Typically six or more mud pits of various sizes are located at the surface of most wells, but at any one time all mud pits are not necessarily “active”, i.e., contributing volume to the closed loop system of {HEAD ~ the circulating mud} going into {TAIL ~ the wellbore} WB and returning to the surface S 
going into
Typically six or more mud pits of various sizes are located at the surface of most wells , but at any one time all mud pits are not necessarily “ active ” , i.e. , contributing volume to the closed loop system of { HEAD ~ the circulating mud } going into { TAIL ~ the wellbore } WB and returning to the surface S
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH