VARIABILITY AND ESTIMATION OF FORMATION WATER DENSITY IN THE ALBERTA BASIN

Alberta basin strata consist of thick platform carbonates, with intervening shales and evaporites, overlain by foreland basin clastics. Formation waters reach conditions of 200°C and 60 MPa and contain over 350 g/L TDS in the vicinity of evaporitic beds. Previous mapping of the major ion chemistry of formation waters identified: 1) Na-Ca-Cl brines (Group I) in basal carbonate aquifers; 2) Na-Cl brines (Group II) in foreland basin clastics; and 3) Holocene meteoric water (Group III), emplaced into uppermost sediments by local-scale flow systems. Accurate estimation of formation water density and viscosity across such a wide spectrum of salinity, temperature and pressure conditions is essential for prediction of the fate of injected fluids, such as residual water, acid gas and CO₂, and for the delineation of hydrocarbon migration paths and accumulations. A high-quality set of over 8000 water density analyses was selected from more than 150,000 formation water samples collected by the Alberta energy industry. Linear relationships between TDS and water density were identified for each of 23 major aquifer systems in the basin. Measured density values were compared to predicted density values calculated from published density algorithms for NaCl solutions, considered proxies for basinal water. For Group I and II waters, the algorithms accurately predict measured brine density values for salinity values less than 70 g/L TDS at standard temperature and pressure. Measured density values of more saline waters are significantly denser than predicted values, which may explained by high proportions of Ca and K. Correcting for the proportion of CaCl₂ in formation waters allows for better predictions of measured density values of Group I waters. Calculations of kinematic viscosity showed that density and dynamic viscosity variations within a sedimentary basin can significantly affect the magnitude and direction of formation water flow, both on geological and human time scales, thus affecting exploration strategies and environmental assessments.