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What is a Class VI well?

Why do we need an Area of Review?

The Area of Review refers to the region surrounding a CCS project where underground sources of drinking water (USDWs) could be affected by the injection of CO_2Ìýinto deep geological formations.

Drinking water could be endangered if there is a vertical leakage pathway between the CO_2Ìýinjection zone and the USDW.

Additionally, drinking water can be negatively impacted by the movement of CO_2Ìýthrough changes in pH or acidity, contamination by trace impurities, including mercury or hydrogen sulfide, or the leaching of metals and organics.

If non-potable water, such as brine, moves up into a USDW because of the increased pressure in an underlying geological formation (or reservoir) due to the injection of CO₂, the USDW could become contaminated.

What project considerations does the AoR impact?

The AoR sets the boundary area for Class VI permitting and will impact several factors.

1. Site characterization – Gathering extensive information about the proposed site’s local and regional geology and hydrogeology for feasibility studies and planning. Depending on the availability of existing studies, this data may be time-consuming and expensive to obtain.
2. Corrective action – Artificial penetrations such as active and abandoned wells in the AoR must be evaluated for potential leakage before authorization to inject is granted.
3. Testing and monitoring – During active injection, the area must be monitored regularly using 4D seismic and/or monitoring wells. The monitoring period can extend to 50 years after CO_2Ìýinjection stops depending on the jurisdiction governing the proposed site.
4. Emergency and Remedial Response – The AoR will inform the Emergency and Remedial Response Plan for the project’s duration regarding notifications and reporting.

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How is the AoR determined?

The area of review is delimitated and can be mapped using established methods based on the site information. Geologists and hydrologists use complex computer modeling to determine how all phases of the injected carbon dioxide stream (whether supercritical, liquid, or gaseous) and the fluids it displaces, will behave. The model must account for various physical and chemical properties of the CO₂, geological information, site characterization, monitoring, and operational data.

Why size matters – for an AoR

It’s important to talk about right-sizing for the AoR because this area will impact carbon sequestration projects for the entire project lifecycle. If the AoR is larger than necessary, more resources will be needed to characterize the site, more corrective action will be required, and there will be a broader area to monitor. The scope of the emergency and remedial response plan will also be affected.

Depending on the method used to calculate the size, the AoR could become too large, making the entire project unfeasible from a cost and operational perspective. On the other hand, a too-small area of review could mean that artificial penetrations, such as well bores into the reservoir that will be impacted by the CO₂/pressure plumes, are not properly assessed and sealed, resulting in a leak beyond the AoR boundary. Any leaks could potentially halt the injection process, resulting in negative consequences such as contract breach, costly remediation and reputational issues. Such incidents would obviously damage public and regulatory confidence and could slow down the widespread adoption of CCS.