Wells – Packers or plugs – Flow stopping type; e.g. – plug
Patent
1998-05-21
2000-01-04
Tsay, Frank
Wells
Packers or plugs
Flow stopping type; e.g., plug
166153, E21B 3313
Patent
active
060099445
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a plug launching device for use in cementing operations in the construction of oil and gas wells.
During the construction of oil and gas wells a wellbore is bored in the ground. A string of casing is lowered into the wellbore and the annular space between the casing and the wellbore filled with cement.
When the cement is set the wellbore is extended and the process repeated. Conventionally the new length of casing is referred to as a "liner" until such time as it is cemented in place after which it is referred to as "casing".
Typically, the liner is lowered on a work string and is releasably attached thereto by a running tool which, as shown in EP-A-0 450 676 (which forms the basis of the pre-characterising clause of claim 1) may include a plug launching device which accommodates a top plug and a bottom plug for use in the subsequent cementing operation.
Once the liner is in position a liner hanger is set. This secures the liner to the casing. Circulating fluid is then pumped through the tool string and the liner and returns in the annular space between the liner and the wellbore thereby removing the majority of mud and debris therefrom. At this stage a first dart is introduced into the work string and is pumped down the work string by a column of cement. In EP-A-0 450 676 the dart passes down the running tool and engages a plunger which ejects the bottom plug.from the running tool. When operating properly the dart should come to rest on top of the plunger with the cement leaving the running tool above the dart and passing down the liner on top of the bottom plug.
When the required quantity of cement has been pumped down the work string a second dart is introduced into the work string and pumped down with hydraulic fluid. Typically circulating fluid or sea water is used for this purpose.
When the second dart engages the first dart it displaces the plunger further downwardly thus releasing the top plug which is pumped down the liner on top of the cement by the hydraulic fluid.
A float shoe is installed at or near the bottom of the liner. The bottom plug travels down the liner until it lands on the float shoe. Since the top plug is being pumped down the liner by the hydraulic fluid the pressure on the bottom plug increases until a frangible diaphragm therein ruptures and allows the cement to pass through the bottom plug and the float shoe and flow upwards into the annulus between the liner and the wellbore until the top plug lands on the bottom plug. The float shoe incorporates a non-return valve and the cement is allowed to set in the annulus.
The difficulty with the known arrangement is that if the first dart is pumped down the work string too rapidly it can impact the plunger with sufficient force to launch both the top and bottom plugs together.
The present invention addresses this problem by preventing the first darts from mechanically ejecting the plugs.
According to the present invention there is provided a plug launching device for use in cementing operations in the construction of oil and gas wells, said plug launching device comprising a tubular member having an upper section and a lower section which can accommodate at least a bottom plug and a top plug, characterised in that said upper section has a floor and said lower section is movable relative to said upper section, the arrangement being such that, in use, when a first dart is pumped into said upper section by cement said cement is diverted to raise said lower section to release said bottom plug and when a second dart is pumped into said upper section by hydraulic fluid it lands on said first dart and said cement is diverted to raise said lower section further to release said top plug.
Preferably, said upper section and said lower section are generally cylindrical and said lower section is telescopically movable with respect to said upper section.
Advantageously, the wall of that part of said lower section which accommodates said top plug and said bottom plug is imperforate. This feature, whilst not absolutely essent
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McClung Guy
Tsay Frank
Weatherford / Lamb, Inc.
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