Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo Research paper A comprehensive study on geometric, topological and fractal characterizations of pore systems in low-permeability reservoirs based on SEM, MICP, NMR, and X-ray CT experiments Yuqi Wu a,b , Pejman Tahmasebi b,* , Chengyan Lin a , Muhammad Aleem Zahid c , Chunmei Dong a , Alexandra N. Golab d , Lihua Ren a a School of Geosciences, China University of Petroleum (East China), Qingdao, 266580, China b Department of Petroleum Engineering, University of Wyoming, Laramie, 82071, USA c Faculty of Marine Sciences, Lasbela University, Balochistan, 90250, Pakistan d Thermo Fisher Scientific, Canberra, ACT, 2600, Australia ARTICLE INFO Keywords: Pore system Topology characteristics Fractal dimension NMR X-ray CT ABSTRACT Characterization of pore systems in subsurface systems is of great importance for predicting the properties of rocks and classifying the subsurface systems. Geometric features have been used widely for this aim, but to- pological characteristics of the pore structures are not studied much. In fact, accurate characteristics of pore space should comprise both its geometric and topological properties. In this paper, the above vital characteristics are comprehensively studied based on direct experimental results. Besides, previous studies aiming at linking fractal dimension analysis to pore space are often based on limited sources of information, which are the mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR). In this paper, the scanning electron microscope (SEM), MICP, NMR, and X-ray computed tomography (X-ray CT) experiments are all used to characterize the geometric and topological properties of pore space of several low-permeability porous media. Based on our observations, the advantages and disadvantages of the above techniques in characterizing the pore structure are also summarized. Moreover, the differences of these three experiments are quantified using fractal dimension. The results indicate that the NMR technique is a promising tool for characterizing geometric features of pore systems as it can cover more details than other techniques. Most of geometric, topological, fractal and transport properties of pore space can be obtained from X-ray CT method, which is unique among all the methods. In addition, the 3D fractal dimensions of pore systems obtained from NMR is smaller than that from MICP, which is due more ultra-micropores captured by NMR that smooth the surface of the pore systems. Finally, a novel method constrained by a new pore shape factor for calculating the pore size from 2D images is proposed by which the pore-size distributions are compared more effectively. 1. Introduction In subsurface characterization, finding the high-quality and permeable regions (e.g. sweet spots of reservoirs) is very crucial (Clarkson et al., 2013; Nelson, 2011, 2009; Tahmasebi et al., 2017; Xiao et al., 2017). The quality of the reservoirs was affected by many mac- roscopic factors, such as depth, porosity, and permeability of the re- servoirs, hydrocarbon content, and hydrocarbon quality in the re- servoirs. In fact, most of these macroscopic properties of reservoirs (e.g. porosity and permeability) are mainly controlled by the microscopic petrophysical properties of the rocks, such as the features of pore sys- tems (Blunt et al., 2013; Daigle et al., 2017; Daigle and Dugan, 2011; Fagbemi et al., 2018b, 2018a; Hu et al., 2017, 2012; Hu and Brusseau, 1994; Jiang, 2008; Tahmasebi and Kamrava, 2018; Xiong et al., 2016). As such, the characterization of pore systems is of great importance for porous media modeling. The pore structure properties of porous media include the geometrical and topological characteristics (Jiang, 2008). For the reservoirs with few or no fractures, the geometrical properties of pore space consist of volume, surface area, radius and shape of the pores and throats. The topological parameters comprise the con- nectivity, tortuosity, Euler number, coordination number, and two- point correlation function. As to the fractured reservoirs, such as shales and coal seams, the characterization of pore systems should include the description and measurement of the fractures in the rock because the https://doi.org/10.1016/j.marpetgeo.2019.02.003 Received 28 November 2018; Received in revised form 19 January 2019; Accepted 2 February 2019 * Corresponding author. E-mail addresses: ptahmase@uwyo.edu (P. Tahmasebi), lincy@upc.edu.cn (C. Lin). Marine and Petroleum Geology 103 (2019) 12–28 Available online 06 February 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved. T