Mn3O4 water based drill-in fluids were studied. Mn3O drill-in fluid showed better thermal stability than typical drill-in fluids (KCl/CaCO3/barite and potassium formate drill-in fluids). The plastic viscosity, yield point, and fluid loss did not change after aging the sample for 16 hr at 300°F. The use of CaCO3 with Mn3O4 improved the filtration and reduced the filter cake compared to using Mn3O4 alone. The use of 1:3 weight ratio of CaCO3 (medium/fine) with Mn3O4 showed the best rheological properties and filtration loss. CaCO3 material should be added before Mn3O4 to improve filtration control. In providing good rheological stability and filtration control for Mn3O4 drill-in fluids, PAC-R and starch were better than Resinex and Therma Check polymers. Therma check or Resinex polymers provided high theological properties, which could decrease penetration rate and cause swabbing and gas flow.