## 2 Selection of Parameters Affecting the Sealing Performance of Caprocks
### Indicator Screening of Caprock Sealing Evaluation  
Table 1 lists the main parameters (e.g., lithology, gas breakthrough pressure, porosity, permeability, thickness, fracture development degree, rock brittleness, and lateral continuity) in the sealing evaluation of caprocks by different researchers. It shows that the selection of parameters is strongly subjective.  
In this paper, six parameters, including gas breakthrough pressure (A), permeability (B), thickness (C), brittleness (D), fracture development degree (E), and porosity (F), are selected as the main evaluation indexes. To establish an evaluation standard for the sealing performance of caprocks, the weights of six selected factors should be reasonably assigned. The analytic hierarchy process (AHP) is often used to determine the weights of factors (see detailed steps of AHP in Appendix A.1). The key step of AHP is to establish the judgment matrix. Before assigning the weights of parameters, some related literatures are referenced [40; 41; 42; 43; 44]. The gas breakthrough pressure is four compared with the permeability, five compared with the thickness, five compared with the brittleness, three compared with the degree of fracture development, and four compared with the porosity, see details in Table 2.  
The pairwise comparison matrix is calculated using AHP (see Appendix A.1 for the specific calculation process of AHP). The evaluation parameters and equations of six factors are obtained. When CR = 0.0988, it is less than 0.1, and the consistency test is satisfied, as shown in Table 3.  
Combined with the evaluation results, the evaluation equation for the sealing performance of caprock can be written as:  
\[S=0.4291A+0.2157B+0.1370C+0.0816D+0.0893E+0.0473F \tag{1}\]  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Lithology**} & **Breakthrough Pressure** & **Porosity** & **Permeability** & **Thickness** & **Fracture Development Degree** & **Rock Brittleness** & **Continuity** & **Source** \\ \hline \(\bigvee\) & \(\bigvee\) & \(\bigvee\) & \(\bigvee\) & \(\bigvee\) & \(\times\) & \(\times\) & \(\times\) & [PERSON] et al. [40] \\ \(\bigvee\) & \(\bigvee\) & \(\bigvee\) & \(\bigvee\) & \(\times\) & \(\times\) & \(\times\) & [PERSON] et al. [41] \\ \(\times\) & \(\times\) & \(\times\) & \(\times\) & \(\times\) & \(\bigvee\) & \(\times\) & [PERSON] et al. [42] \\ \(\bigvee\) & \(\bigvee\) & \(\times\) & \(\bigvee\) & \(\bigvee\) & \(\bigvee\) & \(\bigvee\) & [PERSON] et al. [43] \\ \(\bigvee\) & \(\bigvee\) & \(\bigvee\) & \(\bigvee\) & \(\times\) & \(\times\) & \(\times\) & [PERSON] et al. [44] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Selected parameters in the evaluation of sealing performance of caprocks.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Breakthrough Pressure (A)** & **Permeability (B)** & **Thickness (C)** & **Brittleness (D)** & **Fracture Development Degree (E)** & **Porosity (F)** \\ \hline Breakthrough pressure (A) & 1 & 4 & 5 & 5 & 3 & 4 \\ Permeability (B) & 1/4 & 1 & 3 & 4 & 2 & 4 \\ Thickness (C) & 1/5 & 1/3 & 1 & 3 & 2 & 4 \\ Brittleness (D) & 1/5 & 1/4 & 1/3 & 1 & 2 & 2 \\ Fracture development degree (E) & 1/3 & 1/2 & 1/2 & 1/2 & 1 & 3 \\ Porosity (F) & 1/4 & 1/4 & 1/4 & 1/2 & 1/3 & 1 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The pairwise comparison matrix used in the sealing evaluation of caprocks.  
where \(A\) is the breakthrough pressure of caprock; \(B\) is the permeability of caprock; \(C\) is the thickness of caprock; \(D\) is the brittleness of caprock; \(E\) is the development degree of fracture; and \(F\) is the porosity of caprock.