Study On The Permeability And Production Promotion Method Of Hydrate Bearing Sediments

Natural gas hydrate(NGH)is considered to be a promising fossil energy source because of its wide distributions and huge reserves.At present,the main research purpose on NGH deposits is to achieve stable and efficient natural gas production.In order to predict the production characteristics more accurately and obtain better gas production,more efforts should be made to study the permeability of hydrate bearing sediments and the production promotion methods.In this work,four aspects:the permeability of hydrate bearing porous media with different hydrate saturation S_H,pore scale interaction between NGH and porous media,the gas permeability of natural sediments overlying hydrate layer,and the production promotion method focued on the reservoir reconstruction around the production well of marine hydrate deposits are studied.The research results are helpful to predict the gas-liquid flow in the process of NGH exploitation,and provide engineering practical value for the development of NGH deposits in the future.The main results are as follows:(1)A one-dimensional experimental apparatus for permeability measurement in porous media with hydrate is developed.The porous media of galss beads and five quartz sands are employed to synthesize hydrate bearing sediments.The permeability of methane hydrate sediments is obtained by steady-state water injection,and the coupling effects of hydrate saturation,sediment particle size,porous media type,sediment accumulation,hydrate morphology and hydrate growth habit on permeability are quantified.The decomposition process of hydrate in sediments is completely recorded by cold field emission scanning electron microscope for the first time,and the decomposition characteristics of hydrate in pore space are found.In addition,it is confirmed that the hydrate is more likely to form in the center of the pore space,and the hydrate migrates to the minimum pore to block the liquid flow.(2)Four ideal regular packing models of equal diameter sphere are established.The mathematical relations between hydrate saturation and pore internal surface area and hydrate decomposition surface area are quantified under two modes of hydrate coating grains and hydrate occupying pore centers,respectively.Based on the calculation results,the Musuda model and Kozeny model are improved,and the empirical models suitable for the permeability of hydrate bearing sediments with initial porosity from 25.95%to 47.64%are obtained.(3)The gas effective permeability of natural sediments overlying hydrate layer in the South China Sea at different water depths is measured.The coupling effects of particle size,confining pressure and porosity on gas permeability are studied,and the Kozeny-Carman formula for the prediction of absolute permeability is modified.A theoretical model of shape factor is established to describe the irregularity and internal development of natural sediment samples.(4)The production promotion method of enlarged permeable well is proposed for the marine hydrate deposits.The feasibility of the above method is comprehensively simulated and evaluated by using the TOUGH+HYDRATE simulator based on the parameters of W17 site in the Shenhu area of the South China Sea.The characteristics of phase transformation and heat transfer during long-term exploitation of hydrate deposits are analyzed.