Numerical Study on Eastern Nankai Trough gas Hydrate Production Test

Abstract

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.

Utilizing Non-Equilibrium Thermodynamics and Reactive Transport to Model CH4 Production from the Nankai Trough Gas Hydrate Reservoir

“…Two-and three-dimensional seismic surveys have led to the discovery of hydrate-bearing sediments under the Pacific Ocean [16]. Based on the agreement under the Research Consortium for Methane Hydrate Resources in Japan (MH21 project), the eastern Nankai Trough had been assigned as a target for future gas production from hydrate deposits [16].…”

“…Thus, the production well was drilled to 20 m above the BSR, and the production interval was selected to be 40 m from the top of the MHCZ [15]. Due to effects of the layering formation, the horizontal permeability is higher than the vertical permeability [16]. The total average porosity has been estimated from the density log to be in the range of 40% to 50% [22,27].…”

“…Formation pressure and fluid mobility measurements indicated the water permeability ranges from 0.1 to a few millidarcy (mD) both within the sealing layers and the MHCZ [15]. The sea-bottom temperature is in the range of 2.8 to 3.0 • C (1000 m below sea level) [16,28]. The borehole pressure was set to 5 MPa [23].…”

“…Figures 6 and 7 show the increase and accumulation of effective stress in the vertical and horizontal directions inside the reservoir model as the result of depressurization and hydrate decomposition. Hydrate dissociation reduces the matrix structure's strength [16]. …”

“…The pressure in the production well was set to 4.5 MPa [25]. The value of the geothermal gradient was set to 0.03 • C/m, and that of the hydrostatic pressure gradient, to 0.01 MPa/m [16,28]. The water depth was 998 m, with the lower part of the BSR located at 1335 m below the sea floor [23].…”