Methane hydrate is one of the unconventional natural gas resources, which exists typically in unconsolidated or weakly consolidated reservoirs. Hence, it is necessary to consider not only flow behavior but also geomechanical behavior such as compaction and deformation of a reservoir to predict the methane hydrate reservoir performances rigorously. In the past study (Iwata et al., 2019), the program that couples the methane hydrate flow simulator (MH21-HYDRES) and geomechanics simulator (COTHMAs) was developed incorporating several coupling methods such as “iterative coupling method”, “explicit coupling method”, “spatial hybrid coupling method” and “dual grid coupling method”. However, this program has never been tested with field scale models, which have a huge number of grids and complexity. The objective of this study is to examine new coupling methods and expand the program so that it can be applied to field scale simulation.
First, we verified the existing coupling program by referring to the reports of the Second International Gas Hydrate Code Comparison Study (IGHCCS2), (White, M.D. et al, 2020). As a result of the verification, the results simulated by this program for the problems of given in IGHCCS2 agreed with those of other simulators, which validated the functions of this program at a certain level.
Second, we investigated the applicability of the existing coupling program to field scale problems. In this examination, the coupling program was executed using the reservoir model constructed for one of the methane hydrate reservoirs located in the eastern Nankai Trough. This reservoir model has more than 20,000 grids and high degree of heterogeneity in the vertical direction. As a result, this coupling program could predict the methane hydrate reservoir performances, but took an enormous amount of computational time, especially in the calculation by the geomechanics simulator.
Third, to shorten the computational time, the new coupling methods, “temporal hybrid coupling method” and “time-step integration coupling method” were formulated and incorporated into the above coupling program. Then, we confirmed that these methods could provide the solutions satisfying both the calculation accuracy and speed.
Finally, the expanded coupling program was applied to the field scale simulation using 7 coupling methods of “iterative”, “explicit”, “modified explicit”, “spatial hybrid”, “temporal hybrid”, “time-step integration”, and “dual grid”. It was revealed that the coupling methods of “spatial hybrid”, “temporal hybrid”, “time-step integration” and “dual grid” could provide solutions similar to the solutions by “iterative coupling method” with much shorter computational time.
