The VISAGE^TM System

Basic Description of the Model

The VISAGE^TM System provides for two software options in studying hydraulic and thermal fracture propagation during reservoir simulations in both small (around wells) and large scale reservoir simulations. The first option caters for partially coupled simulations, whereby VISAGE^TM when linked to ECLIPSE, VIP, ATHOS and FRONTSIM forms the SIM2VIS System. The second option is to use a fully coupled stress/fluid/thermal multiphase flow module VIRAGE.

SIM2VIS

The SIM2VIS system offers can be used to study the effect of thermal gradients on preferred waterflood directionality, stress magnitude and orientation. This simulator assesses the effect of porous media deformation on fluid flow characteristics and is linked directly to ECLIPSE, VIP, ATHOS and FRONTSIM. Sophisticated 3-D reservoir models with complex pre-defined distributions of faults and a large number of gridcells (>500,000) are readily accommodated. During waterflooding, faults and fractures may become conduits of flow or transmissibility barriers if sealing occurs. The evolution of fractures is constantly traced with hydraulic parameters being updated, incorporating experimental data obtained from core samples to update permeabilities and rock fabric characteristics, as such fracturing develops. SIM2VIS accounts for changes in the effective stress state and rock fabric.

For example, in a SIM2VIS simulation, a staggered solution scheme is adopted where ECLIPSE performs the fluid flow and temperature calculations, using permeability fields that have been determined from a non-linear stress analysis using VISAGE^TM. ECLIPSE determines pore-pressure and/or temperature distributions which are used in the stress calculations to determine equilibrium levels of effective stress. If hydraulic fracturing takes place, normal and shear plastic fault/fracture strains are determined and used to enhance or reduce levels of permeability in the reservoir. If thermal fracturing takes place VISAGE^TM will take into account thermal gradients in the reservoir in determining distributions of plastic strain. The frequency of the non-linear stress calculations and the associated permeability enhancement calculations are at the discretion of the user when using the SIM2VIS system.

Stress Sensitive Reservoir Simulator

The VISAGE^TM System also offers a Fully Coupled Stress Sensitive Multiphase Flow Simulator, the VIRAGE module, for fracture propagation studies in a compressible non-linearly deforming porous media. The simulator is based on the finite element method and uses sophisticated Galerkin based numerical discretisation techniques to obtain fully coupled solutions to the mass balance, force equilibrium and plasticity equations of continuum mechanics. Incorporating experimental data from core samples the multiphase simulator updates the full permeability tensor as fault activation and/or fracture initiation develops during thermal injection. The thermal front is determined from coupled solutions of the advection diffusion equation governing thermal energy transport.

Porosity levels, together with system compressibilities may be continually updated in response to changes in volumetric stress and strain. Direct, symmetric and asymmetric solvers enable large problems, which may involve complex distributions of faults and fractures, to be solved effectively and efficiently. VIRAGE is also linked to ECLIPSE, FRONTSIM, ATHOS and VIP. Pre- and post-processors, FEMGEN and FEMVIEW, provide a wide range of mesh and visualisation techniques for complex, structured or unstructured model generation and interpretation of predicted results, which include deformations, stress/strain distributions, levels of induced pressure and saturation and vector plots of all the velocity components of all fluid phases.

Fault/Fracture Model

Fracture models in the VISAGE^TM System are available for both 2-D and 3-D simulations and use the finite element method to determine changes in fault and fracture apertures. The approach incorporates constitutive models for the rock fabric operating under the fundamental principles of visco-plasticity. In this manner the ‘intact’ rock, fracture sets and faults can obey independently different constitutive laws. Upon fracturing and/or fault activation, the simulator calculates normal and shear strains for each fracture set or fault. Normal strains represent the potential for fault opening and permeability enhancement, whereas shear strains represent the potential for fault sealing. The visco-plastic approach to solving ‘intact’ material non-linearity and/or fracturing is based on iterative procedures which determine successive solutions until any effective stresses that violate the imposed constitutive models are returned to the ‘yield’ surfaces within strict tolerances.

For soft sands a ‘damage’ mechanics theory, with or without a ‘cap,’ and based on multi-plane theories may be invoked to assess the potential for micro-fracture initiation and fracture evolution and propagation during thermal injection. Permeability and porosity changes in the rock fabric can be ascertained, which in turn alter the pressure and/or thermal fields. Both the VIRAGE and the SIM2VIS Systems have ‘access’ to these procedures in an attempt to assess the effects of fracturing in soft sands and in reservoir performance in general.