What does "lost circulation" refer to in drilling?

"Lost circulation" refers specifically to the situation where the drilling fluid fails to return to the surface, which directly indicates that some or all of the fluid is being lost to the surrounding formation. This can occur when the pressure in the wellbore exceeds the pressure within the surrounding geological formations, leading to fractures or gaps that allow the drilling fluid to escape into the rock layers instead of flowing back up to the surface.

This phenomenon can significantly impact drilling operations, as it may lead to inadequate cooling and lubrication of the drill bit, problems with maintaining wellbore stability, and potentially increased costs due to the need for additional drilling fluids or interventions to rectify the loss of circulation. Understanding this concept is crucial for drilling engineers and personnel involved in managing drilling operations effectively.