| Coalbed methane is an important clean energy source in China.The development of unconventional resources is of extremely important to alleviate the energy crisis,reduce the geological disasters of coal mines and improve the greenhouse effect.Coalbed methane reservoir commonly,has the characteristics of easy to break,easy to compress,complex pore structure,and filled with adsorbed gas.With the conventional experiment methods,it's hard to accurately test the coal sample,thus it is needed to try new experimental techniques for opening up ideas for scientific research.Low field nuclear magnetic resonance technique(LFNMR)is a rapid and,nondestructive method,having abundant detectable information.Thus,this thesis uses the nuclear magnetic resonance cryoporometry technique(NMRC)and LFNMR method to analysis six coal samples in different coal ranks.The feasibility of the application of nuclear magnetic resonance(NMR)in the study of physical properties and adsorption of coal reservoirs is discussed comprehensively through comparison with ordinary experiments.The pore structure test results of different coal samples between NMRC and low temperature nitrogen adsorption experiment(BJH)showed that the total pore volume values measured by NMRC method was significantly larger than that of the BJH method.For one possible reason,the test range of NMRC technique is extensive reaching up to 1nm?1?m and involving information of many closed pores.For another,the theoretical model of BJH method has limitation itself,which is more suitable for the measurement of pores within 10nm?100nm.The pores below 10nm are not easy to measure.In addition,the total pore volumes tested by NMRC appeared a trend of decrease followed by a slight increase with the increase of metamorphic grade of coal ranks,which is in good agree with the regular relationship on porosity.While the result of the BJH method is not in conformity with the common recognition because of a certain abnormal data point.Based on volumetric and NMR methods,the adsorption test results of dry coal samples showed that the NMR technique did well on high rank coal samples as well as low rank coals.The Langmuir volumes(V_L)measured through both methods is no more than 3cm~3/g deviation of absolute error and less than 10%deviation of relative error.While as to coal samples in middle rank,the standard error of NMR fitting values tended to be high which may be caused by the interference of paramagnetic minerals.Compared with coal samples in the condition of equilibrium water,with the increase of the vitrinite reflectance in coal rocks,the adsorption capacity of dry coal samples has the characteristic of U-type curve,however,the equilibrium water ones increase linearly.Among them,methane adsorption capacity of the low rank coal was affected most by water which leaded to a decline of almost 2/3,yet the coal sample in middle rank only reduced about 1/10,being affected least.In conclusion,the NMRC method has obvious advantages in the quantitative identification of all pores in coal reservoir than conventional methods and this method is less influenced by error factors during experiment like sample preparation,which is an experimental method worth applying to the pore structure test of coal reservoirs.Moreover the NMR isothermal adsorption method at high pressures contains a good potential too owing to its convenient operation,visual effects,less time consuming and other advantages,which can be a new method as well worthy of deep development on adsorption of methane gas. |
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