Permeability evolution during triaxial compaction of an anisotropic porous sandstone

In this paper we report results from an experimental study into the influence of pre-existing structural anisotropy on deformation localization and permeability evolution during triaxial compaction of a porous sandstone. Diemelstadt sandstone was selected for this study because it exhibited significant structural anisotropy as measured by AMS and P and S-wave velocities. This resulted in significant mechanical anisotropy, with samples deformed parallel to bedding being always stronger in the compactant regime than samples deformed normal to bedding. Permeability decreases were also larger (by about an order of magnitude) and far more abrupt in samples deformed normal to bedding. Microstructural analysis of deformed samples revealed that both the mechanical anisotropy and the differences in permeability evolution can be explained by the different geometrical attributes of the compaction bands that propagated in the two orientations. Bedding-parallel compaction bands developed at lower stresses and were more tabular and less tortuous than bands that developed at oblique angles to bedding. Bands appear to have a similar thickness regardless of orientation, and we estimated a permeability contrast of about 3 orders of magnitude between the host rock matrix and the compaction bands. Our new data suggest that pre-existing structural anisotropy in a sandstone formation may play a major role in the development of compaction localization and its influence on fluid flow. Our laboratory results generally compare well with attributes of compaction bands observed in the field.