Managed pressure drilling operations pose several challenges to the upstream oil industry. While tripping the drillstring, the bottomhole pressure of the wellbore must be maintained at the nominal formation pressure, and the solutions currently used have less than optimal success rates in high angle wellbores. A balanced mud cap displaced into the well before a trip can supply the bottomhole pressure required, but the cap is prone to fall through the lighter drilling fluid below. Viscosifying the mud cap is seldom successful, and placing a high viscosity spacer between the two fluids has not been a reliable solution, until now. The spacer chemistry presented in this paper successfully addresses those limitations. This barrier spacer meets the operational tripping requirements because it is easy to pump and spot, and robust enough to hold the heavy mud cap above the drilling fluid, yet easy to remove when it is no longer required. Since its introduction, some minor shortcomings of the spacer were identified, and a modified formulation has been developed to improve the mixing and deployment of the spacer.