## 3 Cyclic Depressurization Simulation Study
### Bottomhole Pressure during Primary depressurization Stage  
The bottomhole pressure during the primary depressurization was varied (6, 9 and 12 MPa). The case of the lower bottomhole pressure caused a high gas production rate and cumulative gas production (Figure 9). The gas-production rate of all cases steadily increased until 60 days, and it maintained a certain level, with the exclusion of the 6 MPa case (Figure 9a). The gas-production rate of the 6 MPa case consistently decreased after 60 days, and that of the 12 MPa case was very low compared to other cases. The cumulative gas production was between 1.64 \(\times\) 10\({}^{4}\) m\({}^{3}\) (in the case of 12 MPa) and 7.47 \(\times\) 10\({}^{5}\) m\({}^{3}\) (in the case of 6 MPa) (Figure 9b). There was little difference in the cumulative gas production between the non-cyclic case and the 9 MPa case; the difference was only 12.1%. Specially, the cumulative gas production of the 6 MPa case was three times higher than that of the 9 MPa case. During the secondary pressure stage, a small amount of gas production cyclically decreased in all cases because of the relatively high bottomhole pressure condition.  
Figure 10 represents the vertical subsidence at the top of the HBS. The amount of vertical displacement increased according to the production time. In the case of low bottomhole pressure, the amount of vertical displacement was high due to the low pore pressure; the range of vertical displacement was from \(-\)1.09 m (in the case of 12 MPa) to \(-\)2.39 m (in the case of 6 MPa). From the aforementioned cumulative gas-production results, we confirmed that high cumulative gas production resulted in a high amount of vertical subsidence. In all cases, the amount of vertical displacement increased during the primary  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No.** & **Stage** & **Property** & \multicolumn{3}{c}{**Value**} \\ \hline \multirow{4}{*}{Case study 3 (Figures 13 and 14)} & Primary production stage & Bottomhole pressure (MPa) & 9 & 9 & 9 & 9 \\ \cline{2-6}  & Primary production stage & Production time (days) & 8 & 8 & 8 & 8 \\ \cline{2-6}  & Secondary production stage & Bottomhole pressure (MPa) & 12 & 16 & 20 & Shut-in \\ \hline \multirow{4}{*}{Case study (Figures 15 and 16)} & Primary production stage & Bottomhole pressure (MPa) & 9 & 9 & 9 & - \\ \cline{2-6}  & Secondary production stage & Bottomhole pressure (MPa) & 8 & 8 & 8 & - \\ \cline{2-6}  & Secondary production stage & Bottomhole pressure (MPa) & 16 & 16 & 16 & - \\ \cline{2-6}  & Production time (days) & 1 & 2 & 4 & - \\ \hline \hline \end{tabular}
\end{table}
Table 4: _Cont._  
Figure 9: Results of gas production by use of different bottomhole pressure during primary depressurization stage: (**a**) gas production rate and (**b**) cumulative gas production.  
depressurization stage, while it decreased during the secondary depressurization stage. The reason is that relatively low gas production during the secondary depressurization stage caused the increment of pore pressure as compared to the primary depressurization stage. In addition, the amount of vertical subsidence in the case of 9 MPa was low compared to that of the non-cyclic case; the difference was only 16.6%. This value was higher than the difference of the cumulative gas production. Moreover, the result of the 6 MPa case was similar with that of the non-cyclic case. Accordingly, geomechanical stability increased significantly by using the cyclic depressurization method, and this is a key parameter for predicting geomechanical stability.