Migration-assisted storage: opportunities are (almost) endless
21st November 2018
Topic(s): CO2 storage
In the pursuit of geological CO2 storage sites, migration assisted storage (MAS) offers the largest storage resource potential and spatial availability, globally.
MAS can trap and immobilise CO2 in a formation without a physical structure or stratigraphic closure to confine the CO2. A permeable reservoir overlain by a non-permeable seal are the only requirements for MAS. This pattern of permeable to non-permeable layers of rock is a standard cyclic pattern of the geological record associated with falling and rising sea levels which has happened continuously for eons. Geological structures suitable for MAS are common in the world’s sedimentary basins.
Storing CO2 with MAS
The main process for immobilising CO2 in a MAS scenario is residual capillary trapping. Residual trapping occurs in a few steps.
On injecting the CO2, the ‘free phase’ CO2 moves through the formation. The CO2 is pushed and pulled away from the injection well due to buoyancy forces (rising up the formation), injection pressures and pressure gradients. Once the CO2 is in the pore space (a pore space can be around 0.05-0.15 mm, but the range is huge, many times larger, or many times smaller) of the formation:
At this micron-scale, the storage of CO2 does not seem significant. However, typically, storage operations target rocks with greater than 10% porosity, with most current sites’ porosity being 20%. In a MAS scenario, the formation thickness could typically be over 50m (some up to 500m thick), and span hundreds of meters. That is a lot of little pore spaces and a lot of CO2.
The process of residual trapping is well known to the oil industry. As oil migrates from the ‘kitchen’ where it is produced to the field where it is stored, the oil leaves a trail of residual oil. Residual oil zones have low oil saturation and are challenging to produce. Ironically, CO2 enhanced recovery could unlock the full potential of these zones.
Residual trapping is the dominant process that immobilises the CO2 in a MAS formation. In addition to residual trapping are three additional trapping mechanisms that are active:
- Physical trapping along the reservoir-seal boundary keeps the CO2 from migrating out of storage area.
- As the migrating CO2 moves through the brine-filled formation, the CO2 dissolves in the unsaturated brine. Diffusion then happens over a longer scale.
- The CO2 reacts with the formation and mineralises forming solids.
MAS scenarios do not have a known confining accumulation point. However, knowing the geology and storage system (pressure gradients, water flow) means the movement of the plume can be predicted. For example, if the reservoir is slightly slanted (called dipping), the CO2 migrates up from injection point and then along the reservoir-seal interface, up-dip. Given the footprint of the CO2 plume, the area of monitoring needs to be larger, extending beyond the plume and within the predicted path of the plume. Alternative predicted migration scenarios need to be also assessed under MAS scenarios, and accounted for in monitoring plans.
The behaviour and movement of liquids and gases in the subsurface is very well understood courtesy of over a century of experience and research associated with maximising oil and gas production. Today, it is possible to predict with a very high degree of certainty how a given mass of CO2 will migrate through a reservoir before all further migration ceases due to MAS. Thus, the final extent and location of the CO2 plume can be predicted with confidence, especially if an appropriate margin of error is applied to represent the worst credible scenario. MAS has not been robustly tested from a regulatory perspective. It is likely that further research around the trapping mechanisms of MAS, along with demonstration of MAS scenario will be required before regulators are prepared accept it as the primary mechanisms for containing injected CO2.
MAS opens up a major portion of the world’s sedimentary basin for CCS, with the potential to very significantly increase the global distribution of CO2 storage resources and in many cases, reduce the distance between CO2 source and sink. In the absence of large structural traps, which most projects today have targeted, pursing MAS is a viable alternative as wide-scale CCS deployment gathers pace.