## 4 Results and Discussion
### Application of the Adsorption Characteristic Curve  
A previous study [33] has shown that the methane adsorption characteristic curve for a specific coal sample is unique. The absolute adsorption capacity at any temperature and pressure can be calculated based on the equation of the methane adsorption potential characteristic curve. In the present study, the methane isotherm adsorption curves of the four coals at 45, 60, and 75 \({}^{\circ}\)C were predicted (Figure 5).  
As shown in Figure 5, the adsorption capacity of methane on coal continuously weakened with the increase in temperature, which is completely consistent with previous studies. Meanwhile, it was found that predicting the adsorption isotherm curve at high temperatures still followed the Langmuir model. Then, the Langmuir constants were calculated using the Langmuir model based on the predicted results of the amounts of methane adsorbed under certain pressure values (Table 3).  
Figure 5: Measured and predicted methane isotherms at different temperatures: (**a**) XW1; (**b**) XW2; (**c**) WH; (**d**) ZK.  
Meanwhile, the data on ground temperature and pressure at different burial depths were obtained based on the measured geological parameters of the ground temperature gradient and pressure gradient in the southern Sichuan region. In this study, the geothermal gradient of 2.45 \({}^{\circ}\)C/100 m and the pressure gradient of 0.98 MPa/100 m were used. The volumes of methane adsorbed on coals were obtained as mentioned earlier. Finally, the relationship between the volume of methane adsorbed on coal and the burial depth of the coal seam was obtained (Figure 6).  
As shown in Figure 6, the amount of methane adsorbed by coal first increased rapidly with the increase in burial depth, tended to plateau to reach its maximum value at a depth  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **T ( \({}^{\circ}\)C)** & \(V_{\text{L}}\) (cm\({}^{3}\)/g)** & \(P_{\text{L}}\) (MPa)** & \(\mathbf{R^{2}}\) \\ \hline \multirow{3}{*}{XW1} & 45 & 29.79 & 1.38 & 0.9999 \\  & 60 & 29.28 & 1.74 & 0.9989 \\  & 75 & 29.20 & 2.05 & 0.9985 \\ \hline \multirow{3}{*}{XW2} & 45 & 32.88 & 1.61 & 0.9998 \\  & 60 & 32.66 & 2.12 & 0.9990 \\  & 75 & 32.53 & 2.50 & 0.9980 \\ \hline \multirow{3}{*}{WH} & 45 & 33.89 & 1.55 & 0.9996 \\  & 60 & 33.42 & 1.96 & 0.9992 \\  & 75 & 32.94 & 2.25 & 0.9988 \\ \hline \multirow{3}{*}{ZK} & 45 & 26.21 & 1.58 & 0.9993 \\  & 60 & 25.80 & 2.00 & 0.9992 \\ \cline{1-1}  & 75 & 25.24 & 2.20 & 0.9985 \\ \hline \hline \end{tabular} Note: Dry basis.  
\end{table}
Table 3: The prediction results of the methane isothermal adsorption on the samples at different temperatures.  
Figure 6: Predicted relationship between the methane adsorption capacity and the burial depth.  
of approximately 1400 m, and then slowly decreased with a further increase in burial depth. This indicates that the influence of pressure was more important in methane adsorption than that of temperature in shallow coal seams. Furthermore, with the increase in burial depth, the control effect of the temperature on the adsorption capacity was more significant. The predicted results are consistent with the methane adsorption characteristics of the deep coal reservoirs [14]. Due to the small geothermal gradient, the negative effect of the adsorption temperature on adsorption does not fluctuate significantly with the change in burial depth. This is the difference in the adsorption characteristics of deep coalbed methane in the southern Sichuan region and certain other regions.