Geostatic_data

Contents

Data Structure: Geostatic_data
Description	geostatic set data structure
Usage	Geostatic_data NUM=ival where ival is the data structure number

Description

Overview

Geostatic_data may be used, in conjunction with Geostatic_control_data, Spatial_variation_definition and Spatial_state_set data structures, to specify an initial material and stress state. The objective is to specify a state that is approximately in equilibrium at the start of the simulation, typically requiring distributions of stress, pore pressure porosity and state variables to be defined or evaluated.

For simple cases, the geostatic state may be defined in a single analysis stage (step), and Geostatic_control_data may not be required. In more complex cases, e.g. with sliding contact, a multiple step initialization procedure may be optimal, or other features; e.g. displacement re-initialisation, may be required. In this case Geostatic_control_data must also be specified.

Initialisation with Geostatic_data is quite flexible and different initialisation data can be assigned to each element group. Initialisation can be either:

•Constant stress and pore pressure via Initial_stress and Pore_pressure respectively, or

•Dependent on gravity, in which case stress is defined via K-values ( K_value_x, K_value_y, K_value_z)

Porosity may be either:

•Constant,

•Defined as a function of depth (via Porosity_spatial), or

•defined via a spatial grid (see Spatial_state_set)

Note that if porosity is defined using Spatial_state_set then a two step initialisation procedure is recommended (see later)

Pore pressure may be defined as:

•"None" if no pore pressure data is specified

•"Constant" if Pore_pressure or Pore_pressure_spatial is specified.

•"Hydrostatic" if Overpressure or Overpressure_spatial is specified without specifying either Pore_pressure or Pore_pressure_spatial.

For geomechanical only drained and undrained simulations the default pore pressure is "Hydrostatic" if gravity is specified.

An overpressure may also be specified if the pore pressure distribution is "Hydrostatic" (see Overpressure or Overpressure_spatial keywords for further clarification).

Stress Initialisation with constant porosity

Stress initialisation with constant porosity may be performed for cases with and without gravity.

•No Gravity: Stress and pore pressure are specified using Initial_stress and Pore_pressure respectively.

•Gravity active: Stress and pore pressure may be initialised by specifying K-values ( K_value_x, K_value_y, K_value_z) and pore pressure using the Pore_pressure_distribution set to "Hydrostatic" together with over-pressure options if required.

Stress and porosity initialization using porosity vs. depth curve

In this case porosity and stress are both initialised using the data specified on Geostatic_data. The steps in the evaluation are:

•Evaluate the porosity as a function of depth for all elements

•Evaluate the vertical stress using density derived from porosity depth curve assuming uniaxial burial assumptions (no stress arching).

•Set horizontal stresses using K-values

•Initialise element plastic volume strain using stress state (if poroelastic), reference and initial porosity using uniaxial burial assumptions.

For this case:

•If the porous flow flag corresponds to a case with pore pressure then the pore pressure is also computed.

•The Porosity state is always initialized on the first step.

•The stress state can be either initialized instantaneously or applied gradually by defining a time curve ( Time_curve ). The latter is recommended if a fully geostatic state cannot be defined; e.g. sloping sediment surfaces, usage of contact surfaces (e.g. faults and detachments), etc.

Porosity Initialization via Spatial Grid

If the porosity is initialised using a spatial grid, this operation is performed prior to geostatic initialisation. The recommended initialisation procedure is then dependent on how stress is initialised; i.e.:

Stress defined via geostatic data:

•Perform a geostatic step assuming elastic material to evaluate the spatial distribution vertical effective stress. If k-Values have been specified then they will be applied to ensure that the horizontal stress state approximately satisfies these values

•Perform a second geostatic step with:

1 - Strength initialisation based on the elastic stress state and the reference and initial porosity (unless the state variables defining the yield surface have also been specified using the spatial grid)

2 - Relax the elastic stresses if the stress state lies outside the yield surface

Stress defined via spatial grid (including vertical stress). Perform a geostatic step with:

•Strength initialisation based on the elastic stress state and the reference and initial porosity (unless the state variables defining the yield surface have also been specified using the spatial grid)

•Relax the elastic stresses if the stress state lies outside the yield surface

Notes

•Several geostatic_data sets may be specified. This may be desirable for example when different geostatic properties are required for different geometry entities.

Name of the geostatic data set

Flag indicating whether the set is currently active

List of element groups (Optional)

List of group names

List of unit names

Initial constant stresses for assigned groups

Initial saturation for assigned groups

Phase IDs to identify target fluid phases of prescribed initial saturation in 3-phase flow cases

Name of spatial grid containing initial stresses

Type of pore pressure distribution

Type of saturation distribution

Gas-oil contact

Water-oil contact

Initial pore pressure for group

Initial pore pressure spatial variation assignment

Name of spatial grid containing initial pore pressure

Initial saturation spatial variation assignment

Name of spatial grid containing initial Saturation

Initial overpressure for assigned groups

Initial overpressure spatial variation assignment

Initial temperature for group or surface temperatue

Temperature gradient as a function of depth

Initial temperature spatial variation assignment

Name of spatial grid containing initial temperature

Time curve name or number

K-value in X direction

K-value in Z direction (2D) or Y Direction (3D).

Horizontal K-value spatial variation assignment in X direction

Horizontal K-value spatial variation assignment in Y or Z direction

Initial Porosity

Assigns spatial variation data to define the initial porosity

List of material names corresponding to facies ID

Assigns spatial variation data to define the initial porosity on a facies basis

Name of spatial grid containing initial porosity

Pre consolidation pressure flag

Flag denoting use of geostatic data with newly deposited groups

Flag defining whether the subsequent steps should use the initial stresses as a target

Flags defining whether the subsequent steps should use each initial stress component as a target