Energy & Minerals

Schematic diagram showing how CO2 is captured, utilized, and stored. Modified from Gray, 2012, U.S. Department of Energy, Office of Fossil Energy, Carbon Utilization and Storage Atlas, fourth edition.

Once the CO₂ is captured, it can be injected into geological formations, or reservoirs, deep in the ground. These reservoir rocks are selected because they are porous, meaning they have space between the grains of the rock to enable them to hold fluids and gasses. Reservoir layers lie well below any potential drinking water aquifers and tend to hold naturally-existing saline brines (salty water). To get the CO₂ into the ground it is first compressed into a supercritical fluid, which behaves like a liquid, and can be injected into the reservoir rock formation using a well like those drilled for oil and gas production. In fact, decommissioned oil and gas wells are considered for conversion into CO₂ injection wells since the porous rock layers they penetrated for the extraction of petroleum also have the potential to act as CO₂ reservoirs. Compressing CO₂ into a supercritical fluid allows for the storage of larger quantities of CO₂ per volume.

Once injected, CO₂ may be permanently trapped within rock formations through a variety of mechanisms. Primarily it is held at great depth in the subsurface by the impermeable seal rock that overlies the reservoir. Further entrapment may occur as CO₂ dissolves in brine, or salty water, which typically fills most deep underground rock layers. Depending on the reservoir rock type and the chemistry of the brine, the CO₂ can also precipitate (or crystalize) to form carbonate minerals in the pore space of the reservoir rock formation. This process traps the CO₂ as a mineral and decreases the ability of fluids to move through the reservoir in the future, increasing the capacity of the rock to further trap non-mineralized CO₂. These processes are well understood because CO₂ has been injected into reservoirs for Enhanced Oil Recovery (EOR) for decades. EOR refers to injecting CO₂ into existing oil fields to aid in extracting residual hydrocarbons.

Because CO₂ injection uses the same technology as oil and gas production, the technology and expertise from those industries can be leveraged and transferred to CCUS. The technical expertise geologists and engineers use to understand how hydrocarbons flow through the subsurface can instead be applied to assessing how CO₂ moves through similar rocks.