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.