The effect of intra-reservoir and non-reservoir shales on 4D seismic signatures

Shales are often regarded as inactive barriers in the reservoir simulation model and the surrounding rocks. Whilst this appears the correct approach for fluid flow modeling purposes, it is inaccurate for the pressure component of this process. In most clastic reservoirs experiencing pressure depletion, the sands naturally compact. This leads to the well documented extension of the non-reservoir rocks, but also the extension of the intra-reservoir shales. Less well known is that the shales have a finite, but small, permeability and pressure equilibration will occur with the reservoir sands. This diffusion process opposes the geomechanical effects. Numerical computation for a range of shale permeabilities suggests that intrareservoir shales of 1m to 10m thickness should be considered as active when quantitatively assessing the 4D seismic signature. Also it is observed that pressure depletion in the reservoir can `propagate' distances of as much as 50m into the shale over/under burden during the production time scale. The integration of these coupled mechanisms into forward modelling of time lapse seismic shows vertical time shift profiles different from those proposed for geomechanics alone.