They are having trouble maintaining a fluid column in the well...

They are having trouble maintaining a fluid column in the well due to downhole "losses". This in a common occurance when drilling through formations (normaly ones with high perm) where the pressure exerted by the column of fluid is greater than the formation pressure therefore the fluid doesn't return to surface as it's easy for it to go into the formation.

The fluid column needs to be maintianed to allow the drilled (cuttings) formation to be returned to surface and also as the primary method to control the inflow of formation fluids ie hydrocarbons.

hope this helps

cheers
otaw