Lost Circulation

the loss of quantities of whole mud to a formation, usually in cavernous, Fissured, or coarsely permeable beds, evidenced by the complete or partial failure of the mud to return to the surface as it is being circulated in the hole. Lost circulation can lead to a blowout and, in general, reduce the efficiency of the drilling operation. It is also called lost returns.

perda de circulação

A condition which occurs when the fluid pressure within the wellbore acting at the formation face exceeds reservoir pressure, resulting in loss of fluids from the wellbore into the formation. This loss of fluids into the formation may result in reduced or interrupted fluid circulation at the surface.