Geomechanics and Geophysical Processes in Naturally Fractured Reservoirs

In the case of domal reservoirs, production may cause down-dip shearing on conjugate dipping fractures. This mechanism was deduced a long time ago from rock mechanics modelling and from new slickensiding in the case of the Ekofisk reservoir in the North Sea (Barton et al., 1988). There may also be changes in the stretching over-burden in the case of the compacting reservoir, causing the over-burden to subside. This will cause temporal changes to the strength of shear-wave anisotropy and attenuation, due to intra-bed joint opening and shear. It is insufficient in each of the above cases to refer to ‘stress or strain’ effects, as if a continuum alone was reacting to the multiple effects of production in a multi-km3 fractured reservoir, with a multi-km3 overburden. The relatively ‘early’ consideration of discontinuous behaviour at Ekofisk is presented in this classic ‘carbonates region’ of the Middle East, in the hope of stimulating the modelling of fracture deformation, and coupled behaviour, which still seems to be rare, despite being needed for realistic 4D interpretation. The possibilities of making good use of geomechanics understanding has improved a lot since LOF monitoring/interrogation of reservoirs was slowly introduced in this last decade, starting in the North Sea.