Oil-based muds have shown great success in preventing borehole instability in shale formations. The presence of a threshold capillary entry pressure between oil-based muds and low permeability shales is considered to be the main factor in preventing chemical interactions between the drilling mud and the shale which can lead to shale failure. This article presents experimental data for the capillary entry pressure of several shales when exposed to oil-based muds. It is clearly shown that the entry pressure depends on the interfacial tension between the oil and water phases and shale permeability and cation exchange capacity. Data suggests that incorporating "emulsifiers" in oil-based mud formulations reduces the minimum capillary entry pressure. Non-polar oils show significantly higher entry pressures, indicating the importance of surfactants present in the oil-based mud.