Basin Modeling on Formation of Methane Hydrate Including Generation and Migration of Microbial Methane

Akihiko Okui¹, Ryosuke Aoyagi², Martin Schoell³, and Tetsuya Fujii^4
1Idemitsu Oil and Gas Co., Ltd., Tokyo, Japan
²Mizuho Information and Research Institute, Inc., Tokyo, Japan
³GasConsult International Inc., Berkeley, California
⁴Japan Oil, Gas and Metals National Corporation, Chiba, Japan

Exploration and development for gas hydrate is hot issue in Japan now. Estimation of its resources is essential in initial stage of investigation. Understanding of generation and migration of methane as well as formation of methane hydrate is very helpful for this kind of study like petroleum system for oil and gas exploration. A basin model, which can simulate the formation of methane hydrate as well as generation and migration of microbial methane was developed and then was used for the evaluation of methane hydrate in Nankai Trough area in Japan.
New model was developed by upgrading of ordinary basin model, which was used for the exploration of oil and gas. Actual model based is SIGMA, which was developed by Japan National Oil Corporation in 1990’s. SIGMA can model generation, migration and accumulation of oil and gas, by involving the burial and compaction of formations, heat transfer by conduction and convection, oil and gas generation by first-order kinetic model, three-phase fluid flow by Darcy’s law and trapping of oil and gas by capillary pressure. Furthermore, following functions (modules) was added to SIGMA.
   1  Generation of microbial methane
   2  Migration of microbial methane by dissolution into water
   3  Formation and destruction of methane hydrate
   4  Change in rock properties due to formation of methane hydrate
Generation of microbial methane is modelled by empirical relationship derived from head space analysis on actual samples such as collected by ODP. The relationships of organic accumulation rate with maximum concentration of methane and its depth were derived, which are used for the model. Dissolution of methane into water is modelled by the functions of temperature and pressure, and published relationship was used. Formation and distraction of methane hydrate is also modelled by the functions of temperature and pressure using published relationship. However, the reaction was assumed to occur quickly in geological time scale (so called “static” model), when temperature and pressure conditions reached, according to laboratory experiments. Regarding the change in rock properties due to formation of methane hydrate, we found that relative permeability model used for SIGMA can be also applicable by replacing oil phase to hydrate phase. Since the viscosity for hydrate phase are given as extremely large value, after the formation, hydrate can not move. Under these assumptions, effective permeabilities for other phases such as free methane and water should decrease with increasing saturation of methane hydrate. In this model, methane hydrate is treated as non-wetting phase like oil, which corresponds to the case that hydrate precipitates and grows in central part of pore spaces.
This new model (SIGMA-MH) was applied to geological section hypothesized Nankai Trough located in the Pacific side of Japanese Islands, where big research program is going on and actual methane hydrate was sampled in wells. The results of SIGMA-MH modelling indicated that microbial methane was generated just beneath sea bottom and then dissolved into water. The water dissolving methane was at first buried with sediments, but then expelled to near surface and free gas phase was created, which was converted to methane hydrate in SIGMA-MH modelling. In addition, the scenario including destruction of hydrate, re-migration of methane and re-formation of hydrate as well as the combination of methane hydrate with free methane beneath it can be modelled by SIGMA-MH. Then, the system for the formation of methane hydrate was evaluated by checking the sensitivities to input parameters such as water depth, dipping of formations, sedimentation rate and rate of methane generation. Water depth was supposed to affect the depth at which methane hydrate forms. Dipping of formations as well as sedimentation rate should affect the migration of water which dissolves microbial methane.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands