DYNAMICS OF SUBSURFACE FLOW OF SALT WATER AND BRINES

Building an understanding of the actual physics of subsurface flow is a confusing experience at the best of times, exposing one to conflicting statements from the sides of engineers, hydrogeologists, and, for a decade or more, by the followers of free convection and density-driven flow. In case of variable density flow involving salt water and brines, the confusion is magnified. It is generally assumed that, due to their higher density two systems of forces act upon salt water and brines, namely piezometric head forces and buoyancy forces. Presently, the buoyancy forces are always assumed to be directed vertically downwards for fluids heavier than the host fluid, or upwards for lighter fluids.

These assumptions are widely applied in engineering hydraulics (i.e. Bear 1972) and have, for example, been utilized in predicting the fate of CO₂ injections into brines leading to the widely accepted - but erroneous - conclusion that, due to higher density and associated weight increase, brine with the injected CO₂ would migrate downwards coming to rest at the bottom of the geological layer package. Hubbert (1953) has shown, however, that vertical buoyancy forces (balanced by gravitational forces) exist only in the hydrostatic case but not under the hydrodynamic conditions. In the hydrodynamic case forces due to density differences are directed along the piezometric pressure potential force of the host fluid and integrated into the resultant force calculation.

Hydrostatic (no-flow) conditions require the same boundary conditions for mechanical forces at the bottom of oceans (off-shore). Hydrodynamic flow conditions exist on land (on-shore).

This presentation will shed light on the maze of conflicting statements issued within engineering hydraulics and groundwater dynamics and will help foster the understanding of the correct physics involved and how this physics can be beneficially applied to practical cases regarding subsurface flow in general, hydrodynamic migration of contaminants, variable density flow, migration of hydrocarbons and CO₂, and to scientific processes in the present and within the geological past. It will also refer to a practical field case involving the numerical modelling of variable density flow at a major industrial landfill site.