Slug
A volume of mud pill that is denser than the mud in the drillpipe and wellbore annulus. A slug is used to displace mud out of the upper part of the drillpipe before pulling pipe out of the hole and is mixed in the pill pit by adding weighting material (barite) to a few barrels of mud from the surface pits. The pill is pumped into the top of the drillstring to U-tube mud downward, out of the pipe, thus keeping the upper stands of pipe empty during trips.