Evaluation of pore development in different coal reservoirs based on centrifugation experiment Shuang Wu a, b, * , Dazhen Tang a, b , Song Li a, b , Yanjun Meng c , Wenji Lin d a School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China b Coal Reservoir Laboratory of National Engineering Research Center of Coalbed Methane Development & Utilization, Beijing 100083, China c Department of Earth Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China d China United Coalbed Methane National Engineering Research Center Co., Ltd, Beijing 100095, China ARTICLE INFO Keywords: Pore development Coal reservoir Capillary pressure data Centrifugation Gas storage/transportation mechanism ABSTRACT Pore development in a coal reservoir is closely related to the coalbed methane (CBM) occurrence. Therefore, an experimental study on the properties and characteristics of coal pores is critical for CBM exploration and pro- duction. In this paper, capillary pressure data from six coal samples with coal ranks ranging from medium volatile bituminous to anthracite were obtained using centrifugation, which is fast, convenient, non-toxic and harmless compared with other experimental methods. The centrifugation capillary pressure curves can be classified into three types. Type A refers to the medium-low volatile bituminous with a wide platform on its capillary pressure curve, indicating the pores have a good connectivity. The fractal analysis show that the fractal structure of macro- and mesopores are more complicated than that of transition pores. Thus the pore structure has a good capacity for gas seepage. Type B and C refer to the semi-anthracite to anthracite with steeper capillary pressure curves, suggesting that they both contain large numbers of micropores. However, the maximum and average pore di- ameters of type B are larger than those of type C, indicating that type B has more macropores than type C. Therefore, type B has both gas flow and adsorption capacities, while type C is only favorable for methane storage. Mercury porosimetry and scanning electron microscopy (SEM) were also performed on the six samples to verify the results of centrifugation. The capillary pressure curves obtained using mercury porosimetry also indicate the three types of pore structures. The pore size distributions evaluated from mercury intrusion show a consistency with those from centrifugation. As the assessment of pore development using centrifugation is limited to quantity, SEM can help to visually reveal the genesis of pores in different types of coal reservoirs. 1. Introduction China attaches unprecedented importance to the exploration and development of coalbed methane (CBM) due to coal mining safety, greenhouse gas emissions and demand for natural gas (Luo and Dai, 2009; Tao et al., 2012). For coal reservoirs, a special porous media, the characteristics of pores (quantity, size, proportion of different type pores, and connectivity between pores), can influence not only gas trans- portation behavior, but also the mechanisms of gas adsorption and storage in coal seams (Clarkson and Bustin, 1996; Karacan and Okandan, 2001; Mastalerz et al., 2008). The matrix pore structure, specifically the relative abundance of micro-/transition-/meso-/macropore volume, has been demonstrated to be a function of coal composition and rank (also known as maturity or level of metamorphism which is commonly quantified by vitrinite reflectance) (Clarkson and Bustin, 1996, 1999; Gan et al., 1972). Therefore, the development of pore systems in different rank coal reservoirs is an important physical property having great in- fluence on CBM occurrence (Meng et al., 2014; Zhao et al., 2015). Experimental studies, which aim to determine the pore size, shape, dis- tribution, connectivity and genesis at different rank coals, are useful in understanding pore properties, gas content, porosity and permeability of coal reservoirs (Levy et al., 1997; Rahman et al., 2007). Recently, many researchers have characterized the complex coal pore systems at different scales using mercury intrusion, N 2 adsorption/ desorption analysis, low-field nuclear magnetic resonance (NMR), computed tomography (CT) and scanning electron microscopy (SEM) (Liu et al., 2009; Yao et al., 2008, 2009, 2010a, 2010b; Xu et al., 2015; Karacan and Okandan, 2001). But very few papers have discussed the application of centrifugation experiments on characterizing pore devel- opment for the study of coal reservoirs. Similar to mercury intrusion, * Corresponding author. Shuang WU School of Energy Resources, China University of Geosciences (Beijing), NO.29 Xueyuan Road, Haidian district, Beijing 100083, China. E-mail address: wusg62@163.com (S. Wu). Contents lists available at ScienceDirect Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol http://dx.doi.org/10.1016/j.petrol.2017.08.027 Received 26 August 2015; Received in revised form 11 July 2016; Accepted 9 August 2017 Available online 12 August 2017 0920-4105/© 2017 Elsevier B.V. All rights reserved. Journal of Petroleum Science and Engineering 157 (2017) 1095–1105