Retrieving well tools under pressure

Abstract

A shut-off valve for use in a downhole string of tools adapted to be retrieved from a well under pressure is disclosed. The valve has a housing and a piston slidably disposed within an axial bore of the housing. The housing has upper and lower ends configured for attachment to upper and lower portions, respectively, of the tool string. The valve defines an internal passage for hydraulic communication between the upper and lower tool string portions. The piston is arranged to, in first and second positions, permit and block, respectively, hydraulic communication along the internal passage. The housing has an outer surface defining upper and lower outer surface regions for engagement by two spaced apart seals of a retrieval head (such as a BOP). The housing also defines an outer port disposed between the upper and lower outer surface regions and arranged for hydraulic communication between the piston and the outer housing surface. The piston is adapted to be moved to its second position by an elevated pressure applied to the outer port, 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 is particularly useful for retrieving long strings of tools without killing the well. Methods of using the valve to uncouple upper and lower portions of a string of tools exposed to elevated well bore pressure are also disclosed.