Pore and core-scale investigation of CO2 wettability and residual trapping

Project Summary

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.

Key conclusions:

• 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.