Geomechanics is the study of how subsurface rocks deform or fail in response to changes in stress, pressure and temperature. Therefore, geomechanics is an important input for optimal exploration and development of hydrocarbon reservoirs. The Zechstein formations exhibit different rock deformation behaviours due to varying sequences of evaporites that cause significant drilling challenges, increasing drilling time and associated costs. Commonly used 1D analytical geomechanical models have been inadequate for explaining the drilling experiences in the Zechstein formation. We therefore examine a dynamic 3D geomechanical model to explain the behaviour of these formations.
The Zechstein sedimentation sequence is typical for an evaporite basin (from top): clay, salt, sulphatic rocks, carbonatic rocks and salty clay. The salt is known for its rapid hole closure behaviour, and stresses are often modelled as lithostatic. Although sulphate rocks like anhydrite creep more slowly, they are often considered viscoelastic over long time horizons, and laboratory measurements show creep behaviour. However, the behaviour of anhydrite is strongly dependent on water saturation, and the dynamic 3D geomechanical model results suggested that there is a stress transition zone through the anhydrites between the Zechstein salt (halite) and Rotliegend sandstones. The inclusion of a stress transition zone explained the drilling complications encountered in earlier wells, which could not be adequately explained using 1D geomechanical modelling.
Geomechanics is the study of how subsurface rocks deform or fail in response to changes of stress, pressure and temperature, and therefore geomechanics is a key aspect of the exploration works. The sequences of different evaporate rocks through the Zechstein formations group result in formations being able to exhibit a number of behavioural mechanisms. Without careful geomechanics analysis and risk assessment, one should expect, numerous drilling complications, as well as health and safety risks, excessive time to complete operations and increased well cost.
