# Pore Structure of Coals by Mercury Intrusion, N\({}_{2}\) Adsorption and NMR: A Comparative Study
## 3 Results  
### Determination of Pore Structure by Mercury Intrusion  
The plotted curves and pore size distribution (PSD) characteristics corresponding to 16 coal samples can be divided into four pore categories as shown in Figure 1. Type I is represented by coal sample BS, shown in Figure 1a. Curve characteristics: This kind of coal has high mercury saturation and the mercury injection process is continuous. The pores are dominated by macropores and mesopores; the micropores are less developed and the mean value of pore diameter is relatively large; and the low efficiency of mercury-ejection represents the poor pore connectivity. Type II is represented by the coal samples TB, SW, TA, FL, and BD, shown in Figure 1b. Curve characteristics: the mercury saturation is not high and the porosity saturated by mercury is between 20% to 50%. Under the pressure less than 100 MPa, the amount of mercury into the interval rises with increasing the pressure. When the pressure is larger than 10 MPa, there is a small platform segment indicating that the pores are mainly microporous, with a small number of macropores and mesopores, in which the content of mesopores is greater than that of macropores, and the content of micropores is greater than or slightly greater than that of transition pores. The efficiency of mercury ejection is relatively high, basically reaching more than 50%, indicating better porosity connectivity. Because of the fracture development of coal samples FL and BD, a large amount of mercury has entered when the pressure is below 0.1 MPa, causing the high cumulative mercury saturation. Type III is represented by the coal samples SL,WJY,TL,DEP, and WK, shown in Figure 1c. Curve characteristics: The integrated intrusion mercury saturation is not high, and the porosity saturated by mercury is 20% to 30%. Similar to type II, the interval mercury saturation has increased with increasing pressure, and the difference is that the amount of mercury injection between 0.1 and 1 MPa is very low, indicating that the pore development is dominated by transition pores and mesopores, while the macropores are not very developed or basically not developed. Among the five samples, the final cumulative mercury saturation of samples DEP and WK are slightly higher than that of the others because of fracture development. Type IV is represented by the coal samples QLT,HS,DP,CYH, and SK, shown in Figure 1d. Curve characteristics: the cumulative mercury intrusion is less than 20%. When the pressure is less than 100 MPa, the amount of mercury intrusion is very low, indicating that the pore development is dominated by micropores, while the other pores are basically undeveloped. The mercury ejection curve is closed to the mercury intrusion curve. The efficiency of mercury ejection is low and the porosity connectivity is poor. Among this kind of coal, due to the development of fractures in QLT, a large amount of mercury entered into fractures at the beginning of mercury intrusion, showing the high cumulative mercury saturation.  
Based on the above four kinds of mercury curve type, the characteristics are substantially as follows: with the increase of \(\rm{R_{o}}\), the mercury intrusion curves are gradually transformed from type I to type II, Type III, and Type IV. In addition, the pore connectivity is becoming worse.