Permeability
and Petrophysical Properties of Reconstituted
Mudstones – An Experimental Study
Mondol, Nazmul
Haque1, Knut Bjørlykke1, Jens
Jahren1 (1)
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.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California