This study investigates permeability anisotropy in well characterized synthetic mudstones. A total of 10 brine-saturated synthetic mudstones of varying textural and mineralogical compositions were compacted in a triaxial cell (K0-consolidation) while the vertical and horizontal permeability were measured at different stress levels. The results show that the vertical and horizontal permeabilities of synthetic mudstones differ by as much as 5-6 orders of magnitude as a function of mineralogy at the same stress level. Permeabilities were isotropic in pure silt but adding about 30% clays to the silt strongly reduced the permeability and increased the anisotropy. The horizontal-to-vertical permeability ratio in silt-clay and clay-clay mixtures varies from 0.5 to 3.5, demonstrating the importance of clay mineralogy, grain size and silt content on permeability anisotropy development in mudstones. Natural mudstone may have much higher permeability anisotropy than found in this study due to a primary depositional lamination and bedding on larger scale than produced in the laboratory. Permeability distributions and horizontal-to-vertical permeability ratios derived in this study can readily be tested on cap rocks and used to model fluid flow where the compaction is mostly mechanical. At greater depth chemical compaction may increase the permeability anisotropy of mudstone further.


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