Storage of CO₂

Utilization and Storage

So, what do we do with CO₂ once it has been captured? As previously discussed, CO₂ has value as a commodity in a number of different industries, and revenue from selling this commodity can help defray the costs of capture and transportation of the gas. But current demand for CO₂ is not significant enough to match all of the gas we might target for capture. For the rest of the captured CO₂, a technological solution exists to put it back in the ground. Current projects to put the CO₂ into rock formations in the subsurface use different strategies.

Carbon Capture and Storage (CCS) Projects

Some projects capture the CO₂ and then store it in the subsurface. These projects often are referred to simply as storage projects, carbon capture and storage (CCS) projects, or geological carbon sequestration projects. In these cases, the CO₂ is not commercially used.

Carbon Capture, Utilization and Storage (CCUS) Projects

However, other projects capture the CO₂, use the CO₂ as part of oil and gas operations, and then eventually store the CO₂ underground. These projects are referred to as carbon, capture, utilization and storage projects, or CCUS.

CO₂ Injection – Expertise from Oil and Gas Operations

The technology is not new—the technology for injecting CO₂ into existing oil and gas reservoirs has existed for decades, used as a method of enhanced oil recovery (EOR). Subsurface reservoirs exist—suitable reservoirs for storing CO₂ include saline aquifers, depleted oil and gas reservoirs (some associated with EOR), possibly hydraulically fractured shale formations, and thick formations of igneous rocks (basalt and peridotite). We can use these same principles to inject CO₂ into depleted reservoirs, or we can leverage existing well infrastructure to inject CO₂ into nearby saline aquifers that have similar gas-retention properties.

The illustration here from the U.S. Environmental Protection Agency (EPA) displays an injection well used for EOR. The EPA has six classes of injection wells the agency regulates under its Underground Injection Control (UIC) Program. Or, in some cases the authority for regulating these wells has been granted to the state (primacy). The injection wells shown here (one for injection of CO₂ for EOR and the other for disposing of produced water, or brine) are categorized as Class II wells under the U.S. Safe Drinking Water Act. These well are therefore regulated under the UIC Program. Texas has primacy over regulating Class II wells, and regulatory authority falls under the Railroad Commission of Texas. Note that oil and gas production wells do not fall under the UIC Program, because they produce rather than inject. The Railroad Commission of Texas also has regulatory authority over the oil and gas production wells in the state.