Contents lists available at ScienceDirect Magnetic Resonance Imaging journal homepage: www.elsevier.com/locate/mri Original contribution The downhole circumferential scanning magnetic resonance imaging tool Wei Liu a , Lizhi Xiao a,b, ⁎ , Guangzhi Liao a , Yan Zhang a , Sihui Luo a 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 ABSTRACT The downhole circumferential scanning magnetic resonance logging is able to image saturation distribution and fluid properties of stratum around a borehole, thus providing relevant and abundant information for formation evaluation. The device employs a phase-controlled excitation device based on combined array structure to accomplish three dimensional data acquisition from axial, radial and circumferential directions. This paper focuses on the design principle of device and the structure of electronic control system. A mutual coupling analysis with array antenna was carried out using inductance coupling principle, and realize the decoupling and energy discharge compensation of array antennas. The circumferential scanning nuclear magnetic resonance technique has a potential of overcoming the weakness of two dimensional measurements and raising new applications that it determines the azimuth of the fluid in the borehole and realizes the imaging measurement of the pore structure and the reservoir fluid. 1. Introduction With the increasing difficulty of exploration, conventional logging technology has been unable to meet the needs of accurate oil and gas evaluation, especially in the evaluation of complex oil and gas re- servoirs such as low porosity, low permeability, low resistivity and unconventional reservoirs [1]. Nuclear magnetic resonance (NMR) logging as one of Imaging logging technology can provide important rock physical parameters such as porosity, bound water, permeability and pore size distribution, and can effectively identify and evaluate the fluid properties and so on. It is widely used in the comprehensive evaluation of complex oil and gas reservoirs such as complex lithology, special lithology, low porosity and low permeability and low resistivity. The nuclear magnetic resonance imaging (MRI) logging has been developing rapidly for decades, and it is still the hot topic of oil and gas exploration technology [2]. For example, relaxation and diffusion methods from NMR logging can be used to obtain porosity, fluid properties, flow properties and saturation distribution of stratum. NMR logging mainly includes the following three parts. First is hardware which is refers to magnet structure, antennas, electronics and surface system. The second is the method which includes pulse sequences and data processing. The third is the explanation of data from which we can acquire information about fluids and pore structure in porous media. The NMR logging signal detected in down hole NMR probe is from hydrogen nuclei of pore fluids, which are located several kilometers underground. So the logging instruments work in a harsh environment where it has the high temperature and high pressure. The size of in- struments is also restricted, and the motion effect of probe complex the measurement. This means that NMR logging measurement needs high reliable and efficient instruments. At present, several obvious devel- oping trends have been formed: ➢ Modularized probe magnet structure ➢ Array antenna structure ➢ Exploration space azimuth ➢ Multi-Parameterization of detecting object attributes However, with the exploration objects from conventional reservoirs to deep stratum, deep water and unconventional oil and gas reservoirs, higher requirements have been put forward for downhole oil and gas exploration technology. The existing two-dimensional nuclear magnetic resonance instruments do not have the resolution of circumferential reservoir properties. At present, nuclear magnetic resonance logging technology needs to be continuously upgraded and replaced. Therefore, on the basis of the existing technology, we designed a novel cir- cumferential scanning magnetic resonance imaging (CSMRI) logging tool, which has the feature of centric and eccentric instruments to realize the performance of both scope of 360-degree detection round the borehole and sectional region detection. The development of this technology will bring an important change for downhole oil and gas exploration technology. It truly has a potential of achieving three-di- mensional magnetic resonance imaging for complex reservoirs. The main focus of this paper is on the design and the structure of the electronic control system and shows how to overcome the technical challenges of decoupling array antennas and switching the radio fre- quency quickly. https://doi.org/10.1016/j.mri.2018.09.031 Received 31 July 2018; Received in revised form 26 September 2018; Accepted 26 September 2018 ⁎ Corresponding author at: State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China. E-mail addresses: lizhi_xiao@fas.harvard.edu, xiaolizhi@cup.edu.cn (L. Xiao). Magnetic Resonance Imaging xxx (xxxx) xxx–xxx 0730-725X/ © 2018 Elsevier Inc. All rights reserved. Please cite this article as: Liu, W., Magnetic Resonance Imaging, https://doi.org/10.1016/j.mri.2018.09.031