Residual Trapping and Solubility Trapping in Geological Carbon Storage

Abstract Carbon dioxide (CO2) storage in saline aquifers holds promise as a viable strategy for mitigating greenhouse gas emissions. T his study investigates two primary trapping mechanisms, solubility trapping and residual trapping, crucial for the long-term, stable containment of injected CO2 within saline aquifers. Solubility trapping and residual trapping are rapid and pervasive mechanisms in CO2 storage within saline aquifers. The effects of various reservoir properties, fluid transport properties, and engineering parameters on these two trapping mechanisms were studied using simulation-assisted sensitivity analysis. This study elucidates their spatiotemporal evolution and interactions under varied geological, fluid, and engineering conditions. Residual trapping emerges as predominant in long-term CO2 storage, with residually trapped volumes typically doubling those of solubility trapping after 50 years of monitoring, despite solubility trapping initially exceeding residual trapping at the end of the injection phase prior to monitoring. Water imbibition post-injection notably enhances residual trapping, which more than doubles with an injection rate increase from 1 MMT/y to 10 MMT/y. Initial higher solubility trapping in higher porosity aquifers transitions to significant residual trapping as injection progresses and concludes. Temperature influences CO2 trapping dynamics, with a six-fold increase in residual trapping observed at 40°C post-injection. Enhanced rock compressibility by a factor of ten elevates solubility trapping by 33% and residual trapping by 20%, while lower vertical permeability increases solubility trapping by 42% and residual trapping by 47%. Over the monitoring phase, solubility trapping and residual trapping increase by 56% and 60%, respectively. Thicker aquifers exhibit pronounced post-injection residual trapping increases, contrasting with stable solubility trapping levels. High-pressure aquifers sustain greater residual trapping, particularly evident 15 years post-injection. Reducing residual gas saturation (Sgr) from 0.35 to 0.2 decreases residual trapping by 54%. During the monitoring phase, residual trapping stabilizes for residual gas saturation lower than 0.2 but continues to rise for residual gas saturation higher than 0.2. Notably, high-pressure, high-permeability, high-anisotropy, or low-temperature aquifers exhibit drastic residual trapping increase when the injection stops, highlighting their significant role in the long-term, stable CO2 storage. A change in the Reservoir Quality Index (RQI) of the aquifer from 0.03 to 0.99 enhances solubility trapping by 2.46 times and residual trapping by 3.76 times. Conversely, salinity and relative permeability and capillary pressure exponents minimally affect trapping efficiencies.