Contents lists available at ScienceDirect Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse Impact of inherent moisture on the methane adsorption characteristics of coals with various degrees of metamorphism Ming-yi Chen a,b , Yuan-ping Cheng c,* , Hao-ran Li d,** , Liang Wang c , Kan Jin c , Jun Dong c a College of Civil Engineering, Shijiazhuang Tiedao University, Shijiazhuang 050043, China b Cooperative Innovation Center of Disaster Prevention and Mitigation for Large Infrastructure in Hebei Province, Shijiazhuang Tiedao University, Shijiazhuang 050043, China c National Engineering Research Center for Coal and Gas Control, China University of Mining and Technology, Xuzhou 221116, China d Structure Health Monitoring and Control Institute, Shijiazhuang Tiedao University, Shijiazhuang 050043, China ARTICLE INFO Keywords: Moisture correction term Oxygen-containing function groups Pore structure Coal rank Water-holding capacity ABSTRACT Gas and water coexist in coal reservoirs. Many scholars have examined the effect of moisture on the gas ad- sorption property of coal. In this paper, the oxygen-containing functional groups and pore characteristics of five coal samples with different degrees of metamorphism were studied by Fourier transform infrared (FTIR) spec- troscopy and CO 2 adsorption method, and methane adsorption tests were carried out on coal samples with different inherent moisture contents. The water-holding capacities of different rank coals are related to their oxygen-containing functional groups and micropores characteristics, and the former play a decisive role. This is probably the main reason why low rank coals have stronger water-holding capacity than medium and high rank coals. The Langmuir volume tends to decrease with increasing inherent moisture content for all of the samples, which can be attributed to hydrogen bonding between the coal macromolecules and water molecules. The re- lationship between the amount of adsorbed methane and the inherent moisture content can be characterized well using the linear and exponential models. And it can be found that the moisture effect coefficient is de- pendent on the coal rank. Further analysis indicates that the influence of inherent moisture on the methane adsorption capacity is more prominent for low rank coals, which is probably due to its greater hydrophilia and water-holding capacity. Whereas, the reduction in methane adsorption capacity is mainly controlled by the pore- blocking effect of adsorbed water for high rank coals, and the extent of the influence of inherent moisture is a little. 1. Introduction In coalbed methane (CBM) recovery or coal mine gas extraction, improving the prediction accuracy of the coal seam gas content is key for estimating the CBM reserves and the occurrence of coal seam gas (Clarkson and Bustin, 2011; Cheng et al., 2017; Peng et al., 2014). The gas adsorption capacity of coal is a key parameter for determining the gas content of a coal seam. Conversely, the coal properties and coal reservoir environment affect the gas adsorption capacity (Clarkson and Bustin, 1999; Crosdale et al., 2008; Laxminarayana and Crosdale, 1999; Yao et al., 2008). Gas and water coexist in coal reservoirs, and water has a significant influence on the gas adsorption property of coal (Guo et al., 2015; Pan et al., 2010; Wang et al., 2017). For example, Krooss et al. (2002) reported that the methane adsorption capacity of moisture- equilibrated coal is ∼20–25% lower than that of dry coal. Therefore, the presence of water should not be neglected in estimates of the gas adsorption capacity, which is significant to the evaluation of the gas content in coal seams and the prevention and control of coal seam gas disasters. As early as 1936, Coppens observed that the methane adsorption capacity of Belgian coals decreases due to the presence of moisture. Since the 1970s, many scholars have investigated the gas adsorption properties of moist coals (Busch and Gensterblum, 2011). It is generally believed that the gas adsorption capacity of coal decreases with in- creasing moisture content; however, when the moisture content ex- ceeds a critical value, the gas adsorption capacity remains constant (Joubert et al., 1973, 1974; Gensterblum et al., 2013). Joubert et al. (1973) further revealed that the critical value of moisture content is https://doi.org/10.1016/j.jngse.2018.05.018 Received 7 December 2017; Received in revised form 8 May 2018; Accepted 10 May 2018 * Corresponding author. National Engineering Research Center for Coal and Gas Control, China University of Mining and Technology, Xuzhou 221116, China. Tel.: +86 516 83885948; fax: +86 516 83995097. ** Corresponding author. College of Civil Engineering, Shijiazhuang Tiedao University, Shijiazhuang 050043, China. Tel./fax: +86 311 87936452. E-mail addresses: chenmingyi@stdu.edu.cn (M.-y. Chen), ypcheng@cumt.edu.cn (Y.-p. Cheng), lihaoran@stdu.edu.cn (H.-r. Li), liangw1982@126.com (L. Wang), jinkan@yeah.net (K. Jin), cumtdongjun@163.com (J. Dong). Journal of Natural Gas Science and Engineering 55 (2018) 312–320 Available online 12 May 2018 1875-5100/ © 2018 Elsevier B.V. All rights reserved. T