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Researchers from Southwest Research Institute (SwRI) and The University of Texas at Austin (UT Austin) have evaluated a promising approach to improving long-term carbon storage in depleted oil and gas reservoirs. The team proposes using foam-entrapped supercritical carbon dioxide (sCO₂) to prevent stored and captured carbon from moving back to the surface.

Carbon capture, utilization and sequestration (CCUS) involves using captured CO₂ in processes such as enhanced oil recovery, where CO₂ is injected underground to help extract additional oil instead of being released into the atmosphere. This extends the CO₂'s usefulness before it's stored underground.

"CO₂ plays a crucial role in enhanced oil recovery, because it can perform work underground while being captured and stored," said SwRI's Angel Wileman, a co-principal investigator on the project. "Unlike fresh water or oil, which we prefer to avoid using, CO₂ is a substance we already aim to keep underground."

Wileman will discuss this research in her presentation, "Supercritical COâ‚‚ Foam for Enhanced Oil Recovery and CCUS: Insights from Simulated Reservoirs," at the in Houston on June 26.

To ensure that the stored CO₂ remains stable underground and doesn't migrate to the surface, SwRI applied principles from traditional CO₂-enhanced oil recovery methods to investigate the stability and behavior of foam-entrapped CO₂ at high-temperature and high-pressure reservoir conditions. Under these conditions, the CO₂ reaches its supercritical state, exhibiting gas-like viscosity and liquid-like density, which influences its mobility and storage behavior.

Supercritical CO₂ foams exhibit a behavior known as shear thinning, meaning their viscosity decreases under higher shear rates. This allows them to flow more easily through high-permeability zones while limiting flow into low-permeability regions. As a result, they improve sweep efficiency for oil recovery and help reduce the risk of COâ‚‚ migration by limiting channeling and preferential flow through fractures.

"One of the significant findings from our project was observing that foam viscosity increased up to a certain pressure point, after which it suddenly dropped. This variation in viscosity plays a pivotal role in how the foam behaves underground, affecting its distribution and effectiveness," Wileman said.

The SwRI and UT Austin researchers tested the sCO₂ foams under high temperatures and pressures in fractured sandstone rock, as well as on larger scales using a heterogeneous sand pack that mimics the underground formations that foams would encounter in oil fields.

"These laboratory experiments are crucial for understanding the behavior of sCO₂ foams under various conditions before attempting field applications," said SwRI's Dr. Raouf Tajik, who oversaw the laboratory testing. "By collaborating and comparing different testing methods and scales, we aim to develop a comprehensive understanding of the utility of sCO₂ foams while addressing the challenges associated with their field use."

The project has expanded SwRI's small foam generator facility, making it possible to evaluate unconventional technology on a much wider scale and offer testing and other services to a range of industry clients.