| Natural gas hydrate(NGHs),one of the novel energies with the most commercial exploitation value in the 21 st century because of its clean combustion,high calorific value,and huge resources,has attracted the worldwide attention.As a hydrate resource-rich country,it is of historic significance to realize the industrial exploitation for alleviating the energy shortage,optimizing the energy structure in China.NGHs exploitation is a complex process of multifield coupling including phase transition and multi-phase seepage et.al.Therefore,analyzing the change rule of permeability characteristics in reservoir during hydrate dissociation,exploring the influencing mechanism of fluid flow on hydrate phase transition,has important theoretical significance for the exploitation of NGHs resources.Focusing on the above scientific problems,this study has explored the interaction mechanism between fluid migration and phase transition of hydrate in porous media.Firstly,the permeability characteristics of porous media and seepage law of water in continuous hydrate phase transition process was studied in this paper.The multi-point and continuous testing system of gas-water two-phase seepage characteristics was self-designed and self-built,then the real-time permeability measurement of the sediment containing hydrate phase transition was firstly realized by using this system.The exponential effect law of realtime hydrate phase transition on permeability of porous media was obtained via using temperature increase method with a constant step.The critical hydrate saturation in porous media is determined to be 32%when the hydrate occurrence mode changes from grain coating to pore filling.Comparing these two modes,the difference of specific value between permeability reduction and hydrate saturation increment is up to 2 times.Moreover,the bipolar gradient fast-spin-echo pulse sequence was improved based on the technique of magnetic resonance imaging(MRI).The characteristics of velocity mapping containing the positive velocity area and the negative velocity area were found.The phenomenon that the new water dilutes the old water in the flow process is observed by tracing heavy water,and the higher water flow rate caused the more obvious dilution effect.It provides the direct evidence that high water flow rate can accelerate the hydrate dissolution process.Then,the hydrate dissolution behavior was found that it was the fundamental reason for permeability increase during gas-unsaturated water flow process by using visualization system of hydrate phase transition.The mechanism of hydrate dissolution was determined based on thermodynamic(potential difference)and kinetic(mass transfer)theory:the chemical potential difference between water phase and hydrate phase provided the dissolution driving force,and the gas mass transfer velocity at the interface between water phase and hydrate phase determined the dissolution rate.It is suggested that the formation of the third phase(bubble)is the macroscopic fundamental sign between hydrate dissolution and hydrate decomposition.A kinetic model of hydrate dissolution with the main parameter of water flow rate is established.It was found that the faster flow rate caused the faster interfacial mass transfer and the smaller gas concentration in aqueous environment,then further caused the greater dissolution driving force and the faster dissolution rate.The characteristics of hydrate dissolution during the undersaturated water flow process in porous media were observed by MRI system for the first time.A prediction model for the spatial-temporal evolution of hydrate dissolution was established.The radial dependent characteristics of the water-hydrate interface changes were illustrated.The spatial distribution and evolution law of hydrate dissolution was confirmed,which contains stable,preferential dissolution in low hydrate saturation area,formation of flow channel,expansion of flow channel and disappearance of flow channel.Based on the mechanism of hydrate dissolution,an enhanced exploitation technology of water flow erosion was proposed to supplement the traditional hydrate exploitation methods.Considering the icing problem in single depressurization process,three modes of staging depressurization,continuous decompression and gas production with a constant rate were performed to control the depressurizing rate,and introducing the water flow erosion process at the same time,to accelerate the heat-mass transfer and improve the hydrate dissociation rate.It was found that the faster depressurization rate and the higher water flow rate accelerated the hydrate dissociation process,and caused the smaller pressure drop range of the reservoir,which is more conducive to the protection of reservoir stability.An energy return on investment model with the main parameter of water flow is established,and the economy of different mining methods was analyzed.The combination mode of staging depressurization,continuous depressurization assisting with water flow erosion could achieve higher energy return on investment.The temperature-pressure regulation and optimization were realized by introducing coupled factors such as temperature,pressure,salinity and flow velocity.The hydrate exploitation characteristics via three-method-combination-modes of seawater flow erosion,depressurization and thermal assistance were systematically analyzed,that provided a new idea for the safe and efficient exploitation of NGHs. |
