A Study Of Shale Adsorption Characteristics By Using Low Field Nuclear Magnetic Resonance Technology

Shale gas is a kind of unconventional natural gas which is completely different from conventional natural gas in the mechanism of the gas reservoir and production.There are many key scientific problems in the evaluation of shale gas reservoirs,and one of them is the accurate evaluation of the adsorption property and gas content.Currently,thetraditional laboratory testing methodof shale gas content is mainlyrefer to the"Experimental method of high--pressure adsorption isothermal to coal--volumetric method"?GB/T19560-2004?or"Gravimetric method".However,there are some limitations when the two methods were applicated toshale gas.It isneeded to explore a new method for quantitative analysis of the volume of methane adsorption onshale.In this thesis,Low Field Nuclear Magnetic Resonance relaxation time analysis theory-?LFNMR?was firstly introduced into the study of Shales.Based on a series of comparative experiments,the NMR response characteristics and the relaxtion mechanismsof the free and adsorbed methane were analyzed,a newLFNMR-based method was proposed to instantly?quantitatively evaluation of the methaneadsorbed capacity of shale.Petrological and geochemical characteristics of four typical marine shale were evaluated.These shale samples were characterized by low TOC content,high Ro?>2.0%?,lacking of vitrinites,and high sapropelinites contents.All these shales beyong to type-I kerogen.The brittle mineral content is high,mainly by quartz.Clay mineral content is medium,and dominating byillite mineral.The pores in the shales are mainly microporous.The pore specific surface is mainly contributed by the pores of<10nm,and the pore structure typeis dominated bythe slit like pore.The above analysis results provide key reference to investigate the NMR response characteristics of methane gas in shales.A newshale isothermal adsorption experimental setupwasdeveloped,and used to analysis the shale adsorptioncharacteristics.Using the setup,methane adsorption capacity tested by LFNMR method can be compared with the results by conventional method.Totallyfour sets of shale samples were analyzed.Results show that the T₂spectrum of the methane in powerdedshale has typically four spectrum peaks?P1,P2,P3,P4?.The P1 represents the adsorped methane gas component,whose relaxation time is less than 1ms,and it is mainly controlled by the surface relaxation.TheP2 represents the bulk methane gas in pores of shales,whose relaxation time is 1-50ms,and it is controlled by the surface relaxation and diffusion relaxation.The P3 meams the bulk methane gas between the particles of shale sample,whose relaxation time is between P2 and P4.The P4 represents the bulk methane gas in the annular space ofthe sample cell,whose relaxation time is of300-2000ms,and it is controlled by the bulk relaxation.Based on the understanding of the NMR relaxation characteristics and the NMR relaxation mechanism of methane in the shales,this studydevelopeda LFNMR-based method for characterizing the methane adsorption capacity of shale.when the results by LFNMR-basedmethod were compared with those by the conventional gravimetric method,it was found thatthe absolute deviation of the adsorption capacity from these methods is less than 0.09 cm³/g,and the relative deviationswasabout 3%.The calculated Langmuir volumes have absolute deviations of<0.16 cm3/g and relative deviations of<3.37%.The adsorption isotherms from the LFNMR and gravimetric methods are nearly identical.Therefore,it is concluded that the LFNMR method is applicable to effectively evaluation the methane adsorption capacity of shale.