Contents lists available at ScienceDirect Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol Wettability characterization of low-permeability reservoirs using nuclear magnetic resonance: An experimental study Can Liang a , Lizhi Xiao a,b,* , Cancan Zhou c , Hua Wang d , Falong Hu c , Guangzhi Liao a , Zijian Jia a , Huabing Liu e a State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China b Harvard SEAS-CUPB Joint Laboratory on Petroleum Science, 29 Oxford Street, Cambridge, MA 02138, USA c Research Institute of Petroleum Exploration and Development, Beijing 100083, China d Earth Resources Laboratory, Massachusetts Institute of Technology,Cambridge, MA, 02139, USA e Beijing Limecho Technology Co., Ltd, China ARTICLE INFO Keywords: Wettability NMR relaxation Low-permeability reservoir Aging Reservoir type ABSTRACT Influence of wettability to petrophysical responses in low permeability reservoirs is increasingly evident, but research of nuclear magnetic resonance (NMR) technology on wettability calibration in such reservoirs is very limited even though it has been used promisingly. The challenge is that the empirical interpretation model only applies to simple pore structure. It is urgent to develop a relatively simple, quick, and practicable model for wettability evaluation in low permeable reservoirs. This paper presents an experimental NMR study, integrating with Amott tests, X-ray diffraction, and scanning electron microscope (SEM) measurements, on wettability characterization of five typical core samples from different low-permeability reservoirs. NMR relaxation mechanisms and the effect of the aging were analyzed in detail. The study showed that all the core samples had a shift of T 2 (spin-spin relaxation time) spectra between before and after aging. According to the shift characteristics, the core samples were divided into two types. The first type is that the wettability alteration relied on the aging and it displaced crude oil into original water-wet movable pore spaces to restore reservoir wettability. This type of formation is generated in distant-source re- servoirs, which mainly involves fluid transport through buoyancy. Another type is that the wettability alteration was attributed to the natural oil-wet property of the rock matrix. This formation is mainly found in inner-source or near-source reservoirs, which may generate abundant oil-wet materials. Thus, the wettability alteration is the result of hydrocarbon accumulation modes. Based on the observations, a potential method for obtaining the wettability index was proposed. The method uses both the ratio of oil-wetted to water-wetted pore surface areas and diffusion-relaxation diagrams. The estimated results corresponded reasonably to the independent Amott tests. This study demonstrates that NMR relaxation is an effective solution to analyze and quantify wettability in low permeability reservoirs. 1. Introduction Tight formations with low permeability have been actively studied worldwide due to their abundant hydrocarbon resources. Low perme- able tight reservoirs include the low permeability reservoirs (matrix permeability of 0.1–10 mD) and tight reservoirs (permeability less than 0.1 mD) according to the latest criteria (National Energy Administration, 2011; Hu, 2009). The characterization and modeling of the petrophysical properties are the primary key objectives for the re- serves estimation and reservoir engineering calculations during the development of such reservoirs. In particular, wettability character- ization is an important parameter as it is directly linked to oil recovery (Abdallah et al., 2007). Wettability refers to the tendency of a fluid to spread or adhere to a solid surface during the immiscible phase (Tiab and Donaldson, 2004). Reservoir rock wettability is a vital factor as it affects the microscopic distribution of pore fluids and determines oil recovery efficiency (Anderson, 1986). For reservoir crude oil/brine/rock (COBR) system, wettability can be divided into two types: homogeneous and hetero- geneous. Homogeneous wettability refers to strongly water-wet and https://doi.org/10.1016/j.petrol.2019.03.014 Received 4 January 2018; Received in revised form 9 December 2018; Accepted 5 March 2019 * Corresponding author. College of Geophysics and Information Engineering, China University of Petroleum (Beijing), 18 Fuxue Road, Changping, Beijing, 102249, China. ; E-mail addresses: xiaolizhi@cup.edu.cn, lizhi_xiao@fas.harvard.edu (L. Xiao). Journal of Petroleum Science and Engineering 178 (2019) 121–132 Available online 12 March 2019 0920-4105/ © 2019 Published by Elsevier B.V. T