Study On Microscopic Mechanism And Dynamic Response Characteristics Of Parameters During CO₂ Hydrate Formation In Porous Media

Fossil fuels are still the main energy for human survival and development at present,and the continued growth of carbon emissions has posed a serious threat to the global environment.Carbon dioxide geological storage（CGS）is one of the most promising technologies for reducing greenhouse gas emissions and mitigating climate change.CGS is expected to contribute with at least 20%of CO₂ emissions reduction by 2050 and 55%by the end of this century if managed properly.The hydrate-based CO₂ storage is a promising technology for controlling CO₂emissions,which could play a key role in CO₂ storage.In this thesis,the gas storage performance of porous media is evaluated through experiments and theoretical calculation based on hydrate formation kinetics.The pore structure characteristics and seepage evolution of porous media during CO₂ hydrate formation are studied by using a combination of different techniques,including the low-field NMR technology,CT scanning technology,acoustic technology and numerical simulation.The results reveal the hydrate pore habits and provide a scientific basis for understanding the formation mechanism of hydrate in porous media.The evolution characteristics of key physical properties during hydrate phase transition are analyzed,and the dynamic analysis models between phase transition characteristics of hydrate and key physical parameters are established.The results provide a scientific support for CO₂ geological storage.The main results are as follows:（1）Based on the basic physical properties of gas hydrate formation,artificial cores of hydrate reservoir were prepared with different cementing agents.The porosity of low permeability cores is from 4.24 to 15.04%,and the permeability of that is from 0.25 to 40.8 m D.The porosity of high permeability cores is between 24.21 and 35.56%,and the permeability is between 237 and 1030m D.A higher content of coarse-grained quartz sand means higher density.The porosity and permeability of low permeability cores and high permeability cores firstly increase and then decrease with decreased particle diameter of quartz sand.There is a good exponential function for the relationship between permeability and porosity.According to the distribution of T₂ spectrum,the pore structure characteristics of samples are analyzed.The low permeability cores are mainly composed of small holes and micropores,and the high permeability core are mainly composed of medium and large pores.（2）The formation of CO₂ hydrate in quartz sand and artificial core is revealed by using hydrate simulation test system.The effect of particle diameter,pore size and distribution,and permeability on the induction time,gas storage capacity and formation rate are investigated.The variations in temperature and pressure with time can be discussed as four stages:initial phase,induction phase,generation phase and stable phase.The temperature and pressure increase sharply in generation phase.The specific surface area of quartz sand is increased with decreased particle diameter of quartz sand.The induction time is decreased gradually and the gas storage capacity is linearly increased due to the increase in surface contact between the CO₂ gas and water in quartz sands.The induction time is decreased slightly and the gas storage capacity increases gradually with the increased porosity or permeability of low permeability core and high permeability core.（3）The pore structure characteristics and permeability evolution during the formation of CO₂hydrate in quartz sands and artificial cores are studied by using low-field NMR technique.Hydrate is mainly formed in mesopores,macropores and macropores in quartz sands.The mesopores in high permeability core play a decisive role in hydrate formation.Hydrate in low permeability core is mostly formed in micropores.The pore structure of porous media is gradually complicated due to the hydrate formation.Hydrate is generated earlier in large pores than in small pores.Change time at which the fractal dimension changes from a steady decrease to sharply increase is later than the induction time,which reflects the hysteresis of pore fractal dimension.The Coates model for T₂ cutoff value of 10ms provides accurate approximations of permeability of porous media.Change time at which the permeability changes from a steady trend to sharply decrease is consistent with the time corresponding to the curve break for saturation curve over time.The induction time,generation rate and hydrate saturation are affected by the distribution state of water and gas in porous media.The induction time is decreased with increased porosity or permeability of porous media in excess-gas systems,and the hydrate saturation is increased with increased porosity or permeability of porous media in excess-gas systems.However,the induction time is increased and the hydrate saturation is decreased with increased porosity or permeability in excess-water systems.（4）Hydrate growth habits and occurrence patterns are clarified,and the dynamic relationship between phase transition characteristics and acoustic wave velocity during hydrate formation is analyzed based on multi-slice spiral CT scanning technology.The hydrate occurrence characteristics can be discussed as five patterns:contact cement,grain-coating,non-cementing pore-filling,load-bearing,and mixing pattern according to the contact pattern between hydrate and sediment.In quartz sand and high permeability core,hydrate is preferentially generated in large pores and initially appears in non-cementing pore-filling pattern,and then the occurrence characteristics changes to contact cement pattern with increased saturation hydrate.The hydrate saturation in high permeability core is higher than that in quartz sand,and the hyarate occurrence characteristic is finally load-bearing pattern.The wave velocity is rapidly increased in generation phase and then slowly increased with increased hydrate saturation.A good negative exponential function is found for the relationship between P-wave velocity and hydrate saturation:=₀+₁0)^-?1.The wave velocity barely changes in post-generation phase,while the sound wave amplitude continues to increase.Water would migrate to the place where temperature and pressure conditions are most conducive to hydrate formation.（5）The digital core of porous media containing hydrate is established by using CT image,and the seepage characteristics during hydrate formation are revealed by numerical simulation.The hydrate occurrence characteristics in quartz sand are mainly non-cementing pore-filling pattern and contact cement pattern due to low hydrate saturation,and an annular flow path for water is formed in quartz sand during hydrate formation.The hydrate saturation of high permeability core is higher than that of quartz sand,and the occurrence pattern is mainly load-bearing.The Kozeny grain model for hydrate occupying the pore center is applicable to evaluate the seepage evolution characteristics of quartz sand,and can be expressed as:6)=（1-_?）^{（3-0.0770）（1-?）/0.45})/(1+_?^0.5)².The hydrate occurrence characteristics in high permeability core are mainly load-bearing pattern and contact cement pattern.The Kozeny grain model for hydrate coating the grain surfaces could better reveals the seepage evolution mechanism of high permeability core.Based on digital core,the relationship between permeability and saturation can be described by the equation:6)=+*0)^-?⁽?（6）.（6）Based on seepage simulation,the variation characteristics of fluid streamline and velocity in porous media during hydrate formation are illustrated.The streamline density is decreased and the fluid velocity is decreased as a whole with increased hydrate saturation in quartz sand.The streamline distribution changes little.The streamline and fluid velocity changes greatly due to high hydrate saturation in high permeability cores.The formation state and spatial heterogeneity of hydrate can be reflected by the streamline and velocity of fluid.The relationship between permeability and P-wave velocity of quartz sand and high permeability core can be described by the equation:=(6-（76）.These findings indicate that the generalized relationship between permeability and P-wave velocity is effective for predicting the permeability of porous media during hydrate formation in practice.There are 141 figures,62 tables and 285 references in this dissertation.