Page 135 - Well Control for Completions and Interventions

Completion Equipment 127

3.19.1.2 Ambient pressure operated valves
This type of valve uses a nitrogen charge to hold the valve open. If ambi-
ent pressure at the valve drops below a predetermined value, as may occur
during a large volume leak at the surface, the valve will close. This type
of valve has the advantage of not needing a choke to control its operation,
and is therefore more suited to high velocity wells. However, it may
require regular replacement in locations where there is a rapid decline in
flowing pressure.

3.19.1.3 Injection valves
Not all injection wells will be fitted with a safety valve. However, where
injection is into a hydrocarbon bearing formation, and where reservoir
pressure is high enough for the well to flow back to the surface, a safety
valve will often be used.
These are usually a simple “normally closed” valve that allows the
injection of fluids or gas, but closes if the direction of flow reverses. Ball
and flapper closure mechanisms are both used. However, they are to some
degree flow sensitive. At low injection rates the valves can be prone to
throttling. This will damage the valve if it goes on for too long. For this
reason, the “poppet” type valve is now becoming increasingly common.
This is a simple dart type check valve, and is not flow sensitive (Fig. 3.33).
3.19.2 Surface controlled valves

Modern SC-SSSVs operate on the fail-safe principle, and form an integral
part of a production facilities emergency shut-down system. Where a
number of wells are clustered together, for instance on an offshore plat-
form, individual well shut-down systems will be interlinked.
During normal operating conditions, a surface panel supplies hydraulic
pressure to the downhole safety valve via a hydraulic control line. The
pressure drives down a piston, compressing a powerful return spring. At
the same time the piston also moves the inner flow mandrel, or flow
tube. Flow tube movement opens the valve; either by rotating a ball, or
by pushing a flapper off seat and in to a recess away from the flow path
(the flow tube will fully cover and protect the flapper). As long as control
line pressure is maintained, the valve remains open. If control line pres-
sure is lost, the spring pushes the flow tube up, allowing the flapper (or
ball) to close. Closure can be triggered by pressure sensing pilots attached
to the flowline, flame and heat detection equipment, gas detection equip-
ment, or by manual activation (Fig. 3.34).