Application Of Low Field Nuclear Magnetic Resonance In Characterizing The Reservoir Physical Properties And Adsorption Capacity Of Coalbed Methane Reservoir

Coal-bed methane?CBM?is an important fossil fuel energy.Recovering of CBM is significant for adding underground coal mining safety and reducing the air pollution from hydrocarbons.Gas in the coals is in the phases of water-dissolved,free and adsorption.Compared with conventional reservoirs,coal has low mechanical strength,prone to be compressed and complicated pore structure,which makes the conventional analysis method is hard to be used in coal reservoir analysis.Compared with the conventional test methods,Low field NMR technology is fast,nondestructive,and it can give much more detailed reservoir fluid information.This thesis performed series of experiments of low field NMR spectrometry analyses,low temperature nitrogen adsorption analyses,and volumetric gas adsorption analyses,and obtained the surface relaxation rate and the coal conversion coefficient of middle and high rank coals,and then evaluated several influence factors on coal adsorption capacity.Based on the low-temperature liquid nitrogen adsorption and low field NMR experiments,this paper analyzed the characteristics of adsorption/desorption curves and pore size distribution,then calculated the surface relaxation rates of the selected coals,and finally obtained a conversion coefficient?C?,using which the NMR T₂spectra can be transformed into a pore size distribution of the micro-and meso-pores.The results show that high-rank coals have much more complex pore structures and more developed micro-and meso-pores than those of middle-rank coals.The surface relaxation rates of high-rank and middle-rank coals are approximately of0.54·10^-8m/ms,and 1.25·10^-8m/ms,respectively.Furthermore,the paper calculated the C values of middle-and high-rank coals.It is concluded that the pore size distribution that was transformed from NMR T₂ distribution by using the calculated C values,shows a distinctly agreement with that from the BJH calculation model using the N₂adsorption data.The relative error distribute is of 2.2%-10.9%.Therefore,the low field NMR method can provide an advanced characterization of pore structure of miro-and mesopores in coals.This paper developed a pair of non-magnetic samples cells and a NMR measurement system.The methane adsorption characteristics of powdered coals with different coal rank were analyzed by the conventional volumetric method and simultaneous with the low field NMR method.Results show that the methane T₂spectrum of powdered coal sample has distinctly three peaks,i.e.the peaks for the adsorbed methane,the methane in porous medium,and the bulk methane with increasing relaxation time.Results show that both methane isothermal adsorption curves and Langmuir volume from the NMR method agree with those from the volumetric method.The methane adsorption capacity of coals increases with increasing vitrinite reflectance,vitrinite content and fixed carbon content.With increasing particle size,the peak for methane in porous medium decreases,whereas the peak for bulk methane steadily increases.Meanwhile,the coal adsorption capacity for methane decrease with increasing particle size of coals.There is a negative relationship between coal adsorption capacity and temperature when the adsorption pressure is constant.