Geosim

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GEOSIM® is a coupled reservoir, geomechanics, fracturing and reservoir damage software for the analysis of reservoir problems which include strong coupling between reservoir flow and formation stress, deformations, compaction or stress-dependent properties, or interactions with fractures resulting from stimulation treatments, waterflooding or waste injection. The system is currently capable of modeling both single well and full field problems.

GEOSIM was originally developed at SIMTECH Consulting Services in early 1990s and later at Duke Engineering & Services (DE&S). It is now owned, developed, used for studies and leased by TAURUS. New products that have been released since 2003 include the version using Eclipse® simulator as the reservoir module, version with treatment of formation damage for sea water and produced water injection, and a pseudo-continuum rock mechanical model suitable for coupling to a dual-permeability reservoir model for naturally fractured reservoirs.

Typically, the reservoir grid is a subset of the finite element stress grid, which will cover the overburden, flanks and possibly base rock. Each component has its own input and, correspondingly, the user can build the model input data in several parts pertaining to the reservoir model (Eclipse or TRS), the stress-strain model and its interface (FEM3D and GEOINT), the fracture interface (GEOFRAC) and the damage module. Depending on the problem solved, only the relevant parts of the system need to be used.

GEOSIM has been used extensively for full field compaction studies, fault reactivation studies, waterflood and waste injection studies, and geomechanical modeling of conventional and thermal fracturing for a variety of reservoirs throughout the world.

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. It also utilizes a Newton algorithm with variable stiffness scheme to handle general non-linear and elasto-plastic problems. Its features include:

  • Choice of linear, hyperbolic, hypo-plastic, orthotropic or tabular constitutive model for linear or non-linear type material
  • Elasto-plastic constitutive model with Mohr Coulomb or Drucker Prager shear failure criteria with nonlinear friction function, elliptical cap for compressive failure (compaction) with smooth transition to cone, non-associated plasticity on the cone with hardening and associated plasticity with hardening on the compaction cap
  • Regionally dependent constitutive models
  • Three methods of solving plasticity equations
  • Brick elements compatible with corner point geometry in the reservoir module
  • Direct or iterative solvers
  • 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
  • Special technique for tensile fracture modeling
  • Cavity or void space modeling option
  • Out of shear failure plane stress correction option with the aid of a special stress rebalancing scheme
  • Flexibility in specifying stress initialization
  • Automatic generation of the mesh extension outside the reservoir
  • Different loading options for initialization of stresses (gravity, water load, tectonics,…)
  • Option for faulted CPG grids
  • Option for locally refined grids
  • Parallel option for running on PC clusters

Fracture Modeling

Variety of fracture types and configurations can be represented in the model via the GEOFRAC module, as shown in Fig. 3 Fractures can be specified directly through data or imported from associated simulation through the SIMFRAC family of software. The features available include:

  • Static (propped, acidized) or dynamic (waterflood, waste injection) fractures
  • Full thermal treatment (including steam fractures)
  • Vertical (2-D and 3-D) or horizontal fracture geometry
  • Effects of poroelastic and thermal stresses, and nonlinear rock mechanics with failure
  • Fractures in X or Y direction, in the interior or at the edge of the grid
  • Fractures can be placed in a locally refined grid area
  • Multiply fractured horizontal or vertical wells can be modeled

Fractured Reservoir Option

The above features apply to a continuum representation of the reservoir for the solid modeling. A separate option is also available to model deformation and permeability changes of fractured reservoirs. The main features are:

  • Tensile fracture opening is modeled by extreme softening of the assumed fracture elements at tensile stress region, non-linear Barton-Bandis constitutive model is used to soften or harden the fracture stiffness at different stress regions
  • Any number of distinct regular fracture sets can be specified
  • Joint deformation follows Barton-Bandis joint model
  • Pseudo-continuum properties are derived internally using multiplane theory
  • Cubic model for permeability vs fracture opening is used
  • Deformation model is coupled with dual porosity flow model

Cavity Modeling

A void space within a continuum can be created by the cavity modeling option. The initial stiff material is switched to almost zero stiffness material and the respective confining stress is transferred to the elements surrounding the cavity region. The code also includes a procedure for weak modeling of sand production by combining the cavity, stress rebalancing options and fluid pressure gradient module. The quasi-static finite element model becomes evolutional when the fluid pressure gradient overcomes the confining forces at the sand face, through which propagating cavity is modeled

Modules' 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:

  • 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.
  • 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.

Damage Modeling

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

Post-Procesing

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 XP/7/8 compatibility

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.

subsidence.jpg
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