A downhole tool for connecting and selectively disconnecting within a wellbore first and second portions of a downhole tool string from each other. The downhole tool may include a first connector sub connectable with the first portion of the downhole tool string, a second connector sub connectable with the second portion of the downhole tool string, an internal chamber, and a fastener connecting the first and second connector subs. The fastener may fluidly separate the internal chamber into a first chamber portion and a second chamber portion. The first chamber portion may be fluidly connected with a space external to the downhole tool. The downhole tool may be selectively operable to disconnect the first and second connector subs from each other to disconnect the first and second portions of the downhole tool string from each other.

A downhole assembly includes a downhole tool including a degradable-on-demand material including: a matrix material; and, a unit in contact with the matrix material. The unit includes a core including an energetic material configured to generate energy upon activation to facilitate degradation of the downhole tool; and, an activator disposed in contact with the core, the activator including a triggering system having an electrical circuit and an igniter within the electrical circuit, the electrical circuit having an open condition and a closed condition, the electrical circuit configured to be in the closed condition after movement of an object downhole that engages directly or indirectly with the triggering system, and the igniter arranged to ignite the energetic material in the closed condition of the electrical circuit. In the open condition of the electrical circuit the igniter is inactive, and in the closed condition of the electrical circuit the igniter is activated.

A downhole assembly includes a matrix material and a unit in contact with the matrix material. The unit includes a core having an energetic material, an activator disposed in direct contact with the core, and at least one layer disposed on the core. The activator includes a triggering system having an igniter and a pre-set timer connected in an electrical circuit. The igniter is inactive in an open condition of the electrical circuit, and, after a pre-set time period, the pre-set timer closes the electrical circuit and the igniter is activated.

A back-off tool comprises a magazine cylinder having one distal end of a long mast rod secured to the lower end-face of a magazine cylinder. The magazine cylinder is attached to an electrically detonated firing head. A first plurality of blind hole cavities penetrate the magazine cylinder end-face around the mast rod junction. A second plurality of elongated detonation cord ends are inserted into high temperature grease filled magazine cylinder cavities. From the cavities, the detonation cord lengths are bound to the rod surface along the rod length by non-metallic cord. The tool assembly is secured to the end of a wireline or tubing string for downhole placement and detonation.

Apparatus and methods for creating a downhole impact. The apparatus may be an impact tool operable to be coupled between portions of a tool string conveyable within a wellbore extending into a subterranean formation. The impact tool may include a housing, a chamber within the housing, a piston slidably disposed within the chamber and dividing the chamber into a first chamber volume and a second chamber volume, and a shaft connected with the piston and axially movable with respect to the housing. The first chamber volume may be open to a space external to the housing and the second chamber volume may be fluidly isolated from the space external to the housing. The piston may be maintained in a predetermined position within the chamber to maintain pressure within the second chamber volume appreciably lower than pressure within the first chamber volume while the impact tool is conveyed along the wellbore.

A back-off shot tool system includes a high-pressure resistant charge housing at least partially containing an explosive material. A booster charge is placed adjacent the explosive material to activate the explosive material upon actuation. The high-pressure resistant charge housing is formed from a material that is the material of the high-pressure resistant charge housing is frangible so that it will to withstand high temperatures and pressures, such as a ceramic. In addition, fragment into small pieces that can be process or extracted from the well with fluid that is extracted from the region in which the back-off shot tool is deployed.

An apparatus includes a propellant chamber, an expansion chamber, an exit chamber comprising a port providing fluid communication outside the apparatus, and a longitudinal acceleration chamber. The longitudinal acceleration chamber includes a first end in fluid communication with the propellant chamber, a second end in fluid communication with the expansion chamber, and a longitudinal axis between the first end and the second end. An acceleration mass is slidably contained in the acceleration chamber. The acceleration chamber includes a propellant side between the propellant chamber and the acceleration mass, and a target side between the acceleration mass and the expansion chamber. A transfer piston is slidably positioned between the expansion chamber and the exit chamber. The transfer piston includes a first end projecting into the expansion chamber and aligned with the longitudinal axis of the acceleration chamber, and a second end projecting into the exit chamber.