Flow and Transport Properties of Unconventional Reservoirs Jianchao Cai, Zhien Zhang, Qinjun Kang and Harpreet Singh www.mdpi.com/journal/energies Edited by Printed Edition of the Special Issue Published in Energies Flow and Transport Properties of Unconventional Reservoirs 2018 Flow and Transport Properties of Unconventional Reservoirs 2018 Special Issue Editors Jianchao Cai Zhien Zhang Qinjun Kang Harpreet Singh MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade Zhien Zhang The Ohio State University USA Special Issue Editors Jianchao Cai China University of Geosciences China Qinjun Kang Harpreet Singh Los Alamos National Laboratory The University of Texas at Austin USA USA Editorial Office MDPI St. Alban-Anlage 66 4052 Basel, Switzerland This is a reprint of articles from the Special Issue published online in the open access journal Energies (ISSN 1996-1073) from 2018 to 2019 (available at: https://www.mdpi.com/journal/energies/special issues/unconventional reservoirs2018) For citation purposes, cite each article independently as indicated on the article page online and as indicated below: LastName, A.A.; LastName, B.B.; LastName, C.C. 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Contents About the Special Issue Editors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Jianchao Cai, Zhien Zhang, Qinjun Kang and Harpreet Singh Recent Advances in Flow and Transport Properties of Unconventional Reservoirs Reprinted from: Energies 2019 , 12 , 1865, doi:10.3390/en12101865 . . . . . . . . . . . . . . . . . . . 1 Zhaohui Xu, Peiqiang Zhao, Zhenlin Wang, Mehdi Ostadhassan and Zhonghua Pan Characterization and Consecutive Prediction of Pore Structures in Tight Oil Reservoirs Reprinted from: Energies 2018 , 11 , 2705, doi:10.3390/en11102705 . . . . . . . . . . . . . . . . . . . 6 Xingxing Liu, Jinchang Sheng, Jishan Liu and Yunjin Hu Evolution of Coal Permeability during Gas Injection—From Initial to Ultimate Equilibrium Reprinted from: Energies 2018 , 11 , 2800, doi:10.3390/en11102800 . . . . . . . . . . . . . . . . . . . 21 Xiaohong Li, Zhiyong Gao, Siyi Fang, Chao Ren, Kun Yang and Fuyong Wang Fractal Characterization of Nanopore Structure in Shale, Tight Sandstone and Mudstone from the Ordos Basin of China Using Nitrogen Adsorption Reprinted from: Energies 2019 , 12 , 583, doi:10.3390/en12040583 . . . . . . . . . . . . . . . . . . . . 45 Xinxing Ma, Xianwen Li, Shouwen Zhang, Yanming Zhang, Xiangie Hao and Jishan Liu Impact of Local Effects on the Evolution of Unconventional Rock Permeability Reprinted from: Energies 2019 , 12 , 478, doi:10.3390/en12030478 . . . . . . . . . . . . . . . . . . . . 63 Zhuoying Fan, Jiagen Hou, Xinmin Ge, Peiqiang Zhao and Jianyu Liu Investigating Influential Factors of the Gas Absorption Capacity in Shale Reservoirs Using Integrated Petrophysical, Mineralogical and Geochemical Experiments: A Case Study Reprinted from: Energies 2018 , 11 , 3078, doi:10.3390/en11113078 . . . . . . . . . . . . . . . . . . . 80 Feng Sha, Lizhi Xiao, Zhiqiang Mao and Chen Jia Petrophysical Characterization and Fractal Analysis of Carbonate Reservoirs of the Eastern Margin of the Pre-Caspian Basin Reprinted from: Energies 2019 , 12 , 78, doi:10.3390/en12010078 . . . . . . . . . . . . . . . . . . . . 92 Gang Lei, Nai Cao, Di Liu and Huijie Wang A Non-Linear Flow Model for Porous Media Based on Conformable Derivative Approach Reprinted from: Energies 2018 , 11 , 2986, doi:10.3390/en11112986 . . . . . . . . . . . . . . . . . . . 109 Mingqiang Chen, Linsong Cheng, Renyi Cao and Chaohui Lyu A Study to Investigate Fluid-Solid Interaction Effects on Fluid Flow in Micro Scales Reprinted from: Energies 2018 , 11 , 2197, doi:10.3390/en11092197 . . . . . . . . . . . . . . . . . . . 120 Fanhui Zeng, Fan Peng, Jianchun Guo, Jianhua Xiang, Qingrong Wang and Jiangang Zhen A Transient Productivity Model of Fractured Wells in Shale Reservoirs Based on the Succession Pseudo-Steady State Method Reprinted from: Energies 2018 , 11 , 2335, doi:10.3390/en11092335 . . . . . . . . . . . . . . . . . . . 140 Honghua Tao, Liehui Zhang, Qiguo Liu, Qi Deng, Man Luo and Yulong Zhao An Analytical Flow Model for Heterogeneous Multi-Fractured Systems in Shale Gas Reservoirs Reprinted from: Energies 2018 , 11 , 3422, doi:10.3390/en11123422 . . . . . . . . . . . . . . . . . . . 156 v Pengyu Wang, Zhiliang Wang, Linfang Shen and Libin Xin Lattice Boltzmann Simulation of Fluid Flow Characteristics in a Rock Micro-Fracture Based on the Pseudo-Potential Model Reprinted from: Energies 2018 , 11 , 2576, doi:10.3390/en11102576 . . . . . . . . . . . . . . . . . . . 175 Long Ren, Wendong Wang, Yuliang Su, Mingqiang Chen, Cheng Jing, Nan Zhang, Yanlong He and Jian Sun Multiporosity and Multiscale Flow Characteristics of a Stimulated Reservoir Volume (SRV)-Fractured Horizontal Well in a Tight Oil Reservoir Reprinted from: Energies 2018 , 11 , 2724, doi:10.3390/en11102724 . . . . . . . . . . . . . . . . . . . 189 Chao Tang, Xiaofan Chen, Zhimin Du, Ping Yue and Jiabao Wei Numerical Simulation Study on Seepage Theory of a Multi-Section Fractured Horizontal Well in Shale Gas Reservoirs Based on Multi-Scale Flow Mechanisms Reprinted from: Energies 2018 , 11 , 2329, doi:10.3390/en11092329 . . . . . . . . . . . . . . . . . . . 203 Daobing Wang, Fang Shi, Bo Yu, Dongliang Sun, Xiuhui Li, Dongxu Han and Yanxin Tan A Numerical Study on the Diversion Mechanisms of Fracture Networks in Tight Reservoirs with Frictional Natural Fractures Reprinted from: Energies 2018 , 11 , 3035, doi:10.3390/en11113035 . . . . . . . . . . . . . . . . . . . 223 Muhammad Shahzad Kamal, Marwan Mohammed, Mohamed Mahmoud and Salaheldin Elkatatny Development of Chelating Agent-Based Polymeric Gel System for Hydraulic Fracturing Reprinted from: Energies 2018 , 11 , 1663, doi:10.3390/en11071663 . . . . . . . . . . . . . . . . . . . 251 Shanfa Tang, Yahui Zheng, Weipeng Yang, Jiaxin Wang, Yingkai Fan and Jun Lu Experimental Study of Sulfonate Gemini Surfactants as Thickeners for Clean Fracturing Fluids Reprinted from: Energies 2018 , 11 , 3182, doi:10.3390/en11113182 . . . . . . . . . . . . . . . . . . . 266 Wenxiang Chen, Zubo Zhang, Qingjie Liu, Xu Chen, Prince Opoku Appau and Fuyong Wang Experimental Investigation of Oil Recovery from Tight Sandstone Oil Reservoirs by Pressure Depletion Reprinted from: Energies 2018 , 11 , 2667, doi:10.3390/en11102667 . . . . . . . . . . . . . . . . . . . 277 Chaohui Lyu, Qing Wang, Zhengfu Ning, Mingqiang Chen, Mingqi Li, Zhili Chen and Yuxuan Xia Investigation on the Application of NMR to Spontaneous Imbibition Recovery of Tight Sandstones: An Experimental Study Reprinted from: Energies 2018 , 11 , 2359, doi:10.3390/en11092359 . . . . . . . . . . . . . . . . . . . 294 Shan Jiang, Pingping Liang and Yujiao Han Effect of Clay Mineral Composition on Low-Salinity Water Flooding Reprinted from: Energies 2018 , 11 , 3317, doi:10.3390/en11123317 . . . . . . . . . . . . . . . . . . . 306 Xiting Long, Keneng Zhang, Ruiqiang Yuan, Liang Zhang and Zhenling Liu Hydrogeochemical and Isotopic Constraints on the Pattern of a Deep Circulation Groundwater Flow System Reprinted from: Energies 2019 , 12 , 404, doi:10.3390/en12030404 . . . . . . . . . . . . . . . . . . . . 317 Liming Zhang, Zekun Deng, Kai Zhang, Tao Long, Joshua Kwesi Desbordes, Hai Sun and Yongfei Yang Well-Placement Optimization in an Enhanced Geothermal System Based on the Fracture Continuum Method and 0-1 Programming Reprinted from: Energies 2019 , 12 , 709, doi:10.3390/en12040709 . . . . . . . . . . . . . . . . . . . . 335 vi About the Special Issue Editors Jianchao Cai received his B.Sc. in Physics from Henan Normal University in 2005, and earned an MSc and a Ph.D. in Condensed Matter Physics from Huazhong University of Science and Technology in 2007 and 2010, respectively. He joined the Institute of Geophysics and Geomatics at the China University of Geosciences (Wuhan) in 2010. From July 2013 to July 2014, he acted as a Visiting Scholar at the University of Tennessee-Knoxville, USA. He currently is a professor of Geological Resources and Geological Engineering (since 2015). Furthermore, he is the founder and Editor-in-Chief of Advances in Geo-Energy Research and serves as an Associate/Guest Editor for other international journals. Dr. Cai focuses on the petrophysical characterization and micro-transport phenomena in porous media, as well as fractal theory and its application. He has published more than 110 peer-refereed journal articles, three books, and numerous book chapters. Zhien Zhang is currently a postdoctoral researcher in the William G. Lowrie Department of Chemical and Biomolecular Engineering at Ohio State University. His research interests include advanced processes and materials (i.e., membranes) for CO2 capture, CCUS processes, gas separation, and gas hydrates. Dr. Zhang has published 80+ journal papers and 10+ editorials in high-impact journals, including Renewable and Sustainable Energy Reviews. He has written three Hot Papers (top 0.1%) and 10 Highly Cited Papers (top 1%). He also serves as an Editor or Guest Editor for several international journals, including Applied Energy, Fuel, and Journal of Natural Gas Science and Engineering . He currently works as a Visiting Professor at the University of Cincinnati. Qinjun Kang is a senior scientist at the Earth and Environmental Sciences (EES) Division of Los Alamos National Laboratory (LANL). His current research focuses on the modeling and simulation of transport and interfacial processes in porous media at the pore (nano/meso) scale, and on multiscale models bridging different scales. His work is applied to problems in a broad range of engineering and science disciplines, including the geologic storage of carbon dioxide and nuclear waste, conventional and unconventional hydrocarbon exploration and production, the fate and transport of underground contaminants, and the engineering of energy storage and conversion devices (e.g., fuel cells and batteries). Dr. Kang has coauthored more than 100 publications, which have been cited nearly 6000 times. He has been invited to give 20+ talks at various international and national conferences, universities, national laboratories, and industries, and has organized/chaired sessions at numerous conferences. He is currently a member of the Editorial Board or an Associate Editor for multiple journals, and has served as a reviewer for over 50 journals and numerous funding agencies, including DOE and NSF. Dr. Kang has been a top publisher in the EES Division at LANL since 2010. Harpreet Singh is a postdoctoral research associate at the National Energy Technology Laboratory, Morgantown, West Virginia. He is a reservoir engineer by training and holds Master’s and Ph.D. degrees in petroleum engineering from the University of Texas at Austin. He has published 23 manuscripts, 17 of them as the first author, and authored one chapter in a book published by Elsevier. He currently serves as an Associate Editor for Advances in Geo-Energy Research , and has been a Guest Editor for journals such as Fuel and Energies . He has served as a reviewer for over 20 journals. vii energies Editorial Recent Advances in Flow and Transport Properties of Unconventional Reservoirs Jianchao Cai 1, *, Zhien Zhang 2 , Qinjun Kang 3 and Harpreet Singh 4 1 Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, China 2 William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, USA; zhienzhang@hotmail.com 3 Computational Earth Science Group, Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA; qkang@lanl.gov 4 National Energy Technology Laboratory, Morgantown, WV 26505, USA; harpreet.singh@netl.doe.gov * Correspondence: caijc@cug.edu.cn Received: 4 March 2019; Accepted: 13 May 2019; Published: 16 May 2019 Abstract: As a major supplement to conventional fossil fuels, unconventional oil and gas resources have received significant attention across the globe. However, significant challenges need to be overcome in order to economically develop these resources, and new technologies based on a fundamental understanding of flow and transport processes in unconventional reservoirs are the key. This special issue collects a series of recent studies focused on the application of novel technologies and theories in unconventional reservoirs, covering the fields of petrophysical characterization, hydraulic fracturing, fluid transport physics, enhanced oil recovery, and geothermal energy. Keywords: unconventional reservoirs; petrophysical characterization; fluid transport physics 1. Introduction Unconventional reservoirs, such as shale, coal, and tight sandstone reservoirs, are complex and highly heterogeneous, generally characterized by low porosity and ultralow permeability. Additionally, the strong physical and chemical interactions between fluids and pore surfaces further lead to the inapplicability of conventional approaches for characterizing fluid flow in these porous reservoir rocks [ 1 ]. Therefore, new theories, techniques, and geophysical and geochemical methods are urgently needed to characterize petrophysical properties, fluid transport, and their relationships at multiple scales for improving production e ffi ciency from unconventional reservoirs. Petrophysical characterization covers the study of the physical and chemical properties of rock and its interactions with fluids, which has many applications in di ff erent industries, especially in the oil and gas industries. The key parameters studied in petrophysics are lithology, porosity, water saturation, permeability, and density. Petrophysical characterization is the basis for understanding the special properties of unconventional reservoirs. Fluid transport physics in micropore structures and macro-reservoirs covers a wide range of research studies including hydrocarbon extraction, geosciences, environmental issues, hydrology, and biology. Implementing reliable methods for the characterization of fluid transport at multiple scales is crucial in many fields, especially in unconventional reservoirs and rocks. Hydraulic fracturing is currently considered as one of the most important stimulation methods in the oil and gas industry, which significantly improves the productivity of the wells and the overall recovery factor, especially for low-permeability reservoirs, such as shale-gas and tight-gas reservoirs. Problems that are associated with unconventional oil and gas production in hydraulic fracturing operations include aqueous phase trapping, diversion mechanisms of fracture networks, and fluid incompatibility with the formation. Energies 2019 , 12 , 1865; doi:10.3390 / en12101865 www.mdpi.com / journal / energies 1 Energies 2019 , 12 , 1865 This collection associated with the special issue in Energies emphasizes fundamental innovations and gathers 21 recent papers on novel applications of new techniques and theories in unconventional reservoirs. 2. Overview of Work Presented in This Special Issue The papers published in this special issue present new advancements in the characterization of porous media and the modeling of multiphase flow in porous media. These studies are classified into five categories. The first category focuses on petrophysical characterization. By means of a set of experiments including scanning electron microscopy, mercury intrusion capillary pressure, X-ray di ff raction, and nuclear magnetic resonance measurements, Xu et al. [ 2 ] characterized the pore structure of a tight oil reservoir in Permain Lucaogou formation of Jimusaer Sag and further performed a consecutive prediction for its pore structures. The pore types of this formation were mainly divided into four categories, and the capillary pressure curve and the T2 distribution data were analyzed in depth. A matrix–fracture interaction model was developed by Liu et al. [ 3 ] to investigate the transient response of coal deformation and permeability to the temporal and spatial variations of e ff ective stresses under mechanically unconstrained conditions. The impacts of fracture properties, initial matrix permeability, injection processes, and confining pressure were separately evaluated through the developed model. Base on a low-pressure nitrogen adsorption experiment and fractal theory, Li et al. [ 4 ] studied the characteristics of nanopore structure in shale, tight sandstone, and mudstone, with an emphasis on the relationships between pore structure parameters, mineral compositions, and fractal dimensions. The relationships among average pore diameter, Brunner–Emmet–Teller specific surface area, pore volume, porosity, and permeability were also discussed. Ma et al. [ 5 ] introduced the local force to define the interactions between the matrix and the fracture and derived a set of partial di ff erential equations to define the full coupling of rock deformation and gas flow both in the matrix and fracture systems. Permeability evolution profiles during unconventional gas extraction were obtained by solving the full set of cross-coupling formulations. A comprehensive experiment, including petrophysical measurements (porosity and permeability), pore structure measurements (low-field nuclear magnetic resonance and carbon dioxide / nitrogen adsorption), geochemical measurements (vitrinite reflectance, pyrolysis, and residual analysis), and petrological analysis (X-ray di ff raction, thin section, scanning electron microscopy, and isothermal adsorption measurement), was designed by Fan et al. [ 6 ] to explore the influential and controlling factors of the gas adsorption capacity. By using the data from casting thin section and mercury intrusion capillary pressure experiments, Sha et al. [ 7 ] investigated the pore structure characterization, permeability estimation, and fractal characteristics of Carboniferous carbonate reservoirs. The second category focuses on fluid transport at multiple scales. Based on Swartzendruber equation and conformable derivative approach, as well as the modified Hertzian contact theory and fractal geometry, Lei et al. [ 8 ] developed a novel nonlinear flow model for tight porous media, which manifests the most important fundamental controls on low-velocity nonlinear flow. According to this model, the average flow velocity in tight porous media is a function of microstructural parameters of the pore space, rock lithology, and di ff erential order, as well as hydraulic gradients and threshold hydraulic gradients. Moreover, the relationships between average flow velocity and e ff ective stress, the rougher pore surfaces, and rock elastic modulus were further discussed. Chen et al. [ 9 ] proposed a novel model for characterizing boundary layer thickness and fluid flow at microscales, which has a wide range of applications proved mathematically. Based on this model, the e ff ects of fluid–solid interaction on flow in microtubes and tight formation were analyzed in depth. Two di ff erent productivity models, the steady-state productivity model of shale horizontal wells with volume fracturing and the transient productivity calculation model of fractured wells, were 2 Energies 2019 , 12 , 1865 derived by Zeng et al. [ 10 ]. The former considered the multiscale flowing states, shale gas desorption, and di ff usion, while the latter combined the material balance equation. Furthermore, the horizontal well productivity prediction and the analysis of influencing factors were carried out. In order to describe the pressure-transient behaviors in shale gas reservoirs in a way that considers the stimulated reservoir volume region with anomalous di ff usion and fractal features, an improved analytical model was established by Tao et al. [ 11 ] through introducing the time-fractional flux law. Base on this model, the influences of relevant parameters, such as fractal-anomalous di ff usion, stress sensitivity, absorption, and Knudsen di ff usion, on the pressure-transient response were further analyzed through sensitivity analysis. By introducing an improved pseudopotential multirelaxation-time lattice Boltzmann method, Wang et al. [ 12 ] simulated the fluid flow in a microfracture. The e ff ects of contact angles, driving pressure, and the liquid–gas density ratio on the slip length were discussed. Based on the dual-media theory and discrete-fracture network models, Ren et al. [ 13 ] built a mathematical flow model of a stimulated reservoir volume fractured horizontal well with multiporosity and multipermeability media. The di ff erences of flow regimes between triple-porosity, dual-permeability and triple-porosity, triple-permeability models were identified. Moreover, the productivity contribution degree of multimedium was analyzed. Tang et al. [ 14 ] summarized the flow law in shale gas reservoirs and established a three-dimensional composite model, which uses dual media to describe matrix-natural microfractures and utilizes discrete media to describe artificial fractures. The production of multisection fractured horizontal wells in a rectangular shale gas reservoir was described, considering multiscale flow mechanisms in the matrix, such as gas desorption, the Klinkenberg e ff ect, and gas di ff usion. The third category focuses on hydraulic fracturing. By means of the extended finite element method, Wang et al. [ 15 ] investigated the diversion mechanisms of a fracture network in tight formations with frictional natural fractures. The e ff ects of some key factors, for example, the location of natural fracture, the intersection angle between natural fracture and hydro-fracture, the horizontal stress di ff erence, and the fluid viscosity on the mechanical diversion behavior of the hydro-fracture, were analyzed in detail. Kamal et al. [ 16 ] developed a new smart fracturing fluid system mainly consisting of a water-soluble polymer and chelating agent, which can be either used for proppant fracturing (high pH) or acid fracturing (low pH) operations in tight as well as conventional formations. The optimal conditions and concentration of this fracturing fluid system were determined by performing thermal stability, rheology, Fourier transform infrared spectroscopy, and core flooding experiments. By measuring the solution viscosity, Tang et al. [ 17 ] investigated the e ff ects of hydrophobic chain, spacer group, concentration, temperature, and addition of nano-MgO on the viscosity of sulfonate Gemini surfactant solution. Moreover, their micellar microstructures were observed by Cryo-SEM. Further, the thickening mechanism of sulfonate Gemini surfactant was investigated by correlating the relationship between solution viscosity and its microstructure. The fourth category focuses on enhanced oil recovery. A novel depletion laboratory experimental platform and its evaluation method for a tight oil reservoir were developed by Chen et al. [ 18 ] to e ff ectively measure the oil recovery and pressure propagation over pressure depletion. On this platform, under di ff erent temperatures, formation pressure coe ffi cients, and oil property conditions, the recovery factor as well as the real-time monitoring of the pressure propagation in the process of reservoir depletion were measured to reveal the drive mechanism and recovery factor of tight oil reservoir depletion. Lyu et al. [ 19 ] applied the nuclear magnetic resonance technique to explore the spontaneous imbibition mechanism and the oil displacement recovery by imbibition in tight sandstones under all face open boundary conditions. The distribution of remaining oil and the e ff ect of microstructures on imbibition were analyzed. 3 Energies 2019 , 12 , 1865 Through three groups of core displacement experiments with cores containing di ff erent clay mineral compositions, Jiang et al. [ 20 ] studied the e ff ect of di ff erent clay mineral compositions on low-salinity water flooding. Additionally, the properties of the e ffl uent were determined in di ff erent flooding stages, and the mechanism of enhanced oil recovery e ff ect of low-salinity water flooding was analyzed. The fifth category focuses on geothermal energy. Based on hydrogeochemical and isotopic constraints, the deep circulation of the groundwater flow system was surveyed by Long et al. [ 21 ] to elucidate the origin of the geothermal fluids and the source of solutes and to discern the mixing and hydrogeochemical alteration. The conceptual models and mechanisms for the deep circulation of the groundwater flow system were further discussed. Combining the fracture continuum method and genetic algorithm, a well-placement optimization framework was proposed by Zhang et al. [ 22 ] to address the optimization of the well-placement for an enhanced geothermal system. The optimization e ffi ciency and e ff ect of this framework were further analyzed. 3. Conclusions Many researchers around the world from di ff erent areas, ranging from natural sciences to engineering fields, have been working on the characterization of petrophysical properties for unconventional reservoirs, fluid transport at multiscales, and technologies for the e ffi cient development of unconventional resources. The aim of this special issue is to provide new technologies and theories of characterizing petrophysical properties, fluid transport, and their relationships at multiple scales in unconventional reservoirs. Clearly, the studies covered by this special issue will be helpful to the economic and e ff ective development of unconventional oil and gas resources. Author Contributions: The authors contributed equally to this work. Acknowledgments: The guest editors would like to acknowledge MDPI for the invitation to act as the guest editors of this special issue in “ Energies ” with the kind cooperation and support of the editorial sta ff . The guest editors are also grateful to the authors for their inspiring contributions and the anonymous reviewers for their tremendous e ff orts. The first guest editor, J.C., would like to thank the National Natural Science Foundation of China for supporting his series of studies on flow and transport properties in porous media. H.S. acknowledges the support in part by an appointment to the National Energy Technology Laboratory Research Participation Program, sponsored by the U.S. DOE and administered by the Oak Ridge Institute for Science and Education. Conflicts of Interest: The authors declare no conflicts of interest. References 1. Cai, J.; Hu, X. Petrophysical Characterization and Fluids Transport in Unconventional Reservoirs ; Elsevier: Amsterdam, The Netherlands, 2019; p. 352. 2. Xu, Z.; Zhao, P.; Wang, Z.; Ostadhassan, M.; Pan, Z. Characterization and consecutive prediction of pore structures in tight oil reservoirs. Energies 2018 , 11 , 2705. [CrossRef] 3. Liu, X.; Sheng, J.; Liu, J.; Hu, Y. Evolution of coal permeability during gas injection—From initial to ultimate equilibrium. Energies 2018 , 11 , 2800. [CrossRef] 4. Li, X.; Gao, Z.; Fang, S.; Ren, C.; Yang, K.; Wang, F. Fractal characterization of nanopore structure in shale, tight sandstone and mudstone from the Ordos basin of china using nitrogen adsorption. Energies 2019 , 12 , 583. [CrossRef] 5. Ma, X.; Li, X.; Zhang, S.; Zhang, Y.; Hao, X.; Liu, J. Impact of local e ff ects on the evolution of unconventional rock permeability. Energies 2019 , 12 , 478. [CrossRef] 6. Fan, Z.; Hou, J.; Ge, X.; Zhao, P.; Liu, J. Investigating influential factors of the gas absorption capacity in shale reservoirs using integrated petrophysical, mineralogical and geochemical experiments: A case study. Energies 2018 , 11 , 3078. [CrossRef] 7. Sha, F.; Xiao, L.; Mao, Z.; Jia, C. Petrophysical characterization and fractal analysis of carbonate reservoirs of the eastern margin of the pre-Caspian basin. Energies 2018 , 12 , 78. [CrossRef] 4 Energies 2019 , 12 , 1865 8. Lei, G.; Cao, N.; Liu, D.; Wang, H. A non-linear flow model for porous media based on conformable derivative approach. Energies 2018 , 11 , 2986. [CrossRef] 9. Chen, M.; Cheng, L.; Cao, R.; Lyu, C. A study to investigate fluid-solid interaction e ff ects on fluid flow in micro scales. Energies 2018 , 11 , 2197. [CrossRef] 10. Zeng, F.; Peng, F.; Guo, J.; Xiang, J.; Wang, Q.; Zhen, J. A transient productivity model of fractured wells in shale reservoirs based on the succession pseudo-steady state method. Energies 2018 , 11 , 2335. [CrossRef] 11. Tao, H.; Zhang, L.; Liu, Q.; Deng, Q.; Luo, M.; Zhao, Y. An analytical flow model for heterogeneous multi-fractured systems in shale gas reservoirs. Energies 2018 , 11 , 3422. [CrossRef] 12. Wang, P.; Wang, Z.; Shen, L.; Xin, L. Lattice Boltzmann simulation of fluid flow characteristics in a rock micro-fracture based on the pseudo-potential model. Energies 2018 , 11 , 2576. [CrossRef] 13. Ren, L.; Wang, W.; Su, Y.; Chen, M.; Jing, C.; Zhang, N.; He, Y.; Sun, J. Multiporosity and multiscale flow characteristics of a stimulated reservoir volume (SRV)-fractured horizontal well in a tight oil reservoir. Energies 2018 , 11 , 2724. [CrossRef] 14. Tang, C.; Chen, X.; Du, Z.; Yue, P.; Wei, J. Numerical simulation study on seepage theory of a multi-section fractured horizontal well in shale gas reservoirs based on multi-scale flow mechanisms. Energies 2018 , 11 , 2329. [CrossRef] 15. Wang, D.; Shi, F.; Yu, B.; Sun, D.; Li, X.; Han, D.; Tan, Y. A numerical study on the diversion mechanisms of fracture networks in tight reservoirs with frictional natural fractures. Energies 2018 , 11 , 3035. [CrossRef] 16. Kamal, M.; Mohammed, M.; Mahmoud, M.; Elkatatny, S. Development of chelating agent-based polymeric gel system for hydraulic fracturing. Energies 2018 , 11 , 1663. [CrossRef] 17. Tang, S.; Zheng, Y.; Yang, W.; Wang, J.; Fan, Y.; Lu, J. Experimental study of sulfonate Gemini surfactants as thickeners for clean fracturing fluids. Energies 2018 , 11 , 3182. [CrossRef] 18. Chen, W.; Zhang, Z.; Liu, Q.; Chen, X.; Opoku Appau, P.; Wang, F. Experimental investigation of oil recovery from tight sandstone oil reservoirs by pressure depletion. Energies 2018 , 11 , 2667. [CrossRef] 19. Lyu, C.; Wang, Q.; Ning, Z.; Chen, M.; Li, M.; Chen, Z.; Xia, Y. Investigation on the application of NMR to spontaneous imbibition recovery of tight sandstones: An experimental study. Energies 2018 , 11 , 2359. [CrossRef] 20. 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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http: // creativecommons.org / licenses / by / 4.0 / ). 5 energies Article Characterization and Consecutive Prediction of Pore Structures in Tight Oil Reservoirs Zhaohui Xu 1, *, Peiqiang Zhao 2 , Zhenlin Wang 3 , Mehdi Ostadhassan 4 and Zhonghua Pan 5 1 College of Geosciences, China University of Petroleum, Beijing 102249, China 2 Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, China; zhaopq@cug.edu.cn 3 Research Institute of Exploration and Development, Xinjiang Oilfield Company, PetroChina, Karamay 834000, China; wzhenl@petrochina.com.cn 4 Petroleum Engineering Department, University of North Dakota, Grand Forks, ND 58202, USA; mehdi.ostadhassan@und.edu 5 Wuhan Geomatic Institute, Wuhan 430022, China; cugpzh@outlook.com * Correspondence: xuzhaohui@cup.edu.cn; Tel.: +86-187-0138-0799 Received: 28 September 2018; Accepted: 9 October 2018; Published: 11 October 2018 Abstract: The Lucaogou Formation in Jimuaser Sag of Junggar Basin, China is a typical tight oil reservoir with upper and lower sweet spots. However, the pore structure of this formation has not been studied thoroughly due to limited core analysis data. In this paper, the pore structures of the Lucaogou Formation were characterized, and a new method applicable to oil-wet rocks was verified and used to consecutively predict pore structures by nuclear magnetic resonance (NMR) logs. To do so, a set of experiments including X-ray diffraction (XRD), mercury intrusion capillary pressure (MICP), scanning electron microscopy (SEM) and NMR measurements were conducted. First, SEM images showed that pore types are mainly intragranular dissolution, intergranular dissolution, micro fractures and clay pores. Then, capillary pressure curves were divided into three types (I, II and III). The pores associated with type I and III are mainly dissolution and clay pores, respectively. Next, the new method was verified by “as received” and water-saturated condition T 2 distributions of two samples. Finally, consecutive prediction in fourteen wells demonstrated that the pores of this formation are dominated by nano-scale pores and the pore structure of the lower sweet spot reservoir is more complicated than that in upper sweet spot reservoir. Keywords: Lucaogou Formation; tight oil; pore structure; prediction by NMR logs 1. Introduction As a major unconventional resource, tight oil reservoirs have received significant attention for exploration and development all around the world [1–3]. Tight oil reservoirs are complex and highly heterogeneous, generally characterized by low porosity and ultra-low permeability [ 4 , 5 ]. Single wells have no natural production capacity, which requires horizontal drilling and hydraulic fracturing to obtain economic flow [ 5 – 8 ]. It is necessary to evaluate various properties of such reservoirs for a better exploitation of the resources. However, macroscopic petrophysical parameters such as porosity, permeability, and saturation cannot satisfy adequate evaluation of the effectiveness of tight oil reservoirs. In this regard, pore structures, in particular determine reservoir storage capacity and control rock transportation characteristics, represent microscopic properties of the rock [ 9 – 12 ]. Therefore, characterization and consecutive prediction of rock pore structure in wells is a key task in the study of tight oil reservoirs. The Permian Lucaogou Formation of Jimusaer Sag, Junggar Basin, China is a typical tight oil reservoir which has been studied previously in terms of the pore structures. Kuang et al. [ 13 ], Energies 2018 , 11 , 2705; doi:10.3390/en11102705 www.mdpi.com/journal/energies 6 Energies 2018 , 11 , 2705 Zhang et al. [ 14 ], Zhou [ 15 ] and Su et al. [ 16 ] used diverse imaging techniques such as CT-scanning, SEM and FIB-SEM image analysis to qualitatively characterize the pore structures. They concluded that pore types include organic matter pores, mineral pores, inter-crystalline pore, dissolved pores, and micro cracks. Zhao et al. [ 17 ] presented that the median capillary radius of this reservoir ranges from 0.0063 to 0.148 μ m with an average of 0.039 μ m. Zhao et al. [ 18 ] studied the complexity and heterogeneity of pore structures based on multifractal characteristics of nuclear magnetic resonance (NMR) transverse relaxation ( T 2 ) distributions. Wang et al. [ 19 ] investigated pore size distributions and fractal characteristics of this formation by combining high pressure and constant rate mercury injection data. However, the limited number of core samples could not reflect general properties of this formation. The NMR logging which is consecutively recording the vertical variations of transverse relaxation time can reveal pore distributions and is widely used to overcome the discrete data points that core sample analysis owns. Researchers have conducted extensive studies on the construction of mercury intrusion capillary pressure curves by NMR T 2 distributions obtained in laboratory [ 20 – 27 ]. The pore structure evaluation methods by NMR technique are based on the fact the rocks are water-saturated and hydrophilic. However, in oil reservoirs, it is necessary to correct the effect of hydrocarbons on T 2 spectra of NMR logging. Volokin and Looyedtijn [ 22 , 23 ] first studied the morphological correction of T 2 spectra of NMR logging in hydrocarbon-bearing rocks. The basic idea is that the bound water of the T 2 distribution is constant, and hydrocarbon would only affect the free fluid portion of the T 2 distribution. Therefore, when performing a hydrocarbon-containing correction on the T 2 distribution, it is only required to correct the T 2 signal of the free fluid portion and remain the bound fluid of T 2 signal intact. Xiao et al. [ 28 ] established a method for constructing capillary pressure curves based on J function and Schlumberger Doll Research (SDR) model. This method used T 2 logarithmic mean value ( T 2 lm ) as an input parameter, which makes it possible for the correction of T 2 distributions regarding hydrocarbons. This is possible because T 2 lm can be calibrated by core values. Hu et al. [ 29 ] proposed a novel method for hydrocarbon corrections where T 2 distribution measured by short echo time ( T E ) was used to construct the T 2 distribution under full-water conditions with long T E time. The difference between the measured and constructed water-saturated state T 2 distributions determines the oil signal and the water signal, thereby the correction of the hydrocarbon-containing state T 2 distribution would become achievable. Ge et al. [ 30 ] proposed a correction method through extracting oil signals from the echoes, which has been already applied to carbonate reservoirs. Xiao et al. [ 31 ] proposed a method to remove the effect of hydrocarbons on NMR T 2 response based on a point-by-point calibration method. However, the application of these methods would be challenging when the wettability of the reservoir appears to be oleophilic or neutral. This is because the bulk transversal relaxation time could not be ignored according to NMR relaxation mechanism [ 32 – 34 ]. Zhao [ 35 ] proposed a new method for evaluating pore structures of reservoirs with neutral wettability and oil-wetting characteristics, but the method is not firmly verified. In this research, the major objectives are to: (a) characterize the pore structures by MICP data and SEM images; (b) further confirm the Zhao method [35] by “as-received” and water saturated state T 2 distributions; and finally (c) predict the global features of pore structures via field NMR logs. 2. Methods 2.1. Samples and Experiments Samples were drilled from the Permian Lucaogou Formation in Jimusar Sag, Junggar Basin. The Junggar Basin is the second largest inland basin in China, which is located in north of the Xinjiang Province, Northwest China. The Jimusaer sag is structurally located in the eastern uplift of the Junggar Basin, adjacent to the Fukang Fault in the south, and the Santai Oilfield and the North Santai Oilfield in the west [ 36 ]. The Permian system is the main source rock strata in the Junggar Basin. The target Lucaogou Forma