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We review the literature data published on the topic of CO 2 wettability of storage and seal rocks. We first introduce the concept of wettability and explain why it is important in the context of carbon geo-sequestration (CGS) projects, and review how it is measured. This is done to raise awareness of this parameter in the CGS community, which, as we show later on in this text, may have a dramatic impact on structural and residual trapping of CO 2 . These two trapping mechanisms would be severely and negatively affected in case of CO 2 -wet storage and/or seal rock. Overall, at the current state of the art, a substantial amount of work has been completed, and we find that:Sandstone and limestone, plus pure minerals such as quartz, calcite, feldspar, and mica are strongly water wet in a CO 2 -water system.Oil-wet limestone, oil-wet quartz, or coal is intermediate wet or CO 2 wet in a CO 2 -water system.The contact angle alone is insufficient for predicting capillary pressures in reservoir or seal rocks.The current contact angle data have a large uncertainty.Solid theoretical understanding on a molecular level of rock-CO 2 -brine interactions is currently limited.In an ideal scenario, all seal and storage rocks in CGS formations are tested for their CO 2 wettability.Achieving representative subsurface conditions (especially in terms of the rock surface) in the laboratory is of key importance but also very challenging.

a b s t r a c tA significant amount of theoretical, numerical and observational work has been published focused on various aspects of capillary trapping in CO 2 storage since the IPCC Special Report on Carbon Dioxide Capture and Storage (2005). This research has placed capillary trapping in a central role in nearly every aspect of the geologic storage of CO 2 . Capillary, or residual, trapping -where CO 2 is rendered immobile in the pore space as disconnected ganglia, surrounded by brine in a storage aquifer -is controlled by fluid and interfacial physics at the size scale of rock pores. These processes have been observed at the pore scale in situ using X-ray microtomography at reservoir conditions. A large database of conventional centimetre core scale observations for flow modelling are now available for a range of rock types and reservoir conditions. These along with the pore scale observations confirm that trapped saturations will be at least 10% and more typically 30% of the pore volume of the rock, stable against subsequent displacement by brine and characteristic of water-wet systems. Capillary trapping is pervasive over the extent of a migrating CO 2 plume and both theoretical and numerical investigations have demonstrated the first order impacts of capillary trapping on plume migration, immobilisation and CO 2 storage security. Engineering strategies to maximise capillary trapping have been proposed that make use of injection schemes that maximise sweep or enhance imbibition. National assessments of CO 2 storage capacity now incorporate modelling of residual trapping where it can account for up to 95% of the storage resource. Field scale observations of capillary trapping have confirmed the formation and stability of residually trapped CO 2 at masses up to 10,000 tons and over time scales of years. Significant outstanding uncertainties include the impact of heterogeneity on capillary immobilisation and capillary trapping in mixed-wet systems. Overall capillary trapping is well constrained by laboratory and field scale observations, effectively modelled in theoretical and numerical models and significantly enhances storage integrity, both increasing storage capacity and limiting the rate and extent of plume migration.

We measure primary drainage capillary pressure and the relationship between initial and residual non‐wetting phase saturation for a supercritical carbon dioxide (CO2)‐brine system in Berea sandstone. We use the semi‐permeable disk (porous‐plate) coreflood method. Brine and CO2 were equilibrated prior to injection to ensure immiscible displacement. A maximum CO2 saturation of 85% was measured for an applied capillary pressure of 296 kPa. After injection of brine the CO2 saturation dropped to 35%; this is less than the maximum trapped saturation of 48% measured in an equivalent n‐decane (oil)‐brine experiment. The dimensionless capillary pressure is the same to within experimental error for supercritical CO2‐brine, n‐decane‐brine and a mercury‐air system. CO2 is the non‐wetting phase and significant quantities can be trapped by capillary forces. We discuss the implications for CO2 storage.

Carbon capture and storage (CCS), where CO2 is injected into geological formations, has been identified as an important way to reduce CO2 emissions to the atmosphere. While there are several aquifers worldwide into which CO2 has been injected, there is still uncertainty in terms of the long‐term fate of the CO2. Simulation studies have proposed capillary trapping – where the CO2 is stranded as pore‐space droplets surrounded by water – as a rapid way to secure safe storage. However, there has been no direct evidence of pore‐scale trapping. We imaged trapped super‐critical CO2 clusters in a sandstone at elevated temperatures and pressures, representative of storage conditions using computed micro‐tomography (μ‐CT) and measured the distribution of trapped cluster size. The clusters occupy 25% of the pore space. This work suggests that locally capillary trapping is an effective, safe storage mechanism in quartz‐rich sandstones.