When it comes to underground CO2 storage there are a number of factors related to the candidate storage medium which needs to be thoroughly investigated before the commencement of the CO2 injection, including: storage capacity, containment integrity, injectivity, the behaviour of the CO2 plume and how it would evolve.
ANLEC R&D commissioned research to assess innovative ways to evaluate these uncertainties.
This project examined the concept of using N2 as a surrogate for CO2 in trial subsurface injections to characterise a storage resource. The technique would make regulatory compliance much simpler.
- Two sets of four conventional unsteady state core-flood experiments were conducted, one set using CO2-water and the other using CO2-N2-water fluid systems.
- Three x-ray imaged core-flood experiments conducted using the CO2-N2-water system.
- Core-flood numerical simulations performed investigating the effect of a number of possible core-scale heterogeneities on the results of the core-flood experiments.
Dispersion coefficients were measured for the N2-CO2 system under reservoir conditions.
Key conclusions are:
- The levels of residually trapped N2 during an N2 trial injection may be an overestimation of that which may be achieved during a subsequent CO2 storage process.
- Buoyancy forces during N2 injection can reveal vertical pathways for subsequently injected CO2 and therefore gain a better site characterisation.
- Sensitivity to capillary pressure and heterogeneity appears to be determined by core properties rather than by invading fluid. Therefore, N2 trial injection provides an inexpensive and low-risk way of understanding the heterogeneity structure of a reservoir, which is a critical unknown for CO2
- The concept of site characterisation using the injection of an inert gas such as N2 may deliver valuable information to any CO2 geo-sequestration site.