Summary During gas production from offshore gas-HBS, there are concerns regarding the settlement of the seabed and the possibility that frictional stress will develop along the production casing. This frictional stress is caused by a change in the effective stress induced by water movement caused by depressurization and dissociation of hydrate as well as gas generation and thermal changes, all of which are interconnected. The authors have developed a multiphase-coupled simulator by use of a finite-element method named COTHMA. Stresses and deformation caused by gas-hydrate production near the production well and deep seabed were predicted using a multiphase simulator coupled with geomechanics for the offshore gas-hydrate-production test in the eastern Nankai Trough. Distributions of hydrate saturation, gas saturation, water pressure, gas pressure, temperature, and stresses were predicted by the simulator. As a result, the dissociation of gas hydrate was predicted within a range of approximately 10 m, but mechanical deformation occurred in a much wider area. The stress localization initially occurred in a sand layer with low hydrate saturation, and compression behavior appeared. Tensile stress was generated in and around the casing shoe as it was pulled vertically downward caused by compaction of the formation. As a result, the possibility of extensive failure of the gravel pack of the well completion was demonstrated. In addition, in a specific layer, where a pressure reduction progressed in the production interval, the compressive force related to frictional stress from the formation increased, and the gravel layer became thin. Settlement of the seafloor caused by depressurization for 6 days was within a few centimeters and an approximate 30 cm for 1 year of continued production.

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Depressurization process is regarded as the most effective process for gas recovery method from the viewpoints of gas productivity and economic efficiency among in-situ dissociation processes of Methane Hydrate (MH) existing in marine sediments. However, it is supposed that consolidation and deformation of the stratum occurs due to MH dissociation and increase of effective stress in the stratum during operation of depressurization. Consolidation and deformation wreak negative friction on the production well. As a result, the production well may suffer large compressive or tensile stress. In the worst case, it may cause shear failure, tension failure and crushing. Therefore, in order to improve the accuracy for evaluation of stress distribution occurring on production well during depressurization, it is necessary to construct the numerical model enable to reproduce unsteady change of the relationship between shear stress and strain occurring on the contact surface between well and layer and introduce into geo-mechanical simulator. In this study, targeting three contact surface locating above depressurization interval such as 1) casingcement, 2) casing-layer and 3) cement-layer consisting of different material, we conducted push-out test in laboratory in order to evaluate the frictional behavior at these contact surface based on the relationship between displacement and axial load. From experimental observation, it was found that shear stress occurring on the contact surface linearly increased at the initial stage in the case of steel-cement specimen. On the other hand, for specimens consisting steel-clay and cement-clay, non-linear increase of shear stress was confirmed in the process leading to the shear strength. In addition, shear strength τ max for each contact surface increased depending on effective stress σ ', effective friction angle δ ' and effective cohesion c' as failure criteria was estimated based on τ max and σ '. Then, constitutive equation of variable compliance type was applied for reproduction of the relationship between displacement and shear stress observed in a series of push-out test. Through numerical simulation by introduction of this constitutive equation, we confirmed the validity of modeling of the frictional behavior.

Depressurization process is regarded as the most effective process for gas recovery method from the viewpoints of gas productivity and economic efficiency among in-situ dissociation processes of Methane Hydrate (MH) existing in marine sediments. However, it is supposed that consolidation and deformation of the stratum occurs due to MH dissociation and increase of effective stress in the stratum during operation of depressurization. Consolidation and deformation wreak negative friction on the production well. As a result, the production well may suffer large compressive or tensile stress. In the worst case, it may cause shear failure, tension failure and crushing. Therefore, for optimization of gas production process by depressurization, it is necessary to perform numerical simulation in consideration of a series of phenomenon during MH dissociation in porous media and evaluate the effect of consolidation deformation of the stratum on MH production well. In this study, using the geo-mechanical simulator named as COTHMA developed under MH21 research consortium, we carried out the field-scale numerical simulation for prediction of deformation and stress distribution around production well during depressurization. On the basis of field data for the Eastern Nankai Trough area and the structure of production well for the methane hydrate first offshore production test in 2013, the detailed model for reservoir and production well was constructed. In addition, we conducted push-out test to evaluate the frictional behavior at the interface between screen-gravel pack as the different materials constituting production well and introduced into numerical model for COTHMA. From calculation results, it was found that Mises stress occurring on base pipe installed into the interval of depressurization reached 420 MPa as yield point of steel due to the effect of friction. However, the original shape was maintained because the occurred equivalent plastic strain was about 2.95 % and this strain value was much smaller than 21 % as failure criterion. Furthermore, the effect of interface between casing and cementing was not large. This result suggested that the well structure above the interval of depressurization acted as unit and the interfacial frictional behavior between well and layer was the dominant factor on deformation behavior and stress distribution of casing and cementing.

： Methane Hydrate (MH) is well known as one of the unconventional resources, which is con rmed to exist abundantly offshore in Japan. A variety of studies have suggested that the depressurization is the most effective method for developing MH from the commercial and technical points of view. In the development of MH by the depressurization method, the dissociation and production of MH must greatly influence the geo-mechanical behaviors such as compaction/deformation of formations and vice versa. Hence, to rigorously predict the MH reservoir performances, it is essential to take account of the ow and geo-mechanical behaviors simultaneously.First, we developed a program for coupling the MH flow simulator (MH21-HYDRES) and the geo-mechanics simulator (COTHMAs) , which were developed under The Research Consortium for MH Resources in Japan (MH21 Research Consortium) , enabling simple explicit coupling (ECM) and iterative coupling (ICM) . As a result of the simulations using this program, it was confirmed that ICM could precisely predict MH reser voir performances in conjunction with geo-mechanical behavior, but the computation became far slower than uncoupled simulations by MH21-HYDRES.Second, we incorporated two functions into the program to improve the practicality, that is hybrid coupling (HCM) and dual-grid system (DGS) . In the simulations using the HCM, the ICM is applied only to the grid blocks (or elements) with noticeable strain/deformation, while the ECM is applied to those with negligible strain. In the DGS, the sizes of ow simulation grid blocks and geo-mechanics simulation elements can be defined independently. The simulation studies using the program revealed that the HCM could remarkably shorten the computational time without compromising calculation accuracy. It was also con rmed that the combination of the HCM and the DGS could dramatically reduce the computational time, although the calculation results were slightly different from those with smaller elements