The movement of water and ions into or out of shale causes physico-chemical and mechanical property alterations, and can lead to wellbore instability related problems. This paper presents experiments used to analyze the effects of chemical osmosis, diffusive osmosis, and capillary suction on water and ion movement when shale interacts with drilling fluids. Results show that water movement is not only controlled by chemical osmosis (water activity), but is also influenced by diffusive osmosis and capillary suction. The immersion of shale into fluids changes the chemical composition of the shale due to ion movement, and thus its physico-chemical and mechanical properties are altered. The gravimetric-swelling test (GST) is used in the laboratory to acquire a better understanding of the relationship between water flow, ion flow, and shale swelling. This insight provides information used to optimize drilling fluids to effectively control or mitigate wellbore instability when drilling through shale.