Study On Clogging Of Sand-Control Media And Sand-Control Optimization For Natural Gas Hydrate Production Wells In The South China Sea

Natural gas hydrate(NGH)reservoirs in the northern South China Sea are usually characterized with shallow burial depth,weakly consolidation,and extremely low permeability.These characteristics give rise to great challenges for safe and efficient gas extraction from the NGH reservoirs.The NGH developments are highly dependent on the integrated production schemes considering geological and engineering factors,among which sand production is of vital importance.Several NGH production trials have been forced to be terminated because of severe sand production.Figuring out working performance of sand control media in a NGH development well and developing targeted sand-control operation schemes are essential to mitigate sand production rate,and to ensure continuous and efficient NGH development.This thesis aims to find out optimized sand control schemes and corresponding sandcontrol parameters for NGH developing wells in the northern South China Sea.Plugging of sand control media induced by secondary NGH would be studied as a unique concern in NGH production wells based on both experimental and theoretical modeling methods.To be more specific,the following topics will be comprehensively investigated.(1)Hydrate reformation characteristics within sand screens and the secondary hydrate induced clogging of sand screens.(2)Secondary hydrate induced clogging of gravel packing layer and its coupling effect with clay-induced clogging.(3)Optimization of sand screens considering the plugging potential,and(4)gravel packing optimization for NGH developing horizontal wells.The results reveal that hydrate reformation induced clogging of sand control media would inevitably occur during NGH development under improper depressurization schemes.Once sand control media are plugged by secondary hydrate,the permeability of the sand control media would be reduced to only 1%?2% of the original value,which would inevitabley affect gas productivity.The existence of clay content is proved to enhance secondary hydrate plugging rate of gravel packing layer.Based on the plugging behaviors of sand control media,a novel sand retention precision optimization method is proposed,which aims to strike a balance between retaining coarser particles and avoiding screen blockage by the relatively fine particles.As for the horizontal for natural gas hydrate production,the extremely narrow safe operational window and high fluid-loss ratio are the main obstacles that affect packing effectiveness in marine NGH production horizontal wells.The safe operational window becomes narrower with the decrease of hydrate saturation within the reservoir;hence,the packing failure risk increases significantly.A joint application of lightweight gravel,low concentration slurry,viscosity reducer additives could prolong the maximum packable length,which enables application of gravel packing operation in a wider range.Based on the plugging behavoirs of sand-control media,the thesis proposed a nove sand-retention precision design method for sand-control screen and an optimization method for key parameters for gravel packing in horizontal wells.The research provides significance on prolonging safe and efficient gas extraction from NGH-bearing sediment in the South China Sea.