核磁共振方法研究煤吸附过程汇总

1、international journal of coal geology 131 (2014) # -40article infoarticle history:received 8 march 2014received in revised form 27 may 2014accepted 2 june 2014available online 8 june 2014keywords: coal adsorption capacity volumetric methodnuclear magnetic resonance (nmr) coalbed methane (cbm)content

2、s lists available at sciencedirectinternational journal of coal geologyjournal home page: www. elsevier. com/locate/ijcoalgeoquantitative characterization of methane adsorption on coal using a low- ?eld nmr relaxation methodyanbin yao ?, dameng liu, songbin xiebeijing key laboratory of unconventiona

3、l natural gasgeology evaluation and development engineering, china university of geosciences, beijing 100083, pr chinato develop a nuclear magnetic resonance (nmr) method for characterizing the methane adsorption capacity of coals, we built an nmr transparent isotherm adsorption experimental setup.

4、proton (1h) nmr measurements were ?rst performed with bulk methane to obtain a hydrogen amplitude index re ?ecting the methane volume concentration. then, dry coals were pressurized with methane to assess the quantity of adsorbed methane at pressures up to 6.1 mpa. the adsorption isotherms obtained

5、by this procedure were compared with the corresponding sorption isotherms determined by the traditional volumetric method under the same experimental conditions. the methane transverse relaxation time (t2) spectra have three distinct peaks at t2 b 7 ms, t2 = 7 n40 ms, and t2 = 240 2000 ms. the peaks

6、 at t2 b 7 ms and t2 = 7 n40 ms both correspond to the surface relaxation mechanism, and individually, they are interpreted as coal-adsorbed methane "and porous medium- con ? ned methane w, respectively. the peak at t2 = 240 n000 ms represents the relaxationof bulk methane.the integrated amplit

7、ude of the porous medium-con ? ned methane ” peak has a positive linear relationship with pressure, whereas the integrated amplitude of the coal-adsorbed methane "peak with increasing pressure follows the langmuir equation. the adsorption isotherms from the nmr and volumetric methods are nearly

8、 identical. the absolute deviations of the experimental data points from the two methods fall within ±2 m 3/t, and the calculated langmuir volumes have absolute deviations of b 0.38 m 3/t and relative deviations of b 1.24%. we suggest that the low- ? eld nmr may be applied as a noninvasive anal

9、ytical technique to characterize the methane adsorption capacity of coals.2014 elsevier b.v. all rights reserved.1. introductionthe quantitative measurement and evaluation of methane in coals are very important for understanding and predicting the gas production performance of a coalbed methane rese

10、rvoir. the most common procedure to determine adsorption isotherms and capacity is the volumetric (/manometric) method (busch and gensterblum, 2011; genserblum et al., 2009; sac, 2008 ). the purpose of this study is to provide a novel method using a low- ?eld nuclear magnetic resonance (nmr) spectro

11、scopy analysis.many nmr spectroscopy methods exist for coal characterization. solid-state (13 cand 1h proton) nmr relaxation measurements are commonly used for analyzing coal molecular structureand petrologicalcharacteristics (genetti and fletcher, 1999; maroto-valerlya et al., 1998 ). a study by ha

12、rmer et al. (2001)illustratedthe potential ofsolid-state nmr as a rapid advanced coal characterization method. in their study, coal components were successfully divided into ?ve different proton classes: two classes representing water in coals and water in a mineral structure and three classes corre

13、sponding to different types ofcorresponding author. fax: +86 10 82326850.e-mail address: yyb (y. yao).organic components. unlike the solid-state nmr relaxation measurements, low- ?eld nmr measurements are suitable for analysis of hydrogen-containing ?uids (e.g., water and methane) in porous media (c

14、oates et al., 1999; yao et al., 2010). for example, in the petroleum industry, low- ?eld nmr measurements have been used as a boreholelogging tool for three decades (coates et al., 1999 ). moreover, the nmr technique provides an alternative method for characterizing the porosity, pore size distribut

15、ion, permeability, and surface wettability of sandstones, carbonates, coals and other rock types (coates et al., 1999; yao et al., 2010 ).a few laboratory measurement studies of methane in sandstones, carbonates and other porous medium using low- ?eld nmr exist (akkurt et al., 1996; gerritsma and tr

16、appeniers, 1971; gerritsma et al., 1971; hari et al., 1998; oosting and trappeniers, 1971a ). most of these studies are based on the assumption that methane gas is present as a non-wetting phase in the pore space of sandstones and carbonates (akkurt et al., 1996 ). this means that in conventional re

17、servoirs methane is assumed to occur as a bulk gas phase. however, coals have a high adsorption capacity for methane. moreover, methane in coals can exist as a free gas, an adsorbed gas, or a solid solution (alexeev et al., 2004a,b; weishauptov a and medek, 1998). therefore, the nmr relaxation of me

18、thane in coals is quite different from that in other rocks.http:/10.1016/j.coal.2014.06.0010166-5162/?2014 elsevier b.v. all rights reserved.y.yaoet al./international journal of coal geology 131 (2014) 32 -4047are relatively few studies regarding of high-pressure adsorbed methane used 1h n

19、mr wide-line spectroscopyneers, there2.2.experimentalfigshowssetupa schematic diagram of the experimentalarrangement,the nmr relaxation characteristics conin coals. alexeev(2004a,bonsistingsetup.of an nmr measurement apparatus and a volumetric sorp-the volumetric setup is composed of a reference cel

20、l, a sam-to detect methane in coals. theyple cell, and a high-pressure gas-delivery system. both the reference andgoueal忸07)particularly at a meth-issed the application ofpressure was evaluated. however,index from bulk methane to quan-found that methane can exist in coals not only in open or closed

21、porosity and fracture systems but ane pressure of wmpa.low- 2ld nmr to detect free and adsorbed methane in two low volatilebituminousals. in their study, three different relaxation mechanismswere identi £d for methane in coals. meanwhile, the change of adsorbed methane with increasing gieal(20q

22、7) the amplitude tify the mass and volume of adsorbed methane, which may cause majoruncertainty in the results because the relaxation property of bulk methane is completelydifferent from that of adsorbed methane. moreover,using low- gld nmr to detect the process of methane adsorption on coals and to

23、 calculate the langmuir adsorption parameters has not been reported in the literature to date. to address these problems, we sample cells were specially designed to be nonmagnetic and non- metallic in the segment located in the magnet coil (fic|. to pressurize the sample, a simple high-pressure gas

24、line, including a vacuum pump, a booster pump, two gas sources, an exhaust collector, and several actuator-driven valves, was constructed. the gas-delivery system incorporated a temperature transmitter, and two high-precision pressure transducers were installed to monitor the gas pressure.the nmr me

25、asurement apparatus is a minimr-60 nmr spectrometer, manufactured by niumag corporation ltd, china. the instrument uses a frequency of 23.15 mhz, a magnetic strength of 0.54 t, and a magnet coil diameter of 60 mm. the magnetic uniformity is as low as 30 ppm, in which relaxation from gas diffusion ca

26、n be ignored. the temperature of the spectrometer and the analyzed samples were held constant at21.5 ± 0.5 ° c.designed an nmr transparent isotherm adsorption experimental setup and conducted conventional volumetric isotherm measurements in parallel with nmr measurements. the results from

27、the two methods were then compared to discuss the relaxation mechanisms of methane contained in coals.2.3. experimental methods2.3.1. volumetric method formethaneadsorption measurements2materiarlmethodsthe theoretical background volumetric method ofandionin detail in the literature200g；enserb200the

28、method is outlined below.experimentalisothermsenoeaieprocedures for the a discusseduses a refercell, including reference vol-experiments are conducted in a .programmedgenster2l01genserb1am2002.1. coal samplesthe coal block sample used in the present experimental work was collected from the no. 3 coa

29、l bed of the lower permian shanxi formation in the houcun mine, southeastern qinshui basin, china. the sample is a typical low volatile bituminous coal, with an average vitrinite re gc- tance of 1.84%, an ash yield of 14.97% (dry basis), a volatile matter content of 11.05% (dry-ash-free basis) and a

30、 moisture content of 0.85%.the collected coal block sample was crushed into powder with a 60 80 mesh size. the powder was vacuum-driedin an oven at 110 ° cfor 1 h. two sets of powder samples (i.e., s1 and s2) were analyzed in this study.the volumetric method for adsorption measurements ence cel

31、l and a sample cell. the volume in the reference the dead volume of the pressure transducer, is used as a ume and is determined by helium expansion in a calibration run. the coal samples, are kept in the nonmagnetic sample cell with a calibrated volume (fig_ the sample cell volume includes the solid

32、 sample (sorbent) volume, the volume of the adsorbed phase and the void volume, respectively.the void volume is determined by helium from the reference cellinto the reference and sample cell, and then volumetric gas adsorptionkroosau2002；pressure tranmitlerreference celltemporal tiretransmitterfig.

33、1. an experimental low- ?eld nmr setup for analyzing methane in coals. the nmr measurements for bulk methane used the reference cell; the nmr measurements for adsorbed methane used both the reference and sample cells.sieerimtemtpe±21.°£一 rroxmat bstance evequi-). 一 (a) 1.04 mpa (b) 1.

34、90 mpa (c) 3.11 mpa 、一(d) 4.07 mpa(e) 5.02 mpa (f) 5.98 mpa(f)rivolumtea(e)1600aherms.1200 _2.3.2. nmr measurements of methanein coalss800ioioa-veesuxngat differentpressures.addcopsota2(d)(c)罂 peri31041010t2(：,. )fig. 2. nmr measurements for bulk methane运ferenaed-第pusur2400e2y0librium press awc(b)(a

35、)sor son iooi 1atchostasuatidhmefthepm andu 帮 sure theiaerimranmoo_ _ _.aseiecobemorevamoo1 epnassournemputed fromheressur新mfef璃hwsureax-eriletbuniei(gradienlusedthistudec(.1)1tst2bt veceafibwpoessure.sow®-t1f融噌鬻npfwnwieeea2.4. basic theory of nmr measurements for methanegasre axatpdoreertcoais

36、dedetvistheurface-to-volum( feoreosig?ca ermsimpbyht tribiosurfaeacotribtotexakonnucle 骷nuncersmerme sur1ccecemccorasffeclreeetcif-esssity.etweenpettoageace2oagm©peg3.1. relaxation properties of bulk methane铲3resandiscussionsanaenaanwradii efexatressltrng1x 1x 1t2t2b t2s t2dwherebsrespeclated b

37、y(c1 ?deg2toon?goi!'shmngoivendistribuisoloserotonerei1se 嘱onshipc?q00326r?0999niewhaocms,gas pressure (mpa)0-22-33-44-55-7wait time (s)1.52.03.03.56.0echo spacing (ms)0.30.3number of trains6464646464number of echoes18,00018,00018,00018,00018,000table 1experimental parameters for nmr me

38、asurements.oluconcentrabe吗瓢centfnthans崛1ntoliteasuetosad喃sm胁etable 2experimentalresults of bulk methane.pressureconcentrationmassaamplitude of t2(mpa)(mg/cm 3)(mg)spectrum1.046.940485.652372.71.9012.879906.814415.83.1121.5501517.346844.84.0728.6952020.418927.25.0235.9982534.6210,878.95.9843.6083070.

39、4413,088.7a the reference cell volume is 70.41 cm3.50fig. 3. relationships between total t2 amplitude and speci?c density of bulk methane instrument -speci ?c calibration curve.where m is the methane mass with unitsmethane density at standard conditionof mg. by substituting the (of 0.714 mg/cm 3) to

40、eq. (5) , the methane volume at standard condition byv ? 0:321468 ? tr2 ? 0:999where一 3 cmv is the methane3.2. relaxation propertiescan be obtainedvolume at standard conditionof methane in coalse6twith units ofmeasuring cell is identical 70.41 cm 3, the methane mass can be obtained bym ? 0:22962 ?tr

41、2 ? 0:999e5tthe methane relaxation spectra of samples s1 and s2 are shown in fig. 4. in contrast to those of bulk methane they exhibit three distinct peaks (fig. 4). relaxation times, t2, for the methane in the coal- ?lled sample cell ( fig. 4) are shorter than those observed for the cell ?lled with

42、 bulk methane (no coal; fig. 2). the reason is that the coal surfaces enhance the relaxation of methane. in addition, pressure effects are also seen in fig. 4, i.e., an increase in pressure results in an overall increase of the mean relaxation times t2.because the dry, methane-free coal shows essent

43、ially no signal, the t2 distributions in fig. 4 are related entirely to the relaxation of methane. the t2 spectra of the two coal samples exhibit three distinct peaks (i.e., p1, p2 and p3). comparison of figs. 2 and 4 suggests that the p3 centered near in the t2 range of 240 n000 ms in fig. 43002502

44、00150100500p1p2(g)(f)t2c21p3(b)5 mpa4 mpa5 mpa d mpa8 mpa 0mpa9 mpa10-210-1100101102103104t2 (ms)350300250200150100500(a) 0.47 mpa(b) 1.09 mpa(c) 2.01 mpa(d) 2.97 mpa(e) 4.05 mpa-(f) 5.05 mpa-(g) 6.03 mpa-dry coal画p2(g)(e)10-210-1100101102t2 (ms)t2c2=240msp3103104fig. 4. t2 spectra for adsorbed meth

45、ane at different pressures for (a) sample s1, and (b) sample s2.orresehttceds axphrro sennhlip m3dsrala«fn5000400030002000 -10000 -01234567pressure (mpa)of t2 spectra for coal adsorbed01234567pressure (mpa).nr«tceds axphrro seamlip m3dsrala«fnmethane (p1), methanein porous medium (p2)

46、, and bulk methane (p3). p1, p2, and p3 arefig. 5. relationships between methane pressures and amplitudes shown in fig. 4. (a) sample s1, and (b) sample s2.eemethonnefrnnstributionsuarlente.orousupect(vejytheee-sctgpodocatoonmecodthemiu* 24ms maopce ancoa 用 mechawaofhmethamainar coeasuresures saium-

47、ne,tumllysurfal.(2004a,b)7 teracqons rees state in cu降 reurfaces,peak.eomatrix. thea忸*/峰山丽ethr rasspheracapey血酗 isinveraxatametholtlewncommatrthrs the tdistri t即研觊蚪- bumeth ainthistu(k 4) atictrmthresnthasubaisocosadsorbed藏atowprarm.ane icsimila匚 ncantrast, 哪n 一 i aa"aann -acaiatiqnemalscularae

48、laxaqiann1fornhe incregnsarcon- surasetblwgihift ieincreasnea wit methanress urcontrast,tog oftdecreas®re.cula butiuo. et al. (2007)inewasoaarausame7 ca cu lergeibutedpectrumisax-u5000medium-edn4500 豳sresejjnercn-arure nethane1.95 mpaiteracp1rocoss 强咿叫必, eak|coal-adsqrbe new(peakrbmallln eaabse

49、nowiress fce4000 6.09 mpa k"5.1 mpa4.07 mpah3.01 mpacahas.astroalniorcoannsurfa 3500esstoaceebe?明ressisthe dominaechawressboa2500vseecem 2000pressa_ thet%distrik witmcreaconcent macroporeproxiorbedpa ina 1500tinn 1000nctl hme stem.t1.04 mpa；one hour0.45 mpa100200300400time after methane additio

50、n (minute)fig. 6. integrated amplitudes of adsorbed methane versus time after methane addition under the experimental pressures, ranging from 0.45 to 6.09 mpa, sample s1.table 3measurements of theexcess amounts of methaneadsorbed on thecoal samples as determinedby the volumetric method.sequence12345

51、67isample s1p(mpa)0.451.041.953.014.075.16.09vs (cm 3)350.868457.448548.685556.422565.818574.416623.718v vol (m 3/t)15.6920.4524.5324.8825.3025.6827.89sample s2p(mpa)0.471.092.012.974.055.056.03vs (cm 3)356.923491.743609.375692.918714.361755.701788.938v vol (m 3/t)14.4919.9624.7328.1329.0030.6732.02

52、p is the experimentalpressure, vs is the standardvolume of adsorbed methane at p, and v vol is the excess adsorbed amountof methane (at standardpressureand temperature) per unitmass of coal.vvolvlp ept pltet.“”the coal-adsorbed methane (peak p1) increased with increasing pressure to a very limited e

53、xtent and reached a limiting value, similar to a langmuir function.3.3. discussion of methane adsorption equilibration timean advantage of using the low- feld nmr method is that the adsorption process can be detected in real time. thus, it is possible to use this method to estimate the adsorption eq

54、uilibration time at a given pressure. this is the time required to achieve a dynamic sorption equilibrium, where the number of adsorbed molecules remains constant as long as the external conditions (e.g., pressure, temperature) remain unaltered. it can be determined by comparing the t2 distributions recorded as a function of time.“fig. 6 shows the integrated t2 amplitudes of the p1 peaks ( adsorbed methane ) as a function of time after methane addition for sample s1. in the?st sequ