Defining a multi-layered aquifer in TOUGH2
I am trying to simulate a sandstone aquifer capped by two sealing layers (Shale and evaporite), so there are totally 3 layers in my model.
I think there are two blocks in the input file that can be used to define the system; ROCK and LAYER blocks. I used the following order to define the lithology of the system with corresponding hydraulic properties:
But I am not sure how to define the thickness of each unit in the input file. The thickness of the units are as follows:
Evaporite: 100m (top)
Snadstone: 500m (bottom)
I wonder if the following format is correct.
500. 150. 100.
I would be so thankful if you could check the input format mentioned above and let me know if it's correct or not.
The basic concept and workflow is as follows:
Block ROCKS simply defines material properties; they can later be referred to either by their names (EVAP, SHALE, etc.) or their ordering number within block ROCKS. Next, you generate a mesh, either by hand, using MESHMaker, or an external program. If you use MESHMaker, the vertical discretization is provided through block LAYER; each discretization layer (or "row" of elements will have the specified thickness (in your case, 500, 150, and 100 m; see the comments about discretization and stratigraphic layer thicknesses below). Terminate the meshgeneration step with ENDFI; the resulting mesh can be found in file MESH, which contains blocks ELEME and CONNE. Edit that file, and assign the material name or number (variable MA1 and MA2 in columns 16-20) to each element, either by hand or using a utility program such as AssignRock (downloadable at http://esd.lbl.gov/research/projects/tough/licensing/free.html).
Note that you need to distinguish between the discretization "layers" you define in block LAYER from the stratigraphic layering, i.e., groups of elements assigned to SHALE or SAND etc. The stratigraphic layer may be thicker than an individual row of elements, i.e., you may want to vertically discretize a stratigraphic layer into more than one model layer. The thickness given in LAYER is the thickness of a row of elements; the thickness of a stratigraphic layer is the sum of the element thicknesses assigned to a given material. Having only a single row of elements for each stratigraphic layer may not be a good idea; just think about the issues related to interface weighting of properties (see MOP(11)); I recommend you subdivide a stratigraphic layer into multiple discretization layers in block LAYER (e.g., replace the first layer of thickness 500 m by 5 discretization layers each of thickness 100 m), all of which later to be assigned to material SAND for a total stratigraphic layer thickness of 500 m).
Hope this helps.
If you would like to get fancy and have access to GMS (Groundwater Modelling System) you can create your geologic model and grid in GMS and then translate all the files in tough2 format using TMT2; see http://tough.forumbee.com/media/q4kbx/download/borgia---tips-and-tricks.pdf. You can also try Petrasim, which for simple problems does it all for you...
You could also have a look at PyTOUGH ( https://github.com/acroucher/PyTOUGH ): it is a set of entirely free open-source modules, written in Python. Several examples on usage are on the github page. Creating a mesh as the one you described above is really simple, but also more complex structures are easily possible. If you are interested, I can give you some help with getting started with it.
--- Florian ---