## 3 Results and Discussion  
### Permeability Behavior of CO\({}_{2}\) under Steady-State Conditions  
CO\({}_{2}\) was first injected into the same siltstone sample at steady-state conditions under four different confining pressures (see Table 1), and the corresponding downstream flow rate was observed and recorded over time. The flow rate development under 20 MPa confinement is shown in Figures 6 and 7. When the injection pressure is not higher than 9 MPa, the downstream CO\({}_{2}\) flow rate gradually reaches a steady state at each injection pressure after around 20 min of injection (see Figure 6). However, when the injection pressure goes beyond 9 MPa, although the flow rate increases with increasing injection pressure, obtaining a steady state flow rate condition under each injection pressure condition was quite difficult due to the highly unsteady flow (see Figure 7). At each unsteady downstream flow behavior step (relevant to each injection pressure after 9 MPa), the flow rate first increased rapidly followed by a slow reduction, although confining and injection pressures remained steady during each injection process and the possibility of leak-off was excluded. This is possibly because some physical process occurs in the sample with existing high flow rates under greater injection pressures, and fine particle migration is one of the suspected causes. Fine particle migration with the high pressure-driven great advective flux in such situations may be responsible for this phenomenon, and a small quantity of fine particles was seen on the filter paper at the downstream after the permeability tests. The mineral particle distribution is shown in a SEM image and the differentiation of particle radiuses is marked (see Figure 8). The size of the kaolinite particles (less than 2 \(\upmu\)m in diameter) is much smaller than that of the quartz particles (less than 62.5 \(\upmu\)m in diameter), and kaolinite particles therefore normally exist between quartz particles and have a high tendency to occupy the flow path, showing the negative influence of clay content on flow ability or permeability of sample [31; 40]. When the injection pressure is higher than 9 MPa, higher advective flux along the rock matrix induces a higher pushing force on some free or loosely-bonded fine particles, extracting them from the rock matrix and mixing them into the fluid flow. While blockage of the available narrow pore throats in the rock sample through fine particle accumulation can cause a significant reduction in flow ability through the rock matrix, once the accumulated pressure at the upstream side reaches a certain value, these blockages can be pushed out by the associated great pushing force, opening the pore throats. This might cause the observed sudden flow rate increments (see Figure 9). Another reason for this phenomenon maybe the presence of small drikold particles near the downstream due to the sharply drop of temperature of CO\({}_{2}\) flow, which will be discussed in detail in Section 3.2. However, for these complex downstream flow behaviors obtained under high CO\({}_{2}\) injection pressure, the average of peak points was taken as the steady-state downstream flow rate to calculate the permeability. These observed flow behaviors of CO\({}_{2}\) under steady-state conditions exhibited similar patterns under the greater confinements of 30 and 40 MPa. The flow rates obtained under each test condition are shown in Figure 10, which confirms the linear variation between the flow rate and injection pressure. Therefore, [PERSON]'s equation was used to calculate the permeability for CO\({}_{2}\) in dry siltstone under each test condition. Figure 8: SEM image of tested siltstone sample with varisized particles. Figure 10: Flow rates for different injection pressures. Figure 9: Schematic diagram of effect of fine particle migration on permeability (solid circles represent mineral and fine particles; dashed circles represent moving fine particles).