Modelling of deep subsurface for geohazard risk assessment.

Exploitation of subsurface natural resources, and subsurface storage of energy residues, may cause subsurface and surface deformation and damage to property. Deformation is generally difficult to assess and prove, although economical, environmental and societal interests are huge in terms of strain on granting concessions, (potential) damage claims etc. For accurate prediction of deformation we developed an integrated workflow for three-dimensional (3D) geomechanical modelling. The workflow integrates the tools for geological- and process modelling and allows efficient transfer of data between the shared earth models. We describe the workflow focussing on the techniques that facilitate construction of 3D structural models and conversion of structural models into quality finite element meshes. With a case study of depleting gas reservoir we illustrate the modelling process and the use of tools for structural modelling and finite element modelling. For the reservoir under consideration, both 2D and 3D geomechanical models were developed to study possible geohazards due to gas extraction. The 2D modelling study investigates the mechanisms of fault reactivation as a cause of induced seismicity in the area. The 3D modelling study focuses on investigation of the interplay between the remote tectonic stress and the man-induced changes in stress due to reservoir production. The 2D modelling study relies on a conventional 2D approach to the geomechanical modelling while the 3D approach makes use of the workflow and an advanced modelling approach. The 3D approach demonstrates that it is technically possible to incorporate both the full complexity of the 3D geological structure of a reservoir including faults and possible variations of the tectonic stress field