Key Factors for Depressurization-Induced Gas Production from Oceanic Methane Hydrates

Oceanic methane hydrate (MH) deposits have been found at high saturations within reservoir-quality sands in the Eastern Nankai Trough and the Gulf of Mexico. This study investigates the key factors for the success of depressurization-induced gas production from such oceanic MH deposits. A numerical simulator (MH21-HYDRES: MH21 Hydrate Reservoir Simulator) was used to study the performance of gas production from MH deposits. We calculated the hydrate dissociation behavior and gas/water production performance during depressurization for a hypothetical MH well. Simulation runs were conducted under various initial reservoir conditions of MH saturation, temperature, and absolute permeability. A productivity function (PF) was introduced as an indicator of gas productivity, which is a function of gas production rate, water production rate, and discount rate. The simulations showed that recovery factors over 36% and maximum gas production rates over 450 000 Sm3/d were expected for the most suitable conditions of a class 3 deposit (i.e., an isolated MH deposit that is not in contact with any hydrate-free zone of mobile fluids). However, gas productivity was affected by formation temperature and initial effective permeability. The values of PF increased with increasing formation temperature when the initial permeability of the deposit was higher than a threshold value (the threshold permeability); however, it decreased for the deposit below the threshold permeability. The threshold permeability was estimated to be between 1 and 10 mD in the class 3 deposit. These results suggest that key factors for the success of depressurization-induced gas production from oceanic MH are as follows: (1) The initial effective permeability of the MH deposit is higher than the threshold value, and (2) the temperature of the MH deposit is as high as possible.