By Jason McIntosh

Simulating fluid flow, mass and heat transfer requires the synthesis of geological models with a multitude of parameters, the process is complex. So how can Leapfrogs modelling functionality be used to streamline it? 

Interoperability with FEFLOW and MODFLOW allows Leapfrog Hydro, Geothermal and Hydrology module users to interpolate initial simulation parameters and apply them to geologically constrained finite element and finite difference grids. For the purpose of this blog I will demonstrate the capabilities by modelling an aquifer system in Leapfrog Geo, simulating it in FEFLOW and viewing the time series in Leapfrog.

Aquifer systems are comprised of permeable porous water bearing aquifers and impermeable aquitards. Both have variable permeability and porosity within the sedimentary units they are comprised of, the units themselves pinch-out and diverge within stratified layers of sediment. Stratified drift aquifers are among the most challenging of such systems, as a result of the complex depositional environments they derive from.

Stratified drift aquifers

Stratified drift aquifer systems are comprised of glacial till, glaciofluvial sediments and glaciolacustrine sediments which are deposited during glacial advances and interglacial retreats. Glacial till consists of unsorted clay to boulder sized clasts which occupy a variety of pore spaces. The clasts are deposited either beneath glaciers or within glacial outwash plains with relatively minimal alluvial transformation during glacial advances. Like most poorly sorted sediments till has very minimal pore space, and thus usually forms aquitards. Glaciolacustrine sediments also form aquitards, they are deposited in low energy glacial lakes where clasts are uniformly very small with very little pore space between them.

Interglacial glaciofluvial sediments are deposited in alluvial environments during glacial retreats when comparatively consistent low energy rivers deposit uniformly sized clasts with ample pore space between clasts. These deposits typically constitute the aquifers in stratified drift systems provided they are saturated. The deposits include:

  • Long sinuous ridges of sand and gravel called Eskers, which are deposited in meltwater channels within glaciers or at glacier margins.
  • Kame terraces which are ridges consisting of sand and gravel deposited by glacial meltwater.
  • Outwash plains formed as meltwater deposits build up reserves of eroded sediment via aggrading rivers.
Lake Pukaki, New Zealand. Numerous sediment rich channels of the tasman glacier outwash plain draining into a glacial lake during the current glacial retreat.

Lake Pukaki, New Zealand. Numerous sediment rich channels of the tasman glacier outwash plain draining into a glacial lake during the current glacial retreat.

Consecutive glacial retreats and advances build up stratified suits of sediments comprised of low permeability and porosity till and high permeability and porosity interglacial sediments to form stratified drift aquifer systems with multiple perched aquifers. The permeability and porosity govern the initial hydraulic parameter values which differ greatly between and moderately within depositional environments. The interdepositional variation is the result of heterogeneous flow rates over time allowing for variability in the range of particles sizes falling out of suspension. Hence it makes sense to interpolate the flow parameters within the individual aquifers and aquitards.

Principal types of aquifers in the United states with stratified drift aquifers in pale grey.

Principal types of aquifers in the United states with stratified drift aquifers in pale grey. [source USGS] http://pubs.usgs.gov/ha/ha730/ch_a/gif/A004_us.gif

Model the geology

Refined aquifer deposits.

Refined aquifer deposits.

Firstly to domain the layers create a Geological Model (GM) and domain the aquitard and aquifer stratigraphy by building surfaces with the ‘Deposit’ or ‘Stratigraphic sequence‘ tools from within the ‘Surface Chronology’. Note FEFLOW grids can only be layer guided by deposits or stratigraphic sequences because the surfaces must continue to the model boundaries. The vein tool’s pinchout, minimum and maximum thickness options are ideal for surfacing aquifer borehole segments. The footwall and hanging wall surfaces can be extracted to the meshes folder where they can be used to create deposits.
To introduce more detail within the aquifers and aquitards refine a layer by right clicking ‘Geological Models’ and selecting ‘New Refined Model’ then add the layer you wish to refine, from the ‘Lithology to refine’ list. New surfaces can be added from the refined models ‘Surface Chronology’ the same way surfaces are added to the parent GM. Dividing the layers allows the unique characteristics of each subunits’ hydraulic parameters to be captured by varying the interpolant settings to suit the unique assumptions made for each.

Refined Geological model with high permeability aquifer units in blue and moderate permeability aquifer units in black.

Refined Geological model with high permeability aquifer units in blue and moderate permeability aquifer units in black.

Interpolate the hydraulic parameters

Once the domains are satisfactory the hydraulic flow parameters; x conductivity, y conductivity, z conductivity, drain/fillable porosity and specific storage due to compressibility effects can be interpolated for each domain. I have used the spheroidal interpolant with no drift within the aquifer units to capture the assumed finite range of the aquifer and applied minimum evaluation limits to insure the interpolants don’t recede below each parameters assumed minimum. For the aquitards I used a constant drift spheroidal interpolant with a query filter applied to take out the aquifer borehole segments. In order to capture the relatively strong inferred continuity in their hydraulic properties I have applied anisotropy to both the aquifer and aquitard interpolants with either a structural trend or a planar trend.

High permeability aquifer specific storage spheroidal interplant with a moderate anisotropy structural trend.

High permeability aquifer specific storage spheroidal interplant with a moderate anisotropy structural trend.