The locally varying thermodynamic driving force dominates the gas production efficiency from natural gas hydrate-bearing marine sediments

Overcoming the gas production efficiency issues from marine gas hydrate reservoirs requires a better understanding of the interactions between the depressurization scheme and hydrate decomposition behavior. In this work, special attentions were paid to the time-varying relationships between the local temperature/pressure conditions and hydrate phase equilibrium. It was found that the routine step-wise depressurization scheme with equal gradient per step will significantly sacrifice the production efficiency. Whereas through dividing the overall production process into two stages according to the evolving boundary and handling them separately, we found that a straightforward depressurization at the free gas release stage would contribute to a maximum of 43.59% increase in the production efficiency. Once hydrate decomposition was intervening, the pressure should be carefully managed to avoid temperature drop problem. This could be improved by carrying out a further step-wise depressurization and regulating the duration of the constant pressure stage. Consequently, a positive dependence was identified between the production efficiency and the phase-equilibrium-based thermodynamic driving force. This could be an indicator to guide the design of the pressure schemes in the field test via simply monitoring the local pressure and temperature conditions to balance between the production efficiency and reservoir temperature.