Reservoir and crustal permeability based on shear stress
|Location||International Geological Congress,oslo 2008|
|Holding Date||08 October 2008|
A standard assumption in the geophysics literature is that shear wave polarization and splitting occurs due to stress-aligned structure. This structure is considered by some to be stress aligned microcracks, by others with reservoir interests, as a desirable ‘open’ set of sub-vertical conducting fractures that are also assumed to be parallel or sub-parallel to the maximum horizontal stress. Geomechanics modeling demonstrates that unless fractures are rather rough and wall strength rather high, or that there is over-pressure, there are likely to be only very small hydraulic apertures at several kilometers depth. On the other hand, deep-well measurements demonstrate that fractures that are under differential shear stress are more likely to be water conducting, and those that are principally under normal stress are less likely to be water conducting.
In this paper, alternative interpretations of shear-wave polarization directions are examined, including the contribution of two, maybe unequal joint sets, intersected by the major stress, having different compliances, and possibly with pre-peak non-linear shear strength and dilation contributions, to their enhanced permeability. Shearing induced by reservoir production and compaction is also considered as a potential source of temporal rotation of seismic anisotropy, as recently recorded in 4D seismic at the Ekofisk and Valhall reservoirs in the North Sea.