Note: Descriptions are shown in the official language in which they were submitted.
2178367
A METHOD FOR INHIBITING HYDRATE FORMATION
FIELD OF THE INVENTION
The present invention relates to a method for inhibiting the formation of
clathrate hydrates in a fluid. More specifically, the invention relates to a method for
inhibiting the formation of gas hydrates in a pipe used to convey oil or gas.
BACKGROUND OF THE INVENTION
Carbon dioxide, hydrogen sulfide, and various hydrocarbons, such as methane,
ethane, propane, normal butane and isobutane, are present in natural gas and other
petroleum fluids. However, water is typically found mixed in varying amounts with
such petroleu~n fluid constituents. Under conditions of elevated pressure and reduced
temperature clathrate hydrates can form when such petroleum fluid constituents or
other hydrate-formers are mixed with water. Clathrate hydrates are water crystals
which form a cage-like structure around guest molecules such as hydrate-forming
hydrocarbons or gases. Some hydrate-forming hydrocarbons include, but are not
limited to, methane, ethane, propane, isobutane, butane, neopentane, ethylene,
propylene, isobutylene, cyclopropane, cyclobutane, cyclopentane, cyclohexane, and
benzene. Sorne hydrate-forming gases include, but are not limited to, oxygen,
nitrogen, hydrogen sulfide, carbon dioxide, sulfur dioxide, and chlorine.
Gas hydrate crystals or gas hydrates are a class of clathrate hydrates of
particular interest to the petroleum industry because of the pipeline blockages that they
can produce during the production and/or transport of the natural gas and other
petroleum fluids. For example, at a pressure of about lMPa ethane can form gas
hydrates at te~nperatures below 4C, and at a pressure of 3MPa ethane can form gas
hydrates at temperatures below 14C. Such temperatures and pressures are not
uncommon for many operating environments where natural gas and other petroleum
fluids are produced and transported.
As ga$ hydrates agglomerate they can produce hydrate blockages in the pipe or
conduit used to produce and/or transport natural gas or other petroleum fluid. The
2178~67
formation of such hydrate blockages can lead to a shutdown in production and thus
substantial financial losses. Furthermore, restarting a shutdown facility, particularly an
offshore production or transport facility, can be difficult because significant amounts
of time, energy, and materials, as well as various engineering adjustments, are often
required to safely remove the hydrate blockage.
A variety of measures have been used by the oil and gas industry to prevent the
formation of hydrate blockages in oil or gas streams. Such measures include
m~int:~ining the temperature and/or pressure outside hydrate formation conditions and
introducing an antifreeze such as methanol, ethanol, propanol, or ethylene glycol.
From an engineering standpoint, m~int~ining temperature and/or pressure outside
hydrate formation conditions requires design and equipment modifications, such as
insulated or jacketed piping. Such modifications are costly to implement and m~int~in.
The amount of antifreeze required to prevent hydrate blockages is typically between
10% to 30% by weight of the water present in the oil or gas stream. Consequently,
several thous~ind gallons per day of such solvents can be required. Such quantities
present handling, storage, recovery, and potential toxicity issues to deal with.Moreover, these solvents are difficult to completely recover from the production or
transportation stream.
Consequently, there is a need for a gas hydrate inhibitor that can be
conveniently mixed at low concentrations in the produced or transported petroleum
fluids. Such an inhibitor should reduce the rate of nucleation, growth, and/or
agglomeration of gas hydrate crystals in a petroleum fluid stream and thereby inhibit
the formation of a hydrate blockage in the pipe conveying the petroleum fluid stream.
One method of practicing the present invention uses gas hydrate inhibitors
which can be used in the concentration range of about 0.01% to about 5% by weight
of the water present in the oil or gas stream. As discussed more fully below, the
inhibitors of this invention can effectively treat a petroleum fluid having a water phase.
~178367
SUMMARY OF THE INVENTION
According to the invention there is provided a method for inhibiting the
formation of clathrate hydrates in a fluid having hydrate-forming constituents. The
method comprises treating said fluid with an inhibitor having a substantially water
soluble copolymer selected from the group consisting of the following N-vinyl
amide/lactam copolymers:
CH2--Cl I H2C Cl I
N CH3 ~o
CH3 n _ y
where n ranges from one to three and the sum of x and y is an average number
sufficient to produce an average molecular weight between about 1,000 and about
6,000,000.
BRIEF DESCRIPTION OF THE DRAWINGS
In drawings which illustrate embodiments of the present invention:
Figure 1 is a graphical representation illustrating (1) a "best fit" curve (solid)
based on the Inini-loop subcooling performance of three different copolymer
compositions of N-methyl-N-vinylacetamide/vinylcaprolactam (VIMA/VCap) having
25%, 50%, 75% mole fractions of VIMA and two homopolymer compositions
including poly(N-methyl-N-vinylacetamide) and polyvinylcaprolactam, and (2) a linear
line (dashed) representing the approximate arithmetic average in subcooling
performance that was expected over the same range of VIMA/VCap copolymer
compositions.
2l7836?
DETAILED DESCRIPTION OF THE INVENTION
INVENTIVE METHOD
The inventive method inhibits the formation of clathrate hydrates in a fluid
having hydrate-forming constituents. Formation of clathrate hydrates means the
nucleation, growth, and/or agglomeration of clathrate hydrates. Such clathrate hydrates
may be formed in a fluid whether it is flowing or substantially stationary, but are often
most problematic in flowing fluid streams conveyed in a pipe. For example, flow
restrictions arising from partial or complete blockages in a fluid stream can arise as
clathrate hydr~tes adhere to and accumulate along the inside wall of the pipe used to
convey the fluid. Nonetheless, the invention can be used for inhibiting formation of
clathrate hydrates in substantially stationary fluids.
In one embodiment of the invention, a concentrated solution or mixture of one
or more of the inhibitors of the type described below is introduced into a petroleum
fluid stream having an aqueous phase. As the inhibitor solution or mixture of this
invention is substantially dissolved in the aqueous phase or dispersed in the fluid
stream it reduces the rate that clathrate hydrates are formed, and thereby reduces the
tendency for a flow restriction to occur.
In a preferred embodiment, the solid polymer is first dissolved into an
appropriate carrier solvent or liquid to make a concentrated solution or mixture. It
should be understood that many liquids may effectively facilitate treatment of the fluid
stream without dissolving the inhibitor. Many liquids, however, will preferably
dissolve the inhibitor and, for convenience, are referred to hereafter as solvents
whether they produce an inhibitor solution, emulsion, or other type of mixture. The
solvent's principal purpose is to act as a carrier for the inhibitor and to facilitate the
inhibitor's absorption into the aqueous phase of the petroleum fluid. Any solvent
suitable for delivering the inhibitor to the fluid's aqueous phase may be used. Such
solvents include, but are not limited to, water, brine, sea water, produced water,
methanol, ethanol, propanol, isopropanol, glycol, or mixtures of such solvents. Other
solvents f:~mili~r to those skilled in the art may also be used.
217~367
It should be understood that the use of a carrier solvent is not required to
practice the invention, but it is a convenient method of introducing the inhibitor into
the fluid. In many applications the use of a carrier solvent will facilitate treatment of
the fluid stream.
Any c~nvenient concentration of inhibitor in the carrier solvent can be used, solong as it results in the desired final concentration in the aqueous phase of the
petroleum fluid. Higher concentrations are preferred, since they result in a reduced
volume of concentrated solution to handle and introduce into the petroleum fluid. The
actual concentration used in a specific application will vary depending upon theselection of carrier solvent the chemical composition of the inhibitor, the system
temperature, and the inhibitor's solubility in the carrier solvent at application
conditions.
The ir~lhibitor mixture is introduced into the aqueous phase of the petroleum
fluid using mechanical equipment, such as, chemical injection pumps, piping tees,
injection fittings, and other devices which will be apparent to those skilled in the art.
However, such equipment is not essential to practicing the invention. To ensure an
efficient and effective treatment of the petroleum fluid with the inhibitor mixture two
points should be considered.
First, an aqueous phase is preferably present at the location the inhibitor
solution is introduced into the fluid. In some petroleum fluid systems (particularly
natural gas systems), an aqueous phase does not appear until the gas has cooled
sufficiently for water to condense. If this is the case, the inhibitor solution is
preferably introduced after the water has condensed. Alternatively, in the event that an
aqueous phasc is not available at the point the inhibitor solution is introduced, the
inhibitor solution concentration should be selected to ensure that the inhibitorsolution's viscosity is sufficiently low to facilitate its dispersion through the fluid and
permit it to reach the aqueous phase.
Second, because the inhibitor primarily serves to inhibit the formation of
clathrate hydrates, rather than reverse such formation, it is important to treat the fluid
prior to substantial formation of clathrate hydrates. As a wet petroleum fluid cools it
217~367
will eventually reach a temperature, known as the hydrate equilibrium dissociation
temperature or Teq~ below which hydrate formation is thermodynamically favored. A
petroleum fluid's Teq will shift as the pressure applied to the fluid and the its
composition change. Various methods of determining a fluid's Teq at various fluid
compositions and pressures are well known to those skilled in the art. Preferably, the
fluid should be treated with the inhibitor when the fluid is at a temperature greater
than its Teq It is possible, but not preferable, to introduce the inhibitor while the
temperature is at or slightly below the fluid's Teq~ preferably before clathrate hydrates
have begun to form.
The quantity of inhibitor introduced into a petroleum fluid with an aqueous
phase solvent will typically vary between about 0.01 wt% and about 5 wt% by weight
of the water present in the fluid. Preferably, the inhibitor concentration will be about
0.5 wt%. For example, a laboratory study has shown that adding 0.5 wt% of a
copolymer of N-methyl-N-vinylacetamide and vinylcaprolactam (VIMA/VCap) to a
petroleum fluid allowed the fluid to cool to a temperature which was about 16.7C
below its Teq without formation of a hydrate blockage. A higher inhibitor
concentration can be used to lower the temperature at which a hydrate blockage is
obtained. A suitable concentration for a particular application, however, can bedetermined by those skilled in the art by taking into account the inhibitor's
performance under such application, the degree of inhibition required for the petroleum
fluid, and the inhibitor's cost.
INHIBITOR I)ESCRIPTION
Compounds belonging to the group of VIMA/lactam copolymers described
below, and mixtures thereof, are effective inhibitors of hydrate nucleation, growth,
and/or agglomeration (collectively referred to as hydrate formation). A generic
structure of the VIMA/lactam copolymers is depicted as follows:
2178367
CH2--Cl I H2C Cl I
C- oCH3
CH3 \ _/)
- x - n _y
where n ranges from one to three and the sum of x and y is an average number
sufficient to produce an average molecular weight between about 1,000 and about
6,000,000.
Where n = 1 the resulting polymer is a copolymer of
N-methyl-N-vinylacetamide and vinylpyrrolidone, VIMA/VP.
CH2--Cl I H2C Cl I
~c- oH3 ~ \~
CH3
x Y
Where n = 3 the resulting polymer is a copolymer of N-
methyl-N-vinylacetamide and vinylcaprolactam, VIMA/VCap.
CH2--CH H2C Cl I
N - CH3 C~
CH3
x _y
These VIMA copolymers may be used in mixture with other substantially water
soluble polymers, including but not limited to, poly(vinylpyrrolidone) (PVP),
poly(vinylcaprolactam) (PVCap), polyacrylamides or copolymers of vinylpyrrolidone,
vinylcaprolactam, or various acrylamides.
Without limiting the scope of the invention, and for the purpose of illustratingthe invention, three different ratios, 75:25, 50:50, and 25:75, of VIMA/VCap
copolymers were evaluated.
2178367
INHIBITOR SYNTHESIS
General Procedure
N-methyl-N-vinylacetamide (VIMA) is commercially available from various
specialty chemical suppliers such as, Aldrich Chemical (Milwaukee, Wisconsin). Afree radical initiator, 2,2'-Azobis(2-methylpropionitrile) (AIBN), used for synthesizing
these copolymers is also commercially available from Pfaltz and Bauer, Inc.
(Waterbury, CT). N-vinylpyrrolidone (VP) and N-vinylcaprolactam (VCap) may be
obtained com~nercially from Aldrich. N-vinylpiperidone may be synthesized according
to procedures well known to those skilled in the art.
Polymers were synthesized using standard laboratory procedures. Benzene or
low molecular weight alcohols were used as solvents. AIBN was used as the free
radical initiator. The polymers were isolated and characterized using well-knowntechniques (13C and 'H NMR and gel permeation chromatography) to confirm their
structures. Some examples of synthesis procedures are provided below for
convenience.
Synthesis Procedures
Synthesis of VIMA/VCap Copolymer
Ethanol was dried overnight over activated molecular sieves and then purged
for about 4 hours with a stream of dry nitrogen gas. A 500 ml flask equipped with an
overhead stirrer, condenser with drying tube, thermometer and nitrogen inlet waspurged with nitrogen. 19.8 g (0.2 moles) N-methyl-N-vinylacetamide (Aldrich) and27.8 g (0.2 moles) vinylcaprolactam (Aldrich) were loaded into the flask with about
250 ml ethanol. 0.4 g (0.002 moles) AIBN (Pfaltz and Bauer) was added and the
reaction heated at 78C for about 8 hours. The reaction was cooled and the product
isolated by vacuum evaporation of the solvent. The product was characterized by 13c
nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography
(GPC).
2178367
Synthesis of VIMA/VP and VIMA/VPip Copolymers
N-vinylpyrrolidone (VP) and N-vinylpiperidone (VPip) can be copolymerized
with VIMA using a synthesis procedure substantially similar to the one describedabove for synthesizing VIMA/VCap.
INHIBITOR EVALUATION
Mini-Loop Testing Procedure
One method for evaluating an inhibitor's effectiveness uses a bench-scale high
pressure apparatus referred to as a mini-loop apparatus. A mini-loop apparatus
consists of a loop of stainless steel tubing with about a one-half inch inside diameter
and about ten feet in length. The loop also has a transparent section for observing the
fluid flow in the loop and the onset of hydrate formation in the loop. Fluid
comprising about 40% by volume SSW (Synthetic Sea Water) solution having about
3.5% total ionized salts, 40% by volume hydrocarbon condensate (i.e., C6+), and 20%
by volume hydrocarbon gas mixture is circulated around the loop at constant pressure.
The hydrocarbon gas mixture is comprised of about 76 mole% methane, 9 mole%
ethane, 7 mole% propane, 5 mole% n-butane, 2 mole% iso-butane, and 1 mole% of
C5+. The inhibitor is typically injected into the loop as an aqueous solution to produce
the desired weight percent concentration of inhibitor in the aqueous sea salt/gas
solution. Generally, many hydrate inhibitors are evaluated at about 0.5 wt% of the
aqueous sea salt/gas solution.
The fluid is circulated at a constant velocity of about 2.5 feet/second. The loop
and its pump lay in a controlled temperature water bath for controlling the temperature
of the fluid circulating in the loop. The bath's water is circulated to ensure uniform
temperature throughout the bath and rapid heat transfer between the bath water and the
loop. As the loop temperature changes or as hydrates form, the gas volume in theloop will change accordingly. Therefore, to m~int~in constant pressure in the loop a
pressure compensating device is required. Such a device can be comprised of a gas
cell and a hydraulic oil cell separated by a floating piston. So as the gas volume in
the loop changes, oil may be added or removed from the oil cell to produce a
~178367
commensurate addition or removal of gas to the loop. Mini-loop tests are typically run
at a pressure of about 1,000 pounds per square inch gauge (p.s.i.g.). However, any
pressure between 0 and 3,000 p.s.i.g. could be selected for evaluating an inhibitor's
performance.
The temperature of the water bath is reduced at a constant rate, preferably
about 6F per hour, from an initial temperature of about 70F. At some temperature,
clathrate hydrates begin to rapidly form. As the dissolved gas is used to form clathrate
hydrates there is an abrupt and corresponding decrease in the volume of dissolved gas
in the aqueous sea salt/gas solution. The temperature at which this abrupt decrease in
the volume of dissolved gas is observed is known as the temperature of onset forhydrate formation (Tos). Recalling from the discussion above, the hydrate equilibrium
dissociation temperature or Teq is the temperature below which hydrate formation is
thermodynamically favored in an aqueous sea salt/gas solution without an inhibitor
present. Therefore, another measure of an inhibitor's effectiveness is the difference
between Teq and Tos which is known as the inhibitor's subcooling, Tsub Therefore, for
a given pressure, the greater the subcooling the more effective the inhibitor.
Typically, an aqueous sea salt/gas solution with no inhibitor present produces a Tsub of
about 6-7UF.
Mini-Loop Test Results
Without limiting the scope of the invention, and for the purpose of illustratingthe invention, three VIMA/VCap copolymers in different ratios were evaluated using
the mini-loop testing procedure described above. The results of these evaluations are
provided below.
2178367
TABLE 1
MINI-LOOP TEST RESULTS WITH POLYMERIC INHIBITORS
i
INHIBITOR RATIO CONC. WT%MINI-LOOP SUBCOOLING
TEMP. (~F)
None --- --- 7.0
PVIMA NA 0.5 12.5
PVCap NA 0.5 22.4
VIMA/VCap 75:25 0.5 26.5
VIMA/VCap 50:50 0.5 29.0
VIMA/VCap 25:75 0.5 30.0
Generally, copolymerizing VIMA with VCap produced an unexpected
improvement in the lactam homopolymer's inhibitor performance. As indicated above,
the VIMA homopolymer's subcooling was nearly 10F below the VCap
homopolymer~s subcooling. Consequently, it was unexpected that copolymerizing
VIMA with VCap would enhance, rather than (liminish the copolymer's hydrate
inhibition activity as compared to the VCap homopolymer.
Figure 1 illustrates a best fit curve produced from the data in Table 1. This
curve shows the synergistic inhibition effect that VIMA has when copolymerized with
VCap. The linear dashed line connecting the subcooling points obtained for the VCap
and VIMA homopolymers approximates the subcooling performance that was expected
for VIMA/VCap copolymers with various mole fractions of VIMA. The dashed line
represents the approximate arithmetic average in subcooling performance that wasexpected when VIMA was copolymerized with VCap. As the dashed line indicates,
VIMA/VCap subcooling performance was expected to (limini~h proportionately with
increasing mole fractions of VIMA.
It is believed that copolymerizing VIMA with other lactam monomers, such as
N-vinylpyrrolidone (VP) and N-vinylpiperidone (VPip), would also demonstrate such a
synergistic effect. However, the extent of the synergism observed for these other
2178367
VIMA/lactam copolymers, VIMA/VP and VIMA/VPip, may vary from that observed
for VIMA/VOap. In any case, the VIMA/VP and VIMA/VPip copolymers also are
expected to produce some synergistic effect. Consequently, they are expected to have
at least slightly improved subcooling performance over the arithmetic average
produced by using the subcooling performance of each comonomer's homopolymer
and the relative ratios of the comonomers comprising the VIMA/VP and VIMA/VPip
copolymers.
The means and method of the invention and the best mode contemplated for
practicing the invention have been described. It is to be understood that the foregoing
is illustrative ~nly and that other means and techniques can be employed withoutdeparting from the true scope of the invention claimed herein.
