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Faults and Fractures: Contact Mechanics

Faults and fractures in ParaGeo may be represented using either continuum or discrete approaches. Robust and flexible contact algorithms enable characterization of the mechanical interaction between two or more contact surfaces as well as simulation of along-fault and cross-fault fluid flow and across-fault thermal transfer. Some of the key features in the contact mechanics framework are:

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  • Contact algorithms for Mechanical, Porous Flow and Thermal Fields

  • Node-to-facet algorithm for large-displacement simulations and node-to-node algorithm for improved accuracy in small-displacement simulations

  • Automated contact facet identification or definition of contact surfaces and contact sets (e.g. fault surfaces)

  • User-controllable contact surface search (e.g. exclusion of specific contact surface interaction) 

  • Contact models in normal and tangential directions defined independently for each field

  • Contact properties for all fields may be defined constant, as a function of depth, as a function of contact normal stress, as a function of time or as a function of the material properties of the adjacent mesh element.

  • Contact state variables are output to the plot file and can be visualised and post-processed in ParaView

  • History output of point values, integrated values and average values for contact state variables in each individual contact surface

  • Specialised treatment of contact surfaces for geostatic initialisation 

Visualisation of contact state variables

Mechanical contact laws focus on simulating the interaction forces between two contacting surfaces interacting with each other. This prevents mesh penetration and captures the frictional interaction of the surfaces when there is tangential displacement. Some features include:

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  • Different models for Normal contact, Tangential contact and Adhesion

  • Tangential models: Elastic, Mohr-Coulomb with and without cohesion, Maximum Stress yield criterion

  • Adhesion models: Elastic, Maximum adhesion stress, Debonding model with damage

  • All model properties may be defined as a function of depth, time, mesh penetration or as a function of the adjacent element material properties.

 

The flow contact algorithm enables simulation of fluid flow across and along contact surfaces with properties defined in the normal and tangential directions which are characterised independently. Some key features include:

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  • Normal flow matrix-to-fracture and fracture-to-fracture conductivities defined independently allowing different pore pressures in the fracture and matrix

  • Contact hydraulic conductivities may be defined constant or as a function of time and depth

  • Contact tangential conductivity may be decreased as a function of normal stress to simulate fracture closure

  • Contact tangential conductivity may be defined as a function of the contact gap

  • Contact hydraulic conductivities may be defined as a function of the underlaying element material (e.g. contact conductivity adjacent to a sand may be defined higher than the contact conductivity adjacent to a shale)

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The thermal contact algorithm enables temperature transfer across contact surfaces. 

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 ParaGeo allows import of FracMan discrete fracture networks into a continuum domain. During the import process, a geometry intersection algorithm is used to automatically resolve the fracture intersections of the tessellated fractures to automatically generate a 3-D mesh for the fractured volume.  The import algorithm can therefore capture the complexity in multiple-intersecting fracture networks. Furthermore, the inverse-modelling optimization workflows facilitate the upscaling of highly fractured domains with large number of fractures into continuum embedded fracture equivalent properties, thus enabling coarser meshes and lower computational times while taking into account the effect of fractures into the bulk response of the formation.

In this simulation a prescribed pore pressure of 10 MPa and 0 MPa are prescribed at the base and the top surfaces respectively. The contact tangential conductivity is defined to decrease with normal stress. At a given time a downward displacement is prescribed at the top surface, leading to a contact stress increase, which in turn decreases the contact tangential conductivity thus modifiying the flow pathways.

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