Page 105 - Handbook Of Multiphase Flow Assurance

Hydrate of natural gas 101

in a plugged pipeline until the regulatory requirements mandate the operator to remove the
blockage or to decommission the pipeline. Some blockages get remediated by operator com-
panies through operating process parameters manipulation, with proper engineering sup-
port. Some blockages get remediated by other service providers. In rare cases, blockages have
been known to get remediated by a confluence of events triggered by a natural phenomenon
like a hurricane as discussed earlier in Chapter 1.
Historically, the number of hydrate blockages has been on an uptrend.

Blockage remediation

Production system should be designed for blockage remediation to enable the removal of
blockage(s) and to avoid secondary re-formation of solids. Access points and topsides deck
space should be designated for regionally available remediation equipment. System equip-
ment should include provisions for flow reversal for blockage remediation such as a stuck
scraper.
Technologies which could be considered for remediation of blockage in produced fluids
include:
• Solvent chemical
• progressive scraping
• depressurization from topsides
• depressurization through gas lift line
• subsea depressurization skid
• dry tree coiled tubing jetting.
Field-proven technologies which may be applicable on a case by case economic basis in-
clude pressure pulsation, long reach coiled tubing, and active electric heating.

Hydrate of natural gas

Introduction
Hydrates are solid crystals combining water molecules and guest molecules in a proportion
of approximately 10 wt% gas and 90 wt% water. Flow assurance deals mainly with hydrates
of light hydrocarbons such as methane, ethane, propane and isobutane present in natural gas.
Hydrates were first observed and reported by Priestley (1778). The compound was named
by Davy.
After hydrates form, they get deposited on pipe wall or dispersed in hydrocarbon liquid or
in water. Hydrate acts as a solid restriction or as viscous slush creating resistance to fluid flow.
Both light and medium gas components can form hydrate crystals but hydrates of
medium-sized molecules are less often encountered in production systems. Medium-small
molecules like neohexane can still fit in hydrate cells (Makogon, 1996) but in general larger
hydrocarbon molecules with 5 or more carbon atoms in a chain are too big to fit as guests in
the hydrate cells.