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Density driven flow in porous media: How accurate are our models?

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  1. Density driven flow in porous media: How accurate are our models? Wolfgang Kinzelbach Institute for Hydromechanics and Water Resources Engineering Swiss Federal Institute of Technology, Zurich, Switzerland

  2. Contents • Examples of density driven flow in aquifers • Equations • Formulation • Special features of density driven flows • Benchmarks • Analytical and exact solutions • Experimental benchmark: Grid convergence • Experimental benchmark: Fingering problem • Upscaling issues • Conclusions

  3. Density driven flows in groundwater resources management • Sea water intrusion • Salt water upconing under freshwater lenses (both on islands and inland) • Salt water fingering under playa lakes and saltpans • Flow around salt domes (nuclear waste repositories) • Brine injection • Leachate from waste deposits • Even the ordinary tracer experiment...

  4. Saltwater Intrusion Salt water Fresh water

  5. Formation of toe Fresh water Salt water

  6. Saltwater Upconing Fresh water Salt water

  7. Example: Salt Water Upconing on Wei Zhou Island Thesis Li Guomin

  8. Freshwater Lens Upconing

  9. Alternative Extraction Strategies

  10. Salinity backflow from Chotts in Tunisia Oasis

  11. 200 km Salt fingers on islands in the Okavango Delta

  12. Islands and salt crusts in Delta

  13. Schematic cross section of an island Transpiration Trona saltcrust Evaporation Increasing salinity of GW Increasing salinity of GW gravity vs. upward flow

  14. Instability on the Islands instable stable kf= 10-5 m/s, uET=10-8 m/s Critical Wooding Number:

  15. t=900 d cmax=11 mg/l t=6000 d cmax=54 mg/l t=8500 d cmax=75 mg/l t=16800 d cmax=235 mg/l t=32500 d cmax=350 mg/l t=66000 d cmax=350 mg/l t=2900 d cmax=30 mg/l t=12400 d cmax=110 mg/l t=25000 d cmax=350 mg/l t=46500 d cmax=350 mg/l Simulation of fingering

  16. Flow in the vicinity of a salt dome Recharge Discharge Salt water - fresh water interface Top of salt dome With density difference No density difference

  17. Basic Equationsexpressed in mass fraction c and pressure p • Mass balance total mass • Mass balance salt • Darcy law • Dispersion tensor • Constitutive relationships • Boundary conditions (many combinations) e.g. Possible simplification: Boussinesq approximation

  18. Features of density driven flow • Non-linearity • Consistency problem of boundary conditions • Rotational flow with closed streamlines • Plus all difficulties known from advective- dispersive transport

  19. Flow in porous media and rotation Darcy-flow in heterogeneous porous media is rotational Example: kf But we still have: In density flow, rotation is non-trivial: closed streamlines For constant k/m Rotational when r not parallel to

  20. Numerical solution and testing of codes • Analytical solutions • Exact solutions • Inter-code comparison • Experimental benchmarks • Grid convergence All computations are made with d3f, a density flow model using unstructured grids, finite volume discretization, multigrid solver, error estimator, automatic local refinement/coarsening, parallel computing

  21. Idea of „exact“ solution (steady state) Pressure equation Salt mass fraction equation Assume any differentiable functions p(x,z), c(x,z) Assume any domain Assign function values as first kind boundary conditions on boundary of that domain

  22. Plug functions into flow equations Pressure Salt mass fraction Right-hand sides are not zero: They are interpreted as source-sink terms So analytical expressions are exact solution for problem with - these source-sink terms and - first kind boundary conditions with given function values Only good if source-sink terms are small and do not dominate the problem

  23. Analytical expressions for „exact“ solution (steady state) Pressure Salt mass fraction Values in example tuned to make sources/sinks small: t=20, s=12, h=.14, b=1, r0=1, g=1, Dx=0.1, Dz=0.02, xs=1, zs=-0.1 In PDE: n=1, g=1, k=10, m=1, Dm=1, r/c=0.1, r(c)=r0+ r/c c=1+0.1c

  24. Analytical Pressure Values between 0 and 1.13 p units Salt mass fraction Values between 0 and 1 c units Plugged into equations for c and p

  25. Source-sink distributions Red: max. input Turquoise: 0 input Total mass Salt mass Red: max. input Light blue: 0 input Blue: output

  26. Computed (with 4 grid levels) Pressure Values between 0 and 1.13 p units Salt mass fraction Values between 0 and 1 c units

  27. Error Pressure Red : computed value too large by 0.004 % Blue: computed value too small by 0.005 % Salt mass fraction Red : computed value too large by 0.007 % Blue: computed value too small by 0.006 %

  28. Experimental benchmark • 3D transient experiment in box with simple boundary and initial conditions • Measurement of concentration distribution in 3D with Nuclear Magnetic Resonance Imaging • Measurement of breakthrough curves Drawback: Test of both model equations and mathematics Way out: Construction of a grid convergent solution inspired by the physical experiments

  29. Experimental setup • Cube filled with glass beads of diameter 0.7 mm • Size of model 20*20*20 cm3 • Injection of dense fluid on bottom center hole • Application of base flow via top corner holes • In unstable case: Injection from below and rotation • All parameters measured except transverse dispersivity, diffusion coefficient

  30. Experimental setup continued 20 cm 20 cm unstable situation stable situation Salt water

  31. Stable situation: Low concentration contrast

  32. NMR images of diagonal section: stable situation at low concentration contrast Injection Equilibration Flushing End

  33. NMR images of diagonal section: stabel situation at high concentration contrast Injection Equilibration „Entraining“ End

  34. Two modes No density contrast Large density contrast

  35. Experimental breakthrough curves Low contrast High contrast

  36. Comparison of computed and measured breakthrough curves Low contrast High contrast Choice of parameters within intervals given through measurements of those

  37. Comparison of concentrations along diagonal section Low contrast case, end of experiment

  38. Comparison of concentrations along diagonal section High contrast case, end of experiment

  39. unit 10-2 Grid convergence: Low contrast case Level # grid points 0 8 1 27 2 125 3 729 4 4,913 5 35,937 6 274,625 7 2,146,689 Dx at level 7: 1.56 mm

  40. Grid convergence: Low contrast case

  41. Grid convergence: High contrast case unit 10-2 Level # grid points 0 8 1 27 2 125 3 729 4 4,913 5 35,937 6 274,625 7 2,146,689 8 16,974,593 Dx at level 8: 0.78 mm

  42. Grid convergence: High contrast case

  43. Error of grid-convergent solutions Low contrast High contrast

  44. Unstable case: NMRI data

  45. NMRI of vertical and horizontal section: fingering experiment Horizontal section Vertical section

  46. Fingering: Experiment vs. Computation

  47. Finger velocity: comparison of experiment and computation

  48. Causes for poor perfomance • Numerical dispersion smoothes out fingers and eliminates driving force • Initial perturbance not known well enough • Start of fingers on microlevel, not represented by continuum equations

  49. Influence of heterogeneity on density flow • Homogeneous Henry problem • Heterogeneous Henry problem

  50. Definition of Henry problem: Homogeneous aquifer Hydraulic conductivity 1E-2 m/s, effective diffusion coefficient 1.886E-05 m2/s Boundary conditions: Left fresh water flux given at 6.6E-05 m/s Right hydrostatic salt water, salt mass fraction 0.0357 kg/kg