A method to classify coal pore system by using cumulative amplitude ratio and its dynamic variation

As coal pore development is decisive for choosing the engineering site and predicting the CO₂ storage capacity, this paper provides a new method to define the double T₂ cutoff values by using cumulative amplitude ratio measured by nuclear magnetic resonance measurements, classifies the coal pore systems, and analyzes the influences on cumulative amplitude ratio. The following cognitions are achieved. The minimum ratio always varies narrowly and ranges from 0.9 to 1.1, which is quite stable and approximately equals to 1. Ranges of maximum and average ratios are 1.2–3.5 and 1.1–1.8, respectively. T_2c1 represents the dividing point of diffusion pore and permeation pore, and its average value is about 4.1 ms. T_2c2 represents the dividing point of permeation pore and cleat, with an average value of about 81.9 ms. The volumetric proportions of diffusion pore range from 1.5% to 76.2%, with an average value of 34.6%; the volumetric proportions of permeation pore are from 14.9% to 98.5%, with an average of 46.8%; while the volumetric proportions of cleat are between 8.4% and 57.5%, with an average of 26.6%. According to the different influencing degrees on maximum and average ratios, three types of parameters can be divided. The first type is strong correlation parameters and includes permeability, volumetric percentage of cleat, and relative volumetric percentage of cleat. The second type is medium correlation parameters, such as volumetric percentage of diffusion pore. The third type is weak correlation parameters, including T₂ cutoff values, porosity, and maximum vitrinite reflectance. © 2023 Society of Chemical Industry and John Wiley & Sons, Ltd.