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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. The invention proposes that reference pressure within the test string be trapped by a novel mechanism wherein a valve (4) drivable into a closed position by a first piston (3) open (at 5) to annulus pressure first defines, and then defines and closes, the open-to-tubing-pressure entrance (6) to a passageway (30/19) leading to a reference-gas-containing chamber (22) via a second piston (20) therewithin. The invention also proposes a new mechanism by which compensation can be made for the effect of downhole temperature changes on the gas in a reference pressure chamber, in which mechanism there is a hydraulic-liquid-containing chamber (27) which is connected at one end, via a piston (25) thereat, to a vent (24) to annulus and at the other end to two 'one-way' passageways (28, 29) linking it to the reference-gas-containing chamber (22) via a chamber-contained second piston (23).

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First Claim