CO₂ Storage and the fast-moving Pressure Front
In conversations revolving around the green shift and CO₂ storage, the concept of ‘fast movers’ is frequently mentioned. However, an element that demands a central place in all discussions on CO₂ storage is the fastest mover of them all: the pressure front.
In ExploCrowd we are dedicated to share our insights and perspective on CO₂ storage with different audiences, fostering a transformation in how our business perceive the challenges associated with geological CO₂ storage. The ambition is to ensure that upcoming large-scale in CO₂ storage is developed at an informed level to safeguard against the wastage of substantial investments.
our simulations of subsurface CO₂ injection reveals that the pressure front travels notably faster through the saline aquifer than the CO₂ plume.
This carries important implications for determining the suitability of a reservoir as a potential CO₂ storage site.
Here’s what unfolds in the video snippet from our simulation model below
We start with a 200-meter thick, homogeneous aquifer without any barriers. CO2 injection takes place on the left side of the profile. The relief of the structure is 200 meters, and the model spans 50 kilometers from the injection point.
The water already occupying the pore space between the sand grains in the reservoir possesses very low compressibility, which means that water needs to be pushed out of the trap to make room for the injected CO₂.
Although the CO₂ plume in the upper left corner of the model is relatively minor, the model above illustrates that pressure front travels much faster than the CO₂ plume. On closer inspection, you will notice that the pressure front travels tens of kilometers in just 5–10 years.
Managing aquifer pressure
These findings have implications for the management of aquifer pressure in areas with multiple CO₂ injection license holders injecting into the same aquifer, and it becomes particularly intriguing when there’s an active oil or gas field operating in the same geological layer hosting the aquifer.
Securing early access for injection of CO₂ into an aquifer essentially translates into a competitive advantage for your business case.
Securing early access for injection of CO2 into an aquifer essentially translates into a competitive advantage for your business case.
- Sveinung Hatløy, geologist
Hunting for ideal reservoirs
Determining the ideal reservoir for CO₂ injection requires careful considerations of both reservoir quality, the project duration and the desired CO₂ volumes to be injected. Part of the process to assess whether a potential CO₂ storage reservoir is suitable or can be excluded early. And you want to spend your time efficiently.
When evaluating a drilled reservoir’s suitability for CO₂ injection, we are checking several aspects. One of them is reservoir thickness. To highlight the significance of sand thickness, two examples are shown below. Let’s start with the less favourable one in Example 1.
In our visualisation of the results from our petrophysical well interpretation method, we have chosen yellow to represent the layers of sand.
On the well log to the right, you can see two layers of ‘good sands’ with porosities ranging from 20-30%.
The issue here is that these two sand layers are relatively thin, each measuring less than 10 meters in thickness. Our experience tells us that even if these sands exhibit strong lateral continuity and internal connectivity away from the wellbore, their thickness is not sufficient to withstand the build up of the pressure front moving through the aquifer.
The consequence is a very limited CO₂ storage capacity.
It’s worth noting that between these two sand layers, there are generally good homogeneous shale layers, addressing that seal is not a concern for this site.
The second example is from another geological level within the same well, with several sand layers in Skagerrak Formation.
However, the issue here is the lack of lateral connectivity as the sands are very thin. The sands in this section are less than 1 –2 m thickness with only a few layers exceeding 5 metres in thickness, and most of them exhibit porosities below 15%.
This geological unit Skagerrak Formation is typically deposited in a fluvial setting, where sands are laid down by meandering rivers across floodplains. This depositional setting often results in poor to very poor lateral and vertical connectivity between the sands, resulting in a limited CO₂ storage capacity.
While some level of heterogeneity and baffles in the reservoir can be advantageous for distributing the CO₂ plumes, in this case, the gaps between the ‘good sand layers’ seems to be too big.
Consequently, if this well is representative for the surrounding geology in the aquifer, our interpretation of currently available data suggests that this specific location could prove to be a highly challenging offshore site with very limited CO₂ storage potential, if any.
In contrast to the challenges highlighted in Example 1 and Example 2, the reservoir in Example 3 could be suitable for CO₂ injection.
The example is from a well located onshore in Denmark.
Here, relatively thick interval of sands with very few barriers in between the sand packages are highlighted in the log section to the right.
It is sections with multiple sand layers with good porosity that geologists are doing their utmost to identify, as pressure can move far and fast through such sections.
The reservoir is currently in use for production of geothermal heating.
CO₂ injection could significantly interfere with geothermal activities here, as a consequence of the pressure front development. This again calls for thorough simulation and evaluation of the impact of CO₂ injection in the reservoir and aquifer.
What now?
To bolster confidence in both current and future CO₂ storage business endeavors, the paramount focus should be on maintaining the highest standards of data quality assurance and highest possible confidence precision in interpretations.
Determining an ideal storage site is undoubtedly a multifaceted challenge, but we firmly believe that by collaboratively sharing knowledge and insights, success is not only attainable but also well within our grasp.