jar - a percussion tool operated manually or hydraulically to deliver a heavy upward or downward blow to fish stuck in the borehole. v: to apply a heavy blow to the drill stem by use of a jar or bumper sub.
jar accelerator - a hydraulic tool used in conjunction with a jar and made up on the fishing string above the jar to increase the power of the jarring force.
jerk line - a wire rope, one end of which is connected to the end of the tongs and the other end of which is attached to the cathead.
jet - 1. a hydraulic device operated by a centrifugal pump used to clean the mud pits, or tanks, and to mix mud components. 2. in a perforating gun using shaped charges, a highly penetrating, fast-moving stream of exploded particles that forms a hole in the casing, cement, and formation.
jet cutoff - a procedure for severing pipe stuck in a well by detonating special shaped-charge explosives similar to those used in jet perforating. The explosive is lowered into the pipe to the desired depth and detonated. The force of the explosion makes radiating horizontal cuts around the pipe, and the severed portion of the pipe is retrieved.
jet cutter - a fishing tool that uses shaped charges to sever casing, tubing, or drill pipe stuck in the hole. See jet cutoff. Compare chemical cutter.
jet gun - an assembly, including a carrier and shaped charges, that is used in jet perforating.
jet-perforate - to create holes through the casing with a shaped charge of high explosives instead of a gun that fires projectiles. The loaded charges are lowered into the hole to the desired depth. Once detonated, the charges emit short, penetrating jets of high-velocity gases that make holes in the casing and cement for some distance into the formation. Formation fluids then flow into the wellbore through these perforations. See bullet perforator, gun-perforate.
journal bearing - a machine part in which a rotating shaft (a journal) revolves or slides. Also called a plain bearing.
joint of pipe - a length of drill pipe or casing. Both come in various lengths.
junk - metal debris lost in a hole. Junk may be a lost bit, pieces of a bit, pieces of pipe, wrenches, or any relatively small object that impedes drilling or completion and must be fished out of the hole. v: to abandon (as a nonproductive well).
junk basket - a device made up on the bottom of the drill stem or on wireline to catch pieces of junk from the bottom of the hole. Circulating the mud or reeling in the wireline forces the junk into a barrel in the tool, where it is caught and held. When the basket is brought back to the surface, the junk is removed. Also called a junk sub or junk catcher.
junk mill - a mill used to grind up junk in the hole. See mill.
junk retriever - a special tool made up on the bottom of the drill stem to pick up junk from the bottom of the hole. Most junk retrievers are designed with ports that allow drilling fluid to exit the tool a short distance off the bottom. This flow of fluid creates an area of low pressure inside the tool so that the junk is lifted and caught in the retriever by the higher pressure outside the tool. See junk, junk basket.