Traditionally, CO2 is injected into a reservoir capped by an impermeable rock layer, known as caprock, which prevents the gas from escaping. This approach, borrowed from petroleum exploration, has been effective for both oil and CO2. However, recent research from The University of Texas at Austin's Bureau of Economic Geology suggests that a collection of smaller, interconnected barriers, known as a "composite confining system," may provide a more reliable method for long-term CO2 storage.
This discovery is promising for the carbon storage industry, particularly in regions like the Texas Gulf Coast, which feature these types of geological formations.
"Directly under what is the largest concentration of emissions in the U.S. we have incredible reservoirs, but few regional seals. What we have instead, is lots and lots of discontinuous barriers to vertical flow," said Alex Bump, a research associate professor at the Bureau's Gulf Coast Carbon Center in the UT Jackson School of Geosciences. "There is a very local motivation for this research but the application is global."
Many experts in carbon storage come from the oil and gas sector, including Bump. While both fields share knowledge in reservoir geology and fluid dynamics, Bump points out that this has led to assumptions about optimal storage scenarios. Specifically, the belief that caprock-sealed reservoirs ideal for oil should also be preferred for carbon storage.
"In petroleum, the goal of production favors large-volume, concentrated, mobile accumulations, so we explore for large traps, and high-permeability reservoirs with an impermeable seal," said Bump.
Although this model works for carbon storage, it carries risks. A leak in the seal, such as an improperly decommissioned well, could release a large volume of CO2. Conversely, composite confining systems not only prevent leakage but also distribute the CO2 plume throughout the reservoir, effectively immobilizing the gas. Even if an escape path exists, the dispersed CO2 minimizes the risk of significant leakage.
Bump compares this to catching a water leak with a bucket versus a pile of towels. While both methods work, the towels absorb the water, eliminating spill risk.
In a paper published in the International Journal of Greenhouse Gas Control, Bump and his colleagues at the Gulf Coast Carbon Center, Hailun Ni and Sahar Bakhshian, advocate for composite confining systems by presenting data from experimental models, numerical simulations, and actual reservoirs. Their research shows that the length and frequency of the barriers are crucial for an effective confining system. Even small differences in grain size between geological layers can redirect a rising CO2 plume, spreading it laterally and minimizing upward migration.
Bump's next step is to promote composite confining systems for carbon storage. He is developing a best-practices guide for identifying and permitting these reservoirs.
"This is really about creating a user's manual," Bump said. "We're figuring out how to take a good idea and apply it. There are already commercial projects moving forward with this on the Gulf Coast. We want to make it a standard part of the global toolkit for carbon storage."
Research Report:An experimental investigation on the CO2 storage capacity of the composite confining system
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