Comprehensive Report of Phase 2 & 3 Research Results

VII. Environmental Studies
For commercial production of methane gas from methane hydrate-bearing sediments, it is essential tounderstand potential environmental impacts associated with methane hydrate development, and to establish
an appropriate assessment procedure for these environmental impacts.
In Phase 1, necessary information for realistic investigation of environmental issues, such as detailed
methane hydrate distribution, and effective methods for gas production and target areas, was uncertain, andvarious basic environmental studies associated with environmental monitoring, surveys and predictions hadbeen conducted.
During Phase 2 and Phase 3, more realistic environmental studies have been conducted based on theimportant findings of Phase 1, namely, identification of the distribution of methane hydrate concentrated
zones, and the effectiveness of the depressurization method.
The offshore production tests conducted twice in the "Japan's Methane Hydrate R&D Program" weremeaningful and important opportunities to acquire real data on environmental impacts and to apply andoptimize the methods for prediction, assessment and monitoring of future methane hydrate development.
Environmental studies had been conducted referring to the procedure of Environmental Impact Assessment(EIA) generally implemented in conventional offshore oil and gas production projects worldwide. The
iterative process of environmental studies is shown in Fig.Ⅶ-1.

Identification of the potential environmental impact

Feedback

Identification of the environmental risk factors
・Specific events to methane hydrate development
・Common activities with the conventional offshore oil and gas development
Identification of the environmental receptors
・Environmental baseline surveys
Feedback

Prediction and assessment
Ecotoxicity tests of the dissolved methane
Prediction by numerical models for possible significant risks
・Dispersion of methane gas from seafloor
・Dispersion of discharged produced water・Dispersion and deposit of drill cuttings and related fluids
・Impacts on marine organisms
Qualitative assessment using risk matrix

Environmental monitoring and survey
Continuous monitoring on seafloor deformation and dissolved methane
using specific monitoring system before, during and after gas production
Regular surveys for the conditions of water, sediment and marine flora and faunaaround the test site

Fig.VII-1 Process of environmental studies

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Comprehensive Report of Phase 2 & 3 Research Results

In summary, four events were identified as potential environmental risk factors that might cause significant
impacts, namely, methane leakage from the seafloor, seafloor deformation, discharge of produced water,and deposition of drill cuttings and related fluids. The first three events are considered to be methane
hydrate-specific events. In addition to the produced water, the difference from that of conventional oil andgas production is that pure water generated from dissociation of methane hydrate is mixed with formationwater. With reference to the last one arising from riser-less drilling, even though this event is common
among conventional offshore oil and gas developments and generally accepted worldwide through the EIA
process, MH21 took sufficient care of this taking into account the uncertainty surrounding the deep seaenvironment.
Regarding the prediction for methane gas seepage from the seafloor and seafloor deformation, specificnumerical models have been developed and applied (Please refer to chapter V for seafloor deformation). On
the other hand, proven models applied within oceanographic, coastal and estuarine environments were used
for the dispersion of produced water and drill cuttings. Additionally an ecosystem model has beendeveloped to enable consideration of environmental impacts on marine organisms in the lower trophic
levels of the food chain. Other environmental risks were comprehensively assessed using a risk matrix. The
results of these predictions and assessments were checked based on the data acquired by environmental
monitoring and surveys that were implemented to extract technical issues.
Regarding environmental monitoring, a newly improved methane sensor was applied to dissolved methane
monitoring, and a highly accurate pressure sensor was used to monitor seafloor deformation. In general,
several technical issues were identified, and necessary conditions and limitations for deep sea long-term use
were clarified.
The results and developed methods mentioned above were only for small-scale gas production tests inlimited areas, and EIA methods should be optimized taking into account environmental characteristics ofactual production fields, production methods and development activities for future commercial methane
hydrate development.

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