UNASSOCIATED GAS - Natural gas occurring alone, not in solution or as free gas with oil or condensate.
UNCONSOLIDATED FORMATION - a loosely arranged, apparently unstratified section of rock.
UNCONSOLIDATED SANDSTONE - a sand formation in which individual grains do not adhere to one another. If an unconsolidated sandstone produces oil or gas, it will produce sand as well if not controlled or corrected.
UNDERGAUGE BIT - a bit whose outside diameter is worn to the point at which it is smaller than it was when new. A hole drilled with an undergauge bit is said to be undergauge.
UNDERGAUGE HOLE - that portion of a borehole drilled with an undergauge bit.
UNIT OPERATOR - the oil company in charge of development and production in an oilfield in which several companies have joined to produce the field.
UNLOADING A WELL - removing fluid from the tubing in a well, often by means of a swab, to lower the bottomhole pressure in the wellbore at the perforations and induce the well to flow.
UPPER KELLY COCK - a valve installed above the kelly that can be closed manually to protect the rotary hose from high pressure that may exist in the drill stem.