Decision making in CO2 resource exploration
At the exploration level, decisions are as a balance between the amount of CO2 which can be injected into an aquifer (the storage capacity) and the risks involved.
Whereas the theoretical storage capacity is easily estimated, it is the effective storage capacity (often much less due to limited storage efficiency) which finally counts.
Some risks are easily identifiable and resolvable (e.g., reservoir and seal presence), whereas the containment risk is a future prediction which needs to be quantified and managed, e,g, through monitoring.
Both storage capacity and risk are highly uncertain, which is why in Ariane EAAE for Carbon Storage we use a probabilistic approach to estimate:
- Pore volume,
- Pressure-volume-temperature (PVT) relationships,
- Pressure and seal balance,
- Containment risk.
Where and how to store CO2 in the subsurface?
For carbon storage resource exploration, we investigate various “containers”, from delineated structures with or without previous well penetration, possibly containing residual oil and gas (e.g., depleted fields), to injection into unstructured aquifers. Aquifers can be open or closed, and different trapping mechanisms are considered.
Structural and stratigraphic traps are delineated prior to injection using seismic data. Those traps might contain some hydrocarbons (non-economic discovery, depleted oil or gas field), or simply water-bearing. In Ariane EAAE, we propose exploration screening workflows to evaluate the risk and storage capacity for structured aquifers.
Unstructured aquifers can be delineated regionally. However, the potential for safe CO2 storage largely depends on the propagation the injection plume, and is generally much more limited (per area or volume) than for structured aquifers. In Ariane EAAE, we propose exploration screening workflows to evaluate the risk and storage capacity for unstructured aquifers.
Open versus closed aquifers
Structured aquifers or traps can be open, i.e., connected to larger aquifers beyond the spill point, and therefore provide large volumes of pore space to accommodate displacement of water during injection. However, structured aquifers can also be closed, and therefore need higher injection pressure to create space for CO2 by compressing the in-place water. Unstructured aquifers are open by definition, but can be more or less suitable to water displacement.
Trapping mechanisms vary from free-phase CO2, CO2 dissolved in water, residual CO2 on injection and migration pathways, to long term mineral reactions. They do not primarily depend on the structured or unstructured nature of the aquifer and can be overlapping.