# Experimental Analysis of Pore and Permeability Characteristics of Coal by Low-Field NMR
## 1 Introduction  
Coalbed methane (CBM), as a type of clean and efficient unconventional energy, is mainly adsorbed in the coal matrix micropores. A coal reservoir is composed of the coal matrix, various scales, and morphological pores and fissures. To describe the migration and output of CBM, some scholars have put forward the triple-porosity/dual-permeability geological model and divided the pores and fissures of coal into adsorption pores, diffusion pores, seepage pores, and fissures. It is generally accepted that the CBM is mainly adsorbed on the micropores surface, diffuses through the mesopores, then flows continuously to the fissures through macropores and micro-fissures, and finally gets to the well bore in the CBM production [1]. The fine characterization of the pore structure plays an important role in coal reservoir research and CBM productivity evaluation. For coal, as a typical porous material, there are many test methods for studying the pore structure. They are divided into three types: fluid injection, image observation, and physical detection. Fluid injection is to study the structure information, such as volume, specific surface area, and fractal dimension of pores in different scales, in coal with fluid molecules, in which mercury intrusion porosimetry (MIP) and gas adsorption are the most commonly usedmethods [2; 3]. Image observation is to observe the morphology, size, and distribution of the pores in coal using an optical microscope (OM), scanning electron microscopy (SEM), and transmission electron microscope (TEM) [4; 5]. Physical detection is to analyze and recognize three-dimensional pore structures in coal by receiving the feedback signal of the external magnetic field or ray, including nuclear magnetic resonance (NMR), computed tomography (CT), and small angle X-ray scattering (SAXS) [6; 7; 8]. Fluid injection can only detect open pores, ignoring the closed pores information. Image observation has higher requirements for sample pretreatment, and the statistical data of the micro-local pores structure cannot fully represent the pores structure parameters in the whole coal. The above two methods generally use powdered or small bulk coal samples, which can only obtain the distribution information of pores smaller than the sample size. However, NMR has the advantages of fast, convenient, non-destructive, and multi-scale pore information representation, and it can be used to analyze the pores in different scale samples, such as laboratory samples and strata in drilling. At first, NMR was used to analyze the fluid composition and pore structure of the conventional reservoir and to determine the reservoir permeability, which was used to analyze the physical parameters of the coal reservoir in recent years [9; 10]. NMR usually uses the transverse relaxation time (\(T_{2}\)) to analyze the pore size distribution (PSD). The pores corresponding to the \(T_{2}\) peaks are divided into adsorption pores, seepage pores, and fissures by [PERSON] et al. [11] and [PERSON] et al. [12]. They think that the patterns of unimodal, bimodal, and multi-peak in the \(T_{2}\) spectra represent different pore types and PSD, and the pore characteristics of NMR were compared with those of MIP and CT scanning [13; 14]. Most of the previous research calculates the PSD and permeability based on the inversion analysis of the NMR signals of the saturated and centrifugal coal samples. The structure, distribution, and connectivity of pores in different mining areas and in different ranks of coal are different, and the centrifugal force, \(T_{2}\) cutoff value (\(T_{2\text{C}}\)), and its corresponding size in centrifuge experiments with different coal samples are also different. Therefore, the PSD of different coal samples cannot be calculated according to the constant \(T_{2\text{C}}\). The movable fluid porosity cannot be obtained directly by using the \(T_{2}\) spectra in the logging process.