A thermal, three-phase, one-dimensional numerical model is developed to simulate two regimes of gas production from sediments containing methane hydrates by depressurization: the dissociation-controlled regime and the flow-controlled regime. A parameter namely dissociation-flow time-scale ratio, R s , is defined and employed to identify the two regimes. The numerical model uses a finite-difference scheme; it is implicit in water and gas saturations, pressure and temperature, and explicit in hydrate saturation. The model shows that laboratory-scale experiments are often dissociation-controlled, but the field-scale processes are typically flow-controlled. Gas production from a linear reservoir is more sensitive to the heat transfer coefficient with the surrounding than the longitudinal heat conduction coefficient, in 1-D simulations. Gas production is not very sensitive to the well temperature boundary condition. This model can be used to fit laboratory-scale experimental data, but the dissociation rate constant, the multiphase flow parameters and the heat transfer parameters are uncertain and should be measured experimentally.

The feasibility of a pilot production project on the North Slope of Alaska was computed to determine the production potential of a hydrate accumulation. The production of gas from a 1 mile by 4 mile reservoir block containing hydrate underlain by an accumulation of free gas was simulated and the resulting production profiles were analyzed. Results of the simulations indicate that depressurization of the free gas zone reduces the pressure at the gas-hydrate interface below that necessary for hydrate stability and causes the hydrate to dissociate into methane gas and water.

No abstract

No abstract