4 hydrates. An additional additive is needed to accelerate the formation of hydrate from injection gas while at the same time reducing the creation of blocking hydrate films. Based on reasonable assumptions and approximations as used in a verified kinetic model it is found that CH4/CO2 swapping is a feasible method for Black Sea hydrates. It is also argued that the technology is essentially conventional petroleum technology combined with learning from projects on aquifer storage of CO2, and a thermodynamic approach for design of appropriate injection gas. It is also argued that the CH4/CO2 swap can be combined with well-known technology for steam cracking of produced hydrocarbons to H2 and CO2 (for re-injection).
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