The effect of intra-reservoir shales on effective stress sensitivity

In most elastic reservoirs experiencing pressure depletion, the sands in the reservoir naturally compact. As a consequence, the much lower permeability reservoir shales may experience extension. This extension is counteracted to some degree by pressure equilibration of the shale. The effective seismic response of the reservoir interval may thus be a mix of both hardening and softening reservoir components, depending on the balance of these phenomena. This effect is predicted to alter the overall stress sensitivity of the seismic properties from that anticipated for a homogeneous, fully connected reservoir interval. However, the final resultant response depends on the time period over which this effect is observed. Numerical computation using simplified geological models indicates shales of 1m to 10m thickness should be taken into account when quantitatively assessing the 4D seismic signature from frequently shot time-lapse surveys with a periodicity of 3 to 12 months, whilst 5 to 10m thick shales could impact conventional 4D seismic surveys shot over 5 to 10 years. These conclusions are strongly affected by the mechanical and transport properties of the intra-reservoir shales, their thickness and distribution, and are hence also a function of the depositional environment.