2003 年 82 巻 4 号 p. 197-207
For offshore methane hydrate deposit, we constructed a conceptual production system model incor-porated with the software named QUE$TOR, and a model for direct and indirect CO2 emission assess-ment by reference to the induced CO2 emission data from the 1995 inter-industry relations table for envi-ronment analysis. Based on the calculated data, we evaluated economics and CO2 emission in the gas production from a hypothetical offshore hydrate deposit.
The three production methods were evaluated: Depressurization using vertical wells; depressuriza-tion using horizontal wells; and hot-water injection using horizontal wells. The gas production cost was evaluated to be 818 and 53 Yen/Sm3 for the depressurization method using vertical and horizontal wells respectively, and 67 Yen/Sm3 for the hot-water injection method. In regard to CO2 emission, the depres-surization method yielded 66.51 g-C/Mcal for the vertical-well case and 58.43 g-C/Mcal for the horizon-tal-well case. These values were lower than the reported value for LNG (69.77 g-C/Mcal). The case of hot-water injection emitted more CO2 at 79.59 g-C/Mcal, but it was lower than the value for oil (83.63g-C /Mcal).
This study demonstrated that the depressurization using horizontal wells was most promising method from economical and environmental concerns, but its production cost is much higher than the domestic natural-gas supply price. For utilizing offshore methane hydrates as an economically feasible resource, the research and development are required in the following area: (1) Completing well with a horizontal section exceeding 1, 000 m at a shallow depth under deep water; and (2) Cost reduction in off-shore well completion and production system. Moreover, gas productivity from offshore methane hydrates must be evaluated more rigorously from field production tests and simulation studies.