A sensitivity study on CO2 storage in saline aquifers

It is recognised that saline aquifers have the potential to store large quantities of anthropogenic CO2. However, there is much uncertainty in aquifer properties which needs to be evaluated to ensure the safety of CO2 storage.

In the CASSEM Project (CO2 Aquifer Storage Site Evaluation and Monitoring), we have studied CO2 storage in two hypothetical near-shore UK sites. The work was undertaken in three phases (or levels), starting with existing data and simple models and moving on to more complex models as more data became available. The sites had contrasting structures: Site A consisted of simple layers, while Site B was more complex with anticlines and synclines.

Here we focus on the models in Levels 2 and 3. In Level 2, we had geological models, but no laboratory data, so typical values for properties such as relative permeabilities were assumed. In Level 3, the results of laboratory measurements were incorporated in simulations. We modified the Level 2 models in stages, changing one parameter at a time, so that we could evaluate its effect on pressure build-up at the wells, CO2 migration, dissolution and residual trapping. Thus the study formed a valuable sensitivity assessment.

In the models for both sites, the most significant difference between the Level 2 and Level 3 results was the use of the relative permeabilities which had been measured in the laboratory. In the Site B model, where there was more uncertainty in petrophysical properties, a reassessment of the absolute permeability, which led to lowering the average value by a factor of four, also had a large effect. We also assessed multiple realisations of the geostatistical model, and found these had little effect in Model A, but had a large effect for Model B due to pressure building up to the limit for rock failure in wells in low permeability regions.