Study On Hydrate Growth And Decomposition Characteristics In Fine-Grained Sediments

Natural gas hydrate is a promising alternative energy source in the future,usually accompanied by sediments.The properties of sediments will directly affect the formation of hydrate reservoirs and the choice of mining methods.The difficulty of its commercial mining lies in the fact that the growth and decomposition characteristics of hydrates in sediments,especially fine-grained sediments,have not yet been clarified.Thus,the fine-grained sediments?silica sand and kaolin?of 300 mesh and 3000 mesh were selected as the research objects in this paper.We studied the composition and particle size of sediments,the growth and decomposition cycles of hydrate,and the effect of salt solution on hydrate growth and decomposition through microscopic observation and mesoscopic observation.Cyclopentane hydrate,which is similar in structure to natural gas hydrate,as a substitute.Microscopic observation method was used to observe the growth and decomposition processes of hydrate in fine-grained sediments under microscopic conditions.The result shows that the crystals of cyclopentane hydrate were staggered irregular flaky,three-dimensional polygonal,columnar,mound-like and gully-shaped.The presence of Na Cl will roughen the crystal.Cyclopentane hydrate first appeared at the three-phase interface of oil-water-solid.Cyclopentane was transferred from the gap between the hydrate and the pipe wall to the aqueous phase,so that the hydrate grew to the aqueous phase.The initial growth patterns of hydrate appeared randomly in suspended and covered,and eventually turned to cemented type.Hydrate nucleated randomly on the surface and in the pores of particles in silica sand,while they tend to nucleate on the surface in kaolin.The particles of silica sand/kaolin with smaller particle size migrated more significantly than silica sand/kaolin with larger particle size,indicating that hydrate growth and decomposition had a greater impact on sediments with smaller particle size,and more cyclopentane droplets could adhere in the water phase after decomposition.Adding sediments could accelerate the growth of hydrate,and its growth rate was from large to small:3000 mesh silica sand>300 mesh silica sand>3000 mesh kaolin>300 mesh kaolin.Using 90%CH₄-10%C₃H₈ as simulated natural gas,mesoscopic observation method was used to study the growth and decomposition process of natural gas hydrate in fine-grained sediments.Natural gas hydrate first appeared at the three-phase interface of gas-liquid-solid.Water migrated from the layer of sediments to the hydrate to form white ice-like hydrate,which were massive in the gas phase region and existed in the pores of the layer of sediments as dispersed,nodular and crack-filled.Hydrate appeared more in the pores of the sedimentary layer in the form of dispersion and nodules in the silica sand with large particle size,while they appeared more in the gas phase area in the form of lumps in the silica sand/kaolin with small particle size,and the particles migration was significant.The decomposition rate of natural gas hydrate was from large to small:300 mesh kaolin>300 mesh silica sand>3000 mesh kaolin>3000 mesh silica sand.With the growth and decomposition cycle,the hydrate showed particle accumulation growth and particle ablation decomposition.Crack-filled hydrate was more likely to appear in the sediments with small particle size.Finally,based on the experiments,a kinetic equation for the decomposition of natural gas hydrates in fine-grained sediments was established.The results show that the activation energies of decomposition of natural gas hydrate were 83.48?102.25 k J/mol in the sediment-pure water system.The activation energies of decomposition of natural gas hydrate were 79.71?94.84k J/mol in the sediment-salt water system.The decomposition activation energies of natural gas hydrate in pure water were greater than that in brine.The activation energies of decomposition of natural gas hydrate in silica sand were larger than that of kaolin.The decomposition activation energies of natural gas hydrate in large particle size sediments were greater than that in small particle size sediments.