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Conceptual Model Development for MODFLOW or FEFLOW models FEFLOW Conference September 2009 Wayne Hesch Schlumberger Water Services

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Page 1: Conceptual Model Development for MODFLOW or …novametrixgm.com/pdfs/brochures/FEFLOW_Conference... · Conceptual Model Development for MODFLOW or FEFLOW models FEFLOW Conference

Conceptual Model Development forMODFLOW or FEFLOW models

FEFLOW ConferenceSeptember 2009

Wayne HeschSchlumberger Water Services

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OutlineIntroduction

What is a conceptual modelGroundwater modeling workflows

Numerical modelingConceptual modeling

Benefits of Conceptual ModelingFuture DevelopmentQuestions

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IntroductionIn order for a groundwater model to be accurate, reliable, and robust, it requires a tremendous amount of information and understanding of the aquifer. The first step in developing a groundwater model, and perhaps the most important, involves the design of a conceptual modelConceptual modeling is often overlooked => modelers constrained by selected simulator, and/or a specific numerical grid or mesh Conceptual modeling can lead to more efficient model development, and opportunity for multiple interpretations and multiple discretizations.

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Build a Conceptual Modela conceptual model is a hydrogeologist’s mental representation of the groundwater flow systemalways sketch the system and augment this representation with:

distribution of hydrogeologic layers,location of boundaries,2D/3D representation of the domain,plan vs. cross-sections,tables of parameter input values,

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Conceptual Model: Definitions“A conceptual model is a simplified, high-level representation of the site to be modeled…”“The conceptual model represents our best idea of how the aquifer works.”“A conceptual model is a basic graphical representation of a complex natural aquifer system that can more easily be adjusted prior to dedicating the effort in developing the numerical model.“

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Why Create Conceptual Models?Simplify the field problemOrganize field data so that the system can be analyzed more easilyThe closer the conceptual model approximates the field situation, the more accurate is the numerical modelStrive for parsimony – simplest is best, but retain enough complexity to adequately reproduce the system behaviorFailure of numerical models to make accurate predictions can often be attributed to errors in the conceptual model

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Numerical Model Developmentthe conceptual hydrogeologic model is the most important step in groundwater model processit forms the basis for developing the numerical modelan increased level of effort in creating the conceptual model reduces the effort calibrating the numerical model

Level of EffortConceptualModel

NumericalModel

“Everything should be made as simple as possible, but not simpler.” – Albert Einstein

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Developing a good conceptual model requires you to compile detailed information on

geologic formationsgroundwater flow directionshydrologic boundaries (recharge, rivers, lakes, wetlands, …)hydrogeologic parameters (conductivity, storage, porosity, …)extraction or injection from wells (location, depth, screens, rates), andobservations of groundwater head and water quality

Conceptual Model

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vast amounts of data generated from numerous sources in a variety of formats

Field, Analytic, SpatialEg: GIS, CAD, Gridded files, spreadsheets, databases

added complexity of multiple projects and changing conditions over timedetermining which data is needed for the groundwater modelgathering the required data from other applications in the correct format to import into the modeling software package

Conceptual Model Challenge

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The Groundwater Modeling Process

Build Conceptual Model

Assign Model Parameters

Collect Data

Design Model Grid/Mesh

Assign Boundary Conditions

Define Objectives

Yes

No

Predictive Simulations

Post Audit?

Calibrate and Validate Model

Sensitivity Analysis

Suitable?Yes

NoSuitable?

Yes

NoSuitable?

Yes

NoSuitable?

After Anderson & Woessner (1982)

Conceptual Modeling

Numerical Modeling

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Traditional Approach - Numerical Modelingwith numerical modeling designing the grid/mesh is the first stepthe disadvantages of this approach include:

The correct grid/mesh must be generated before assigning properties, boundaries, wells, etc. If the grid/mesh is modified after other inputsare defined, you will need to “check andre-work” those input elements, to see thatthey are still in the appropriate locationGenerally the input elements are not easilymodified, typically you need to delete them and then re-assign them

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Numerical Modeling Workflow (FEFLOW)

model?

Another

Define Numerical Model• Develop mesh• Define the Property Zones• Define Boundaries (rivers, wells,, …)

Define SuperElement Mesh Define 2D Mesh

Define Slice Elevations

Input Data• Import shapes, wells, surfaces,

XYZ points, cross-sections• Digitize new GIS layers

Define Property Zones

Define Flow Boundaries

Run Simulation &Analyze Results• Run FEFLOW• Check (visualize) results

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Conceptual Modelingwith conceptual modeling, designing the gridor mesh is the last step

Advantages:define the conceptual model boundary, and model inputs independent of any numerical grid or meshprovides the freedom to design multiple conceptualizations of your site, and easily change your conceptual modeldefine multiple grids or mesh types, each with different resolution and size, and choose the most appropriate onetransfer the conceptual model, and the desired numerical grid/mesh, to the numerical modelAbility to change the simulator, based on the project needs

all model inputs including properties, wells, and boundary conditions are assigned to the selected grid/mesh automaticallyresulting MODFLOW or FEFLOW input files are generated

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Conceptual Modelingother advantages:

if you are not happy with the grid/mesh, you can design a new one and re-generate a new numerical model using this new gridthis flexibility is not possible with classical numerical modeling, as it would require you to build and manage multiple numerical models

Easily change your model after it is createdraw data are left in tact and grid/mesh-independentEasily expand size of the model domain, vertical discretization, and the model inputs can be easily regenerated from the conceptual objectsif the project objectives change, a new numerical model can be easily generated, or existing ones updated, from the conceptual objectsit allows for translating the conceptual model to FEFLOW or MODFLOW, with vertical layers that follow the geology or are layer-independent

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Workflow: Data → Conceptual → Numerical

Run Simulation &Analyze Results• Load the files into VMOD/FEFLOW to

run the simulation• Load results into Hydro GeoBuilder for

visualization and interpretation

Numerical Model (MODFLOW/FEFLOW)• Apply a grid/mesh• Assign the conceptual model to the grid• Create input files for the simulator

(MODFLOW/FEFLOW)

Finite Differences

Finite Elements

Input Data• Import shapes, wells, surfaces,

XYZ points, cross-sections• Digitize new GIS layers

Structure

Define Conceptual Model• Define the Geology: Coverage and Horizons• Define the Property Zones• Define Boundaries (recharge, pumping wells)

Properties Boundary Conditions

Define Model Domain• Define the region where you want to

run a model simulation

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Conceptual Model Structure

define horizons from surfaces horizon truncation rule determines hierarchy; in case of intersections, which will be pushed up/down, or be truncated by surfaces above/belowseveral horizons types accommodate various geological conditions (pinchouts, discontinuous layers)

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Conceptual Model: Generating Geologic ModelDefine surfaces

by interpolating XYZ pointsfrom well unit contactsfrom cross-sectionsImporting .DEM, .GRD, etc.

Convert to horizons

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Conceptual Model: Generating Geologic Modelload fence diagrams, cross-sectionsinterpolate contact points to create surfaces

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Conceptual Model Structure: Benefits

Model AreaEasily modify the size of the model=>Re-generate superelement mesh and slices=>Re-translate .FEM file input.

HorizonsUse native file formats to define surfaces, and resulting horizons (.XLS, XYZ points, ESRI .GRD, Surfer .GRD, cross-sections)Horizon rules simplifies modeling of complex geology

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Conceptual Model: Property Zonesuse shapefiles (*.SHP) or CAD polygons to define property zonesseveral methods for defining property zone values:

constant value (by layer)Use shapefile attributes2D interpolated surface (2D Grid)use 3D Gridded Data

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Conceptual Model Properties: Benefits

Flexible units for flow materialsVarious methods for defining inputNot assigned to a mesh/grid

If mesh changes, can easily re-generate FEFLOW input from conceptual model

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Conceptual Model: Boundary Conditionsuse shapefiles (*.SHP) or CAD polygons/polylines to define boundary geometry and attributesseveral methods for defining boundary conditions:

constant valueuse Surface (river stage from DEM)use time scheduleuse shapefile attributes

Assign values to entire zone or vertices on lines (eg. River gauging stations)Assign geometry to side faces of model domain

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Conceptual Model Boundary Conditions: Benefits

Flexible units for flow rates, heads, etcVarious methods for defining inputWork with combination of data objects and operations minimized pre-processing in GISNot assigned to a mesh/grid

If mesh changes, can easily re-generate FEFLOW input from conceptual modelCan move boundary objects (eg. Groundwater divide)

Pumping wellsScreen locations and pumping rates are mesh-independent: if mesh changes, FEFLOW input can be easily re-generatedDuring translation to .FEM file:

well screens are assigned between appropriate slicesflow rates are distributed accordingly for multi-layered wells(no need to assign wells on layer-by-layer basis)

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Define Numerical ModelSelect simulator and define appropriate grid or meshMODFLOW

Define horizontal grid resolution, rotationRefine grid, or define local grids Define vertical layers

Use HorizonsIndependent of geology

FEFLOWDefine superelement meshDefine 2D Horizontal meshDefine 3D Slice elevations

Using HorizonsIndependent of geology

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Benefits of Grid/Mesh Generation

Deformed layer elevations automatically taken from conceptual modelGenerate model layers independent of the geologic structureMin layer thickness enforced, in pinchout regionsAdvanced vertical refinementIterative approach

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From Conceptual Model to Multiple Numerical grids with MODFLOW properties

Property zones in theconceptual model

Semi-uniform Grid•deformed top and bottom layers, uniform in middle•Useful for discontinuous layers (common in unconsolidated aquifers)

Uniform Grid•Flat layer top/bottoms•Fully respects FD assumptions•More layers, but useful for transport/density dependent simulations

Deformed Grid•Layers follow geology•Easy, few layers•Problems with pinch-outs and cell aspect ratios

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From Conceptual Model to Multiple Meshes

Property zones in the conceptual model

Semi-uniform•deformed top and bottom layers•uniform in middle•Property upscaling is applied•Useful where Deformed mesh fails

Deformed Mesh•Layers follow geology•Easy, few layers•Convergence issues with tight geometry/water table fluctuations

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Property TranslationWith numerical modeling, properties in pinch out layers have to be assigned manually.With conceptual modeling, properties are assigned to 3D Volumes. During translation, for layers that “pinch out”, the properties are automatically assigned from layers above/below (depending on minimum layer thickness and horizon rules)

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Property Upscaling:

Algorithm to Satisfy Darcy’s Law on Element LevelFor each finite element

Calculate all property zones intersected by the element (even the thinnest ones are taken into account)Upscale horizontal conductivity using parallel connection rulesUpscale vertical conductivity using sequential connection rules – using a weighted average of zone values intersected by finite element

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Numerical Property Upscaling

Zone lines

Grid lines

Zone=1

Zone=2

Zone=3

1 2 3

4 5 6

Elements 1, 2, 3 get zone values calculated at their centers.Elements 4, 5, 6 use properties upscaled from all intersected zones (1, 2, and 3)

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Conductivity Upscaling

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Deformed Mesh – 5 layers Semi-Uniform Mesh – 10 Layers

Simple Budget Analyzer: Comparing Meshes

2.75% difference….more in future work

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Future DevelopmentFully conceptual, simulator-independent approach to building a groundwater model Current implementation supports USGS MODFLOW and FEFLOWFEFLOW: supports 3D mesh design, flow materials, and pumping wells

Future support for Type 1,2,3 boundary conditionsAdditional Analytical modelsAdditional Finite Difference/Finite Element modelsIntegration with surface water modelsSupport for Linked simulations using OpenMI technology

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Summarythe classical approach to numerical modeling starts with a grid or mesh and then assigns model properties and boundaries

for better local modeling the grid is refined over a number of iterations, which requires you to re-work property zones and boundariesthis can be a time-consuming/frustrating process

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SummaryA conceptual model improves the efficiencies of these iterations, by housing all data, and providing a visual environment

It helps with the up-front design of the model; more detailed adjustments are done on numerical levelIt can be considered as the common “root” for a family of numerical models, so it can also be used as a version control for modeling projects

the use of a conceptual model builder allows you to define mesh and grid-independent model location, flow properties, and boundary conditions

the model grid/mesh is assigned after these have been designedthis allows more flexibility in choosing grid orientation and discretizationgrid refinement is easy to apply to conceptual objectsit supports multiple conceptual models for determining the best approach to simulating a specific groundwater environment

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AcknowledgmentsCo-authors

Serguei Chmakov, Petr Sychev, Collin Tu, Marconi Lima, Schlumberger Water Services

DHI-WASY: Peter Schatzl and Support TeamThe workflow based approach was strongly motivated by powerful Schlumberger seismic to simulation workflows in the Petrel software (http://www.slb.com/content/services/software/geo/petrel/index.asp?)

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References

Anderson, M.P. and W.W. Woessner (1992) “Applied Groundwater Modeling: Simulation of Flow and Advective Transport”. Academic Press, Inc. New York.Visual MODFLOW 3D-Builder Users Manual: Schlumberger Water ServicesA New Generation of Waterloo Hydrogeologic Software. MODFLOW and More 2008: Ground Water and Public Policy - Conference Proceedings, Poeter, Hill, & Zheng -www.mines.edu/igwmc/ pp. 154-158http://www.twdb.state.tx.us/gam/GAM_GW_model.htmhttp://www.ce.utexas.edu/prof/maidment/GISHyd97/gms/gms.htmhttp://www.indygov.org/For more information on the OpenMI project, please refer to the extensive OpenMI website at www.openmi.orgFEFLOW. FEM File Format

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Thank youQuestions?