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Mechanical Earth Models (MEMs)

Mechanical earth models (MEMs) are constructed to evaluate the change in stress, strain, pore pressure, temperature, etc as function of drilling, completion and production operations.  This provides benefits during well and production planning and can significantly reduce risk and cost, thereby increasing operational efficiency.


Models may vary in terms of size and also in the level of detail contained within the model; e.g. basin-scale models defined by formation stratigraphy or finer-scale models where faults and fractures are represented as discrete features. They may also vary in dimension, from multiple 1D models at different locations in a field, 2D sections covering main areas of interest or full 3D models. 3D models may also be used with sub-models where the stress, displacement and pore pressure history from a large-scale, coarse-grained model is used to drive a smaller-scale model with high definition; e.g. to study deformation in the region of a wellbore.


The workflow for construction of a 3D MEM is generally subdivided into several steps including:

  1. 1D modelling of individual wells to calculate mechanical properties from log data;

  2. Construction of the 3D geometry comprising reservoir and non-reservoir formations from a structural/geological model which is typically based on seismic and borehole data;

  3. Upscaling and spatially assigning properties derived from the 1D MEMs to the 3D MEM;

  4. Estimation of the initial stress boundary conditions either from large-scale models or the world stress map.


Once populated, MEMs are generally calibrated to provide an initial “current-day” state which is evaluated using geostatic initialisation procedures.  This may include specialised initialisation techniques for faults and fractures, and also for materials which are in transient equilibrium due to long-term creep; e.g. salt.   This “current-day” state must be calibrated against field observations; e.g. from seismic data, well logs, leak-off sets or laboratory tests on core or cuttings samples. This calibration is facilitated in ParaGeo by automated optimisation procedures.


The complexity of the geomechanical constitutive models adopted is dependent on the deformation mechanisms active in the formations and simulations may require models to be dependent on the mechanical, chemical, thermal and fluid evolution.


Once initialised, the MEM allows assessment of the complete evolution of the stress, pore pressure and temperature state during drilling, completion and production operations.  This may be evaluated either in coupled thermo-hydro-mechanical analysis performed using ParaGeo, or in conjunction with external reservoir simulators that evaluate the pore pressure change in the reservoir formations.


The constitutive models for the matrix, fractures and faults in the MEM can be very sophisticated, capturing the mechanisms defining the nonlinear response of these elements to the evolving field state.  This allows evaluation of many key production metrics including reservoir compaction, surface subsidence, wellbore stability, fault reactivation, cross-reservoir fluid transfer, leakage of pore pressure through pre-existing faults and fractures as well as production related fracture propagation. 

Reservoir pore pressure and vertical displacement contours in a coupled MEM tutorial example demonstrating production and injection

Quarter-symmetry simulation of excavation phase in an open hole vertical wellbore using a displacement constraint relaxation boundary condition and application of mud pressure loading on the wellbore surface

ParaGeo's workflow includes submodelling (whereby a relatively small-scale model contained within a large-scale model is defined and extracted; the large-scale model providing the initial and/or boundary conditions for the submodel). In the MEM context this facility is used perform wellbore stability analysis at locations of interest using information from the calibrated MEM. To this end:

  • Initial and boundary conditions for the wellbore model are extracted from the MEM results

  • Principal stress orientations relative to the wellbore may also be derived from the MEM

  • The wellbore model geometry and mesh may be obtained from a parameterized Gmsh template that accounts for near vertical wells, inclined wells and horizontal wells with and without casing and cement

  • Initial conditions in the wellbore model are usually applied via displacement or stress constraints on the wellbore surface

  • The excavation phase is simulated via the application of a boundary condition that releases the displacement constraints and gradually ramps down the reaction forces from the initialisation stage

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