Page 51 - Well Control for Completions and Interventions

42 Well Control for Completions and Interventions

Figure 1.18 Hydrates and safety valves (Illustration courtesy of Jonathan Bellarby).

• Equalization of surface controlled subsurface safety valves. There is a risk of
hydrate blockage when opening a SCSSSV, self-equalizing valves are
particularly vulnerable. If pressure is bled off above a closed valve,
phase separation will take place both above and below the closed
valve. (Fig. 1.18). Applying control line pressure to a self-equalizing
valve, opens the equalizing port. Gas from below the valve will imme-
diately mix with water above, allowing a hydrate to form.
To open a nonequalizing valve, fluid is pumped into the well above
the closed valve until it equalizes with the pressure below. If water is
used, the potential for a hydrate exists.
• Completion and workover operations on dead wells. When brine or seawa-
ter is used to maintain a hydrostatic overbalance, there is a risk of a
hydrate if gas enters the wellbore. Gas, migrating through a water-
based fluid, could result in the formation of a hydrate, particularly
close to the surface, where temperature is low.

1.1.9.2 Hydrate prevention
A cliche ´ perhaps, but the expression “prevention is better than cure” is
particularly apt when applied to hydrates. Once formed, they can be diffi-
cult to remove. Hydrates can be prevented by keeping temperature high