| 摘要 |
A holding sub 110 for a liner wiper plug (figure 2) incorporates a retaining latch 114 that is released by movement of a piston 134 in response to an increase in the string fluid pressure. Piston chamber 132 is filled with fluid that communicates with the well annulus via pathway 128 and a metering jet 130 or a restricted aperture 131. Release of the wiper plug is initiated by the introduction of a pump down plug into the string. It lands within the wiper plug and is secured by a second latch (156, figure 2), blocking fluid flow and increasing the string pressure, forcing the piston to move upwards and release the first latch. The limited rate at which fluid exits the piston chamber causes a time delay that allows the pressure build up and its subsequent rapid decay to be recognised at the surface, indicating deployment of the wiper plug.
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