Input Definition
With the Input Definition form (1D MODEL > Horizontal Stress >Input Definition), you specify the input parameters and the calculation method that will be used to generate the horizontal stresses. You can chose from seven different calculation modes depending on the combination of your input parameters, and the rock properties definition that describes the (an)isotropy, external stresses and tectonic strain. You also have the option to calculate the horizontal stresses per rock type that you have defined for the 1D geomechanical model. The inputs options on the form are active or inactive based on your selected calculation method.
Specifying the input data
Overburden Stress Select the log for the Overburden Stress parameter.
Pore Pressure Select the log for the Pore Pressure parameter.
Isotropic Model input data
Static Poisson Ratio Select the log for the Static Poisson Ratio parameter.
Static Young’s Modulus Select the log for the Static Young’s Modulus parameter.
Calculation method Select the calculation mode from the drop down list.
- No tectonic strain (Biot=1) Also known as Eaton equation, this method is based on a bilateral constraint and is represented by the following equation:
- Isotropic tectonic strain (Biot=1) Tectonic strain values must be defined in the fields below, Ep_Shmin and Ep_SHmax.
Defined by the following equation:
- Tectonic strain
For Isotropic model tectonic strain values must be defined in the fields below, Ep_Shmin and Ep_SHmax. Defined by the following equation:
- <rock type dependent> (for isotropic model only) When selected, all the input options on the form are deactivated. Click on the adjacent Edit button to open the Rock Dependent Constants form. On the form, you define the input parameters for each rock type that you have defined in your lithological model.
Ep_Shmin Minimum tectonic strain value. Select a log if available or check the 'use constant' checkbox and enter a value in the adjacent field. Active only for calculation methods that include tectonic strain.
Ep_SHmax Maximum tectonic strain value. Select a log if available or check the 'use constant' checkbox and enter a value in the adjacent field. Active only for calculation methods that include tectonic strain.
Biot Coefficient Defined as ratio. It is the change in volume to the bulk volume under constant pore pressure. For the isotropic models, the default value is 1 and the field is read-only. For anisotropic models, you can select a log from the drop down list or check the 'use constant' checkbox to enter a value in the adjacent field.
Anisotropic Model input data
Calculation method Select the calculation mode from the drop down list.
- No tectonic strain (Anisotropy) With this method, you define the anisotropy for the rock model. Defined by the following equations with stiffness tensors:
- Tectonic strain For Anisotropic model this method requires you to assign the tectonic strain inputs, Ep_Shmin and Ep_SHmax, along with the anisotropic model inputs. Defined by the following equations with stiffness tensors:
- External Stress With this method, you define the anisotropic model inputs along with the external stress inputs for Shmin and SHmax.
- External Stress Gradient With this method, you define the anisotropic model inputs along with the tectonic stress gradient for Shmin, and the SHmax tectonic stress.
Young's Modulus (A) Defines which Young’s Modulus values are used for the anisotropic stress calculation. Select the source from the drop‑down list: Dynamic (calculated) or Static (converted). By default, dynamic values are used. If Static (converted) is selected, the algorithm applies the conversion equation defined for the current rock type in the specific Rock Dependent Constants view. The conversion equation can use standard rock property input parameters as well as the calculated dynamic Young’s Modulus value, which is exposed in the Rock Dependent Constants view as a dedicated variable. This dynamic value is always provided by the system and cannot be replaced by a log or constant. The resulting converted static value is propagated through the calculation chain.
Poisson’s Ratio (A) Defines which Poisson’s Ratio values are used for the anisotropic stress calculation. Select the source from the drop‑down list: Dynamic (calculated) or Static (converted). By default, dynamic values are used. If Static (converted) is selected, the dynamic value is converted to a static value using the equation specified for the current rock type in the specific Rock Dependent Constants view. The conversion equation may include standard rock property parameters together with the calculated dynamic Poisson’s Ratio, which is available in the Rock Dependent Constants view as a predefined variable. This dynamic input is always taken from the calculation and cannot be substituted by a user‑defined log or constant. The converted value is used in subsequent calculations.
Biot Coefficient (A) Defines which Biot Coefficient values are used for the anisotropic stress calculation. Select the source from the drop‑down list: Dynamic (calculated) or Static (converted). By default, dynamic values are used. If Static (converted) is selected, the dynamic value is transformed using the rock type–specific conversion equation defined in the specific Rock Dependent Constants view. The conversion formula can combine standard rock property parameters with the calculated dynamic Biot Coefficient, which is available as a standalone parameter in the equation editor. This dynamic value is system‑generated and cannot be explicitly assigned from a log or constant. The resulting value is then used in the horizontal stress calculation.
Ep_Shmin Minimum tectonic strain value. Select a log if available or check the 'use constant' checkbox and enter a value in the adjacent field. Active only for calculation methods that include tectonic strain.
Ep_SHmax Maximum tectonic strain value. Select a log if available or check the 'use constant' checkbox and enter a value in the adjacent field. Active only for calculation methods that include tectonic strain.
Biot Coefficient Defined as ratio. It is the change in volume to the bulk volume under constant pore pressure. For the isotropic models, the default value is 1 and the field is read-only. For anisotropic models, you can select a log from the drop down list or check the 'use constant' checkbox to enter a value in the adjacent field.
Once you have filled the required inputs for the Stress Contrast model, click Show to open a new Stress Contrast view showing the corresponding output Shmin and SHmax curves along with the input logs that you have selected on this form. Click OK to save the settings and close the form.
Depending on the selected calculation method, you can either proceed to Isotropic Model form , or use the Anisotropic Model button group to further provide additional model inputs and select the corresponding horizontal model outputs.