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.

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Recently, the shortage of landfill site has become a serious environmental problem according to an increase in the amount of waste disposal. A variety of recycling processes about the waste plastics with large capacity have been proposed from the environmental viewpoints. Especially the liquefaction process using the Ga-Si catalyst is able to recover the BTX (benzene, toluene and xylene) from the waste plastics selectively and efficiently. In this study, the inventory of BTX recovery system for the industrial waste plastics was analyzed, and the environmental loads such as energy consumption, air emission and solid waste were compared with the BTX production system from crude oil adding the waste systems of industrial waste plastics.
As a result, it was confirmed that the BTX recovery system was able to reduce 40% energy con-sumption more than the BTX production system adding the waste systems of plastics. It was suggested that this system was effective as the BTX production system, besides the recycling system for the indus-trial waste plastics.

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