Shale instability is one of the ongoing major problems when drilling oil/gas wellbores. The interaction of aqueous drilling fluids or fracturing fluids, with reactive clay minerals in shales causes swelling, in turn causing costly wellbore instabilities. In this study, the swelling of compacted commercial Na-bentonite clay mineral cores was investigated using a novel non-contact displacement meter with various organic solvents and salt solutions as the electrolyte. Swelling results with organic solvents were correlated with the dielectric constant, dipole moment, surface tension and viscosity of the solvent. It was found that swelling rate and total swelling were proportional to dielectric constant and inversely proportional to viscosity, representing the chemical and capillary components of swelling, respectively. Results of swelling tests with salt solutions are discussed in the context of diffuse double layer (DDL) theory. Swelling behaviour of smectite clay minerals were found to be highly affected by cation concentration. Tests with CaCl2 solutions showed that divalent cations were effective at suppressing swelling at low concentrations. At high concentrations KCl and KI solutions were more effective at inhibiting swelling owing to the lower hydration enthalpy of K+ cations. Repeats of selected tests with non-swelling illite rich shale compacted cores were compared with the swelling Na-bentonite clay mineral compacted cores. Comparison between Na-bentonite and illite swelling shows that, for reconstituted compacted cores, the initial stage of clay swelling is dominated by capillary action. Then, depending on the reactivity of the clay and the medium, chemical swelling occurs. Consequently, the swelling behaviour of compacted clay cores, as used by many drilling fluid research laboratories, is highly dependent on the domination of different swelling components in different time periods as an artefact of the sample preparation and care should be taken when using ground and compacted shales to assess either reactivity of shale formations to drilling fluids or the effectiveness of swelling inhibitor systems
