GB2301836A - Inhibiting Clathrate hydrate formation - Google Patents

Description

1 Exxon Production Research Company A METHOD FOR 1 IBITING = RATE FORMAT1

c 2301836 The present invention relates generally to a method for inhibiting the formation of clathrate hydrates in a fluid. More specifically, but not exclusively, the invention relates to a method for inhibiting the formation of gas hydrates in a pipe used to convey oil or gas. The invention also relates to the use of a nonclathrate hydrate forming polymer in a fluid to inhibit the formation of clathrate hydrates.

Carbon dioxide, hydrogen sultide, and various hydrocarbons, such an methane, ethane, propane. normal butane and iacbutane, are preaent In natural gas and other petroleum fluids. However, water Is typ.cally found mixed in varying amounts with such petroleum fluid constituents. Unde= conditions of elevated preezure and reduced temperature clathrate hydrates can form when such petroleum tluid constituents or other hydrate tormars are mixed.wizh water. Clathrate hydrates are water cryatals w.'"ch fjorm a cage-like structure around guest molecules such as hydrate 0 cJng hydrocarbons or gases.

rx,, -Sona hydrate hydrocarbons include, but are not limited to, methane, ethane, propane, isobutane, butane, neopentane, ethylene, propylene, isobutylene, cycloprcpane, cyclobucane, cyclopentane, cyclohexane, and benzene. Some hydrate forming gases include, but are not limited to, oxygen, nitrogen., hydrogen sul!ide, carbon dioxide, E.-,!-!ur clioxide, and chicrine.

Gas hydrate crystals or gas hydrates are a class of claLhrat:e hydrates cf particular interest to c! -- per-role.-.n, induatry because c the pipeline blockages that they car.

prc-',..ce d,-,r-'ng the pr:)duczion and/or tzanspcrz of the natural cas, and other petrzleu.n f luids. For a: z pressure of abcut IMPa ethane can fcrn gas '.ri.,.drazes at.

BAD ORIGINAL A C - 1 2 temperatures below 4 C, and at a pressure of 3M?a ethane can f orm gas hydrates at temperatures below 14 C. Such temperatures and pressures are not uncommon for many operating environments where natural gas and other petroleum fluids are produced and transported.

As gas hydrates agglomerate they can produce hydrate blockages in the pipe or conduit used to produce and/or transport natural gas or other pezrol'sum fluid. The 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 wall as various engineering adjustments, are often IS required to safely remove the hydrate blockage.

..& variety of measures have been used by the oil and gas industry to prevent the formation of hydrate bloc.ages in oil or gas streams. Such measures include maintaining the temperature and/or pressure outside hydrate formation conditions and Lntroducing an antifreeze such as methanol, ethanol, propanol, or ethylene glycol, From an engineering standpoint, malnt; jilning 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 maintain. The amount of antifreeze required to prevent hydrate bioc!,Laan is typically between IOV to 30 by wei;ht of the water present in the oil or gas stream. consequently, several thousand gallons per day of such solvents can be re;,.i-red, Such quantities present handling, storage, recovery, and potential toxicity issues to deal with. Moreover, these solvents arn dtffivalt to completely recover!rcm the production or transpcrtation stream.

Consequently, there is a need for a gas hydrate inhibitor that can be conveniently mixed at iow concentrations in tae proeuced or transported petrcle.-..,m fluids. Such an inhibl-tor should zedace, the rate of n,,:c:Ieat-4or.,. growth, and/or agglomeraticr. of gas hydrate L BAD OMGINAL A 3 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 practising the present invention uses polymers (gas hydrate inhibitors) which can be present in 9 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 used in this invention can effectively treat a petroleum fluid having a water phase.

According to the invention there is provided a method for inhibiting the formation of clathrate hydrates in a fluid having hydrate-forming constituents, comprising: treating said fluid with an inhibitor comprising one or more polymers having the following amide unit (I):

__CH2-CH-1 F C12-CH 1 J. 1 1 y N-CI"13 N 0 1 - C=0 1 n jj13 (I) wherein the sum of x and y is a number of units such that the average molecular weight for said polymer is between about 1 000 and about 6 000 000; and n is an integer of 1 to 3.

A second aspect of the invention involves the use of a polymer or polymer composition comprising one or more polymers having the following amide unit (I):

1 ( -1:

c 4 +CI12-CH 1 r CI42-CI4 1 1 1 X j j y N-CH3 N 0 n wherein the sum of x and y is a number of units such that the average molecular weight for said polymer is between about 1 000 and about 6 000 000; and n is an integer of 1 to 3, in a fluid having hydrate-forming constituents to inhibit the formation of clathrate hydrates.

The invention will be better understood by referring by way of example to accompanying Figure 1 which illustrates (1) a "best fit" curve (solid) based on the miniloop aubcooling performance of three different copolymer compositions of N-nethyl-NvinYlicetain4-de/vinylcaprolactam (VD4A/VCap) haVing 25k, 50k, 75k mole fractions of VIMA and two homopolymer compositions including poly(N-methyl-K-Yinylacetamide) and pclrjinyleaprolacta:n, and (2) a linear line (dashed) representing the approximate arithmetic average in oubcooling per!czmance that was expected over the came range o! VIMA/VCap copclymer Compositions.

P 1 is 3 5 The method of the invention inhibits the formation of clathrate hydrates in a fluid having hydrate forming constituents. Formation of clathrate hydrates means the nucleation, growtl-., and/or agglomeration of clathratte hydrates. Such clathrate hydrates may be formed in a fluid whether it is flowing or substantially. stationary, but are often most problematic iz% flowing fluid streams conveyed in a pipe. ror example, flow restrictions arising from partial or complete blockages in a fluid stream can arise as clathrate hydrates 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 f luilas.

In one aspect of the invention, a concentrated solution or mixture of one or more of the inhibitcre of the type described below in Introduced Into a petroleum fluid stream having an aqueous phase. As the inhibitor solution or mixtire 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 aspect, the solid polymer is first dissolved into an appropriate carrier scIvent or liclud to make a concentrated solu:ion or mixture. It should be understood that many liquids may effec:ively faciiitate treatment of the fluid stream without dissolv,'ng the inhibitcr. many liquids, however, will preferably dissolve the inhibitor and, for convenience, are referred to hereafter as solvents whether they produce an inhibitor solu:ion, emulsion, or other type of inixture. The solvent's principal purpose is to act as a carrier for the inh-,b-,tor and to facilitate the inhibitor's absc--pt-on nto the aqueous phase of the petroleum flu-d. P-ny solvent su-cal-le fcr delivering zhe to the fluid's aqueous pha,t! 1 BAD ORIGINAL T G', 6 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 familiar to those skilled in the art may also be usod.

it should be understood that the use of a carrier solvent Is not required to practi sethe invention, but it is a convenient method of introducing the inhibitor into the fluid. In many applications the use of a carrier solve-it wi1l facilitate treatment of the fluid stream.

Any convenient concentration of inhibitor in the carrier solvent can be used, so long as it results in the desired final concentration in the aqueous phase of the petroleum fluid. Higher concentrations are preferred, since is they reault in a reduced volume of concentrated solution to hancU_q and introdtace into the petroleum fluid. The actual concentration used in a specific application will vary depending upon the selection of carrier solvent the chemical composition of the inhibitor, the system temperature, and the inhibitor's solubility in the carrier solvent at application ccnditiona.

The,inhibitor miXture is introduced into the aqueous phase of the petroleum fluid using mechanical equipnent, such as, chemical injection pumps, piping tees, injection fittings, and other devices which. will be apparent to those skilled In the art. Hcwever, 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 sho%.ld be considered.

First an acraeoua phase is preferably present at the location the inhibitor solution is introduced Into the fluid. In some petroleum fluid systems (particulzrly natural gas systems), aLn aqueous phase does not appear unti the gas has ccoled sufficiently for water to condense.

3S t1h-, s i a the case, the inhibitor sol'ztioa is preferably introduced after the water has condensed. Alternative!y, ia tl-ie event that an acluecus phase is not availlable. at the point the inhibitcr szlution ia introduced, the inhibizoz so!--iticn concentration should be selected to enzare zha: z'-Ie BAD ORIGINAL A W 7 inhibitor solution's ViscositY Is sufficiently low to facilitate its dispersion through the fluid and permit it to reach the aque0UJ.3 phase.

Second, because the inhibitor primarily eer-ves to inhibit the formation of clathrate hydrates, rather than =everse such formation. it Is important to treat the fluild prior to substantial formation of clathrate hydrates. As a wet petroleum fluid coo;o it will eventually reach a temperature, known as the hydrate equilibrium aissociation temperature cr Teq, below which hydrate formation Is thermodynamically favared. A petroleum fluid's Teq will hift as the pressure applied to the fluid and the its omposition change. Various methods of deternining a fluid's Teq at various fluid compositions and praosures are is well known to those skilled dn the art. Preferably, the fluidg.should be treated with the Irkilbitor when the fluid Is at a temperature greater than Its Teq. It la possible, but not preferable, to introduce the Inhibitor while the temperature in at or slightly below the fluid's Teg, preferably before clathrate hydrates have begun to form.

The quantity of InhibItor Introduced Into a petroleum fluid with an aqueous phaze solvent will typically vary between about O.Cl wt to about 5 wtv by weight of th C,Water present in the fluid. Preferably, the inhibitor concentzation will be about 0.5 wtk. For example, a laboratory study has shown that adding 0.5 wtt cf a copoylmer of N- methyl -Nvinylacetamide and v'nylca...)rolaczam (VIMA/VCap) to a petroleum fluid allowed the flul-d to ccol to a temperature which was about 16.7 0C below its Teq without fc.-mat.icn of a hydrate blockage. A higher inhib!tcr ccncentra:ic= can be used to lower the temperature at whi cn a hydrate blockage is obtained. A suitable concentration for a partIcular application, however, can be deter-,n-r.ed by those skilled in the art by taking into zccount the inhibtor's performance under such applica---'cn, the degree c -ni-ibilicn requIred for the pezroleun fluid, and the inhibitcr,s cost.

BAD ORIGMAL L --- - 8 THWTIRTME np-qrRTP-rTnN Coapounds belonging to the group of VIMA/lactam copolymers described below. and mixturee thereof, are effective Inhibitors of hydrate nucleation, growth, andlor agglomerati6n (collectively referred to as hydrate formation). A generic structure of the VIMA/lactam copolymers Is depicted as follows:

--c, - H- N-CH, 1 C=0 1 cS H2c-CH1 N 0 ) n^ _j Y where n ranges troin one to three and the sum of x and V is a number sufficient to produce an average molecular weight between about 1,000 to about 6,000,000.

is Where n a 1 Cle resulting polymer 10 a copolymer of Nmethyl-N-vinylacetamide and vinylpyrralidone, VIMAIVP.

-2CH;- CH - HIC-CH 1 1 N-CH N a 0 c=o (-;M2 L_ _j X L_ _j Y Ehere n = 3 the resulting polymer is a c=polymer of N- methyl -N-vir.ylace:amide and vinylcaprclactat,, VIMA/VCap.

F- - C P- CH1 N- Ct =0 ch X VIMANCac) H7C - ?H GN 0 These VIMA copoly-ners may be used in n..xture wIth ct-her water soluble pol-mer3, bur nct - BAD ORIGINAL R- li0 C-.) cl 9 limited to, poly ( vinylpyrrol 1 done) (PVP). poly(vJ.nyleaprolactam) (PVCap), polyacrylamides or copolymers of PVP, PVCap, or various polyacrylamides.

without limIting the scope of the Invention, and for the purpose of illustrating the invention, three different ratios, 75125, 50:50, and 25:7S. of VIMAIVCap copolymera were evaluated.

1=31TOR OYNTHESIS (It-ng,-=1 Ernt-gdU-1, N-methyl-N-vinylacetamide (VIMA) Is commercially is available from various specialty chemical suppliers such as, Aldrigh Chemical (Milwaukee, Wisconsin). A free radical initiator, 2,21-Azobia(2- methylpropionltrile) (AZEN), used for synthesizing these copolymers is also comm ercially available from Pfaltz; and Bauer, Inc. (Waterbury, CT). N- vinylpyrralidone (VP) and N-vinylcaprolactam (VCap) may be obtained commercially from Aldrich. N-vinylpiparidone may be synthesized according to proceduxes well knowT. to Lhose skilled in the art.

23 Polymers were synthesized using standard laboratory procedures. Benzene or low molecular weight alcohols were used an solvents. 2,21-Azcbis(2-methylpropionitrile) (AIBN) was used as the free radical initiator. The polymers were isolated and characterized using well-known. tectLnicrues (13C and 1H NMR and gel permeation chromatography) to confirm their structures. Some exzmples of synthesis procedures are provided below for convenience.

8=the-4g t---P VTM-/VCnp CorolyTriar Eth.anol was dried ove=night over activated mclecular sieves and he- purged for abc,,:t 4 ho,,:rs with a szrean cf BAD ORIGINAL E- -------- v c, dry nitrogen Sras. A 500 mL flask equipped with an overhead stirrer, condenser with drying tube, thermometer and nitrogen inlet was purged with nitrogen. 19.8 g (0.2 males) F-methyl-IN-vinylacetamide (Aldrich) and 27.8 S (0.2 moles) vinylcaprolactam Mlclrich) were loaded into the flask with about 250 mL ethanol. 0.4 9 (0-002 moles) AMBN (Pfaltz ar.d Bauer) was added and the reaction heated at 78 C for about 8 bourn. The reaction was cooled and the product isolated by vacuum evaporati6n of the solvent. The product was characterized by 13C nuclear magnetic resonance (nmr) spectroscopy and gal permeation chromatography (gpc).

9=thi!tmloR ^f VIMA/32 and JITMANPip rnpmlvmg-q N-vinylMrolidone (M and Nvinylpiperldone (VPip) can be copolymerized with VIMA using a synthesis procedure subst.antially similar to the one described above for synthesizing VIMAIVCap.

TITHTATTMI1R RVALUATT011 One method for evaluating an inhibitor's effeccivenests uses a bench- scale high prescure apparatus referred to as a 21 m-nillcop apparatus. A minlloop apparatus consists of a loop c! otainleso 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 40k by volume SSW solution having about J.5k total ionized salts, 40k by volume hydrocarbon ccndensate (-4 e., Cc+), and 20k by volume hydrocarbon gas mixture is circulated around the loop at constant pressure. Ihe hydxocarbon gas mixture Is comprised of about 7Z mcle 33 metharke, 9 molet ethane, 7 rnolek promane, 5 mole!k n-butane, 2 molel. iso-bur-ane, and 1 mole% c! C,+. The inhibitcr -9 typically Injected into the loop as an aqueous soluzion zo prcduce the desired weight percent concentration ot 1 Jnhibitor in the acrjeous sea salt/gas sclu.:iion. General!-,,, BAD ORIGMAL f-) 11 c many hydrate inhibitors are evaluated at about 0.5 wtk of the aqueous sea salt/gaa solution.

The fluid in circulated at a constant velocity of about 2.5 ieet/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 iw circulated to ensure uniform temperature throughout the bath and rapid beat transfer between the bath water and the loop. An the loop temperature changes or as hydrates form, the gas volume in the loop will change accordingly. Therefore, to maintain coustant pressure in the loop a pressure compensating device is required. Such a device can be comprised of a gas call and a hydraulic oil cell separated by a floating piston. So as the gas volume in the loop IS changes, oil may be added or removed from the oil cell to produge a commensurate addition or removal of gas to the loop. Miniloop tests are typically run at a pressure of about 1,000 pounds per square inch gauge (p.s.i.c.). However, any pressure between 0 to 3,000 p.s.i.g. could be selected for evaluating an inhibitor's performance.

The temperature of the water bath in reduced at a constanE-rate, preferably about 6 'F per hour, from an Initial temperature of about 70 'F. At some temperature, clathrate hydrates begin to rapidly form. As the disfsolved gas is used to 'form clathrate hydrates there is an abrupt and corresponding decrease in E'he volume of di2solved g3s in the aqueous sea salt/gas solution. The temperature at which t'--Is abrupt decrease in the volume of dissolved gas is cbse=jed is known an the temperature of onset for hydrate f or-nation (T.). Recalling f rcm the discuaeion above, the hyd'rate equilibrium diss9ciatica temperature or T,q -8 the temperat-ure below which hydrate formation is thrmodynamically favored in an aqueous sea salt/gas ac'luticn without an inhibitor present. Therefore, another measure of an inhibitcr-'a effectivene-ar is the difference --etwean T., and T., which is known as the StCOC:"-91 7db- Therefore, for a given pressure, the grea:er the aubcooling the mcre effective the irnhitittc=.

BAD Urlluii,41,L 12 Typically, an aqueous sea aalt/gan solution with no inhibitor present produced a T of about 6-7 F.

minil(ncl2 Tgr Rropultn Withoi.t limiting the scope of the Invention, and for the purpose of illustrating the Invention, three VIMA/VCap copolymers In different ratios were evaluated using the miniloop testing procedure described above. The results of these evaluations are provided below.

TABLE 1 MINTLOOP TEST RESULTS WITH POLYMERIC INHIBITORS INHIBITOR Ratio CONC. WT% MINILOOP SUBCOOLING TED1P. (OF) Nona... 7.0 PV.IMA NA C.S 12.5 FVCap MA 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 Ca."rally, copolymerizing VIMA with VCap produced an unexpected improvement in the lactam homopolymer's inhibitcr perfor-mance. As indicated above, the VIM.A homopolyme.-'s ubcooling was nearly 10 'F below the VCap homopolymer, 9 :ubcooling. Consequently, it was unexpected that copolymerizing VIKA with VCap would enhance, rather than diminish the copolymer's hydrate inhibition activity as compared to the VCap homopolymer.

Fig-ure i illustrates a best fit curve produced froin the data in Table I. This curve shows the synergistic inhibiticn effect that VIMA has when copclymerized with VCap. Th.e linear dashed line connecting the subcoolic points obtalned for the VCap and VIMA homopolyners approx-Inztes the subcooling perfo---mance tha': wms expecte-23 for %"-KA/Vcap copolymers with varioum mole fractions of The dashed line repxesents the approximate arizhmet-z 30 averaga -r. subcooling performance that was expected wten V:MA was copciymerized with VCap. As he dasned 1-nt 13AD ORIC51t4AL 13 ún6Icztcz. VIMAIVCaP Rubcooling performmce was expected to CUMInish proportionately with increasing mole fractions of VIMA.

It to believed that =PolYmerizing VIMA with ozher lactam monomers, ouch as N-Yinylpy=clidone (VP) and NvinylpLperidone (VPip), would also demonstrate such a synerglatic effect. However, the extent of the sync--glati, observed for these other VIMA/laccam copolymers, VIMA/VP and VIMA/Wip, may vary from that obiper-ved for VIMAIVCap. I r.

any case, the VIMAIVP and V"IVPip copcly:nsro also are expected to produce some cynergistic -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 ho=polymer and the relative ratios of the comonomero compr:aing the VIMAIVP and VIM/VP1p copolymers.

1 1 14 Exxon Production Research Company claims:

P 12031/r6 1. A method for inhibiting the formation of clathrate hydrates in a fluid having hydrate-forming constituents, comprising:

treating said fluid with an inhibitor comprising one or more polymers having the following amide unit +CH2-CH X 1 N-CH3 1 L=U 1 L13 CH2-CH 1 1 J y N Q), 7 0 n (I) wherein the sum of x and y is a number of units such that the average molecular weight for said polymer is between about 1 000 and about 6 000 000; and n is an integer of 1 to 3.

Claims (1)

1. The method according to Claim 1, wherein the polymer is mixed with a

   carrier solvent prior to treating the fluid.

   The method according to Claims 1 or 2, wherein the fluid is netroleum fluid.

   A method of inhbiting the formation of clathrate hydraes -7. a fluid having hydrate-formi-nQ r- c C) is constituents, comprising treating said fluid with an inhibitor comprising one or more polymers substantially as hereinbefore described with reference to the inhibitor description examples.

   W 5.

   The use of a polymer or polymer composition comprising one or more polymers having the following amide unit (I):

   CH2-CH CH2-CH 1 1 X 1 J y N-CH3 N 1 L=U C 0 n (I) wherein the sum of x and y is a number of units such that the average molecular weight for said polymer is between about 1 000 and about 6 000 000; and n is an integer of 1 to 3, in a fluid having hydrate forming constituents, to inhibit the formation of clathrate hydrates.

   The use according to Claim 5, wherein the fluid is a petroleum fluid.

   16 Amendments to the claims have been filed as follows A method for inhibiting the formation of clathrate hydrates in a fluid having hydrate-forming constituents, comprising:

   treating said fluid with an inhibitor comprising a polymer containing:

   +CH2-CH 1 F CH2-CH J. 1 1 y N ))r-- 0 n N-CH3 1 C=0 1 k13 wherein the average molecular weight of said polymer is between about 1 000 and about 6 000 000, x and y are independent, positive integers signifying the respective numbers of monomeric units, and n is an integer of 1 to 3.

   A method according to Claim 1, wherein the polymer is.rr'ixed with a carrier solvent prior to treating the fluid.

   3. A method according to Claim 1 or Claim 2, wherein the fluid is petroleum fluid.

   4. A method of inhibiting the formation of clathrate hydrates in a fluid having hydrate-forming i -? constituents, comprising treating said fluid with an inhibitor comprising one or more polymers substantially as hereinbefore described with reference to the inhibitor description examples.

   5.

   Use of an inhibitor comprising a polymer containing:

   N-CH3 1 (.i=U 1 +CH2-CI4 1 F CH2-CH 1 J x 1 1 J y N n 1.3 wherein the average molecular weight of said polymer is between about 1 000 and about 6 000 000, x and y are independent, positive integers signifying the respective numbers of monomeric units, and n is an integer of 1 to 3, in a fluid having hydrate forming constituents, to inhibit the formation of clathrate hydrates.

   Use according to Claim 5, wherein the fluid is a petroleum fluid.