Numerical Simulation Of Decomposition Process Of Natural Gas Hydrate In Reactor By Depressurization Combined With Well-wall Heating

The imbalance between global energy supply and demand has become increasingly prominent,which is stimulating countries to promote their energy security to a national strategic level.Natural gas hydrate is an ice-like non-stoichiometric crystal compound which shows a series of advantages,such as large reserves,clean combustion.As a substitute for traditional fossil energy,it is popular among the energy science workers.The study of gas hydrates in porous media especially the decomposition process of hydrates is an essential preliminary work of large-scale exploitation and utilization of natural gas hydrate.As a new method,depressurization combined with well-wall heating can effectively solve the problem of the insufficient heat supply in the late phase of hydrate decomposition and the formation of the second hydrate.In this paper,the methane hydrate deposition in a laboratory-scale 1.2L cylindrical reactor by depressurization combined with well-wall heating is studied by using TOUGH+HYDRATE simulation software.The effects of different wellhead pressure,different well-wall temperature,and different hydrate deposit permeability on the hydrate dissociation process are analyzed.Five different wellhead pressure?2.4MPa?2.5MPa?2.6MPa?2.7MPa?,five different well-wall temperature?10??20??30??40??50??,and five different permeability(5×10^-10m²?5×10^-11m²?5×10^-12m²?5×10^-13m²?5×10^-14m²)are considered.The dynamic change of important parameters including wellhead gas production rate,wellhead accumulative gas production,free gas production,total decomposition gas production,residual hydrate in the reactor and the gas-water ratio,and the spatial distributions including temperature field,pressure field,hydrate saturation,gas saturation and liquid saturation are studied.The study shows that when the well-wall temperature is the same and the wellhead pressure is reduced from 2.8MPa to 2.4MPa,the peak time of the gas production rate is shortened by 59.1%,the cumulative gas volume at the wellhead is increased from 0.01511m³to 0.01654m³,and the time for complete hydrate decomposition is shortened by 29%.When the wellhead pressure is the same and the well-wall temperature rises from 10°C to 50°C,the peak gas production rate increases from 4.66×10^-7m³/s to 9.64×10^-7m³/s,the hydrate complete decomposition time is shortened by 48%.When the permeability increases from 5×10^-14m² to5×10^-10m²,the hydrate complete decomposition time is shortened by 16.3%.The pressure distribution in the reactor at different time is almost uniform.At the same time,the lower the wellhead pressure is,the lower the pressure level in the reactor is,and the pressure level at different well-wall temperatures is basically at the 3.6MPa level when the time is 240min.The decomposition of hydrates moves from the upper left side near by the well to the lower right area away from the wellbore area.Higher intrinsic permeability plays a beneficial role in hydrate decomposition,and intrinsic permeability affects the distribution of liquid saturation and gas saturation.During the decomposition,there is a small amount of decomposition of hydrate near the boundary region,and the decomposition moves toward the interior as time goes on.