Page 52 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 43

and pressure low. For example, nitrogen in the annulus, or even vacuum
insulated tubing, can be used to insulate the tubing and retain more heat
in the produced fluids. However, for many phases of a completion or
intervention, the combination of high pressure and low temperature is
unavoidable. If gas and water cannot be separated, then chemical inhibi-
tion is the only option.
Several chemicals are used to inhibit (and remove) hydrates. Those
that remove or prevent the accumulation of ice also work well with
hydrates. The general class of chemicals that are effective for de-icing and
hydrate control are alcohols. Methanol (CH 3 OH) is widely used and eas-
ily available. Its relatively low molecular weight enables it to permeate the
hydrate, making it an effective inhibitor. However, there are some safety
concerns:
• Toxicity.
• Low flash-point.
• It burns with a near invisible flame.
• It is a contaminant in oil as it interferes with catalysts in refineries, and
adversely affects produced water treatment and discharge.
• It is a cause of stress corrosion cracking in titanium components such
as heat exchangers.
Monoethylene glycol (C 2 H 3 OH) is also widely used. Although not as
effective as methanol, since it has a higher molecular weight, it is still a
useful inhibitor and is much less toxic than methanol. It is routinely used
to prevent the formation of hydrates when pressure testing intervention
equipment (Fig. 1.19).
Methanol or glycol mixed with water at varying concentrations lowers
the hydrate free temperature at a given pressure (Fig.1.19). For example,
at 3000 psi, hydrates can form in seawater at below 72 F. Mixing the

seawater 50/50 with glycol lowers the temperature at which hydrates can

form from 72 Fto 35 F. Mixed with methanol, the temperature is reduced
further still, to well below the freezing point of fresh water (32 F).

Glycol is routinely used to inhibit when testing well intervention pres-
sure control equipment. There is anecdotal evidence to suggest that, if
left for several hours, the heavier glycol (1.113 SG) separates out from the
water and hydrate problems can occur. Some operators advocate the use
of neat glycol for pressure testing. However, methanol and glycol are sol-
uble in water, so providing they are properly mixed and agitated a prob-
lem should not occur.