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.
In this study, the applicability of Microbial Induced Carbonate Precipitation (MICP) to sand control in high-pressure environment assuming methane hydrate reservoir is investigated. The cell culture solution(S.aquimarina, S.newyorkensis) and cementation solution passed through the specimen until the hydraulic conductivity decreased by two order of magnitude. As a result, it was revealed that S.newyorkensis decreases the hydraulic conductivity lower than S.aquimarina in an early stage of injection. In late stage, both S.newyorkensis and S.aquimarina decrease hydraulic conductivity and increase mechanical strength. Our results suggested the applicability of MICP under high confining pressure condition.
In this study, the lithological properties and gas hydrate saturations in pressure-cores recovered during the India’s National Gas Hydrate Program Expedition 02 (NGHP-02) in the Krishna–Godavari Basin (K–G Basin) off the eastern margin of India were investigated. Grain size, mineralogy, and grain density and employed scanning electron microscopy (SEM) were analyzed in these investigations. The gas hydrate saturations were estimated from P-wave velocities and gas volume measurements. The results of the pressure-core analyses confirm that natural gas hydrates can occur in silt-rich sediments, and the gas hydrate occurrence is controlled by the lithology of the sediments, especially their clay content in the NGHP- 02 pressure-core sediments.