Experimental And Numerical Study Of Methane Hydrate Dissociation By Depressurization In Porous Media

Energy shortage and environmental pollution are serious problems we have to cope with in the world. Developing efficient and clear energy source has become an urgent task to solve these problems. Since Methane Hydrate (MH) has many advantages including a vast amount of reserve, high energy capacity and less pollution after combustion, it has been viewed as a potential energy source for the 21st Century. The studies on the basic properties and production techniques of MH are very important in theoretical and practical aspects.The physical modelings of MH formation and dissociation in porous media were conducted using a self-designed apparatus. The phase equilibrium curves of MH in porous media were measured, which are very consistent with those obtained by other researchers. Under our experimental conditions, the MH formation and dissociation in porous media by depressurization were studied and the production performance and influencing factors were analyzed. The results show that higher boundary heat can increase largely MH dissociation rate.MH in porous media was formed with constant volume method by injecting excess methane gas which can improve the water conversion rate as much as possible. The permeabilities of hydrate-bearing porous media were measured with methane, which decrease the effect that MH formation or dissociation during measurement on experiment data. The experiment results were compared with existing models and it was found that they were good fitting with Masuda's model.A three-dimensional cubic pore network model was developed to model the phase shift of MH in porous media with different pore sizes. The permeability variation of hydrate-bearing porous media was studied. The simulation results are consistent with experiment data well, which shows that pore network model can be used to study MH formation and dissociation in porous media at micro scale.An advanced 2-D axisymmetric simulator including three phase (water, gas, and hydrate) and three components (water, gas, and hydrate) was developed based on the mass and energy conservation theory. The thermodynamic and intrinsic dynamic of MH in porous media are considered in the simulator. The governing equations are dicretized with finite difference method and solved with fully-implicit manner. The data matching was conducted using the mathematical model and the experimental results, which is consistent with each other and proves the accuracy of the mathematical model. The sensitive factors were studied for laboratory-scale MH dissociation by depressurization. The results suggest that intrinsic kinetic constant, permeability reduction index, gas-water relative permeability, rock heat conductivity, initial temperature, initial phase saturation, outer pressure and boundary heat transfer are sensitive for MH production by depressurization. The increase of gas relative permeability causes higher hydrate dissociation rate than the corresponding increase of water relative permeability does; the combination of depressurization and thermal stimulation is a better technique to explore MH in the MH reservoirs with high initial hydrate saturation; The initial water saturation has an optimum value for increasing hydrate dissociation rate and the value is about equal to the effective residual water saturation; Under the condition of higher boundary heat transfer, the increase of cumulative gas production is less than the increase of cumulative heat absorbed, the formation in some regions and dissociation in other regions maybe occur simultaneously.