CCS - What to consider when storing CO2 underground

What to consider when storing CO2 in an underground reservoir?

Can we inject CO₂ into the reservoir?

Though CO₂ is injected into wells like those used for conventional oil and gas, there are additional requirements for storage wells. In the presence of water containing fluids, CO₂ forms carbonic acid that can cause severe corrosion to the well completion. Additionally, if a CO₂ storage well is not designed, completed and operated correctly, the temperature of the CO₂ can rapidly reduce to significantly below zero centigrade as it enters the well. The consequences can adversely affect the well components, such as iron, steel, and elastomers. The ability to effectively manage corrosion and temperature throughout the lifespan of the well is a crucial determinant in well construction and operation.

Does it have the capacity?

Capacity describes the volume of CO₂ that a reservoir can contain. Geologists and reservoir engineers working together determine the porous rock volume measured in tonnes of CO₂ that the reservoir can hold.

Can the CO2 be contained?

Like gases found in deep reservoirs, CO₂ is contained and prevented from entering the atmosphere by the large volumes of rock above the reservoir section. It is important to ensure that neither the reservoir nor the wells leak CO_2. Poor well construction or geological faulting can create CO₂ leak paths. It is important to ensure the layers of rocks above where the CO₂ is stored, the overburden, contain shales or salts, which act as a seal trapping mechanism.

Can it be monitored?

Monitoring the movement of the CO₂ after it has been injected is essential to identify any leak paths. A combination of surface monitoring and geophysical techniques, including 4D seismic surveys, is used to identify any safety concerns. Monitoring also establishes the effectiveness of the well and reservoir in accepting the CO₂, maps any movement of the CO₂ plume, and helps detect any leaks so that corrective action can be taken.

What are the safety risks?

CO₂ is a hazardous gas. It is heavier than air and will displace air from contained spaces and, as such, presents a danger to human health and activity. Identifying, understanding and quantifying the safety risks of large-scale projects is critical to ensuring the safe injection and storage of CO₂ underground. The most significant risk is CO₂ migrating out of the storage reservoir and the physical and chemical effects CO₂ may have on the subsurface and groundwater. When identifying a potential storage site, an accurate and complete dynamic model of the reservoir will significantly improve decision making. Safety risks are also addressed during the well design phase. With proper site selection, well design and ongoing monitoring, CO₂ storage has proven safe. For example, the Sleipner-West Carbon Capture and Storage facility has been operating safely since 1996.

Deep expertise in underground CO2 storage

RPS has significant technical and commercial experience in underground gas storage, including the geological storage of CO₂. Across the CCS lifecycle, we assist and assure clients as diverse as CO₂ emitters, CO₂ transporters, financial investors, licencing and regulatory authorities, government, and the public. Our teams of geologists, reservoir engineers, compliance and permitting specialists, economists, and ecologists apply their experience to manage the uncertainty related to CO₂ storage effectively.