Arabian Journal of Geosciences (2019) 12:785 https://doi.org/10.1007/s12517-019-4808-z ORIGINAL PAPER Magnet design of nuclear magnetic resonance system for well logging and geoscience applications B. Asadi 1 · M. Mohamadian 1 · V. Esmaeili Sani 1 · H. Afarideh 1 · Z. Sadeghdoust 1 Received: 19 October 2018 / Accepted: 5 September 2019 © Saudi Society for Geosciences 2019 Abstract Nuclear magnetic resonance measurement (NMR) is an important technology to assess the characteristics of oil reservoirs which provides information on the type of fluid in the formation. The analysis by logging tool is based on using relaxation time measurements to understand the structure of the reservoir. In this research, the Computer Simulation Technology (CST) software is used to design a typical permanent magnet for NMR device and simulate it for different types of materials and different dimensions. The effects of temperature inside the well along with the magnetic response of the magnet which is a very important and effective parameter in well logging were considered. In addition, the effects of the cement wall on the field were studied. Due to the fact that the relative magnetic coefficient of ferrite material is in the range of 1.45 to 1.65, the effects of this parameter in the field of magnets have been investigated. Increasing the relative magnetic permeability coefficient reduces the amount of magnetic field in the considered depth. The influence of the magnet radius in the produced field has also been analyzed which indicates that, with a 6-inch diameter device, the penetration depth was greater than that using a similar 4.5-inch tool, thus making it an efficient tool for investigating the depth of the formation. Keywords NMR · Permanent magnet · Relaxation time · Static magnetic field · Well-logging tools Introduction Different techniques exist for studying the composition of the earth in different areas and for different applications. In geological topics, finding specific alloys in mines or deep underground is important (Kaya et al. 2018). In some cases, the layout of the different layers of the earth and its material is crucial (Cetin 2013). To study the environmental effects of different materials and how they return to nature and get recycled, these techniques are used to analyze natural formations (Cetin 2015). Meanwhile, in exploration of oil and gas, it is important to recognize the layers of the earth and to find deep ground oil and gas reservoirs. In such explorations, using various material identification methods, they look for hydrocarbons, which are the main constituents of petroleum products. The size and porosity of identified Responsible Editor: Abdullah M. Al-Amri  M. Mohamadian mohamadian@aut.ac.ir 1 Energy Engineering and Physics Department, Amirkabir University, Tehran, Iran hydrocarbons need to be determined so as to specify the size of oil reservoirs in deep ground (Chen et al. 2019; Aqel 2016). It is also important to estimate the saturation of these materials (Huang et al. 2015). Each method of extracting earth formation attributes can only specify one or more limited properties of that structure. Therefore, it is sometimes necessary to use several methods to ensure the discovery of an oil reservoir. One of these techniques is nuclear magnetic resonance (NMR). NMR is attributed to a physical principle, namely the nuclei response to the magnetic field. The quantities measured in NMR logging include amplitude and signal loss. The amplitude of the NMR signal is proportional to the volume of available hydrogen and is calibrated to obtain porosity. This porosity is devoid of the effects of radioactive sources and lithology. However, the deterioration of the NMR signal provides the best information on petro-physical properties throughout each measurement cycle (called the relaxation time). The relaxation time depends on the pore size. For example, small porous spaces reduce the relaxation time and the shortest relaxation time is related to the water adhering to the clay and matrix surfaces. Large porous spaces have a long relaxation time and contain fluids that are easy to