HYDROGEOLOGICAL EVALUATION OF POSSIBLE CO2 LEAKAGE THROUGH THE SEDIMENTARY SUCCESSION OVERLYING THE REDWATER LEDUC REEF IN ALBERTA: PROOF OF HYDRAULIC SITE ISOLATION

Carbon dioxide capture and storage (CCS) is a climate-change mitigation option actively supported by the Alberta and Canadian governments for reduction of CO₂ emissions from large final emitters. Several CCS projects are now in various phases of site selection and characterization in Alberta. The Heartland Area Redwater Project (HARP) aims to store CO₂ in a very large Devonian-age reef, located northeast of Edmonton, overlain by a succession of Devonian and Cretaceous shales, carbonates, sandstones and unconsolidated Quaternary deposits. A detailed characterization of the hydrogeological regime, from the aquifer at the base of the reef to the ground surface, was undertaken to evaluate the potential for upward migration and leakage of stored CO₂.

Sodium-chloride (Na-Cl) brines are present in the Devonian and Lower Cretaceous aquifers, with TDS ranging from 40 to 110 g/L. Sodium-bicarbonate (Na-HCO₃) type waters, found in Upper Cretaceous aquifers, are of meteoric origin with TDS less than 20 g/L. Modern meteoric freshwater, predominantly Ca-HCO₃, is present in surficial and upper bedrock aquifers. Three dominant natural flow systems are evident: 1) gravity-driven flow controlled by topography in the Upper Cretaceous and surficial aquifers, 2) flow driven by erosional and glacial rebound in the Cretaceous Viking aquifer, and 3) flow driven by basin-scale topography and tectonic compression in the Lower Cretaceous and Upper Devonian aquifers. Clear distinctions in water salinity, hydraulic head distributions and variation of pressure with depth between the Upper Devonian, Lower Cretaceous, Upper Cretaceous and surficial aquifers indicate that the respective intervening aquitards, including two coal zones, are strong and competent, thus preventing hydraulic communication and cross-formational flow. This confirms, using hydrogeological evidence that no open faults or fractures exist in the sedimentary succession overlying the Redwater Leduc reef, which is the intended CO₂ storage unit. The system of aquifers and aquitards overlying the Redwater Leduc reef forms a major barrier comprised of secondary traps and barriers to CO₂ leakage through the geological system in the area.