Geological and geophysical features of and controls on occurrence and accumulation of gas hydrates in the first offshore gas-hydrate production test region in the Shenhu area, Northern South China Sea

Permeability of hydrate-bearing sediments

The new scenario involving the rapid energy supply transition from oil-based to natural gas-based undoubtedly affects the future carbon capture and storage (CCS) and offers an opportunity for the use of natural gas hydrates (NGHs). NGHs account for one-third of the mobile organic carbon on Earth, acting as a carbon storage reservoir in the carbon cycle. Hydrate-based technologies including CO2 capture, CO2 separation, and natural gas storage and transportation can also be used to reduce greenhouse gas (CO2, CH4) emissions and have excellent application potential. However, the implications of the energy supply transition, NGH extraction, and hydrate-based technologies for future climate change mitigation have not been deeply recognized. This paper comprehensively discusses the global energy supply and environmental challenges and transitions, and NGHs and their role in the energy supply, carbon cycle, and historical and future climate change, and summarizes the state-of-the-art developments in hydrate-based technologies and NGH exploitation methods and their potential environmental impacts, thereby providing a perspective on the roles of NGHs and their related technologies in the future energy supply and climate change mitigation. In all of these areas, we focus on identifying future CCS challenges and the technological development risk imposed by a natural gas-based global economy and NGH utilization, which should be highlighted in the next several decades.

Natural gas hydrate resources and hydrate technologies: a review and analysis of the associated energy and global warming challenges

Prospect of marine natural gas hydrate stimulation theory and technology system

Tuning the fluid production behaviour of hydrate-bearing sediments by multi-stage depressurization

Following the first attempt at producing gas from a naturally occurring methane hydrate (MH) deposit in the Daini–Atsumi Knoll in the eastern Nankai Trough area off Honshu Island, Japan in 2013, a second attempt was made in April to June of 2017 at a nearby location using two producer wells sequentially and applying the depressurization method. The operation in the first borehole (AT1-P3) continued for 12 days with a stable drawdown of around 7.5 MPa and 41 000 m3 of methane gas being produced despite intermittent sand-production events. The operation of the other borehole (AT1-P2) followed, with a total of 24 days of flow and 222 500 m3 of methane gas being produced without sand problems. However, the degree of drawdown was limited to 5 MPa because of a higher water production rate than expected in the second hole. The pressure and temperature sensors deployed in the two producers, along with the two monitoring holes drilled nearby, gathered reservoir response data and information about the long-term MH dissociation processes in the vicinity of the production holes in the temporal and spatial domains. Although the ratio of energy return to the input was considerably larger than that for the depressurization operation, some observations (e.g., the high contrast in the production rates between the two holes and the almost constant or slightly reduced gas production rates) were not predicted by the numerical models. This failure in prediction raises questions about the veracity of the reservoir characteristics modeled in the numerical simulations. This paper presents the operation summaries and data obtained with thought-experiment based-anticipated production behaviors and preliminary analysis of the obtained data as the comparison with expected behaviors. Detailed observations of gas and water production, as well as the pressure and temperature data recorded during the gas flow tests, indicate that the heterogeneous MH distribution within the reservoir was mainly responsible for the discrepancies observed between the anticipated and actual behaviors. Furthermore, the motion of the water that does not originate from MH dissociation introduces complexity, such as the occurrence of concentrated water-producing intervals and unexpected gas production responses to decreases in pressure, into the production behavior. The influence of heterogeneity should be clearly understood for the accurate prediction of gas production behavior based on MH reservoirs.

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X.-X. Wang

Papers

The second offshore production of methane hydrate in the Nankai Trough and gas production behavior from a heterogeneous methane hydrate reservoir