Scale-span pore structure heterogeneity of high volatile bituminous coal and anthracite by FIB-SEM and X-ray μ-CT

Visualizing and quantifying the pore structure at the nano-micro scale is critical for understanding the micro fluid transport and enrichment in coalbed methane (CBM) reservoirs. In this work, the detailed micro-nano scale pore parameters such as pore counts, pore area, pore volume and pore size distribution can be acquired by the focused ion beam-scanning electron microscopy (FIB-SEM) and X-ray computed micro-tomography (X-ray μ-CT) techniques. Meanwhile, the pore network model (PNM) was adopted to describe and quantify the pore throat characteristics, which found that the pore throats of the sample LHG well developed and were conducive to seepage. Additionally, the three-dimensional fractal dimension (D ₃) by the box-counting method was used to evaluate the pores spatial heterogeneity. The D ₃ of sample LHG and sample L-1 are 2.23 and 2.04 (for FIB-SEM with pore size of 10 nm–~1000 nm), 2.69 and 2.51 (for X-ray μCT with pore size over 500 nm), respectively. The results indicate that the pore network has self-similarity with a secondary development. The variable trends from tens of nanometers to micrometers through the FIB-SEM and X-ray μCT images. For the relationship between porosity and D ₃, two opposite trends have emerged. The positive correlation trend should be related to the complex pore structure. The more complex the pore structure is, the higher the porosity is. The negative correlation should be contributed by a lot of mineral-filled pores. Pores filled with minerals that will increase the proportion of small pores and decrease the porosity, which causes that the spatial complexity of the pore networks is increased, and the D ₃ is increased. Therefore, this work may provide insights into the gas storage and seepage capabilities of CBM reservoirs, and thus will be favorable for enhancing CBM recovery.