Page 227 - Well Control for Completions and Interventions

Well Barriers 221

service. Typically, at least 10% more. There may be a requirement for
some barrier elements to be tested to their working pressure.
• Duration: A barrier test must last long enough to allow pressure to sta-
bilize, and for any pressure changes or flow to be observed. When
testing a large volume component, e.g., a casing test, a small leak
would not be noticeable on a short duration test.
• Documentation: Most operating companies require barrier tests to be
recorded. Plots and print-outs should be retained for the well
records.
• Frequency: Some well barrier elements will be tested at specified inter-
vals, e.g., the SCSSSV, wellhead, and Christmas tree valves. Other ele-
ments might only ever be tested at the time of installation. Items of
well intervention pressure control equipment and drilling BOP equip-
ment are tested each time they are rigged up and following any repairs
or break of containment. They are also tested to working pressure at
regular intervals to obtain the necessary certification.
• Verification: Where downhole mechanical barriers are close together, it
can be difficult to verify the integrity of the upper barrier. The lower
barrier will normally be inflow tested. With the lower barrier in place,
the only way of testing the upper barrier is to test from above.
However, if the space between the plugs is small (low volume) it will
be very difficult to determine if the upper plug is leaking. Plugs are
now available that allow pressure and temperature between the plugs
3
to be monitored. This enables both plugs to be verified.

6.4 INFLOW TESTING

Inflow tests in high temperature wells can be difficult to interpret
because of fluid expansion. It can be difficult to determine if flow from
the well is a leak or fluid expansion, as the cold test fluid warms. In very
high temperature wells it can take many hours for the fluid temperature
to stabilize. One method of determining if flow is diminishing is to create
a Horner plot of flow rate against time, with Horner Time on the x axis
and flow rate on the y axis.
After plotting a series of rate against time points, draw a straight-line
interpolation between the sample points. If the line intersects the x axis