Key failures to avoid in carbon storage site characterisation

Risk assessment of CO₂ storage

World-wide plans to store vast quantities of CO₂ under ground is emerging. Hence, risk management for CO₂ containment is a key issue.

The efficiency and safety of the CO₂ storage depends on expert assessment of several risks.

In this article, our experts highlight just a handful of the risks that must be assessed as part of an investor’s due diligence process.

Risks of CO₂ leakage in legacy wells

The choice of storage location usually includes legacy wells for substrate information. In turn, legacy wells present a risk for the safe containment of CO₂. It’s well known that many – perhaps even most – P&A’d wells are leaking gas and oil to the surface.

Another legacy well problem is that over time, reactions between CO₂ and cement can alter cement characteristics. Also, CO₂ is likely to cause corrosion of casings, possibly generating new conduits.

Wells are usually plugged and abandoned with the aim of preventing hydrocarbons leaking out of reservoirs. The plugs are set to seal hydrocarbons according to oil and gas standards at the time of P&A. So, it’s not surprising that when evaluated today, their integrity for CCS purposes is uncertain – the lack of documentation and low-quality log data simply adds to the challenge.

CO₂ invasion of reservoirs with hydrocarbons and aquifers for industrial and human consumption

CO₂ is stored in reservoirs next to oil and gas fields because of the convenience of existing infrastructure and geological well and seismic data.

The control of CO₂ migration requires good geomodels with reliable lithology, porosity, permeability, pressures, and barriers to flow. While monitoring CO₂ plume migration allows us to control these situations, it won’t prevent build up of the CO₂ columns and over-pressure.

In general, there is a strategic choice of whether to store CO₂ in a trap (similar to an oil and gas accumulation) or allow it to percolate a saline aquifer. In the latter case, risk of top seal failure is less, compared to accumulation of a CO₂ column that builds over-pressure.

When focusing on CO₂ invasion, there’s also the potentially catastrophic risk of it infiltrating water resources. This makes water unusable for irrigation, industrial and human consumption. Ultimately, CO₂ leakage into the atmosphere is a total failure of a project and lethal to public health. For this reason, many countries have prohibited storing of CO₂ onshore.

Containment of CO₂

Long term containment of CO₂ requires a full review.

Geomechanical review and analysis with respect to:

• Maximum and minimum horizontal stress
• Faulting, tectonic history, regional tectonic forces
• Lithological barriers and formation strength/integrity
• Potential for building a CO₂ column inducing over-pressure

Economic risk – judicial questions

Most countries have regulations and targets in place to reduce CO₂ emissions, which means transport of CO₂ across borders may create complex hurdles.

Judicial questions on how to treat CO₂ as ‘waste’ need to be understood and a strategy agreed among all parties before transportation.

The lack of a robust legislative global framework presents considerable economic risk. Most CO₂ emission capture and storage projects are driven by authorities to meet emission reduction targets, and so when CO₂ is shipped across borders, the question arises – who will take responsibility and credit for meeting CO₂ emission goals?

International standards

There is an international standard (ISO 27914), which describes the procedures for screening, qualifying and certifying CO₂ storage. It’s a comprehensive set of guidelines and requirements, that must be satisfied as a prerequisite for issuing a certificate of operations.

Each phase of CO₂ storage, from screening to selection, characterisation and qualification, is subject to a certificate being issued. This is the framework that AGR’s highly experienced team of experts in CCS use as the basis for their work with clients across the world.

For more information on our risk evaluation work in CCS, please contact us.