ABSTRACT
Shales are sedimentary rocks that have distinct laminated layered characteristics and high clay content, and therefore subjected to phenomena such as hydration, swelling, shrinking, and strength reduction when exposed to water and ions. These reactions result from the hydrophilic feature of the clay particles, which are somewhat altered by both the chemical and mechanical environment. The chemical effects are due to the intermolecular forces between the clay particles, the ionic pore fluid inside the shale, and the composition of the drilling fluid.These chemical effects result in a continuous change in shale pore pressure and composition. The osmotic pressure pπ is induced as a result of the chemo-mechanical shale/mud interactions according Equation (2) (Hou et al. 2004):
where R = 8.314 kg.m2/(s2.g-mole.K); V = 1.8*10−5 m3/g-mole (partial molar volume of water); T =Temperature, K; Amud = mud water activity, –; and Ashale = shale water activity, –; Im = Membrane efficiency, –.
