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