Quantification of capillary suction induced by brine evaporation and salt precipitation during CO2 injection in saline aquifers (Flexible Funding 2024)

Professor Vahid Niasar, University of Manchester, was awarded funding in the UKCCSRC’s Flexible Funding 2024 call to look at “Quantification of capillary suction induced by brine evaporation and salt precipitation during CO2 injection in saline aquifers”.

As countries ramp up efforts to meet net zero targets, geological carbon storage in saline aquifers is emerging as a crucial strategy for long-term CO₂ sequestration. When dry CO₂ is injected into brine-filled porous rocks, salt crystals can form and block the tiny pathways that fluids use to move, a process that can dramatically reduce the injectivity of storage site and long-term performance.

Thanks to support from the UK Carbon Capture and Storage Research Centre (UKCCSRC), our team at the University of Manchester has zoomed in – quite literally – on this phenomenon using microfluidics and high-resolution optical techniques. By watching salt crystals grow in real time under a microscope, we were able to uncover how and why pore-scale salt precipitation happens, and what it means for CO₂ storage.

Why Salt Matters in CO₂ storage

When CO₂ is injected into deep saline aquifers, it displaces brine in the pores of the rock. But not all the water is pushed out. Some gets trapped and starts to evaporate into the dry CO₂ stream, especially near the injection well where the CO₂ is driest. This sets the stage for salt precipitation, especially halite (NaCl), which crystallises once the brine becomes supersaturated. These salt crystals can clog the pore space, reducing permeability and making it difficult for more CO₂ to be injected.

What makes things more complex is the role of capillary suction in porous media that pull liquid back toward drying zones. This capillary-driven backflow can amplify evaporation, accelerating the cycle of brine evaporation and salt growth. But until now, we lacked real data on how these forces work at the pore scale.

What We Did

We conducted microfluidic experiments to simulate brine evaporation and salt precipitation under CO₂ flow using our custom-fabricated glass micromodels. Using a state-of-the-art imaging system including μPIV (micro-Particle Image Velocimetry), we captured the behaviour of residual brine, salt crystals, and tracer particles over time.

Microfluidic chip with porous structure and the dual-field imaging system used for simultaneous brightfield and fluorescence imaging

We tracked:

• The displacement of brine by CO₂
• The fragmentation of brine into isolated clusters
• The onset of salt precipitation
• The motion of tracer particles drawn toward drying zones

We observed that there is often a sustained backflow of brine through thin films along pore walls or around salt crystals, driven by capillary pressure. This creates feedback loops where salt growth boosts capillary suction, which in turn fuels further evaporation and precipitation.

Key Findings

• Evaporation and precipitation are temporally distinct: After the CO₂ displaces most of the brine, evaporation and crystal growth continue over a much longer period.
• Brine clusters break into smaller pockets that evaporate independently, leading to dispersed single- and polycrystalline salt formations.
• Capillary suction increases over time, especially in confined zones, driving water towards the evaporation front.
• Thin-film flows were visualised and confirmed to contribute to additional salt growth from distant sources.
• Most importantly, salt and capillarity are deeply linked. Salt crystals increase the effective surface area for evaporation, which enhances capillary suction—a self-reinforcing cycle.

Brightfield images show salt formation; fluorescent tracer particles reveal capillary-induced fluid motion toward the growing crystals

These insights shed light on why injectivity loss can worsen over time, even after initial brine displacement has stabilised. Understanding this evolution is vital for both predicting long-term performance and designing smarter injection strategies.

What’s Next

The experimental results from this study provide a valuable dataset for validating pore-scale models of salt precipitation, which in turn can inform reservoir-scale simulations. We also evaluated a potential mitigation strategy, which showed promise in reducing localised clogging.

Further work will explore:

• Scaling the findings to heterogeneous or fractured systems
• Integrating pore-scale physics into larger simulation frameworks
• Developing operational guidelines to avoid or reverse salt blockage

Sharing the Science

As part of the UKCCSRC-supported project, the team visited Shell’s Energy Transition Campus in Amsterdam and Delft University of Technology in June 2025. These collaborative visits sparked new conversations around cross-scale modelling, experimental design and field applications. We are grateful to UKCCSRC for enabling these exchanges, which are critical for bridging academic research and real-world deployment.

Visit to Shell’s ETCA and TU Delft

Read more on Vahid’s Flexible Funding 2024 project page.