Getting the numbers right
One of our main takeaways from a CO₂ storage efficiency conference in London in September 2023 was the shared concern in the geoscience community about correct communication of CO₂ storage capacity estimate ranges. In this article we explain why it matters to more than the geologists.
Within the technical subsurface world, our scientists have a standard way of communicating potential and confidence. These numbers represent a theoretical potential of how much CO₂ that can be stored safely underground. Yes, the important keyword here is theoretical. And the situation right now, in many parts of the world, is that policy makers, business developers and investors have taken the theoretical numbers for face value. It is quite possible that the technical subsurface experts haven’t been too successful in communicating the underlying assumptions and resulting uncertainty, and that’s really the reason for this.
Seeing these estimates as a reality would be like a business developer juxtaposing market potential and short-term earnings. Put differently: The storage capacity estimates often communicated is a best-case scenario with 10% likelyhood of happening, for which there is no guarantee.
In contrast, banks provide loans based on the low-case scenario with 90% likelyhood of happening in the future. Read more about these aspects in the description of the SRMS classification system found in this article.
Many CO₂ storage capacity evaluations stop at the definition of pore space available in the trap alone.
We assess the project lifetime injectable capacity. Read further to learn how it is done and why this is important for your project finances.
Estimating potential CO₂ storage capacity is generally about three steps:
STEP 1 : National CO₂ atlases - static domain
The country-based CO₂ atlases are presenting a very large hypothetical estimate. The potential is based on very rough assumptions and does not necessarily consider real traps where the CO₂ can be stored.
The numbers are large: UK has estimated a CO₂ storage potential of 75 gigatonnes (Gt), Denmark has estimated 22 Gt, while Norway has estimated 73 Gt. It is also worth noting that the estimates are without time constraints, which means that a poor reservoir can still be relevant if you have 10 000 years to inject a certain amount of CO₂. It is irrelevant for a project with a 25 year lifetime.
STEP 2: Identifying traps and storage complexes - static domain
It is up to each company to identify traps and CO₂ storage complexes consisting of the following: sealing rock, reservoir rock and aquifer with enough porosity and permeability that the resulting pressure pulse from the CO₂ injection can move far.
OPEN AQUIFER VS. CLOSED TRAP: A SIMPLIFIED ILLUSTRATION
The advantage of an open aquifer is greater storage capacity. However, the injected CO₂ can migrate long distances over time. In a closed trap, you have full control.
Please note that the simplified illustration is for informational purposes only.
STEP 3: Reservoir simulation modelling - dynamic domain
In the last step of the maturation phase, a reservoir simulation model is built to model how fluids will flow and pressure will build up in the reservoir. This is done to ensure that the sealing rock will not break. Within the constraints of the seal, the reservoir and aquifer, it is possible to calculate how much CO₂ can be injected into a potential storage site within a given time period, which is often set at 25 years.
The data availability, data quality and hence confidence is considered in connection with relevant business aspects, such as distance to infrastructure and preferences of strategic business alliances.
A full-scale reservoir simulation model, where the number of injection wells are optimised, is often both expensive and time consuming. Because of this, only the most promising CO₂ storage sites are modelled in the dynamic domain within the standard 25 years’ time frame for a business case.
Realistic storage potential: A Norwegian example
Let us explain this further with an example from a project we did for a client of ours. To be clear: We have Intellectual Property rights to the data, which is why we can share this information.
STEP 1: Hypothetical estimate
The Norwegian CO₂ storage atlas presents the estimated potential in this area to be 4.4 Gt (4.400 million tonnes).
STEP 2: Evaluating well and seismic data
An independent evaluation of the well data and seismic data available was carried out, integrating all aspects into a comprehensive understanding of the geology underground that forms the elements of a CO₂ storage complex:
the sealing shales and how much pressure they can hold before they break,
the reservoir, its quality and how much CO₂ it potentially can store with an endless time frame.
the aquifer, its quality, connectivity and how large it potentially can be.
The estimate is now 470 million tonnes, about 11 percent of the estimate in Step 1.
STEP 3: Reservoir simulation model
With all elements forming the static part of the CO₂ storage complex defined, we computed the storage capacity of the best prospects based on injectivity. This let us predict how the CO₂ will flow in the reservoir and how it will push the existing brine water already present underground out into the aquifer.
When we used our simulation model to predict the full CO₂ injection capacity over 25 years in the dynamic domain, we ended up with a final estimate of 200 million tonnes distributed over several closed traps. This is about 5 percent of the estimate in Step 1.
Is it all a guessing game?
Absolutely not!
Even though the more realistic potential is merely a fraction of the theoretical storage capacity communicated in national CO₂ atlases, the storage potential of both UK, Norway and Denmark is still considerable. When the storage capacity estimates at the different levels of maturity are fully understood by more people, it is likely that some business cases will be calibrated. But still, hundreds of millions of tons of CO₂ emitted and captured in Europe can be stored:
For instance, if only 5 percent of the estimated storage i UK, Norway and Denmark turned out to be realistic, then we would be talking about 8 Gt (8.000 million tonnes) CO₂ storage capacity.
Our core message is this: Finding quality storage sites require detailed insights at a local scale and in a regional context for the subsurface. This will make you able to rank the possibilities and select the best business opportunity. There are companies with experts already ready to target the best and commercially most attractive CO₂ storage sites. And as we all now: Time is of the essence to secure the best acreage.
