Exploring the effect of meso-scale shale beds on a reservoir's overall stress sensitivity to seismic waves

Intra-reservoir, sub-seismic shale beds with a thickness range from 1–10 m are present in most clastic reservoirs. They are often studied to investigate their effect on fluid flow and reservoir performance. Here, it is found that shales with such thicknesses can also strongly affect the effective elastic wave behaviour of the composite reservoir, particularly when observed with frequent time-lapse surveys. More specifically, for reservoirs experiencing pressure depletion, the effective impedance of the reservoir interval is usually expected to harden, however our results indicate that shales can reduce this hardening effect or perhaps unexpectedly soften the overall impedance. The overall in situ stress sensitivity of the reservoir is reduced below that predicted from taking laboratory core plug measurements of sand stress sensitivity alone. These predictions are based on a combination of geomechanical and pressure diffusion phenomena that are in turn controlled by the shale thickness, permeability and the mechanical properties. As sub-seismic shale layers of approximately 1 m thickness take less than three months to pressure equilibrate whilst thicker shale layers of 10 m can take over 20 years, in the context of repeated seismic surveying our predictions require accurate knowledge of the shale properties and statistics and hence a good description of the sedimentology. Based on our default property values, it appears that reduced or anomalous stress sensitivity is likely to be more important for 4D projects with frequent acquisitions of 3–12 months but is of less concern when seismic is repeated on conventional time periods of every 5–10 years. The critical set of parameters required to carry out accurate calibration of these predictions is not yet fully available from published literature.