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Among the several problems encountered in drilling, wellbore instability has been regarded as the most adverse and the problem becomes more peculiar when the drilled formation is shale. Because of the predominance of clay minerals in shale there is much tendency for water absorption and swelling in clays which causes wellbore instability. Oil-based mud has good potential as a remediation for wellbore instability but its cost and environmental implication has affected the choice of its usage. In this study, three shale samples from shale formation of Niger Delta were utilized. Three mud samples were formulated: mud sample SK1 without nanoparticles (NPs), mud samples SK2 and SK3 with 6 wt% and 12 wt % of NPs addition respectively. Mud rheology, measurement of density, filtration loss, spontaneous imbibition (SI) and rate of swelling tests were conducted on the prepared mud samples. The results showed that, addition of NPs to water-based drilling muds (WBDM) did not significantly changed the mud density before and after barite addition but addition of NPs to WBDMs increased yield point and plastic viscosity, and gave better filtration control with improved mud capacity to lift extra cuttings and enormous shear stress provided for circulation in the space of the annulus.. Also addition of NPs to the WBDM decreases the amount of imbibition of shale. NPs showed great capacity to swiftly plug the pore throats and decrease rate of swelling of shale when added to WBDMs. [ABSTRACT FROM AUTHOR]