Permeability anisotropy and resistivity anisotropy of mechanically compressed mudrocks

Permeability anisotropy (the ratio of the horizontal to vertical permeability) develops in mudrocks at the macroscale due to heterogeneities such as layering and at the element scale (i.e., within a single homogeneous layer or unit) due to decreasing porosity and increasing platy particle alignment. This work describes new experimental methods and results from a laboratory program using cubic specimens to investigate the evolution of mudrock permeability and resistivity at the element scale. A systematic study analyzes the evolution of the permeability anisotropy and electrical resistivity anisotropy using resedimented Boston blue clay (RBBC) over a stress range of 0.4-40 MPa; additional measurements are presented for three other mudrocks within varying clay fraction and clay mineralogy within the stress range 1.2-10 MPa. The permeability anisotropy and the conductivity anisotropy (inverse of the resistivity anisotropy) of all four mudrocks studied ranges from 1 to 2.2 over the porosity range 0.55-0.30. A nearly 1:1 correlation between the electrical conductivity anisotropy and the permeability anisotropy is observed to be independent of clay fraction or clay mineralogy.