Numerical modeling of coupled behavior of gas production and mechanical deformation of gas hydrate reservoir in Shenhu area, South China Sea: Enlightenments for field monitoring and model verification

All the field tests of hydrate exploitation in the Shenhu area, South China Sea recorded gas production data but did not monitor formation deformation, which leads to a deficiency in understanding mechanical behavior and its effect on gas production performance. This study established a thermo-hydromechanical-chemical coupling model for gas production in the Shenhu hydrate reservoir according to the literature data. Simulation results show that the effective stress increases quickly during field tests, leading to measurable seafloor subsidence (up to -0.4 m) near the well, and then basically keeps unchanged half a year later. The principal effective stress will not evolve towards causing yield failure, which ensures long-term production security. Ignoring mechanical behavior can double the predicted production performance indices, which, however, may be counterbalanced by the existing uncertainty in reservoir permeability. This results in multiple solutions of the inverse modeling, highlighting the importance of acquiring measurement data of formation deformation and reservoir permeability. We suggest that monitoring the subsidence of seafloor at a point near the well (horizontally -1 m away) in a short production duration (50-100 days) could be sufficiently effective to roughly understand the mechanical response to gas production and provide a basis for strict model verification.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

This study establishes a coupling model for gas production in the Shenhu hydrate reservoir and finds that the effective stress rapidly increases during field tests, leading to seafloor subsidence, but remains mostly unchanged half a year later. The principal effective stress will not cause yield failure, ensuring long-term production security. Ignoring mechanical behavior can double the predicted production performance indices, but this can be counterbalanced by uncertainty in reservoir permeability. Therefore, obtaining measurement data of formation deformation and reservoir permeability is crucial.