The two primary CO2 trapping mechanisms active during the first several hundred years of a storage project are structural and residual trapping. In the case of residual trapping the CO2 plume is split into many micro-scale “bubbles” which are surrounded by brine and held in place by capillary forces.
In order to estimate leakage risk and storage capacities for a particular formation, buoyancy forces – which push the CO2 upwards – need to be compared with the capillary forces that hold the CO2 in place for such residually trapped CO2 .
Until now, such estimates and related reservoir models, which predict reservoir multi-phase flow, assume that all storage rock is strongly water-wet, which means that water spreads completely on the rock surface. This means that water can surround CO2 and trap it by pore-scale snap-off processes leading to residual trapping.
However, recent evidence suggests that under certain conditions the storage rock may not be strongly water-wet but can be intermediate wet or even CO2-wet. Moreover, pore-network modelling studies have predicted that CO2-wettability also strongly influences the efficiency of residual trapping i.e. with increasing CO2-wettability the amount of CO2 that can be stored by residual trapping rapidly decreases.
This project utilised several different techniques in order to reduce the uncertainty in measurements of the CO2 wettability of storage and seal rock and how this wettability is influenced by various parameters.
The nuclear magnetic resonance (NMR) response for a sandstone at reservoir conditions, at different CO2 saturation stages, was measured for the first time. In addition, the project also measured supercritical CO2 / sandstone/brine capillary pressures.
- Understanding the wetting characteristics of in-situ reservoir rock is important if structural and residual storage capacities are to be known accurately.
- Structural and residual trapping are likely to be viable storage mechanisms in clean quartz, and siliciclastic rock reservoirs that exhibit weakly water-wet characteristics.
- Certain parameters have a greater effect on wettability than others. Rock surface chemistry is judged to have a very important effect, with pressure and brine salinity having important effects and temperature and surface roughness effects being significant.
- It was proven by micro-computed tomography experiments that residual trapping is a viable storage mechanism at the pore-scale in clean sandstone.