Study On The Increasing Of Methane Production By Marine Natural Gas Hydrate Reservoir Reformation With Injecting CO₂/H₂O Emulsion

Natural gas hydrates(NGHs)are widely distributed in marine environment and the reservoir are commonly characterized of weak-cemented and unconfined,reservoir instability and methane uncontrolled escape may occur during hydrate exploitation process,which cause low energy recovery efficiency.Currently,the production increase technologies are mainly focused on the optimization of drilling equipment and exploitation methods,of which can not fundamentally solve the possibly geological and environmental problems.In response to these problems,an innovative technology was proposed in this paper from the view of exploiting hydrates safely and efficiently,which is form low permeable CO₂ hydrate cap by injecting CO₂/H₂O emulsion and construct a confined reservoir environment for depressurization.Related basic research works have been conducted and the main content are as follows:(1)Methane hydrate bearing sediment samples characterized by clayey silt sand were prepared using a one-dimensional visual hydrate simulator,gas production properties and the effect of clay content on gas recovery ratio were investigated in both confined and unconfined reservoir environments,the migration of gas-liquid-solid phases during exploitation was revealed.The experimental results showed that in confined system with free gas,the increase of clay content in sediment resulted in the decrease of gas production rate,but have little impact on methane recovery ratio.Water-sensitivity effect and particle migration caused by hydrate decomposition were not obvious.In unconfined system with free water,the increase of clay content in sediment resulted in the decrease of methane recovery ratio,the infiltration of seawater induced by depressurization will cause the migration of clay particles,and consequently sediment compaction phenomenon occurred.(2)The feasibility of several commercial surfactant to be the liquid CO₂ emulsifier was evaluated,the CO₂/H₂O interfacial tension was measured in different systems,the CO₂/H₂O emulsion stability was evaluated by visual sapphire reactor.The influence of several factors,such as water-cut,salinity and stirring rate on emulsion stability time were investigated.The results showed that the decrease of the CO₂/H₂O interfacial tension was irrelevant to the functional groups.The increase of water cut in the system will accelerate the delamination of emulsion,the higher salinity and lower stirring rate will reduce the particle dispersibility,resulting the instability of the emulsion.(3)The experiments of“reservoir reformation”by injecting CO₂/H₂O emulsion and methane hydrate exploitation by depressurization were conducted,the optimum volume ratio of CO₂ to H₂O was investigated based on the experimental data.Moreover,the permeability of CO₂ hydrate bearing sediment was measured.By comparing the results of that without reformation,the low permeable CO₂ hydrate cap formed after reformation exhibited excellent compactness and mechanical strength,which can not only prevent the infiltration of overlying water and enhance methane recovery ratio,but also can help to maintain the mechanical stability of the reservoir and sequestrate CO₂under seabed.The permeability of CO₂ bearing sediment formed by injecting CO₂/H₂O emulsion was five orders of magnitude lower than that of pure sediment.The technology of“reservoir reformation”by injecting CO₂/H₂O emulsion was proved to be feasible(4)The influence mechanism of emulsion injection technology and overburden pressure on the quality and stability of the hydrate cap were investigated.It was found that higher injection rate will cause the excessive dispersion of the emulsion and the inhomogeneous distribution of hydrate,and thus resulted in the decrease of sealing capacity.Low injection rate was benefit for forming high-quality hydrate cap and a hydrate cap formation mechanism of“hydrate formation-fracture-regeneration”was proposed based on the analysis of experimental data.The increase of overburden pressure will accelerate the corrosion rate of seawater to the interior CO₂ hydrate cap.The combined effects of both methane hydrate decomposition and the increase of vertical stress resulted in the instability of the hydrate cap.(5)The macro morphology and degeneration process of the hydrate film formed on liquid CO₂/H₂O interface in unsaturated water were observed.A CO₂ hydrate dissolution rate model in porous medium was established based on CO₂ diffusion mechanism,which can provide theoretical support on further study of CO₂ hydrate cap degeneration process and instability mechanism under seabed.