Geomechanical responses during depressurization of hydrate-bearing sediment formation over a long methane gas production period

Abstract

The geomechanical behaviour of hydrate-bearing sediments during methane gas production is complex due to the spatial and temporal changes in stress, pore pressure, temperature, and phase change. In order to evaluate the geomechanical risks during methane gas production, it is necessary to understand the thermo-hydro-mechanical (THM) responses in the production region that recovers methane gas from the hydrate-bearing sediments. In this study, a fully coupled THM numerical simulator code was used to examine the reservoir scale field behaviour observed during the gas production trial conducted at the Eastern Nankai Trough, Japan in March, 2013. Using the available field investigation data, history matching of the gas production test was conducted to evaluate the methane gas production process. The fully coupled model allowed examination of the mechanical response using the critical state based constitutive model proposed by Uchida et al. (2012). The model parameters were determined from the results of triaxial compression tests conducted on recovered core samples. Based on the reservoir scale simulation results, this paper investigated the mechanical responses of five selected elements at different locations in the hydrate gas production region. The mechanical responses of hydrate-bearing sediments at specific locations within the production region are related to their hydrate dissociation status, which typically can be divided into before, during and after stages of hydrate dissociation. The 260 days gas production simulation suggests an increase in effective stresses accompanied by shearing deformation, which makes the soil more plastic. Potential geomechanical risks (such as wellbore stability and formation compaction) associated with the observed changes in stress/strain were also identified.