FIELD OF THE INVENTION

The present invention relates to a method to inhibit gas hydrate formation in the field of oil and natural gas extraction, transportation and processing.

STATE OF THE ART

Gas hydrates (or clathrate hydrates, gas clathrates, clathrates, etc.) are crystalline water-based solids physically resembling ice, in which small non- polar hydrocarbon molecules (typically gases) are trapped inside "cages" of hydrogen bonded water molecules. In other words, gas hydrates are clathrate compounds in which the host molecule is water and the guest molecule is typically a hydrocarbon gas.

Gas hydrates cause problems for the petroleum industry because they can form solid crystals inside oil/gas pipelines, transfer lines, valves and other equipment. Since they have also a strong tendency to agglomerate and to adhere to the pipeline walls, the formation of gas hydrates may even result in obstructions of the pipelines. Preventing gas hydrate formation is therefore desirable in the art of producing, transporting and processing crude oil and natural gas.

One method to control the growth of gas hydrates is by employing chemicals that can lower the hydrate formation temperature and/or delay their formation (gas hydrate inhibitors). Different kinds of gas hydrate inhibitors exist: thermodynamic inhibitors and kinetic inhibitors/anti- agglomerants.

The most common thermodynamic inhibitors are lower alkyl alcohols and glycols.

Kinetic inhibitors and anti-agglomerants are also known as Low-Dosage- Hydrate-Inhibitors (LDHI), because they require much smaller concentrations than the conventional thermodynamic inhibitors. While kinetic inhibitors act by slowing down the kinetics of the nucleation, anti- agglomerants prevent the agglomeration (self adhesion) of gas hydrate crystals. Kinetic inhibitors are usually synthetic polymers or copolymers, while anti-agglomerants are often quaternary ammonium compounds (RiR ₂ R ₃ R ₄ N ⁺ A ^" where all of Ri, R ₂ , R ₃ and R ₄ are organic groups and A ^" is an anion) having surface active properties. These quaternary ammonium compounds and trialkyl amine salts with various substituents are described in many patents, such as in US 5,460,728 US 5,648,575, US 6,214,091 (Shell Oil Company, US), US 6,595,911 (Baker Hughes Inc., US), US 7,381 ,689 (Champion Technologies, Inc.), US 8,034,748 (Clariant Produkte, DE)

In particular, US 7,381,689 discloses a method of controlling gas hydrate blockage through the addition, among the others, of amino or quaternary ammonium amide salts of formula :

where : A is N ; R _lf R ₂ , R ₄ , and R ₅ are organic moieties; Ri is an alkyl having from 4 to 5 carbon atoms; R ₂ is hydrogen or an alkyl having from 1 to 4 carbon atoms; R ₄ is -(CH2),-, wherein t is an integer 2 to 4; R ₅ is an organic moiety, for example an alkyl or alkenyl group, having 4 to 20 carbon atoms; X ^" is an anion ; and a is 0 or 1. When a is 1, then R ₃ is selected from hydrogen, organic moieties having from 1 to 20 carbon atoms, and combinations thereof.

The X ^" anion can be selected from hydroxide, carboxyiate, halide, such as chloride e bromide, sulfate, organic sulphonate, and combinations thereof, but only bromide ammonium salts are really disclosed .

These quaternary ammonium amide bromides, have many advantages : they perform well at very low dosages, may be prepared from largely available, highly reactive, low cost, raw materials, such as alkyl and alkenyl bromide. Moreover they are ecologically friendly: in fact they are easily biodegraded in alkaline environments and exhibit low fish toxicity upon degradation. Unfortunately, quaternary ammonium halides have some drawbacks too.

First of all, quaternary ammonium halides undergo thermal decomposition. Two types of decomposition reactions usually take place simultaneously: the removal of one of the N-alk(en)yl groups as an alk(en)yl halide with formation of tertiary amines, and elimination of hydrogen halide through extraction of an hydrogen atom from one of the N-alk(en)yl groups with formation of mixture of tertiary amine halide salts and olefin. Although tertiary amine salts have been described as being effective as LDHI too, the unselective thermal decomposition often leads to low performing mixtures of compounds.

Moreover, halide ions in the presence of water are potentially damaging to metals because they may lead to the formation of hydro halogenic acid and to its accumulation. This can be an enormous problem in a field in which metal equipments constantly come into contact with water or with oil/water (possibly acidic) two-phase systems. This is particularly true for equipments which were not built in stainless steel or an high alloy steel or which were not treated for resisting to corrosive fluids, such as brines or seawater. Drums, transfer-lines, valves, tanks and injection systems, which are used for the storage, the preparation and the addition of the additives, are examples of these equipments.

The absence or the almost complete reduction of halide ions and organic halides in additives that are used at producing sites, pipelines or tanks is therefore highly desirable in order to mitigate corrosion problems.

Finally, quaternary ammonium amides, also known as amidoquats, are notorious surfactants and they are used in many field as foaming and emulsifying agents.

In the field of producing, transporting and processing crude oil, not only foaming is a problem which can slow down and reduce the efficiency of the processes, but also it can reduce the drainage, i.e. separation of water from the oil phase. In addition, in the presence of a surfactant, oil and aqueous fluids may form emulsions that undesirably increase the viscosity of the mixture and thereby increase the power required to transport the oil. Moreover, the produced hydrocarbons and the aqueous fluids must generally be separated, and where an emulsion has formed such separation may be very difficult.

It is an object of the present invention to provide a gas hydrate inhibitor based on a quaternary ammonium amide salt which do not contain halides and have very little tendency to metal corrosion and to stable foam/emulsion formation in comparison with the gas hydrate inhibitors of the prior art.

Now, it has been surprisingly found that using an alkyl sulfate or alkyl carbonate or carbonate salt of a quaternary ammonium amide with a relatively short fatty chain, it is possible to obtain an effective gas hydrate inhibition without the above mentioned problems.

As far as the Applicant knows, the use of these salts as gas hydrate inhibitors have never been described before.

DESCRIPTION OF THE INVENTION

It is, therefore, an object of the present invention, a method for inhibiting formation of gas hydrates in systems comprising mixture of hydrocarbons and water, said method comprising the addition to the mixture of a quaternary ammonium amide salt of formula I :

wherein

Ri(CO)- is the residue of a saturated or unsaturated, linear or branched, aliphatic carboxylic acid containing from 6 to 24, preferably from 6 to 20, more preferably from 8 to 18, carbon atoms;

R ₂ and R ₃ are, independently of each other, a butyl or a pentyl group; R ₄ is linear or branched alkyl group having from 1 to 5, preferably from 2 to

4, more preferably 2 or 3, carbon atoms;

X can be R ₄ OS0 ₃ ^" , R ₄ 0(CO)0 ^" , bicarbonate and carbonate;

a can be 1 or 2;

with the proviso that at least 50 % by weight of the aliphatic carboxylic acid contains less than 16 carbon atoms.

DETAILED DESCRIPTION OF THE INVENTION

Preferably, Ri(CO)- is the residue of a saturated or unsaturated, linear or branched aliphatic carboxylic acid wherein at least 60 % by weight of said acid contains less than 16 carbon atoms.

In a preferred embodiment of the method of the invention, R ₂ and R ₃ are the same and are a butyl group.

The quaternary ammonium amide salt of formula I of this invention can be prepared by quaternization of a tertiary amino amide of formula II :

II

wherein R _lf R ₂ and R ₃ have the same meaning as reported above.

The tertiary amino amide of formula II can be obtained by condensation of a saturated or unsaturated, linear or branched aliphatic carboxylic acid having formula RiCOOH and a Ν,Ν-substituted propylene diamine of formula R ₂ R ₃ N-CH ₂ CH ₂ CH ₂ -NH ₂ , wherein R ₂ and R ₃ are, independently of each other, a butyl or pentyl group.

Specific examples of saturated or unsaturated, linear or branched aliphatic carboxylic acids, suitable for the realization of the present invention, are hexanoic acid, 2-ethyl hexanoic acid, n-octanoic acid, n-nonanoic acid, n- decanoic acid, n-undecanoic acid, n-dodecanoic acid, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, linoleic acid, linolenic acid and the like. Also mixtures of saturated or unsaturated, linear or branched aliphatic carboxylic acids can be used for the realization of the present invention. Suitable examples are mixtures of carboxylic acids derived from natural oils, such as coco fatty acids, palm kernel fatty acids and palm fatty acids.

Preferred aliphatic carboxylic acids are mixtures of fatty acids from natural oil and particularly preferred are palm kernel fatty acids and coco fatty acids, the latter being the most preferred .

The preferred Ν,Ν-substituted propylene diamine is Ν,Ν-dibutyl propylene diamine.

The preparation of carboxylic acid amides of substituted diamines is well known in the art. In an exemplary preparation process, the N,N-substituted propylene diamine is reacted with about 0.95 to about 1.1 molar equivalents of the carboxylic acid, ester or acid chloride, at a temperature ranging from about 110 to about 220 °C for about 1 to about 30 hours.

The process of preparation of the quaternary ammonium amide salt of formula I requires a further reaction step wherein the amino groups present in the molecule are substantially all quaternized . Quaternization is a reaction type that is well known in the art: typically it contemplates the reaction of a substrate with an alkylating agent.

For the quaternization step of the present invention, the alkylating agent can be selected from the group consisting of dialkyl sulfate and dialkyl carbonate, wherein the alkyl group has from 1 to 5 carbon atoms. Specific examples of these alkylating agents are dimethyl sulfate, diethyl sulfate, dimethyl carbonate and diethyl carbonate, dipropyl sulfate, etc. The most preferred alkylating agents being diethyl sulfate and diethyl carbonate.

In one embodiment of the present invention, the tertiary amino amide of formula II is melt or dissolved in a suitable solvent, such as a Ci-C ₄ alcohol or diol, and quaternized with about 0.95 to about 1.5 molar equivalents of a dialkyl sulfate to form the quaternized ammonium amide salt. The temperature is normally comprised between 70 and 140 °C. Isopropanol, propylene glycol and methanol are the preferred solvents for the quaternization as they exhibit the best ability at reducing the viscosity of the quaternary salt solution. Isopropanol and propylene glycol are preferred over methanol because of the toxicity issues associated with use of methanol.

The aforementioned quaternary ammonium amide salts of alkyl carbonates, carbonates and bicarbonates can be prepared by methods known in the art, such as those described in US 5,438,034 and WO 03/006419.

It must be pointed out that the quaternary ammonium amide carbonates and bicarbonates of the invention are in equilibrium. The ratio bicarbonates/carbonates varies depending on the pH of the solution in which they are contained .

In one embodiment, the method of the present invention comprises the addition to the mixture of hydrocarbons and water of the quaternary ammonium amide salt as such, without any further diluents or additives. In another embodiment, the method of the present invention comprises the addition of a gas hydrate inhibitor composition, comprising the quaternary ammonium amide salt as herein described, a solvent (e.g. a liquid solvent) and other optional additives.

The gas hydrate inhibitor composition of the invention can comprise between 20 and 95 % by weight, preferably between 45 and 90 % by weight, more preferably between 55 and 85 % by weight, of the quaternary ammonium amide salt.

Representative solvents suitable for formulation with gas hydrate inhibitor include polar solvents such as water, alcohols (including straight chain or branched aliphatic alcohols such as methanol, ethanol, 2-ethoxyethanol, propanol, isopropanol, butanol, isobutanol, hexanol), glycols and glycol ether derivatives (including ethylene glycol, propylene glycol, hexylene glycol, ethylene glycol monobutyl ether, ethylene glycol dibutyl ether, or diethylene glycol monomethyl ether), ethers (e.g ., tetrahydrofuran), amides (e.g., N-methyl-2-pyrrolidinone or Ν,Ν-dimethylformamide), ketones (e.g. methyl ethyl ketone, cyclohexanone, or diisobutyl ketone); apolar solvents, such as aromatic hydrocarbon solvents (e.g . toluene and xylene); and mixtures thereof. Preferred solvents are methanol, propylene glycol and isopropanol .

Suitable optional additives are paraffin inhibitors, asphaltene inhibitors, scale inhibitors, corrosion inhibitors, oxygen scavengers, hydrogen sulfide scavengers, non emulsifiers and emulsion breakers.

The quaternary ammonium amide salts, according to the present invention, are particularly suitable as gas-hydrate inhibitors when added to hydrocarbon fluids containing water.

They may be used by simple addition to the hydrocarbon fluids to be treated .

In the preferred procedure of this invention, the quaternary ammonium amide salts are added to a flowing hydrocarbon fluid which may contain both oil and water, at any point in a flow line upstream of the point or line that is intended to be protected . The dosage of gas hydrate inhibitor of the invention needed to obtain a sufficient protection varies with the application, but it is advantageously added in such an amount that the concentration is between 0.1 and 8.0 % by weight, preferably between 0.5 and 5.0 % by weight and more preferably between 1.0 and 3.5 % by weight.

EXAMPLES

Gas Hydrate Inhibition Test

The performances of the gas hydrate inhibitors of the invention were evaluated with a Rocking Cell RC5 by PSL Systemtechnik.

Test Fluids

• Oil Phase: Diesel

• Aqueous Phase: 4% wt seasalt water or deionized water

• Gas: Mix of methane, ethane, propane and butane (various isomers)

Test Procedure

The sapphire test cells, containing a stainless steel ball, were filled with the fluids (see Table 1) and 2 % by weight of inhibitor and pressurized with the gas mixture. Table 1

Each cell was the subjected to a cycle of cooling and rocking consisting of three steps: 1) flowing condition, 2) shut-in and 3) re-start flowing condition .

1) The pressurized cells were cooled down to 4° C over a period of 5 hours while rocking . After reaching 4° C, the cells were rocked for 12 hours.

2) The rocking was stopped and the test cells were kept at 4 °C in horizontal position (shut-in) for 16 hours.

3) At the end of the shut-in period, rocking was re-started for 4 hours.

Finally the cells were heated back to room temperature.

At the beginning of the third step, the content of the cells was visually evaluated.

Each cycle was replicated three times and the results registered.

Results

The results of the gas hydrate inhibition tests are reported in Table 2 according to the following scale :

- FAIL: The ball is stuck and/or large agglomerations and/or solid crystals and/or visible deposits on the cell walls.

- PASS : The ball is free; solid crystals might be present, but agglomerates (large or small) break up under rocking.

Table 2

Comparative Foaming Power Tests

The foam volume (FV) and the foam stability (FS) were determined by stirring for 30 seconds at high speed (8000 rpm) with a Waring Blender 100 mL of a 1 % by weight solution of the inhibitors in deionized water (Test 3) or in a 4 % sea salt water solution (Test 4). The foamed composition was then immediately transferred into a graded cylinder for the determination of the foam volume and the stability of the foam.

FV represent the volume in mL of foam at the end of the stirring . FS is the time in seconds required to the foamed solution to regenerate 50 mL of liquid. The longer the time, the higher the stability of the foam.

Table 3 shows the results of the foaming power test.

Table 3

* Comparative

The results demonstrate that the quaternary ammonium amide salts of formula I of the invention produce less foam than ammonium salts of the prior art.

Corrosion Tests

The Linear Polarization Resistance (LPR) measurements were made with a Gamry Electrochemical Instrument system.

The LPR corrosion tests were conducted in 1 L Pyrex jacketed cells. 900 mL of synthetic brine (50/50 v/v 4.0% Seasalt water/Fresh Water) were loaded in the cell placed on a magnetic stirrer, deaerated overnight with C0 ₂ and, finally, saturated with 200 ppm H ₂ S gas just before testing . A clean C1018 Mild Steel rod was inserted in the corrosion cell assembly as sample probe. A graphite rod was used as the counter electrode. The temperature of the solution was brought to 80 °C for the duration of the tests and C0 ₂ was continuously purged at a constant flow rate. The gas hydrate inhibitors were added at 10 ppm by volume of test solution .

The results are reported in Table 4 as % of protection after a fixed period of time compared to the blank, the test solution without any inhibitor, which is considered 100 % corrosion.

Table 4

* Comparative

The results demonstrate that the quaternary ammonium amide salts of formula I of the invention produce less corrosion than ammonium chloride salts of the prior art.