## 3 Results and Discussion
### Pore-Scale CO2 Dissolution and Flow Development  
The time-lapse images of CO2-brine dissolution were rendered for selected slices, as shown in Figure 2. It is demonstrated that the overall dissolution rates are sensitive to the aqueous flow field, which was affected by the CO2 distribution and the resulting reductions in relative permeability [20; 21]. In the first few pore volumes (PV), the brine invaded along the side of the core and formed a continuous flow path. With the continuous injection of brine, the flow path expanded radially and stretched to the surrounding pores. The flow development in the downward imbibition was severely affected by the gravity effect, and flow bypassing occurred. A small amount of brine intruded into other areas of the core and formed film flow on the surface of the wetting porous media. The film water and corner flow in the former PV resulted in the swelling of thin film within pores and subsequent fragmentation of CO2 droplets (shown in Figure 2 R1). The water flow between CO2 bubbles and pore walls was found to affect the CO2 dissolution rate [22]. In this period, the dissolution rate was controlled by the extension of the flow path. The fundamental reason for this is that further dissolution based on flow development was hindered by the pore structure under the gravity effect. The critical time of the dissolution rates was observed to be extremely sensitive to the position and size of the heterogeneous region, suggesting the disruption of the pore structure to the phase equilibrium [23]. The dissolution expansion was initiated at about 9.48 PV, and the duration was about 3 PVs. The brine began to invade the pores in the center of the core and formed multiple fingerings. With the branching and expansion of the fingerings, the large CO2 cluster dissolved rapidly in the brine, significantly enhancing the CO2 dissolution rate (shown in Figure 2 R2). Dissolution fingering may occur at CO2-brine interfaces because of instabilities in CO2 saturation and flow development during dissolution, as observed in a homogeneous pore network [24] and heterogeneous porous media [25]. High contact between the injected brine and the CO2 surface (i.e., low bypassing) resulted in a greater CO2 dissolution rate (shown in Figure 2 R3). Particularly for supercritical CO2 under larger buoyancy forces, the faster growth of perturbations existed over time, which can provoke a more complicated structure of fingers and also more CO2 dissolution into brine [17]. In contrast, the CO2 bubbles located parallel to the flow direction led to more bypass trapping and lower average effluent concentrations [26]. The CO2 bubbles, which were bound by the bypass flow, generally had low connectivity to the surrounding brine, or were isolated in the large pores through the connected pore throats (shown in Figure 2 R4). The flow bypassing was found to restrain the dissolution rate in experiments with CO2 confined to the central region in the sand-filling core. Simultaneously, the brine that already occupied some corners of the pore gradually filled the pore space owing to the diffusion of the CO2 in the brine (shown in Figure 2 R5). When the brine filled the upstream pores, some of the downstream bubbles happened to snap-off, and the detached part then rapidly dissolved (shownin Figure 2 R6 and R7). The flow development and the diffusion and the coupled water flow and dissolution at the pore scale enhanced the dissolution rate at the pore-network scale, shortening the total depletion time of CO\({}_{2}\)[9; 27]. After the major CO\({}_{2}\) bulk dissolved, the remaining CO\({}_{2}\) bubbles were trapped in the unconnected pores. The effect of the diffusion mechanism on the dissolution rate was enhanced during this period. CO\({}_{2}\) clusters trapped in the large pores were split by brine and changed in size and shape (shown in Figure 2 R8). The newly isolated CO\({}_{2}\) singlet existed for 1-3 PV, while the bubbles that inherited from the period of rapid dissolution expansion sometimes persisted for more than 4 PV.