Page 49 - Well Control for Completions and Interventions

40 Well Control for Completions and Interventions

Figure 1.16 A hydrate in a “pig receiver” on an offshore platform.

Under specific conditions, hydrates form rapidly, blocking tubing, and
flowlines, and trapping pressure. They can be extremely difficult to
remove, and are a potential hazard unless handled correctly (Fig. 1.16).
Knowing where and when there is the potential for a hydrate to form
is essential. If temperature is low, pressure high, and water and gas are pres-
ent in the system, preventative measures will be needed to stop hydrates
forming. If hydrates do form, they will need to be safely removed.
Hydrates form and exist for indefinite periods inside the hydrate sta-
bility zone. This can be plotted on a hydrate disassociation curve. Outside
the zone hydrates do not form. However, if the pressure/temperature
combination is above the stability curve (Fig. 1.17), the potential exists
for hydrates to form if gas and water mix.
Predicting closed-in pressure will normally be based on the pressure
record for a recent shut-in. Predicting temperature against depth can be
more problematic, particularly if an intervention requires intermittent
periods of flow, shut in, and possibly injection. Temperature modeling
using proprietary software such as WellCAT is useful in this regard.

1.1.9.1 Hydrate risk during well interventions
During production, a combination of high temperature and relatively low
pressure will normally prevent hydrates from forming. However, most
interventions require the well to be closed in for some or all of the time.
After shut-in, pressure increases and temperature decreases. If the post
shut-in combination of temperature and pressure fall in the hydrate for-
mation zone, the potential for water/gas contact must be evaluated.