The fully coupled methane hydrate model developed in Cambridge was adopted in this numerical study on gas production trial at the Eastern Nankai Trough, Japan 2013. Based on the provided experimental data of hydrate soil core samples, the soil parameters at Eastern Nankai Trough were successfully calibrated. With calibrated soil parameters and site geometry, a 50 days gas production trail was numerically simulated using the fully coupled simulator CMHGS (Cambridge Methane Hydrate Geomechanics Simulator). The geomechanical behavior of hydrate bearing sediments and production history results under 3 different depressurization strategies were explored and discussed. With the latest gas production site data, several input parameters for the numerical study were calibrated and an updated numerical simulation of the gas production test was carried out. The comparison of gas production history and vertical displacement along the wellbore between the updated simulation and the previous simulation results suggest large discrepancy, which highlights the importance of parametric study for numerical simulation of the gas hydrate production test. Therefore, parameter sensitivity of production history and vertical displacement were investigated and concluded the relative permeability curve; temperature profile, sea water salinity and permeability anisotropy all influence production results and mechanical responses.
Japan Oil, Gas, and Metals National Corporation (JOGMEC) conducted the first offshore methane hydrate production trial on March 2013, in the eastern Nankai Trough, Japan. Field work for the gas production trial began in early 2012. Coring and logging operations were conducted to prepare for the gas hydrate production trial. On March 12th, 2013, JOGMEC succeeded in extracting methane gas from the hydrate-bearing sediments by the depressurization method and approximately 120, 000 cubic meters of methane gas were produced totally. The production finally terminated due to the increase in sand production. It indicates the importance of evaluation the state of wellbore in order to produce methane gas safety and efficiently. It calls for an analysis to predict the stress changes and plastic strain evolution of wellbore during the life of well. Hence, a wellbore model, specifically designed for methane gas production, is necessary to assess the potential risks associated with the construction and production stage. This paper describes a finite element approach, which mainly takes into account of (a) all the typical processes associated with wellbore construction and gas production and (b) the interaction between cement-casing-formation. Based on this approach, a finite element model, which includes the layered soil profile, cement, casing, gravel and sand screen, was developed in ABAQUS to simulate the gas production trial at East Nankai Trough in March, 2013. The Methane Hydrate Critical State (MHCS) model (Uchida et al., 2012), which was developed to simulate the geomechanical response of the methane hydrate bearing sediment, was incorporated into this finite element model. It presents results of coupled ground deformation and pore fluid flow analysis of a gas production wellbore from a methane hydrate formation. The study focused on the short-and long-term mechanical behaviour of the critical physical components at and near the wellbore by simulating the construction processes of the wellbore as well as the production stage. The interaction between cement-casing-formation was examined in detail.
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