Flow of variable-density formation water in deep sloping aquifers: review of methods of representation with case studies

Abstract

Distributions of freshwater hydraulic heads have been, and continue to be, used in the representation and analysis of groundwater flow, including the flow of variable-properties formation water in deep sloping aquifers. Because of variations in pressure, temperature and salinity, the flow of formation waters in sedimentary basins is driven by potential forces resulting from pressure and elevation (topography) differences, by buoyancy resulting from density differences, and by osmosis caused by temperature and concentration gradients. Because of variations in both fluid and rock properties, it is not possible to define a flow potential to be used in representing and analysing the flow. Neither is there a force potential from which a flow-driving force can be derived, and which could be used in flow representation and analysis. Even if osmotic effects are neglected, buoyancy still plays a role which in certain cases may be important. In horizontal or near-horizontal aquifers, the lateral flow of variable-density water is correctly represented by freshwater potentiometric surfaces, and the vertical flow may be analysed using `environmental' hydraulic-head distributions. However, the flow of formation waters in sedimentary basins is mostly along bedding in confined sloping aquifers. In this case, short of calculating and representing either the velocity or the impelling force distribution, freshwater hydraulic heads remain the only tool available for representing and analysing the flow. By neglecting buoyancy, errors in flow strength and direction are introduced, which become significant if a dimensionless driving force ratio exceeds a threshold value estimated to be around 0.5. The review of representation and analysis methods for the flow of variable-properties formation water in deep sloping aquifers is illustrated with two case studies from the Alberta and Llanos sedimentary basins.