Page 48 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 39

After reducing pressure in the production tubing, high pressure remained
trapped below the hydrate (above the closed safety valve). The high differential
pressure across the hydrate caused it to release. It was propelled up the tubing,
probably taking the perforating gun with it. As soon as pressure equalized, the
gun and hydrate dropped back down the well. The falling gun parted the wireline,
allowing the gun to drop onto the closed safety valve where the impact sheared the
hinge pin in the flapper valve, damaging it beyond repair.
Following a fishing operation to recover the lost toolstring, the well had to be
recompleted to replace the damaged safety valve.
Gas hydrates (or clathrates) are a solid structure of water and gas that
closely resemble dirty ice or snow. Under the right conditions they can
form rapidly, blocking production systems. Hydrates form when low
molecular weight gases mix with water at relatively low temperature and
high pressure. Hydrates are characterized by a rigid network of water
molecules that cage in gas molecules of another substance. Gas stabilizes
the crystalline structure of water (Fig. 1.15).
Since methane (CH 4 ) is the most abundant gas in hydrocarbon pro-
ducing wells, nearly all hydrates are methane hydrates. Although having
similarities with ice, if pressure is high enough, hydrates will remain
stable well above the freezing point of water. For example, at 2000 psi,

a typical natural gas hydrate can remain solid at up to 70 F. 4
Approximately 85% 90% (by weight) of a hydrate is water, with the
remainder being gas. When a gas hydrate dissociates, a large volume of
gas is released. One cubic foot of hydrate contains between 160 180 ft 3
3
of gas, and 0.8 ft of water.

Figure 1.15 A methane hydrate. A methane molecule (in the center of the structure)
is trapped inside a latticework of water molecules.