Numerical simulation of gas recovery from a low-permeability hydrate reservoir by depressurization

A successful gas production trial in the South China Sea in 2017 proves that low-permeability hydrate reservoirs have great gas production potential. To improve the efficiency of energy exploitation in the future commercial hydrate production, it is necessary to predict the gas/water production rate and the multiphysical responses of the low permeability reservoirs to the gas production induced by depressurization. In this study, a Mohr-Coulomb geomechanical model based on the test data of the low permeability silty hydrate deposits is embedded into a fully coupled thermo-hydro-mechanical model, which is applied to systematically investigate the responses of hydrate reservoir including the pore pressure, temperature, and hydrate saturation, as well as the mechanical behaviors of the hydrate reservoir during one year of gas production by depressurization using a horizontal wellbore at GMGS3-W19 site in the South China Sea. The simulation results show that the gas production rate increases at the beginning of production then rapidly decreases after no further reduction in the downhole pressure, finally remains at 100 m3/day. Due to the low permeability, the decreased pressure propagates to 100 m, which is not very far from the wellbore. One year of gas production by horizontal wellbore does not severely disturb the reservoir. However, the stress relaxation due to the hydrate dissociation may induce stress redistribution leading to the deposits displacement of 0.35 m around the wellbore, which potentially results in the deformation of production well, further affecting the safety and efficiency of the gas production.