The use of high density "mud cap" tripping pills to provide adequate hydrostatic pressure downhole for well control is a common practice on managed pressure and underbalanced drilling operations. However, placing a high density tripping fluid on top of a lower density drilling fluid is an attempt to defy gravity, and too often the result is large volumes of the two fluids comingling, ruining the potential reuse of both. Viscosifying the tripping pill itself has been tried with some success, but problems encountered have included more difficult movement and storage of the tripping fluid, along with the risk of increasing the well’s bottomhole pressure while pumping the large, viscous pill into place. A better solution to the comingling issue is to place a physical barrier between the two fluids to prevent their mixing.

A novel fluid with robust thixotropic properties was developed to serve this barrier fluid function. The shear thinning properties of this fluid allow for ease of placement in the wellbore prior to tripping, and when the high density "mud cap" is displaced on top of the barrier fluid the comingling of the tripping fluid and the drilling fluid is prevented. After tripping the drillstring back to the top of the barrier pill, the pill can be washed through and disposed of at surface, leaving a clean drilling fluid in the wellbore. The development and initial use of this barrier fluid are the subjects of this paper.

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