Table of Contents
The first step in creating a model is to define the boundary. Select Model->Define Boundary... or 
to open the Define Model Boundary dialog, Figure 7.1. In this dialog, give the grid dimensions and select the material to be used as the default material for the model.
If the model is to be an RZ axisymmetric model, then set Y min to 0.0 and Y max to 1.0.
Internal boundaries can be used to divide a model into regions. Material properties and initial conditions can then be defined by region. Select Model->Add Internal Boundary... or 
to open the Add Internal Boundary dialog, Figure 7.2.
The internal boundary is a plane that extends to the boundary of the model. Several options: Strike + Dip, Three Points, and Point + Normal are provide to define this plane. Figure 7.2 shows the input using the Strike + Dip option. The resulting internal boundary is shown in Figure 7.3.
For the Strike + Dip option, the data include:
Point on Plane - The coordinates of a point on the boundary plane.
Strike Azimuth - The degrees from North (the positive Y axis) in a clockwise direction.
Dip Angle - Degrees from horizontal of the plane. If the viewer is facing the azimuth direction, the dip is to the viewer's right.
For the 3 Points on a Plane option, the data include:
Points on Plane - The coordinates of three points on the boundary plane.
For the Point + Normal option, the data include:
Point on Plane - The coordinates of a point on the boundary plane.
Normal to Plane - The components of a vector normal to the plane. They do not need to be normalized.
You can now use the mouse (or Tree View) to select a region and define a material or initial condition to that region. PetraSim uses a hierarchy when writing the TOUGH input file properties for a cell. If a property has been defined for a specific cell in the Grid Editor, that is used. Next, the region in which the cell lies is used to define properties. Finally, the default model properties are used.
PetraSim has the capability to define the topography of the top and bottom of the model. Any cell whose center is above the top or below the bottom surface will be disabled (not included in the analysis).
Boundaries are useful if the user is modeling the surface of the earth or a deposit that varies with depth. Three options can be used to define the top and bottom surfaces: a constant Z depth, a linear function, or a contour file. The format of the contour file is described in the chapter "Petrasim Basics." Contour files for download are provided with the example manual.
To define the surface, select Model->Set Top and Bottom.... In the Set Top and Bottom dialog choose the option for defining the surfaces, Figure 7.4.
The solution grid is the discretization used to solve the analysis. You have the option of creating a "regular" grid or a "Meshmaker" grid. In a regular grid, the cell sizes are uniform or increase geometrically. In a Meshmaker grid, the size of each cell is specified.
Select Model->Create Grid... or 
to open the Create Grid dialog, Figure 7.6.
If top or bottom surfaces have been defined, the user is provided two options: either to disable cells above and below the top and bottom or to shift the grid to conform to the top and bottom. The first option is recommended, since it preserves the convergence properties of the rectangular grid. However, the shifted grid is an option that is often used in reservoir models if the flow is primarily in the XY plane. It uses the cells efficiently and the error introduced is usually relatively small.
The user can also select an XYZ (full 3D) or RZ (axisymmetric) grid. If the RZ grid option is selected, then there will be only one layer of cells in the Y direction. The model will be displayed as a plane, however, when the cell data is written for the solution, the correct cell volumes and connection areas are calculated that represent an axisymmetric model.
The required input is the number of cells in each direction and a factor used to increase or decrease element size. If the factor is 1.0, then all cells in that direction have the same size. If not, the relations in Figure 7.7 can be used to calculate the cell size that will result for a given initial size and factor.
The user defines the size of each cell when creating a Meshmaker grid. The cells are defined in the direction of increasing coordinate (X, Y, or Z) and the user gives the direction, number of cells, and size of those cells. This must be defined for each direction and the sum of all cell sizes must match the boundary length in each direction.
The user can edit the properties of a cell in the Grid Editor. Open the Grid Editor and right-click on a cell to display the context menu, xref linkend="Cell_01" xrefstyle="select: label"/>. Select Properties, Figure 7.10. The user provides:
Cell Name - A decriptive name that can be used to access cell results for plotting.
Cell ID - This is calculated by PetraSim, not editable.
X, Y, and Z Center - The center of the cell, not editable.
Volume - The volume of the cell.
Volume - The volume of the cell calculated based on dimensions, not editable.
Volume Factor - A multiplier on the volume that is used to obtain the final volume sent to the TOUGH input file.
Material - The material for the cell.
Type - Enabled (included as standard cell), Disabled (not included in analysis), or Fixed State (set as fixed boundary condition). See PetraSim Basics for further details.
Z Thickness - Allows the user to change the Z dimension of the cell. This is calculated by PetraSim for "shifted Z" grids.
Z Base - Allows the user to change the Z coordinate of the base of the cell. This is calculated by PetraSim for "shifted Z" grids.
The Sources/Sinks and Initial Conditions tabs will be described in the Boundary Condition and Initial Condition chapters.
The Print Options tab is used to output cell data every time step for the selected cell (this is the FOFT file as used by TOUGH). In addition, connection data can be written (this is the COFT file as used by TOUGH).
There are times when the capability to add non-geometric "extra" cells is useful. These extra cells can be used to define special boundary conditions or in other ways to "trick" the model into representing some special feature.
Support for "extra" cells is provided in PetraSim through dialogs. Since these cells are not geometric, the user must define the volume and connections of these cells to the regular grid cells. To create an extra cell, select Model->Add Extra Cell.... Figure 7.11 illustrates the definition of the basic cell properties. The user provides:
Cell Name - A decriptive name that can be used to access cell results for plotting.
Cell ID - This is calculated by PetraSim, not editable.
Volume - The volume of the cell.
Material - The material for the cell.
Type - Enabled (included as standard cell), Disabled (not included in analysis), or Fixed State (set as fixed boundary condition). See PetraSim Basics for further details.
The Sources/Sinks, Initial Conditions, and Print Options for an extra cell are the same as a standard cell.
The connections of the extra cell to the grid are specified by selecting the Connected Cells tab, Figure 7.12. The user must manually specify the connection data required by TOUGH. This includes:
To Cell - This is the cell to which the extra cell is connected. This will be the cell ID of a cell in the mesh. You can find the cell ID of any cell using the Grid Editor and then viewing the cell properties.
Orientation - This is must be 1, 2, or 3 and corresponds to the PermX, PermY, or PermZ definitions in the material data.
Dist. This - The distance of the connection in the extra cell. See TOUGH concepts "Spatial Discretization."
Dist. To - The distance of the connection in the connecting cell.
Area - The cross-sectional area of the connection.
Gravitational Acceleration - The cosine of the angle between the graitational acceleration victor and the line between the two elements. If positive, the extra element is above the connecting "To" element.
Radiative Heat Transfer - "Radian emmittance" factor for radiative heat transfer. Usually left as 0.0.
To edit an extra cell, double click on the extra cell in the Tree View.