2 Introduction

Hydrogeologists work with other geologists and engineers to develop a framework that provides the basis for defining the properties of a groundwater flow model in a two-step process; static modeling followed by dynamic fluid flow modeling. Unchanging properties of the geology are characterized in the static model, while the dynamic model simulates flow and adjusts hydraulic properties if changes in the flow system alters the properties.

For example, declining pressure during fluid extraction causes geologic materials to compact, reducing hydraulic conductivity and porosity (dynamic properties). Meanwhile, the amount of sand relative to shale in the reservoir will not change (a static property). A static model is built using geometric tools and is used to populate a flow simulator with hydraulic properties. A dynamic model is built using a flow simulator (for groundwater, MODFLOW is commonly used) and is used to adjust hydraulic properties by matching the simulation with measured data from the field so that the simulator will reasonably predict changes in the flow system in response to stresses. Traditionally the static model was developed by geologists then handed over to the engineers for dynamic simulation. However, experience has shown that this one-way process has some limitations as the calibration of the dynamic model can provide additional insight into the geology, and understanding of the geology can constrain the parameters required for calibration. Today the process tends to be more iterative. In the groundwater industry the modeler is often skilled in mathematics, engineering and geology, or a group of experts work together to develop the model.

Building groundwater models tends to be a geographically intensive process, with attention on map-view details and simplification of the subsurface into a series of aquifers or hydrostratigraphic flow units. This is certainly the most appropriate approach to regional or watershed scale modeling, and many sophisticated GIS tools are available for such work. However, these tools and methods can be cumbersome when dealing with site-scale models, which may have an area of only a few thousands or hundreds of square meters.

This book focuses on building contaminated-site-scale groundwater models using workflow concepts employed for static modeling of petroleum reservoirs. No particular software programs are advocated, as a large part of the work can be done with paper and pencil or generic contouring software. Many of the core concepts pre-date modern computers with graphical capability and haven’t changed with improved software.

The static modeling workflow is presented here as a series of steps building a model for a hypothetical contaminated site (the Test Site). This starts with the thought exercises needed to build a useful conceptual model, moves on to techniques for transforming the conceptual model into a three-dimensional (3D) numerical model, and culminates in the process of using the framework to create a flow model.


Geologic Frameworks for Groundwater Flow Models Copyright © 2020 by J.P. Brandenburg. All Rights Reserved.