Capillary trapping for geologic carbon dioxide storage – From pore scale physics to field scale implications

Abstract

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.

Direct Measurement of Static and Dynamic Contact Angles Using a Random Micromodel Considering Geological CO2 Sequestration

“…When CO2 injection stops, water tends to move back into smaller pore throats due to wetting characteristic [6], causing smaller pore throats to mostly become under the advancing condition. Therefore, SCAs measured during both imbibition and drainage processes are similar to each other when pore throat sizes are small.…”

“…According to Figure 6, when pore throat size is small (i.e., less than 90 µm), the values of SCA during drainage and those measured during imbibition become close to each other. When CO 2 injection stops, water tends to move back into smaller pore throats due to wetting characteristic [6], causing smaller pore throats to mostly become under the advancing condition. Therefore, SCAs Sustainability 2017, 9, 2352 9 of 17 measured during both imbibition and drainage processes are similar to each other when pore throat sizes are small.…”

“…According to Equation (6), by an increase of the dynamic contact angle hysteresis (defined as the difference between RCA and ACA), higher water pressure difference is required before CO2 migration. In other words, mobility of the CO2 decreases as the hysteresis increases.…”

“…However, as the depth of CO2 storage increases, pores throat size decreases due to higher effective stress [50]. According to Equation (6), when deeper aquifer site is selected for CO2 storage, higher capillary pressure is expected due to smaller pore throat size. However, the negative effect of lower hysteresis on the contact angle should be considered for the mobility of trapped CO2 bubble.…”

“…Connected CO 2 plume is trapped under a sealing caprock by capillary pressure during structural trapping. Disconnected CO 2 bubbles are immobilized among rock pores by water/brine for residual trapping as a result of hysteresis and local capillary trapping [6]. Capillary or residual trapping is the more favorable between the abovementioned two short-term storage trappings.…”