One area where water-based muds need improved performance is in shale
inhibition. However, before existing mud systems can be improved, the
mechanisms by which water invades shales and how present-day inhibitiv
e additives operate must be fully understood. An experimental techniqu
e has been developed that uses radioactive tracers to monitor the prog
ress of water and selected dissolved ions through a shale core plug. B
y varying experimental parameters, such as water composition and appli
ed pressure drop, the dominant mechanisms by which water is transporte
d through shales have been identified. Under conditions of zero applie
d pressure, diffusion processes control water and ion movement through
shales. Concentration gradients are the driving force for mass transf
er of ionic species through shales. We observed no evidence to indicat
e that osmosis caused mass transfer of water. Applied pressure caused
an increase in water and ion transport rates. Above a threshold pressu
re, water and dissolved ions travel at the same rate irrespective of t
he ion concentration.