Multiphase flow and geomechanical responses of interbedded hydrate reservoirs during depressurization gas production for deepwater environment

The coupled multiphase flow and geomechanical responses are very important considerations in planning offshore hydrate production. In this work, a novel thermo-hydro-mechanical (THM) coupled simulator has been developed to investigate the THM responses of interbedded hydrate reservoirs using depressurization method in the offshore India. Based on the detailed geological data at the NGHP-02-16 site, a more realistic hydrate reservoir model is constructed to analyze the unique multiphase flow and geomechanical responses induced by depressurization. The results indicate that the average gas production rate of this work is 6370 ST m3/d, which is significantly lower than the previous numerical result of about 9000 ST m3/d using the same 3 MPa production pressure. This is mainly because the geomechanical response gives negative feedback to flow through the stress -dependent porosity and permeability. The reservoir heterogeneity results in the non-uniform hydrate dissocia-tion and effective stress changes in the marine sediments. The depressurization results in significant increase of shear stress and vertical compaction in the hydrate reservoir. The maximum shear stress is no more than 2 MPa and is mainly concentrated in the top and bottom regions of the production interval. The maximum seafloor subsidence is 1.15 m during 100 days depressurization test. Sensitivity analysis results suggest that the pro-duction pressure and perforation length are two important parameters affect the gas productivity and mechanical stability. The low production pressure is recommended under the premise of mechanical stability for gas pro-duction from hydrate reservoirs. Perforation in the underlying water-saturated sand layer will lead to more serious water production and seafloor subsidence. The coupled multiphase flow and geomechanical results presented in this work are helpful to design the safe hydrate production scheme in the future.