Upscaled Anisotropic Methane Hydrate Critical State Model for Turbidite Hydrate-Bearing Sediments at East Nankai Trough

Turbidite formation is a common feature of natural hydrate-bearing sediments that has been observed and reported at several hydrate exploration sites. It is therefore important to incorporate this anisotropic geological feature into the constitutive modeling when evaluating the geomechanical risks involved during hydrate-based gas production. To date, a number of constitutive models have been proposed to capture the isotropic geomechanical behavior of homogeneous hydrate-bearing sediments. Since the turbidite formation contains soil layers at a scale much smaller than the size of the numerical element used for reservoir scale simulations, it is necessary to upscale the geomechanical behavior of a layered system to an equivalent anisotropic continuum model by adopting some homogenization techniques. In this study, an anisotropic methane hydrate critical state model is developed by modifying the original isotropic version of Uchida et al. (2012, https://doi.org/10.1029/2011JB008661). The calibration methodology of the anisotropic model parameters for a given set of hydrate heterogeneity and the turbidite formation at the Eastern Nankai Trough is proposed and demonstrated. The upscaled parameters are calibrated by curve fitting the numerically simulated stress-strain curves of the layered system with the original isotropic constitutive model at the layered scale. Forty-two sets of model parameters are calibrated from different site element models of this site. They are used to develop empirical correlations between the model parameters and the site input properties within the turbidite formation. This paper presents the details of the new anisotropic constitutive model and the performance of the proposed upscaling procedure for the Eastern Nankai Trough case. Plain Language Summary The prospects for gas production from methane hydrates have greatly improved since hydrate deposits were found to exist in sandy sediments. Heterogeneity is a very important feature of natural hydrate-bearing sediments. In order to carry out accurate numerical simulations of the Eastern Nankai Trough gas production test, an advanced soil model is proposed in this study to better represent heterogeneous structure of the hydrate-bearing sediments. The numerical simulations using the advanced model suggested more accurate results than the results determined from previous models. This study, as part of MH21 research program, Japan, offered important insights for offshore methane hydrate gas production in Eastern Nankai Trough, Japan.