4D geomechanics: Inverting for shales R-factors using seismic time-strains in the shearwater field, central North Sea

Since production-induced stress changes in the reservoir lead to deformation and velocity variations in the overburden, understanding the geomechanical behaviour of the caprock has become important for reservoir characterisation. We utilise a semi-analytical Geertsma approach that linearly relates subsurface displacements to geomechanical compacting reservoirs, to invert for reservoir pressure changes from 4D time-strains measured within the overburden shales (Heather, Kimmeridge Clay and Kromer Knoll Gr.) from the Shearwater Field. This solution aims to recover average R-factors from a scaling process where the inverted pressure and the pressure from the simulator predictions are statistically analysed. We validate the technique by generating multiple simulations for several ranges of mechanical properties and forward modelling the inversion results; we found that average R-factor of 4-5 for the shales closely matches both distributions of pressure change. We also demonstrate that the R-values recovered from this method are a function of mechanical properties and lithology, which have significant impact on the computation of realistic time-shift from predicted vertical strain. The inversion method has shown to be an adequate analytical approach for strain and pressure modelling of the reservoir and overburden caprock at Shearwater, improving the current understanding of how shales deformation occur in geomechanically active reservoirs.