The vertical permeability of CO2 storage formations, and specifically those of confining layers, are key parameters influencing the effectiveness of structural CO2 trapping or the risk of leakage.
As a potentially attractive method to examine the integrity of a large CO2 storage site, accurate and long-term passive monitoring of pressure variations in response to barometric pressure fluctuations and earth tide effects may provide a means to assess the continuity of the confining units and their hydraulic properties.
The analyses suggest that the pressure fluctuations observed in deep boreholes may be used to infer hydrogeological and geomechanical properties. However, pore pressures induced by barometric and earth tide loading are controlled by the local hydro-geomechanical properties rather than the large-scale hydrostratigraphic features of the CO2 storage system. This includes medium scale heterogeneity due to the deposition of high and low energy facies as well as small-scale heterogeneities within facies. It is concluded unlikely that reliable estimates of vertical permeability and/or continuity of the confining layer can be obtained by analysing pressure fluctuation data.
Key conclusions were:
- Loading effects from earth tides and barometric pressure fluctuations are instantaneous everywhere in the sub-surface and not due to fluid flow and pressure diffusion from the surface.
- Although these fluctuations are large-scale forcings, the induced pressure perturbations are controlled by local geomechanical properties and pressure dissipation, which is related to permeability and is determined by local hydro-geological properties.
- Hence, even with sensitive sensors and regionally induced gradients, the permeability estimates are related to the local conditions.
- Accurate long-term pressure measurements may be useful to estimate local permeabilities if analysed by a hydro-geomechanical forward simulator embedded in a robust inversion framework.