Abstract: Once a new oil well has been drilled and cased, a test string is set in place for the purpose of evaluating the production potential of the chosen formation. One way of controlling the operation of the various tools included in the downhole test string, including the opening and closing of the downhole valve itself, is by changes in the pressure differential between the tubing and the annular space which surrounds it in the well, but this requires the provision and maintenance of a fixed "reference" pressure within the tool, and a convenient such pressure is the hydrostatic (annulus) pressure experienced by the string after it has been lowered down the well bore and set into the packer.

Abstract: For use as part of a test string employed to evaluate the production potential of a chosen formation through which an oil well has been drilled, an annulus-pressure-operated sub-surface control valve, typically a ball valve, wherein to deal with the problem of the high pressure of fluid acting on the upstream side of the ball resulting in the frictional forces caused by the ball being pressed up against the seating on its downstream side making the ball stick, the utilization of a differential thread force multiplier by which the actuating force may be applied to the ball.

Abstract: Apparatus for the venting and isolation of an oil well test tool drill string comprises reference pressure gas release apparatus having two spaced pistons (7, 11) located at opposite ends of the reference gas chamber (10) and blocking both a gas vent (17) to annulus and a hydraulic liquid passageway (22) extending further up the test string, the pistons being held together by a shear pin (13) until the application of a predetermined higher pressure across those pistons causes the pin to shear, allowing sequential movement of the two pistons toward each other, firstly opening the gas vent to annulus, and secondly opening the passageway (22) to a chamber (24) of hydraulic liquid. The hydraulic liquid pressure within this passageway then causes actuation of ball valve apparatus for isolating the upper section of tubing, which in turn enables transfer of hydraulic pressure to apparatus for venting the contents of the tubing to annulus.