Investigation Into The Dissociation Of Natural Gas Hydrate In Porous Media

Natural gas hydrates have been viewed as a potential low-carbon energy due to its globally distributed huge reserves,the cleanness during burning of methane as well as its high energy density.The scientifically reasonable,safe and efficient exploitation and utilization of hydrate accumulations has been critical for the sustainable development of the uncertain energy future.Due to the feasibility,economic problems and energy efficient,depressurization is believed to be one of the most promising methods and can be used either alone or in combination with other techniques.Considering the fact that most hydrate accumulations are in marine sediments and the sediments have high water saturation or even being water-saturated,this work firstly experimentally investigate the dissociation behavior of methane hydrate in water-saturated sediment induce by depressurization method.The water and gas production,as well as the heat transfer in the porous media is studied.The temperature and pressure data is employed to study the ice generation phenomenon,as well as it causing element and effect.The Ste number is further employed to study the effect of reservoir sensible heat for hydrate dissociation.By analyzing the dissociation P-T with respect to hydrate equilibrium curve,the gas production from water-saturated hydrate-bearing sediment can be divided into three main stages.Thus the heat and mass transfer driving mechanism during methane hydrate dissociation in water-saturated sediments is studied.This work further conduct MRI visualization and analysis for ice generation during methane hydrate dissociation under rapid depressurization and low back pressure condition.At the beginning of depressurization methane hydrate dissociate spatially because the core pressure decreased to production pressure rapidly.Then the core pressure fluctuates and MRI images darken,indicating large amount of ice being generated spatially in the vessel.Based on the energy and mass conservation equations,the model for calculation of the generated-ice saturation is established,and the minimum ice saturation is estimated.Ambient heat transfer drove ice melt and hydrate dissociation from the surrounding wall in,thus the melt of ice may absorb much heat which was supposed to support hydrate dissociation.Moreover,MRI study for methane hydrate dissociation by controlling the depressurizing rate gradually to designed production pressures is conducted.Heat transfer characteristics during hydrate dissociation may be transformed by this method,and the ambient heat transfer effect would be strengthened.Obvious hydrate reformation and ice generation can be avoided through this method.One of the key requirements for any production technique is to supply the heat necessary for hydrate dissociation.Based on the fact that ice generation may occur due to the lack of sufficient and timely heat supply in depressurization method,this work further conduct optimizing dissociation simulation of methane hydrate.Firstly,a methane-hydrate-dissociation model with considering for microwave absorption and conversion is established.Simulation of microwave stimulation for the production of gas from methane hydrate sediment is studied: The simulation results show that microwave stimulation provides timely energy conversion sufficient for promoting the dissociation of hydrates,with rapid,continuous gas production.Temperature gradients caused by the decrease of the microwave penetration depth appear in the reservoir,leading to a rapid dissociation rate in the upper part of the sediment.Secondly,the effects of initial water saturation,initial hydrate saturation,and sediment thermal properties on gas production induced by microwave stimulation are studied.Moreover,the energy efficiency ratio is employed in the simulation.The energy efficiency ratio for all simulations ranges between 3.752 and 6.452,which is in consistent with experimental studies.Hydrate saturation and the specific heat capacity of porous media are two factors that significantly affect energy efficiency.