Carbon sequestration for arid regions overcomes water constraints

A process to trap carbon dioxide underground in solid minerals, without large volumes of freshwater demanded by existing techniques, could provide a promising carbon removal solution even in the world’s driest regions, expanding the tools available to address climate change. A new study from KAUST, published in Nature, now shows how to store CO₂ underground using far less freshwater, opening the door to applications in arid regions^[1].

One way to deal with carbon emissions is to sequester them underground. To prevent CO₂ from leaking back into the atmosphere it needs to react with rocks and form carbonate minerals, such as calcite, which store the carbon in a stable solid form. This can be accomplished by dissolving CO₂ in water and pumping this mixture down into natural fractures in suitable rock formations, where the mineralization reactions then occur. The carbon gets bound into the minerals instead of being released into the atmosphere, but the freshwater cost is high.

“Previous estimates suggested that carbon mineralization required 20 to 50 times more water than the mass of CO₂ being stored,” explains Eric Oelkers, who led the study alongside Hussein Hoteit, Abdulkader Afifi, and Thomas Finkbeiner of KAUST. That renders existing approaches poorly suited to arid regions like Saudi Arabia which hosts major carbon-emitting industries, including petroleum extraction and refining, power generation, and desalination plants.

The key insight for the new approach is to use the site’s groundwater and recirculate it through wells, creating a fully cyclical process. “In our study, the CO₂ gas was dissolved directly into water that is pumped up from deep underground,” says Oelkers.

“This CO₂-charged water was then re-injected into the subsurface naturally fractured basaltic rock formation through a neighboring well. As the carbonated water traveled through the rock, it reacted chemically with the basalt, triggering a natural mineralization process that locked the carbon into stable solid carbonate minerals, trapping it.”

The team — which also included scientists from Saudi Aramco and the University of Iceland — tested their new process at a site near the Jizan Economic Complex and Refinery in western Saudi Arabia, where the combination of industrial CO₂ sources and suitable basaltic rocks made it an ideal test site. They drilled two wells, each several hundred meters deep, and recirculated water through them while pumping CO₂ into the water. Over the course of ten months, 70 percent of the 131 tons of CO₂ injected into the water was trapped in solid subsurface minerals.

“This research addresses a major logistical barrier to deploying this technology globally, which is the need for vast quantities of water,” says Afifi. With this trial successfully demonstrating the approach, the technology could be scaled up and applied along the Jizan coastal plain, where up to 4 billion tons of carbon could theoretically be sequestered in volcanic rock.

The success of the trial has implications beyond Saudi Arabia. The process can be readily adapted for use in other regions with similar rock formations. “By demonstrating that CO₂ can be permanently mineralized in older basalt formations using recycled groundwater, this work opens the door to global deployment of a promising permanent carbon disposal technology,” says Hoteit.