RESEARCH NEWS STORY
November 1, 2023
Chiba University

A Mathematical Model for Studying Methane
Hydrate Distribution in the Nankai TroughScientists develop a one-dimensional flow model for simulating methane hydrate generationand distribution processes in the Nankai Trough of Japan
Naturally occurring methane hydrate deposits are potential sources of energy. However,their observation in natural environments remains challenging. Researchers have now
developed a one-dimensional mathematical flow model to indirectly observe the formation
and accumulation of methane hydrates under different lithological and fluid conditions in
the Nankai Trough of Japan. Numerical simulations using this model revealed important
insights that could help in the extraction of methane hydrates and the prevention ofgeohazards in the future.

Image title: Researchers conduct numerical simulations to explore methane hydrate
formation in Nankai Trough, Japan
Image caption: Methane hydrate has garnered considerable research attention owing to its
potential as an energy source. Now, researchers from Japan use one-dimensional modelbased numerical simulations to understand the nature of methane hydration saturation andgeneration process under different reservoir and fluid conditions in the Kumano Forearc
Basin, Nankai Trough, Japan.
Image credit: Peka from Wikimedia Commons
https://commons.wikimedia.org/w/index.php?curid=2867384
Image license: CC BY-SA 3.0

Methane hydrate is a naturally occurring ice-like crystalline solid that forms when methane
and water are subjected to geological high-pressure and low-temperature conditions. It isoften found trapped in continental margin sediments and permafrost. Owing to its immense
potential as a possible energy resource, researchers have attempted to get a better
understanding of the geochemical and geophysical factors that control the distribution of
methane hydrate reservoirs. However, these deposits take thousands of years to form, andtheir reservoirs are often found in geologically heterogeneous conditions. This makes it
difficult to conduct a real-time or thorough evaluation of the formation, distribution, oraccumulation of methane hydrates.
In a new study published in Island Arc on September 19, 2023, Associate Professor Hitoshi
Tomaru, along with Chao Xu from the Graduate School of Science and Engineering, Chiba
University, have now developed a mathematical model to circumvent this problem. Theyinvestigated the distribution characteristics and accumulation mechanisms of methane
hydrates in the Nankai Trough of Japan indirectly with the help of numerical simulations using
a one-dimensional flow model.
The researchers carried out numerical simulations of the hydrate generation process bymodifying the mathematical model proposed by Liu and Fleming in 2007 to investigate
methane hydrate saturation (Sh). They used logging and geochemical data from the Kumano
Forearc Basin of the Nankai Trough, which is located off the shore of the main island of Japanand is home to considerable hydrate reserves.
“With this model, we aimed to obtain insights into the distribution regularity of methane
hydrate and the factors controlling hydrate generation. So, building on the original model, we
explored the effects of deep fluid supply and reservoir lithology on the distribution andquantity of methane hydrates. We employed a smoother flux configuration to better observe
the impact of methane flux variation on the simulation results,” explains Dr. Tomaru.
Considering the importance of methane flux and water flow for the formation and distributionof methane hydrates, the researchers investigated three distinct scenarios corresponding tolow-, moderate-, and high-methane flux. They observed that the hydrate distribution in theNankai Trough area coincided with the results when the methane flux was relatively small.
Prior studies have shown that in low-methane flux environments, Sh is highest near the baseof the gas hydrate stability zone. In contrast, the formation of hydrates is significantly faster,and Sh is highest at the sea floor in high-methane flux regions. In other words, low-methane
flux reduces Sh upward from the bottom, and high-methane flux increases it downward. The
simulation results obtained in this study add to these observations and suggest that geologicalfactors such as the distribution of fractures and a deep fissure in a rock formation also
influence the sediment stress conditions and constrain the flow regime.
Furthermore, the team used typical permeability values of sand and mud to run simulations
and understand the impact of permeability changes caused by lithological units on hydrate
distribution. The results further revealed that Sh substantially increased in the sand layer but
drastically decreased in the mud layer.

Elaborating on these results, Dr. Tomaru says, “Our simulation model provides valuable
insights into the location, saturation, and distribution of natural methane hydrate resourcesin deep-sea sediments without the need for energy- and labor-intensive processes like physicaldrilling or coring of the sea floor.”
Going ahead, these findings can enable the development of more efficient strategies formethane hydrate generation and contribute to the development of better geohazard
prevention plans.
About Associate Professor Hitoshi Tomaru
Hitoshi Tomaru is an Associate Professor at the Department of Earth Sciences of the Graduate
School of Science and Engineering at Chiba University, Japan. He received his Doctor ofScience degree from the University of Tokyo in 2004 and joined the University of Rochester
as a postdoctoral fellow. He was a Japan Society for the Promotion of Science (JSPS)
postdoctoral fellow at the University of Tokyo and Kitami Institute of Technology. His researchinterests include geochemistry, energy resources, and iodine isotopes. His team currently
works on the geophysics of methane hydrates and the development of 129I geochronology formaterial transport via fluid in the shallow geosphere.
Reference:
Title of original paper: Simulation of methane hydrate formation in coarse- to fine-grained
sediments in the Nankai Trough, JapanAuthors: Chao Xu and Hitoshi Tomaru
Affiliations: Department of Earth Sciences, Graduate School of Science and Engineering,
Chiba University
Journal: Island Arc
DOI: 10.1111/iar.12502
Contact: Hitoshi Tomaru
Graduate School of Science, Chiba University
Email: tomaru@chiba-u.jp
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