Stress strain model -- FEM3D
Modeling of geomechanical response of the formation is performed by the GEOSIM module which consists of the stress analysis modules and the interface to TERASIM. The modular design allows optional stress codes to be utilized. Currently the principal module is the FEM3D code with the ENHANS 3 code as an option. FEM3D is a poroelastic and thermoelastic finite element code which treats elasticity and plasticity. Its features include:

· choice of linear, hyperbolic or tabular constitutive model for elastic analysis
· Mohr Coulomb or Drucker Prager shear failure criteria with nonlinear friction
· elliptical cap for compressive failure (compaction) with smooth transition to cone
· non-associated plasticity on the cone with hardening
· associated plasticity with hardening on the compaction cap
· regionally dependent constitutive models
· two methods of solving plasticity equations
· brick elements compatible with corner point geometry in the reservoir module
· direct or iterative solver
· time dependent surface loading to simulate construction or landfill
· temperature dependent constitutive models for compaction in steam projects
· Special joint elements with stress-strain behavior according to Barton-Bandis model
· Associated permeability model for joint elements
· Flexibility in specifying stress initialization
· Automatic generation of the mesh extension outside the reservoir. back to top

Module coupling
Several methods of coupling between the host reservoir model and GEOSIM stress model can be employed in order to optimize the performance of the system while representing the essential physics of the coupled processes. These range from the rigorous coupling between stress, flow and heat to loosely coupled treatment. Both the pore volume coupling (compaction or porosity enhancement) and coupling through flow properties (permeability changes due to stress or creation of fractures) can be represented. Some of the features available are:

· reservoir porosity (f) and/or permeability (k) function of effective stress
· loading/unloading hysteresis of f and k in the reservoir model
· loading/unloading hysteresis of f and mechanical properties in the stress model
· rigorous coupling between stress and flow or time step-explicit (lagged) coupling
· monitoring of fracture initiation, closure and reopening
· stress grid can overlap reservoir grid in all directions
· local grid refinement in reservoir model can be collapsed in stress model
Two methods of fracture coupling are available:

a) Time-dependent fracture growth can be generated by SIMFRAC or another conventional fracturing software. The time history of fracture geometry then becomes an input to GEOFRAC. In this method, fracture mechanics can be solved rigorously, but the coupling to flow and stress field is weak.
b) The fracture growth is modeled by transmissibility multipliers in fracture plane, which are a function of effective stress. This option is a part of the GEOFRAC module and does not require using a separate fracture mechanics code. It provides rigorous coupling with reservoir flow and stress, and is preferred for waterflood and thermal fracturing problems. back to top

Damage Model
The damage caused by sea water or produced water is modeled by a relationship between permeability reduction and injected water throughput. Its features include:

· flexible representation of damage law, which can be matched to field or lab data
· each well can inject different water of different quality
· any well configuration modeled by the host can be used. back to top

TERAPRO
The TERAPRO PC postprocessor is used to visualize the results of the simulation for the stress solution, and in case of the TERASIM reservoir model, also the flow solution. Its features include:

· plots for individual wells, groups, regions and aquifers
· case comparisons
· pressure, saturation and temperature 2-D and 3-D visualization
· stress and deformation visualization in 2-D and 3-D displays
· extensive editing and formatting controls
· Windows 95/97, NT product using MFC to provide PC look and feel

An example of the visualization of a subsidence study is shown below.

For visualization of the Eclipse reservoir flow results, user has a choice of using either the Eclipse postprocessing or TERAPRO, which can accept some Eclipse format files.


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