| 摘要 |
Various jars are provided for delivering axial blows to a well string. In one aspect, a jar is provided that includes a mandrel and a housing telescopically positioned about the mandrel. A piston is positioned between the mandrel and the housing and closes a substantially sealed chamber in the housing. The piston has a first flow passage and a second flow passage which enable the selective flow of a fluid into and out of the substantially sealed chamber. A collet is positioned in the housing for selectively engaging the mandrel. A sleeve is positioned around and axially moveable relative to the collet. The sleeve has a reduced inner diameter portion at which the collet selectively expands radially to disengage the mandrel. Axial movement of the mandrel engages the collet, which in turn, moves the piston and pressurizes the chamber. When the reduced diameter portion of the sleeve is reached, the collet releases the mandrel, enabling the mandrel to impact an anvil surface of the housing. |
