Carbon Nanodots for Improving CO₂ Geo-Storage in Harsh Carbonate Reservoirs: Solubility and Capillary Trapping Quantifications

Understanding the CO₂–water–rock interactions and their influences on capillarity plays a key role in CO₂ geo-storage and its containment security. In the current study, we investigated the influence of carbon nanodots (CNDs) on CO₂ geo-storage capacity in carbonate reservoirs by studying both the solubility and the capillary (residual) trapping mechanisms under high-temperature, high-pressure, high-salinity (HTHPHS) reservoir conditions. Initially, we conducted capillary pressure–water saturation (P_c–S_w) relationship experiments on the carbonate samples to screen different permeabilities (13, 185, and 689 mD) and various test fluids, including deionized water (DIW), seawater (SW), formation water (FW), and their CND-added solutions. The addition of CNDs consistently shifted the P_c–S_w curves more toward the water-wet state and increased the overall irreducible water saturations. This indicates the wettability alteration establishments, which strengthens the capillarity of the reservoirs and increases the capillary entry or threshold pressure of the core samples. Later, we quantified the CO₂ solubility at 70 °C and 1850 psi and observed that the neat DIW, SW, and FW systems dissolved 2.1486, 1.1588, and 1.0082 m/L of CO₂, respectively, and observed that the solubility was further increased significantly with the addition of CNDs into these solutions. However, we observed that the incremental solubility was more dominant in the SW and FW solutions rather than in DIW. Subsequently, we also ran HTHP coreflood experiments and quantified the residual CO₂ saturation for various test fluids. The results showed that the addition of CNDs increased CO₂ residual trapping from 15.7% to 30.7%. This clearly implies that treating the reservoirs with CNDs altered the reservoir's wettability more towards the water-wet state, which eventually contributed to the increase in the residual fraction of CO₂, even under HTHPHS conditions. Overall, the results showed that, beyond the wettability shifts, the CNDs also contributed to improving both solubility and capillary trappings; therefore, we concluded that the CNDs have strong potential to increase the secured (capillary) CO₂ geo-storage capacity in harsh carbonate reservoir environments.