Defining geomechanics (GEM™ only)

The Geomechanics form (simulate > GEM > Geomechanics) is used to assign a 3D grid and select the appropriate properties and settings for a geomechanics simulation which can be coupled to a reservoir simulation using the CMG simulators. JewelSuite can be used to set-up and define a linear elastic case both for one way or two way coupling.

The selected 3D grid for geomechanics is used to define an independent geomechanics grid (GEOGRID) in the deck file. Following CMG terminology, this defines a dual grid system, where reservoir simulation is carried out on the host grid (for JewelSuite, this refers to the 3D grid selected on the Create Case form), while the geomechanics calculations are done on the independent geomechanics grid. The dual grid system has several advantages for simulation, such as posing less restrictions on the reservoir simulation grid and generally simulating faster.

There are some restrictions for the independent geomechanics grid that can be selected in JewelSuite. These are mainly to ensure the geomechanics grid and the reservoir simulation grid occupy the same area in space, so that the CMG simulator does the mapping correctly between the two. The following are the restrictions on the geomechanics grid:

• It has to be unfaulted. (Faulted 3D grids are not available for selection.)
• It has to have the same origin (i.e., easting and northing location values) as the host grid.
• It has to have the same orientation (i.e., azimuth(GN)) as the host grid.
• It has to have the same total length in I and J directions as the host grid.

For the reservoir simulation grid/host grid (selected on the Create Case form), these geometric restrictions do not apply (it can for example be faulted). See Reservoir simulation using a JewelGrid for more information on how a faulted JewelGrid is used in reservoir simulation.

To setup geomechanics with an existing GEM case

1. Open the Geomechanics form.
2. The active simulation case is automatically selected as the Simulation case. Please ensure the correct simulation case is set to active, e.g., by selecting it on the Create Case form or in the Case Explorer.
3. From the 3D grid drop-down list, select the 3D grid suitable for the independent geomechanics grid (see the restrictions listed above). Only unfaulted 3D grids are available for selection. By default, None is selected meaning no geomechanics calculations will be carried out with the simulation.
4. From the Young’s modulus (matrix) and Poisson’s ratio (matrix) drop-down lists, select the 3D grid properties of interest.
5. Optionally, from the Biot's coefficient (matrix) drop-down list, select an appropriate 3D grid property. This property can either be real Biot's coefficient data, if available, or the Biot's coefficient (Reservoir Zone based) property generated by the geomechanical model when a Reservoir Zone is set.
   Important  Setting the Biot's Coefficient on the geomechanics 3D grid -in particular setting the Biot's Coefficient to 0 in non-reservoir regions, as the Biot's coefficient (Reservoir Zone based) property does- helps the simulator map between the flow simulation grid (host grid) and the geomechanics grid. These grids can differ in geometry, which can cause artifacts when mapping between the two grids. The Biot's coefficient can be used to indicate the simulator where the reservoir zone is located on the geomechanics grid, ensuring that flow simulation results such as pore pressure are not mapped outside the reservoir zone (in regions where Biot's coefficient is set to 0) on the geomechanics grid. This prevents artifacts like undesired additional deformation in the geomechanics simulation.
   The Biot's coefficient is represented by the *BIOTMAP keyword in the CMG deck, and is supported correctly from CMG version 2025.30 onward.
6. From the Coupling option (GCOUPLING) drop-down list, select the coupling option of interest. The options are:
• 0 (one way): By default this option is selected. One way coupling where the reservoir simulation is not updated with geomechanics results.
• 1: Two way coupling where porosity is a function of pressure, temperature and volumetric strain.
• 2 or 3: Two way coupling where porosity is a function of pressure, temperature and total mean stress.

See the CMG documentation for more details on the coupling options.

7. For Initial Stress Tensor, select the six components of the initial stress tensor in the JewelSuite convention. These have to be total stresses. When you select the S_NN component, the form will automatically try to select the other tensor components if they follow the same naming convention.
Geomechanics properties on the 3D grid can be populated using the MODEL > Geomechanics > Modeling workflow and MODEL > Geomechanics > Tools > Output Model > Output to 3D Grid. An initial stress state can be created using parts of the MODEL > Geomechanics > 3D Grid Geomechanical Properties workflow. The required information can also come from other sources or workflows. MODEL > Geomechanics > Tools > Convert Tensors can be used to convert other representations of the stress state (e.g., principal stresses and an azimuth from previously mentioned workflows) to a consistentJewelSuite tensor representation.
8. Optionally, you can define Deformation Rock Types for the Matrix and/or Fractures (when relevant). These are properties of the property type ‘Region’. An example of a property that can be used here is the Reservoir Zone property that can be created using the geomechanical model. The default selection for both is ‘None’.

When selecting a deformation rock type property, its IDs (e.g., visible in the JewelExplorer context menu of the property > Properties > Edit Classes) will be written to the simulation deck’s geomechanics section/include as an array of *GEOTYPE values, followed by a *GEOROCK entry for each ID with active values. For convenience, the original JewelSuite class names of the property are written as a comment to the deck as well. Note a separate *GEOTYPE array is written out for Matrix and Fractures when selecting both.

Without further specification, the *GEOTYPE and *GEOROCK entries in the deck will have no effect on simulation, but they provide a framework for the user to apply e.g., different material models to different zones, while defining these zones in JewelSuite. See the CMG help documentation for more information on using *GEOTYPE and *GEOROCK.

Important  When selecting both a Matrix and a Fracture property, their IDs will be combined. This means for example that if there is an ID ‘2’ for Matrix, and an ID ‘2’ for Fracture, they will end up in the deck as a single *GEOROCK 2. In some cases, such overlap will be desired. If not, you have to ensure unique IDs where needed.
Important  For the IDs, only positive integer values are allowed. This means, no negative values and no undefined values are allowed in the Deformation Rock Types properties. The range does not need to be continuous (e.g., IDs 1, 2, 4 – where 3 is skipped – is allowed).
9. When all parameters and properties are set, click Apply or OK. A validation will be run on the grid geometry and selections, and if any issues exist, error and/or warning messages are displayed explaining the issue.