Permeability anisotropy of resedimented mudrocks

Abstract

Permeability anisotropy (ratio of horizontal to vertical permeability) is an important but uncertain parameter used in characterizing underground formations. While it is a fairly unknown parameter, it is integral for the petroleum industry, where a greater permeability anisotropy understanding can greatly aid in basin modelling, pore pressure prediction, and borehole stability. This research experimentally characterizes the permeability anisotropy of several mudrocks, which are clay-rich sedimentary formations, using re-sedimentation, a process of homogenization of naturally-occurring soils and recreation of the sedimentation environment in a controlled laboratory setting. The permeability anisotropy of resedimented Boston Blue Clay (RBBC), an illitic lean clay (CL), increases from 1.5 to 3 when mechanically compressed from 0.1 to 40 MPa, corresponding to porosities ranging from 0.55 to 0.26. Resedimented Gulf of Mexico - Eugene Island mudrock (RGoM-EI), a smectitic fat clay (CH), exhibits permeability anisotropy increasing from 1 to 5 when compressed to the same stresses, corresponding to porosities ranging from 0.6 to 0.25. Not only does smectitic RGoM-EI mudrock transition to greater anisotropy with compression, but the rate of increase accelerates with compression. These measurements are made using a commercially-available Trautwein® constant rate of strain (CRS) consolidometer with vertical drainage combined with a novel radially-outward draining CRS device. The combination of the vertically and radially draining CRS devices produces permeability anisotropy data quickly, with very little scatter. For RGoM-EI, the effects of horizontal shearing to 29.5% shear strain at 0.14 MPa on permeability anisotropy are negligible. Permeability anisotropy of the homogeneous resedimented mudrocks tested, using a cubic specimen constant head permeameter within a triaxial cell, is directly correlated to their electrical conductivity anisotropy. The permeability anisotropy values measured using this technology, however, are lower than those measured using CRS testing.