Drilling Unayzah-B reservoir (shale and sandstone) in Saudi Arabia requires high mud density (± 95 pcf) to mechanically stabilize the shale and balance the reservoir pressure. Two fluids are used to drill this reservoir: KCl/ BaSO4/ CaCO3 whereas the second is potassium formate/CaCO3. Barite is added with CaCO3 to reduce the amount of solids needed to formulate the drill-in fluid. However, BaSO 4 is insoluble in acids and requires chelating agents to remove it from the formation. Formate drill-in fluids with low solid content can then be used, but they are expensive and corrosive at pH values less than 7-8. To overcome some of the problems associated with these two systems Saudi Aramco developed water-based drill-in fluids using Mn3O4. No similar formulations were developed before to the best of the authors' knowledge. The objective of this study is to determine formation damage induced by KCl/Mn304 mud at 95 pcf. A mini-flow loop was used to assess formation damage induced by the three the drill-in fluids. The experiments were conducted using reservoir cores at bottom hole conditions. Another set of experiments were conducted using a dynamic HPHT cell to determine the thickness and composition of the filter cake filter cake, spurt loss and chemical analysis of filtrate. Compatibility tests between mud filtrate and produced water were performed at 300°F and 500 psi using see through cells. The KCl/Mn3O 4 showed the least formation damage compared to the other two fluids. The return permeability was significantly higher than those obtained with potassium formate/CaCO3 or BaSO4/CaCO3 mud systems. Potassium formate filtrate was not compatible with Unayzah-b formation brine, which resulted in its low return permeability (42%) in spite of its low solids content. Unlike formate based fluids, mud filtrate of KCl/Mn 3O4 was compatible with the formation brine. BaSO 4/CaCO3 drill-in fluid resulted in 40% because the barite solids were trapped inside the core. KCl/Mn3O4 drill-in fluid resulted in 70% return permeability. The main reason for this high result is the spherical shape and small size of Mn3O4 particles, which allowed them to be removed by flow of hydrocarbons.