7/23/2019 E-966-JPT
1/2
F I E L D D E V E L O P M E N T
966 SEPTEMBER 1997
Gas hydrate formation during deepwateroffshore drilling and
production is a well-recognized operational hazard. In waterdepths
greater than 1,000 ft, seabed condi-tions of pressure and
temperature becomeconducive to gas-hydrate formation. In
awell-control situation, although the kickfluid leaves the
formation at a high temper-ature, it can cool to seabed temperature
withan extended shut-in period. With highenough hydrostatic
pressure at the mudline,
hydrates could form in the blowout-preven-ter (BOP) stack and
choke and kill lines, ashas been observed in field operations.
The current practice in deepwater drillingis to suppress the
hydrate-formation tem-perature by use of highly saline drilling
flu-ids formulated from NaCl or other salts.This solution is
applicable for the Gulf of Mexico but insufficient for the
conditionsencountered in Norwegian deep waters. Atextreme water
depths or extremely lowmudline temperatures, this
thermodynamicinhibition alone may not be sufficient toprevent
hydrate formation. Instead, the useof kinetic inhibitors or crystal
modifiers, inconjunction with thermodynamic in-hibitors, may allow
successful operations insuch an environment. The definition of
kinetic inhibitors (to distinguish them fromthe classic
thermodynamic inhibitors, suchas polar compounds and
electrolytes)comes from the effect of the chemicals onthe
nucleation and growth of natural gashydrates, both of which are
time-depen-dent, stochastic processes.
DEEPWATER DRILLING FLUIDS
A literature review indicated that 20 to 23wt% NaCl/polymer
drilling-fluid systems arethe most commonly used fluid
formulationsfor deepwater drilling. Drilling with thesesystems has
been successful in water depthsup to 7,500 ft. A major operator in
the Gulf of Mexico drilled wells in water depths rang-ing from
2,000 to 7,500 using mainly 20wt% NaCl- and PHPA-based drilling
fluids.They experienced an average of one or moregas kicks per
well. In one incident, at a waterdepth of 2,900 ft, difficulties
disconnectingthe BOP were suspected to have been caused
by hydrate formation. It is clear that thehigher the density,
the shallower the water
depth where hydrates could exist, whichprompts the question of
why it has been pos-sible to drill in water depths up to 7,500
ftwithout encountering hydrates. The follow-ing can be cited as a
possible explanation:(1) good well-control procedures, (2) littleor
no gas kicks encountered, (3) the gas kickis of very low-gravity
gas (mostly methane),(4) the effect of metastability, (5) high
circu-lation rates and short shut-down periods,and/or (6) the
presence of other inhibitors
that are unaccounted for.POLAR AND SURFACE-ACTIVE
AD DI TI VE S
Surfactants or alcohols are known todecrease the surface tension
of water, whichwill enhance the rate of gas diffusion in thebulk
water during hydrate formation.Hydrate-crystal growth is controlled
by therate of gas diffusion from the bulk of thewater to the
crystal surface. Consequently,the presence of these compounds in
thewater results in a rapid hydrate growth.Anionic and nonionic
surfactants werefound to cause a dramatic increase in the rateof
gas consumption on hydrate formation.Alcohols and glycols are
well-knownhydrate thermodynamic inhibitors. Alcoholsand glycols,
when dissolved in aqueous solu-tions, normally hydrogen bond with
thewater molecules and make it difficult for thewater molecules to
participate in the hydratestructure. However, their effect on the
kinet-ics of hydrate formation is somehow differ-ent. Results show
that the addition of alco-hol or glycol actually enhances the rate
of hydrate formation, meaning that the pres-sure of small amounts
of polar compounds isworse than having none. One explanationfor
this behavior was that polar moleculesact as templates for
nucleation when theyexist at low concentration. Also, the pres-ence
of drilling-fluid additives like ben-tonite, barite, and lignite,
with their tremen-dous surface area, could provide a surface
onwhich gas molecules could be adsorbed.This behavior could cause
the physicalentrapment of gas in quantities larger thanthe amount
of gas dissolved in the water.The adsorbed gas that is dispersed
within the
bulk water with the solid particles is readilyavailable for
encapsulation in the hydrate
structure. This probably explains earlierobservations about the
increase in the rateand amount of hydrate formation in
drillingfluids formulated with these additives.
KINETIC INHIBITION AND
CRYSTAL MODIFIERS
The kinetic inhibition of gas hydrates gen-erally refers to the
process by which thenucleation and growth of hydrate crystalsare
altered (modified) by use of a low con-
centration of mostly polymeric- and surfac-tant-based chemicals.
The inhibitors maycause one or more effects: (1) delay
theappearance of the critical nuclei (kineticinhibitor), (2) slow
the rate of hydrate for-mation (crystal modifier), and/or (3)
pre-vent the agglomeration process (crystalmodifier). These
chemicals do not disruptthe thermodynamic equilibrium of
thehydrates. Currently, there is no clear under-standing of how
some of these chemicalsaffect the growth or the morphology of
thehydrate. However, it is possible for thesechemicals to control
the growth and theagglomeration of the hydrate crystals byadsorbing
at the crystal/liquid interface. Ananalogous approach is used by
the ice creamindustry to control the growth of the icecrystals. In
both of these processes, mixing(shear) plays an important role in
mechani-cally smoothing the texture of the slurry.
Working with high-salinity drilling flu-ids, as in deepwater
drilling, reduces theoptions in selecting a good kinetic
inhibitorand crystal modifier. Another challengeworking with these
inhibitors is their sur-face activity. Because of all the surface
areaspresent in drilling fluids, unless theinhibitors have a
specific affinity toward thehydrate surface, one most likely will
haveto cover all the solid surfaces before theinhibitor starts to
work. More work isrequired before suitable kinetic inhibitorsor
crystal modifiers for drilling fluids canbe recommended.
LABORATORY MEASUREMENTS
A thermodynamic testing program was con-ducted in two stages. In
the first stage, aque-ous solutions of selected thermodynamic
inhibitors formulated in seawater as the basefluid were tested.
This stage was necessary to
HYDRATE CONTROL INDEEPWATER DRILLING
7/23/2019 E-966-JPT
2/2
968 SEPTEMBER 1997
F I E L D D E V E L O P M E N T
establish a baseline for the best inhibitors tobe used in the
subsequent drilling- and spot-ting-fluids formulations. In the
second stage,drilling-fluid formulations and two solidsfree
spotting fluids with the best combina-tions of the thermodynamic
inhibitorsscreened in the first stage were tested.
RESULTS
The tests produced a number of conclusionscovering the
suitability and performance of various fluid formulations. The
followingare the generalized conclusions. The most common drilling
fluids used in
deepwater drilling are the 20 to 23 wt%NaCl/polymer systems.
Drilling withthese systems has been successfullyachieved in water
depths up to 7500 ft.
On a weight basis, NaCl is the most effec-tive hydrate inhibitor
followed by KCl,
CaCl2, NaBr, sodium formate, and calci-um nitrate. Ethylene
glycol had the best performance
among the glycols tested in this work. Sodium formate increases
the degree of
suppression over that of NaCl but has thetendency to precipitate
at low tempera-tures and high concentration.
A drilling fluid formulation was formulat-ed from 5 wt% KCl+15
wt% NaCl+10wt% ethylene glycol that has a maximumhydrate
suppression of 32.1F.
Two new solids-free spotting fluids weredeveloped. The first
fluid did not formhydrates under an extreme degree of sub-cooling,
while the second suppressed thehydrate temperature by 45.4F.
The synthetic drilling fluid with 30 wt%CaCl2 in the internal
phase did not formhydrates under extreme subcooling.However,
hydrates did form when the con-centration of CaCl 2 was reduced to
15 wt%.
The organic phase of the syntheticdrilling fluid slightly
suppresses thehydrate equilibrium temperature but, atthe same time,
increases the rate andamount of hydrate formation resultingfrom the
higher gas solubility.
This article is a synopsis of paper SPE 38567, Hydrate Control
During Deep- water Drilling: Over view and New DrillingFluids
Formulations, by H. Ebeltoft,Statoil; M. Yousif, IITRI Westport
Tech-
nology Center; and E. Soergaard, NorskHydro, scheduled for
presentation at the1997 SPE Annual T echnical Conferenceand
Exhibition, San Antonio, Texas, 58October. Please read the
full-length paperfor additional details, illustrations, and
ref-
erences. The paper from which this synop- sis was taken has not
been peer reviewed.
