Permeability and Petrophysical Properties of Reconstituted Mudstones – An Experimental Study

Mondol, Nazmul Haque¹, Knut Bjørlykke¹, Jens Jahren¹ (1) University of Oslo, Oslo, Norway

Mudstones are heterogeneous with respect to mineralogy and grain size. Their physical properties including permeability vary greatly with depth. Permeability of mudstones is critical for modeling of fluid flow in sedimentary basin. This study determines permeability and petrophysical properties of reconstituted mudstones and their interrelationships as a function of vertical effective stress. Six brine-saturated clay slurries were compacted in the laboratory under controlled pore pressure and drained conditions. Porosity, permeability, density and velocity were measured continuously under vertical effective stress up to 50 MPa. Results show that kaolinitic clays compact more and have higher density, velocity and permeability than smectitic clays at the same stress. Permeability of pure kaolinite ranges from 0.1 to 0.001 mD, while in pure smectite it varies from 0.004 to 0.00006 mD at stresses from 1 to 50 MPa. Permeability decreases logarithmically with increasing stress, density and velocity and decreasing porosity. Permeability differs up to 10⁵ at the same porosity for different clay minerals. Applications of the Kozeny-Carman equation, relating porosity to permeability in clays, would therefore produce highly erroneous results. Laboratory tests demonstrate that the smectite content is critical to build up overpressure in mudstones and reduce the permeability in cap rocks. The ratio between velocity and permeability may vary by a factor of 10⁴ in pure kaolinite and pure smectite. Velocities can therefore not be used to estimate permeability of mudstones. The present results help to constrain mechanical compaction in mudstones as a function of mineralogy in parts of the basins where mechanical compaction is the predominant process.