Evaluation of coal bed methane content using BET adsorption isotherm equation

Global Geology, 15(1): 74-77(2012doi:10.3969/jisn.1673-9736.2012.01.11Article ID:16739736(2012)01-0074-04Evaluation of coal bed methane content using BETadsorption isotherm equationZHANG Yi, FAN Xiaomin, HAN Xue, NAN Zeyu and XU JunCollege of Geo-Exploration Science and Technology, Jilin University, Changchun 130026, ChinaAbstract Coal bed methane is unconventional raw natural gas stored in coal seam with considerable reserves inChina. In recent years, as the coal bed methane production, the safety and the use of resources have been paidmore attentions. Evaluating coal bed methane content is an urgent problem. A BET adsorption isotherm equan is used to process the experimental data. The various parameters of Bet equation under different tempera-tures are obtained; a theoretical gas content correction factor is proposed, and an evaluation method of actualcoal bed methane is establishedKey words: BET adsorption isotherm; coal bed methane; geophysical well logging; gas content; evaluationmethodIntroduction1938, Brunauer, Emmett and Teller extended Langmuir monolayer adsorption theory to multi-molecularSince 1970s, as the development of coal bedlyer adsorption isotherm type I( brunauer et almethane industry, people have a profound under-1938). In 1977, Kim proposed an adsorption iso-standing of the advantage of coal bed methane explora-herm based on approximate analysis result ( Kimtion on solving the coal mining disasters in gas explo-1977). In 1992, Hawkins derived a Langmuir adsion and providing clean energy for human beingssorption isotherm equation of coal rank, using thethe same time, improving energy structure and redsame data as deriving Kim equation( Hawkins et aling gas emissions on the human environment pollu1992). In 1999, Gao et aL.(1999 )used Langmuirtion. Therefore, exploration and development of coalquation to evaluate coal bed methane content withbed methane resources has attracted wide attention atwell logging datahome and abroad, and the research work on exploraThe adaptation of BET equation to the laboratorytion has been carried outanalytical adsorption isotherm data and the calculationIn 1918, Langmuir derived monolayer adsorptioof the max gas content are discussed, and a correctionisotherm from the kinetic theory, and considered thatfactor is established to correct theory gas content toadsorption sites which can adsorb molecules or atomsactual gas content.exist on solid surface Langmuir et al., 1918). In1930, Freundlich extended Henry adsorption equationI BET equationand found that many kinds of solution adsorption alsoBET theory is suitable for uniform chemical prop-consistent with the equation( Freundlich, 1930). Inerties surface中国煤化工ns are asfoiCNMHGReceived 21 November 2011, accepted 15 December 2011Evaluation of coal bed methane content using BET adsorption isotherm equationisothermal adsorption equation, BET adsorption iso-Two-parameter BET equationtherm equation can fit I and II type adsorption iso-V cptherm curves, and due to the large amount of parame-(o-p)[1+(c-1)(P/b))(1)ters, the results of BET equation are highly consistentThree-parameter BET equationwith the experimental adsorption dataThe three-parameter BET adsorption isothermV(n+1)"+nx"equation is applied in this study, when looking for the(1-x)best accordant adsorption gas content V, to the labora-P(3)urement of adsorption gas content V (leanin Heilongjiang province of China is pro-Where V is adsorbed gas volume(m/t): V is the cessed), the least square method is used, and thusvolume of gas adsorbed when the entire adsorbent sur-the parameters of n, c, Vm and the best fitting adsorp-face is covered with a complete unimolecular layertion isotherm are obtained. The fitting result is shown(m/t); P is pressure(MPa) Po is liquid adsorbed in Fig. 1, three V, curves under different temperaturessaturated vapor pressure of same temperature( MPa);are described as solid. dashes and short dashes linesc is constant related with the heat of adsoron anand the square, circle and triangle points are laborato-condensation. n is the number of absorption layersry measurement data, as it is shown in the figuremited by the adsorption diameter.three-parameter BET isothermal adsorption equationsTwo parameters BEt equation describes the typeare highly consistent with experimental data.II adsorption isotherm of the reverse"S"shape whenThe optimal fitting parameters n, c and Vm at dif-n is infinite. When pressure is very small(P