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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe primary cause of wellbore instability is the interaction of water-based muds with shales. The movement of water and ions into or out of a shale can result in large changes in pore pressure in the vicinity of the wellbore, potentially leading to wellbore failure.A new method, the Gravimetric -Swelling Test (GST), for determining the compatibility between shales and drilling fluids is presented in this paper. An experimental protocol and equations are presented that describe how such measurements can be conducted and interpreted with relative ease. The mass of water and ions entering or leaving shale samples is determined. With additional swelling measurements, the impact of the water and ions uptake on swelling pressures generated can also be obtained.In this paper, results are presented for two preserved shale samples obtained from the field. The influence of different types of ionic solutions on water and ion movement is presented for each shale. It is shown that water uptake and swelling of shales is controlled not only by differences between shale water activity and water activity in the mud (as assumed in the past), but ion type and concentration also play an important role. In these tests the water uptake decreases, while the ion adsorption increases with increasing salt concentration. Different types of cations are shown to have a large influence on water/ion movement. This paper presents a data set showing the influence of ion type and concentration on water uptake by shales. The role of capillary pressure, osmotic effects and ionic diffusion on swelling behavior of shales is also discussed. The technique presented herein may possibly be used at the rigfloor to determine the compatibility of shales with salt-water drilling fluids.