Studies Of Gas&Water Migration And Particle Agglomeration Mechanisms In Natural Gas Hydrate Exploitation

Natural gas hydrates occurrence is mostly found in shallow marine sediments where geological conditions are extremely complex,which brings potential geological and environmental hazards to the exploitation of natural gas hydrates.The exploitation of natural gas hydrate reservoirs is essentially by changing the hydrate-bearing sediment conditions(temperature,pressure etc.)and breaking the clathrate structure to release methane stored in hydrate cages.The key factors affecting the efficiency of exploitation process include gas-liquid migration mechanisms in sediments during hydrate dissocation and hydrate particle agglomeration/deposition mechanisms in flowlines during gas production.In order to study the influence of different factors on gas-liquid-solid migration during hydrate exploitation process from the stage of hydrate dissociation in sediments to the stage of gas/liquid production in flowlines,the dissociation of gas hydrate in sand-packed cores was observed,the gas-liquid migration was monitored and the adhesion and cohesion forces of hydrate particles under differernt conditions were measured in this dissertation.Magnetic resonance imaging(MRI)has advantages in visualizing the distribution and moitoring the migration of liquid water in porous media.By using this technique,methane and methane-carbon dioxide hydrate heterogeneous distributions in sand-packed samples were obtained.Moreover,the formation and dissociation process of was observed.By analyzing the water signal intensity variation in different locations of the sample,it was found that the depressurization method,gas compostion,sediment particle size and initial hydrate saturation were key factors affecting gas-liquid migration in sediments during hydrate dissociation,and the boundary heat-transfer conditions were found to be the major parameter controlling the movement of hydrate dissociation interface.By using the gas-liquid migration visualization system,the water effective permeability of hydrate-bearing samples was measured,and it was found that the presence of gas hydrate in the pore could cause a reduction of core permeability by up to 90%.After acquiring the permeability of hydrate-bearing samples,the cross-sectional water velocity distribution was measured by using phase-contrast method of MRI,and a two-region flow pattern was observed because of the non-uniform distribution of hydrate in sediment pores.It was also found that the heterogentiy of cross-sectional velocity field was related to hydrate saturation and injecting flow rate.The adhesion/cohesion forces of hydrate particles under different conditions were measured by using high-pressure micromechanical force apparatus.The experimental results and the calculation forces by using capillary bridge model revealed how sub-cooling,anealling time,salinity and surface corrosion influenced the adhesion/cohesion forces of hydrate particles.The Camargo-Palermo model was then used to calculate the maximum diameter of hydrate agglomerate,and it was found that the cohesion force between hydrate particles was the key parameter controlling the agglomeration and deposition process o hydrate in flowlines during hydrate exploitation.