Downhole impact apparatus

An apparatus comprising:

• an impact tool operable to be coupled between portions of a tool string conveyable within a wellbore extending into a subterranean formation, wherein the impact tool comprises:
• 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, wherein the first chamber volume is open to a space external to the housing, and wherein the second chamber volume is fluidly isolated from the space external to the housing;
• a shaft connected with the piston and axially movable with respect to the housing;
• and
• a mechanism operable to:
• maintain the piston in a predetermined position within the chamber;
• and
• release the piston to permit pressure differential between pressure within the first chamber volume and pressure within the second chamber volume to move the piston and housing relative to each other ending with an impact between moving and stationary portions of the impact tool.

A method comprising:

• coupling an impact tool to a tool string, wherein the impact tool comprises:
• 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;
• and
• conveying the tool string within a wellbore while:
• maintaining pressure within the first chamber volume substantially equal to pressure within space external to the housing;
• and
• maintaining the piston in a predetermined position within the chamber to maintain pressure within the second chamber volume appreciably lower than the pressure within the first chamber volume thereby forming a pressure differential between the pressure within the first chamber volume and the pressure within the second chamber volume.