Shut-off valve for downhole tool for retrieving said tool from a well under pressure

A shut-off valve (22) has a housing (40), a piston (52) slidably disposed within an axial bore of the housing (40) and upper and lower ends for attachment to a tool string. The valve defines an internal passage (48,50) for hydraulic communication between an upper and a lower tool string portion. The piston (52) is arranged to, in first and second positions, permit and block, respectively, hydraulic communication along the internal passage (48,50). The housing (40) has outer surfaces for engagement by a BOP (28). The housing (40) also defines an outer port (54) arranged for hydraulic communication between the piston (52) and the outer housing (40) surface. The piston (52) is adapted to be moved to its second position by an elevated pressure applied to the outer port (54), thereby blocking the internal passage and enabling the upper end of the housing to be disconnected from the upper tool string portion while the lower tool string portion is exposed to elevated well pressure. The valve (22) is particularly useful for retrieving long strings of tools without killing the well. Methods of using the valve are also disclosed.