After the preliminary site selection step for geological storage of CO2, assessing site performance remains the main objective. Site performance assessments provides us with information on whether the site is safe and how much CO2 can be stored. But how does site performance assessment work?
The main tool for site performance is modelling of the underground. To get started on this, we first need to frame the starting point: the local geological context. The site needs to be described as detailed as possible in computational terms, especially the reservoir layer used to store the CO2. This is the first model representation of the storage complex, or the baseline or reference dataset. As the data on the underground characteristics are present but never perfect, there will be uncertainties in this early characterisation.
Next, we try to figure out what happens when CO2 is injected over a longer time under a set of different scenarios. We do this by telling the preliminary model that CO2 is injected, which then predicts the storage behaviour. By applying different scenarios, we can anticipate the actual behaviour as exhaustively as possible. We figure out CO2 migration and trapping but also potential leakage out of the reservoir. In this way, we get a first idea of the risk profile and capacity of the CO2 reservoir. Remember, at this point no CO2 has been injected in the reservoir yet. Only if the site characterisation and performance assessment indicate that under all imaginable circumstances CO2 injection is safe and leakage-free, should CO2 injection actually be permitted.
Although site performance models are rather complex, there are two essential parameters that it needs to simulate well: the reservoir pressure and the extent of the CO2 plume. The essential step for these parameters is to compare actual monitoring results with the outcome of the modelling.
Reservoir pressure is a system driving force and can induce stress variations in the reservoir that could be at the origin of fault reactivation, or fracture development or opening, and therefore of the creation of undesirable leakage pathways. Downhole pressure measurements can be used to calibrate the reservoir modelling, but other techniques are also possible. At the In Salah project in Algeria, InSAR satellite imaging measuring ground surface displacement was used to indirectly verify the 3D field pressure evolution, leading to a good fit between the computed performance assessment (pressure induced geomechanical effects) and the actual measurements (see Figure 1).
As the reservoir pressure is affected beyond the CO2 plume, we need to apply other techniques to know where the CO2 actually resides in the reservoir. The extent of the CO2 plume modifies the ground acoustic impedance in a more direct way than pressure. This characteristic is used in seismic monitoring, which picks up acoustic changes in the underground. Results from such seismic monitoring can be used to map the CO2 plume. This technique has been used very successfully in the Sleipner project in Norway, where early site performance models had to be adjusted substantially compared to the early versions to match the monitoring results. Monitoring enables better prediction modelling for both the short and the long term, and site assessment and monitoring are therefore an inseparable pair – for responsible CO2 storage, you need both.
In practice, early performance assessment scenarios tend to "pragmatic" predictions to reduce risks and exclude leakage. After a series of iterations fed by new monitoring inputs, models are improved and become more and more realistic. Full certainty on long-term site performance can never be achieved, but through the combination of performance and risk assessment with increasingly better monitoring data we can get as close as possible.
Jean-Pierre Deflandre (IFP Energies nouvelles)
Jean-Pierre is a geoscientist who is in charge of activities in the field of CO2 storage at IFP Energies nouvelles. He is Professor at Center for Exploration-Production at IFP School in the domain of "unconventional" hydrocarbons and in the management of produced CO2. During CO2ReMoVe he has managed the "Site performance assessment and mitigation" Sub-Project and the contribution of IFPEN to the whole European project being personally involved in the technical studies.