Page 106 - Well Control for Completions and Interventions

98 Well Control for Completions and Interventions

• Run completion.
• Install wireline plug in tubing hanger profile (tubing hanger plug).
• Install high pressure internal cap. The cap is usually run with the wire-
line retrievable “crown plug” preinstalled.
• Recover drilling BOP and install debris cap.
Live well intervention work on a well equipped with horizontal tree is
more time consuming and complex than the equivalent operation with a
conventional tree. With a conventional tree, the swab cap is removed and
pressure control equipment connected directly to the tree block. Double
valve isolation is provided by the closed and tested upper master valve
and swab valve. Once the pressure control equipment is tested both tree
valves are opened, giving access to the wellbore. To gain access to the
wellbore through a horizontal tree, both wireline plugs (tubing hanger
plug and crown plug) must be removed. Removal of these plugs requires
the rigging up of gate valves to provide well control barriers. Access to
the wellbore requires the following actions:
• Remove the tree cap.
• Rig up dual valve isolation. This is normally two gate valves. Ideally at
least one of the valves will be actuated and connected to a well control
panel.
• Pull the crown plug.
• Pull the tubing hanger plug.
• Carry out intervention work using the two temporary valves for dou-
ble barrier isolation.
• Replace and test the tubing hanger plug.
• Replace and test the crown plug.
• Replace and test the tree cap.
Since the tubing hanger plug and crown plug are permanent barriers
and an integral part of the Christmas tree, they are normally equipped
with metal-to-metal seals (Fig. 3.10).
Energizing wireline set metal seals can be problematic. To add to the
difficulty, these plugs do not have an equalizing device. Without this criti-
cal feature, pressure differential (force) from above will make pulling the
plug impossible. Potentially more hazardous, pressure differential from
below risks blowing the plug (and the wireline recovery tool string) up
the hole, with the attendant risk of breaking the wire and damaging the
surface pressure control equipment. Pressure must be balanced across the
plugs before recovery can be safely accomplished, and effectively balanc-
ing pressure can be tricky. For these reasons, horizontal trees are not ideal
where frequent mechanical interventions (coiled tubing, wireline,