Page 54 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 45

If it is not possible to reach the hydrate by pumping from the surface,
coiled tubing can be used.

1.1.9.3.2 Pressure reduction
Where possible, pressure should be reduced on both sides of a hydrate,
although this is not normally possible if hydrates have formed downhole.
Bleeding off on one side of a hydrate is hazardous. If pressure differential
across the hydrate is high, the hydrate can partially dissolve, detach from
the tubing wall, and become a projectile that can travel at very high
velocity. The hard, heavy projectile, propelled by gas from below can
cause severe damage to surface equipment.

1.1.9.3.3 Elevate the temperature
If a hydrate is not completely blocking the tubing, flow the well to raise
the temperature and remove the hydrate. Similarly, in dual string comple-
tions where one string is blocked, producing the adjacent string can raise
the system temperature sufficiently to disperse any hydrates present.
Where a hydrate has formed at the surface, and ambient temperature
is high, simply waiting for the system to warm up should be enough to
disperse the hydrate. Heating the area where the hydrate has formed can
be hazardous. Trapped pressure can increase dramatically as the hydrate
dissociates; pipelines have been ruptured (Fig. 1.20).
Hydrates can form single or multiple plugs, and high pressure can be
trapped between plugs.

1.1.10 Hydrogen sulfide (H S)
2
Hydrogen sulfide, sometimes called sour gas, occurs naturally in many
hydrocarbon reservoirs. In some circumstances, “sweet” reservoirs become
“sour,” because of bacteriological action. The primary mechanism for res-
ervoir souring is the reduction of the sulfates present in seawater and other
completion brines to H 2 S through the action of sulfate-reducing bacteria
(SRB) under anaerobic (oxygen-free) conditions.
Hydrogen sulfide is both corrosive and highly toxic. It kills at even
low concentrations. Hydrogen sulfide data is normally recorded in ppm

Figure 1.20 Pressure trapped by hydrates can increase as temperature rises.