Sensitivity and Uncertainty of Petrophysical Models to Predict Storage Properties in Weathered/Fractured HardRock Aquifers

Weathered/fractured hardrock aquifers are complex, heterogeneous groundwater systems with overall low storage. This work develops a methodology using computer coding to estimate hardrock aquifers' storage properties applying Archie (AR) and Waxman & Smits (WS) petrophysical models, and further assess their sensitivity and uncertainty. The data consists in both surface and borehole hydrogeophysical measurements in a micaschist aquifer in Ireland. In this case study, WS appears as most suitable because it allows to account for the significant clay content present in the subsurface as a result bedrock weathering. This property is not accounted for in AR, which therefore largely overestimates the pore space. An important finding is the high sensitivity of both models to the cementation factor. WS models are in addition highly sensitive to the clay properties, namely the cation exchange capacity. Uncertainty analyses determined the higher uncertainty in the deep, high resistivity, massive bedrock, with WS being the most affected. Availability and accuracy of spatial data on the cementation factor and clay properties are key to achieve realistic storage models with high confidence.