Variable-density modelling of multiple-cycle aquifer storage and recovery (ASR): Importance of anisotropy and layered heterogeneity in brackish aquifers

Abstract

Summary Aquifer storage and recovery (ASR) involves injection of freshwater into an aquifer for later removal and use. If the aquifer initially contains brackish or saline water, then the density difference between fluids can cause the injected plume to “float” towards the top of the aquifer. This causes saltwater to intrude the well at the bottom, meaning freshwater recovery must be terminated prematurely. This study examined the density-induced flow phenomena in hypothetical “layer-cake” aquifers (i.e. horizontal layers of alternating low and high permeability). Despite low ambient TDS concentration (2857 mg L −1 ), density-dependent simulations resulted in lower recovery efficiencies than density-invariant simulations. However, results showed that greater permeability ratios (between high and low permeability layers) led to suppression of the vertical buoyancy flow and therefore higher recovery efficiencies. Each stratified heterogeneous aquifer was “simplified” to an equivalent homogeneous, anisotropic medium using the arithmetic mean for horizontal hydraulic conductivity and harmonic mean for vertical hydraulic conductivity. The results showed a very similar effect of suppressing vertical flow as was noted in the heterogeneous models. The simulated recovery efficiencies in the homogeneous cases were in excellent agreement with the explicit heterogeneous cases, with higher anisotropy ratios resulting in higher recovery efficiencies. The important conclusion is that the recovery efficiency of a simulated ASR operation is sensitive to whether or not the model considers density-induced flow and whether the aquifer is considered to be isotropic or not, but relatively insensitive to the representation of heterogeneity as either an explicit heterogeneous medium or an equivalent homogeneous (anisotropic) medium.