Accurate characterization method of pores and various minerals in coal based on CT scanning

Pores, fractures, and mineral compositions in coal have a significant impact on the mechanical properties, seepage characteristics, and water–rock interaction of coal. This paper proposes a method to determine the threshold value of various mineral components and pores inside the coal sample through computed tomography (CT) scanning, X-ray diffractometer (XRD) test and nuclear magnetic resonance (NMR) technology. This method determines the porosity of the coal by NMR, and then determines the pore reconstruction threshold. The type and composition of coal sample minerals were determined by XRD. Combined with the difference in the density of each mineral, the classification threshold of each type of mineral can be determined. We first determined the representative elementary volume through pore and mineral reconstruction of coal with different sizes. It was found that when the edge length of the selected cube is greater than 140 pixels (5 mm) and less than 400 pixels (13.6 mm), the pore threshold is stable. While for the mineral reconstruction, the range of threshold stability is 140 pixels (5 mm) to 580 pixels (20.7 mm). On this basis, we selected three cubes with a side length of 300 pixels and different positions for pore and mineral reconstruction. The reconstruction results show that the coal sample measurement area has pore and mineral composition thresholds of 19804 and 24533, respectively. The pores in coal samples can be divided into connected pores and isolated pores. The equivalent diameter and shape factor of a connected pores are much larger than any isolated pore. The shape of the connected pores is close to a circle or a square. The shapes of isolated pores are close to needles. The threshold values of amorphous, feldspar, quartz, illite and chlorite are 24533, 25740, 25927, 26826 and 28748, respectively. The three-dimensional characterization results show that illite and chlorite are distributed in flakes, and the rest of the minerals are distributed in granular form. The extraction of different mineral components is helpful for the research on the evolution of various minerals before and after water immersion.