Experimental Research On Hydrate Distribution In Porous Media During Hydrate Formation And Dissociation Based On CT

Natural gas hydrate is considered as one of the most important potential clean energy sources in the 21st century because of its large reserves,wide distribution and high energy density.In terms of identifying natural gas hydrate formation and dissociation processes,as a common method for observing the internal three-dimensional structure of sediment samples directly and nondestructively,CT has been widely used in the microscopic observation of hydrate deposits.Optimizing the CT scanning parameters is the key factor to improve the images quality.A method to calculate the porosity of Berea sandstone based on CT images was introduced.Firstly,different CT scanning conditions were set and tested.Then,a group of the optimized parameters were determined to obtain the best CT images based on specifications such as the density resolution,half width,peak intensity and scanning time.The exposure time,voltage,current and number of slices were set as 333 ms,100 kV,90?A and 1500 respectively.The CT image method was used along with the traditional weighing method.The absolute error between them was less than 0.8%.It was shown that the CT image was an ideal method for porosity calculating of Berea sandstone.Based on the CT scanning parameters above,methane hydrate formation and dissociation processes in Berea sandstone were realized in a high pressure and low temperature apparatus.Based on the high resolution computerized tomography?CT?,the pore scale distribution of methane hydrate was observed and discussed.According to the results,there are three kinds of methane hydrate distribution models in the pores.At the first stage of hydrate formation?e.g.,hydrate saturation at 3.66%and 16.38%?,hydrates are mainly growing as a floating model.At the last stage of hydrate formation?e.g.,after hydrate saturation is more than 66.39%?,the hydrate grains connect the sediment particles,described as a contacting model.In the middle stage of hydrate formation?e.g.,hydrate saturation at40.65%?,hydrate distribution is between floating and contacting model.Two methods of hydrate dissociation,i.e.heating and depressurization were used in this study.The hydrate distribution models at heating decomposition process are just opposite to the formation stage.While in the depressurization process,the hydrate distribution at the first stage is between contact and floating model,then the hydrate distribution turns to floating model.Compared with the heating decomposition process,hydrate dissociation rate is higher,indicating that hydrate decomposition is more sensitive to pressure than temperature.Based on the distribution of hydrate formation and dissociation in sediment pore space,to simulate seepage during hydrate formation and dissociation and calculate the absolute permeability during the process.Based on the high resolution computerized tomography?CT?,the pore scale distribution of xenon hydrate in different particle size Al₂O₃ were observed and discussed.According to the results,in the 1.5 mm media,at the first stage of hydrate formation,hydrates are mainly growing as a contacting model.At the last stage of hydrate formation,the hydrate grains cement the sediment particles,described as a cementing model.In the middle stage of hydrate formation,the main hydrate distribution is contacting and cementing model.Compared with the 1.5 mm Al₂O₃,the hydrate grows faster in 500?m Al₂O₃.The meaning of this thesis is to optimize the CT scanning parameters to obtain better CT images.Understanding hydrate formation and dissociation in sediment pore space promotes the research on accumulation and production mechanism of gas hydrate.Measuring the permeability of hydrate-bearing sediments,verifing the relationship between hydrate saturation and permeability,which is of reference value for reservoir evaluation,production capacity calculation and industrial extraction scheme for hydrate exploitation.