Residual water formation during the CO₂ storage process in deep saline aquifers and factors influencing it: A review

Of the various potential CO₂ sinks available deep underground, saline aquifers are dominant due to their huge storage capacity and broad worldwide distribution. CO₂ injected into deep saline aquifers is stored in them through various trapping mechanisms, of which residual or capillary trapping can achieve greater CO₂ storage capacity. However, the formation of residual water that mainly occurs during the CO₂ drainage process has a negative influence on this CO₂ residual trapping process, because it acts as a barrier to the CO₂ flow through the aquifer's pore channels. This study is a comprehensive review to identify the influence of residual water saturation on CO₂ sequestration and factors affecting it.

There are number of influencing factors on the residual water formation process, which are mainly related to the physical properties of the reservoir and the injected CO₂ properties and process. These factors include, CO₂-brine interfacial tension (IFT)-related factors, contact angles in brine-CO₂-rock systems, aquifer rock mass heterogeneity and mineral composition, the permeability and porosity of the aquifer, two-phase equilibrium, and CO₂ injection rate and duration. However, it should be noted that these factors mutually contribute to residual water formation in saline aquifers during CO₂ sequestration and therefore, the overall influence decides the efficiency of the final residual water formation in the aquifer. This work is a state-of-the-art report on the effectiveness and evaluation of reservoirs for CO₂ storage purposes.