# Study on Magnetic Resonance Characteristics of Coal Sample under Progressive Loads
## 4 Testing Results and Analysis
### Pore Size Distribution  
Pores and fissures of various sizes coexist in coal [37; 38]. The evolution law of porosity under the loading is in essence the law of reciprocal transformation of pores and fissures. [PERSON] et al. [39] observed that the pores in coal can be divided into three types: micro-pores (<0.1 \(\upmu\)m), which are \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{Group C} & \multicolumn{6}{c}{**Porosity (\%)**} \\ \cline{2-7}  & NMR-0 MPa & NMR-3 MPa & NMR-5 MPa & NMR-7 MPa & NMR-9 MPa & NMR-11 MPa \\ \hline C-1 & 16.8470 & 16.5783 & 16.6502 & 16.7930 & 17.0519 & 14.1662 \\ C-2 & 16.8593 & 16.5165 & / & 16.6819 & / & / \\ C-3 & 17.5788 & 17.2258 & / & / & / & 17.8763 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Porosity of the coal samples in Group C under different loads. Figure 8: Porosity evolution of the coal samples in Group C under progressive loads. also called adsorption pores (AP); mesopores and macropores (>0.1 \(\upmu\)m, <100 \(\upmu\)m), which are called percolation pores (PP); and crack pores (>100 \(\upmu\)m) (CP). The pore volume ratio of different sizes is obtained by NMR, and then according to the classification standard of the reference [39], the volume ratio of corresponding pores (AP, PP, and CP) are respectively accumulated and obtained. Figure 9 shows the evolution law of pores and fissures of different sizes under progressive loading. From Figure 9a it can be known that AP, PP, and CP all develop in coal sample C-1 under different loads. To be specific, AP develops much better than the other two while CP performs worst in terms of development. When there is no loading, the pore volume ratio of AP is 11.5996%, and that of PP is 5.2398%. When the load reaches 3 MPa, the coal sample is in the compaction stage; the rapid closing of some pores and fissures leads to the volume decline of PP and CP, which justifies the slight rise of pore volume ratio of AP. As the load further increases, the pore volume ratio of AP falls gradually while that of PP rises correspondingly. Since splitting occurs when the load on C-1 reaches 11 MPa, errors would be unavoidable in the data on porosity; in this case, the data collected are not qualified to be used for reference. When the load is 3 MPa, 5 MPa, 7 MPa, and 9 MPa, respectively, the change of pores and fissures of different sizes varies remarkably. To be specific, the pore volume of AP falls while that of PP rises; under progressive loading, AP tends to transform to PP. Meanwhile, PP changes much more noticeably than AP does, which further reveals the rapid expansion of fissures in the latter part of elastic stage. Figure 9b shows the evolution characteristics of pores and fissures in C-2. Compared with that of C-1, AP in C-2 is more condensed and the pore volume ratio of PP is lower. Before loading, the pore volume ratio of AP is 13.5452% and that of PP is 3.3071%. AP and PP take up 80.3% and 19.6% of the total volume, respectively. When the load reaches 3 MPa, the coal sample is in the compaction stage; AP and PP take up 83.6% and 16.4% of the total volume, respectively. When the load reaches  
Figure 9: Evolution law of different pores and fissures of Group C coal samples under progressive load: (a) Sample C-1, (b) Sample C-2, (c) Sample C-3. 7 MPa, the coal sample evolves into the elastic stage; AP and PP take up 76.3% and 23.2% of the total volume, respectively. The change of pores and fissures in coal sample C-2 is similar to that of C-1: the volume ratio of AP increases in the compaction stage, whereas that of PP increases in the elastic stage. Figure 9c displays the evolution characteristics of pores and fissures in C-3. When the load is 0 MPa, 3 MPa, and 11 MPa, AP takes up 73.8%, 80.2%, and 71.2% of the total volume, correspondingly, and PP takes up 18.8%, 15.7%, and 21.7% of the total volume, respectively.