Pore And Core Scale Study Of Gas Hydrate Growth Behavior,Wettability Characteristics And Their Effects On Gas-Water Flow

Natural gas hydrate is an unconventional natural gas resource and has been considered as the most promising alternative future energy due to its advantages,such as abundant reserve and high energy density.There are great amount of natural gas hydrate resources in the South China Sea.The safe and efficient exploitation of natural gas hydrate is of great strategic significance for achieving sustainable energy development goal of our country.However,during the hydrate dissociation process,the changes of hydrate pore habits and wettability variations of the hydrate bearing sediments caused by the phase transition are still unclear,and the gas-water seepage characteristics and its influencing factors are not well understood.As a result,the commercial exploitation of natural gas hydrate is faced with the major bottlenecks such as inefficient and unsustainable gas production.Thus,under this background,this study investigated the hydrate growth behavior in porous medium from pore to core scale,analyzed the wettability evolution of hydrate bearing sediments during the hydrate dissociation process,and finally elucidated the micro-control mechanisms of gas-water flow in reservoir.The specific research contents are as follows:First,the study of gas hydrate time-dependent growth behavior and occurrence pattern at pore and core scale was conducted.Based on the micro-CCD camera,X-ray CT and high-field MRI visualization experimental systems,it was found that the formation process of gas hydrate has obvious stage characteristics at micron pore scale.And with the increasing of hydrate saturation,the preferential hydrate occurrence region changed from gas-water interface to water-hydrate interface;The 3D spatial occurrence patterns of grain-coating and pore-filling hydrate crystals in sand packs were obtained,and the highly irregular microstructure were found based on the calculated shape factor results of independent hydrate clusters.The continuous growth and coating of hydrate would increase the surface roughness of particles in porous medium;The effects of heterogeneity of porous medium and initial gas-water distribution in pores on the formation characteristics of CH₄ hydrate was investigated.And it was discovered that the hydrate preferentially nucleated in pores with small particles and lower initial water saturation.The heterogeneous microscopic growth characteristics of gas hydrate in porous medium were revealed.Second,the study of measuring methods and dynamic evolutions of the wettability characteristics for gas-water-hydrate-grain surface system was conducted.The methods for measuring the local contact angle of gas-water（liquid）-hydrate（solid）system in micro plane and 3D space at pore scale were developed respectively.489 groups of hydrate surface wettability data in porous medium were obtained.The results indicate that the surface of hydrate is hydrophilic.Due to the salt-removing effect during the hydrate formation process,both the surface polarity of hydrate crystal and the adsorption capacity of water molecules decreased.Thus,the surface wettability of hydrate decreased with the increase of water salinity in pores.With the help of low-field NMR analysis technology,the characterization method of dynamic wettability evolution of hydrate bearing sediments was proposed.Based on the relaxation time response characteristics of free water in pores during the hydrate dissociation process,the hydrate bearing cores became more hydrophilic with the decrease of hydrate saturation.A prediction model for the hydrophilicity variation of hydrate bearing core with hydrate saturation was built,which realized the quantitative analysis of dynamic wettability characteristics of gas-water-hydrate-grain surface system during the dissociation process.Finally,based on the aforementioned results,the control mechanism of occurrence pattern and wettability characteristic on gas-water seepage behavior during the hydrate dissociation process was further investigated.By conducting gas-water seepage experiments in porous medium during the hydrate dissociation process,the effects of pore micro-structure and flow barrier on gas/water effective permeability were investigated.The critical hydrate saturation range for the change of permeability variation and hydrate occurrence pattern was found by comparing the experimental normalized gas permeability data with classic Kozeny permeability prediction models,and the micro-control mechanism of gas-water flow behaviors in hydrate reservoir was concluded accordingly.The decomposition behaviors of natural gas hydrates under different exploitation methods and its influencing mechanism were analyzed,the effects of different exploitation methods with the combination of depressurization and thermal stimulation on hydrate recovery efficiency were evaluated.Taking the dynamic wettability evolution of hydrate bearing porous medium into consideration,the classic permeability prediction model was improved,and the influencing mechanism of the change of hydrate occurrence pattern and the wettability variation before and after the hydrate dissociation on gas-water seepage characteristics in reservoir was clarified.