ADDITIVES FOR INHIBITING GAS HYDRATE FORMATION

FIELD OF INVENTION

The present invention relates to clathrate hydrate inhibitors and a method of inhibiting the nucleation, formation, agglomeration, and deposition of clathrate hydrates using them. The invention is useful in inhibiting the formation of clathrate hydrates, for example, in pipelines for production of oil and natural gas and for transporting them, and in drilling, completion and fracturing operations.

BACKGROUND ART

Gas hydrates are clathrates (inclusion compounds) of small molecules in a lattice of water molecules. In the petroleum industry natural gas and petroleum fluids contain a variety of these small molecules, which can form gas hydrates. They include hydrocarbons such as methane, ethane, propane, isobutane as well as nitrogen, carbon dioxide and hydrogen sulphide. Larger hydrocarbons such as n-butane, neopentane, ethylene, cyclopentane, cyclohexane and benzene are also hydrate- forming components. When these hydrate- forming components are present with water at elevated pressures and reduced temperatures the mixture tends to form gas hydrate crystals. For example, ethane at a pressure of 1 MPa forms hydrates only below 4 degrees C, whereas at 3 MPa gas hydrates can only form below 14 degrees C. These temperatures and pressures are typical operating environments where petroleum fluids are produced and transported or in drilling, completion or fracturing operations in the oil and gas industry.

If gas hydrates are allowed to form inside a pipe used to transport natural gas and/or other petroleum fluids they can eventually block the pipe. The hydrate blockage can lead to a shutdown in production and significant financial loss. The oil and gas industry uses various means to prevent the formation of hydrate blockages in pipelines. These include heating the pipe, reducing the pressure, removing the water and adding thermodynamic inhibitors (antifreezes) such as methanol and ethylene glycols, which act as melting point depressants. Each of these methods is costly to implement and maintain. The most common method used today is adding antifreezes. However, these antifreezes have to be

be effective. Recovery of the antifreezes is also usually required and is a costly procedure.

An alternative to the above methods is to control the gas hydrate formation process using nucleation and crystal growth inhibitors. These types of chemicals are widely known and used in other industrial processes. The advantage of using these chemicals to control gas hydrate formation is that they can be used at concentrations of 0.01 to 3 %, which is much lower than for antifreezes.

Examples include a method of adding polyvinylpyrrolidone or a copolymer of vinyl pyrrolidinone and alpha.-olefins (see International Patent Laid-Open No. WO93/25798); a method of adding a copolymer of vinyl pyrrolidinone, vinyl caprolactam and dimethylaminoethyl methacrylate (see International Patent Laid-Open No. WO94/12761); a method of adding a polymer or copolymer containing pyrrolidinocarbonyl aspartate groups (see US Patent 6,232,273); a method of adding a blend of a quaternary ammonium salt and a polyamino acid or salt thereof which has structural units derived from at least one amino acid, at least 50 % of which units have at least two carboxylic acid groups, especially polyaspartic acid (International Patent Laid- Open No. WO96/29502).

Consequently, there is a need for alternate cheap methods for preventing hydrate blockages in oil and gas drilling and production. An additional advantage would be if the hydrate inhibitor is biodegradable.

It is an object of this invention to provide an additive and a method of controlling gas hydrate formation using said additives added at low concentrations to a stream of at least some light hydrocarbons and water.

SUMMARY OF INVENTION

According to the present invention we provide the use of water-soluble polymers containing one or more structural units of Formula I

Formula I

where Ri and R ₂ are H or an organic group with 1-12 carbon atoms, X is a spacer group or a bond, as additives for inhibiting the formation of gas hydrates connection with hydrocarbon production and transportation including production, drilling, completion and fracturing operations. The molecular weight of the polymers are in the range from 500 to 1,000,000.

The water-soluble polymer or polymers can contain 2 or more structural units of Formula I.

The spacer group X is preferably a bond or contains 1-4 carbon atoms such as a methylene or ethylene group.

In one embodiment of the invention the polymers contain 2 structural units of Formula I wherein one structural unit has a spacer group which is methylene and the other has a spacer group which is a bond.

In one embodiment of the invention the polymers are polyaminoacids, proteins, peptides or derivatives of such. Such polymers are not necessarily anti-freeze proteins which are natural proteins designed to inhibit the formation of ice crystals in fish, insects etc. The polymers may or may not be derived from proteinogenic amino acids.

In another embodiment the polymers contain 5-100 Mol-% of structural elements of Formula I.

When X is no atom, the structural unit has Formula II.

Formula II

Such polymers contain 5-100 Mol-% of structural elements of Formula II. A preferred structural unit in a polymer has R ₁ is H and R ₂ contains 1-5 carbon atoms. R ₂ is more preferably isopropyl or isobutyl or Ri and R ₂ are bonded together to make a ring structure such as a cyclopentyl ring.

In one embodiment of the invention the polymers contain one or more structural elements of Formula III

Formula III

wherein each R ₃ and R ₄ are independently H or Ci _-5 -alkyl; and wherein some of the alkyl groups represented by Ri and R ₂ may carry a hydroxy, amino, carboxylic acid or carboxylate substiruent. Such polymers comprising 5-100 Mol-% of structural elements of Formula III.

In a preferred embodiment of the invention the polymers containing structural units of Formula III have R ₃ is isobutyl or isopropyl and R ₄ is H.

In another preferred embodiment of the invention the polymers contain some structural units of Formula III with R ₃ is methyl and R ₄ is H and some of the structural elements of Formula III with R ₃ is isobutyl or isopropyl and R ₄ is H.

Some of the structural units in the polymers containing structural units of Formula III may have Formula IV

wherein R ₅ is H ⁺ or a cation including metal cations and quaternary ammonium cations.

In another preferred embodiment of the invention the polymers contain some structural units of Formula III have some alkyl groups R ₃ or R ₄ containing an amine group containing dimethylamine groups.

In another embodiment of the invention the polymers include one or more structural elements of Formula V

wherein each R ₆ and R ₇ are independently H or Q.s-alkyl; and wherein some of the alkyl groups represented by R ₆ and R ₇ may carry a hydroxy, amino, carboxylic acid or carboxylate substituent.

In a preferred embodiment of the invention the polymers containing structural units of Formula V have R ₆ is isobutyl or isopropyl and R ₇ is H.

In another preferred embodiment of the invention the polymers contain some structural units of Formula V with R ₃ is methyl and R ₄ is H and some of the structural elements of Formula V with R ₃ is isobutyl or isopropyl and R ₄ is H.

Some of the structural units in the polymers containing structural units of Formula III have Formula IV

Some of the structural units in the polymers containing structural units of Formula V may have Formula VI

wherein some of the structural units in the polymer have Formula VI wherein R ₈ is H ⁺ or a cation including metal cations and quaternary ammonium cations.

In one embodiment of the invention, the polymers contain 2 or more different structural units of Formula I. The distribution of the units in the polymer may be random or an exact sequence or alternation.

In one embodiment of the invention, two or more polymers containing one or more structural units of Formula I can be used. The distribution of the units in the polymers may be random or an exact sequence or alternation.

In one embodiment of the invention the polymer or polymers are cross-linked or branched.

In one embodiment of the invention structural elements as in Formula I are made by reacting polysuccinimide or copolymers thereof with one or more amines or diamines. Examples of alkylamines that can be reacted with polysuccinimide and polymers thereof to form the desired product include methylamine, dimethylamine, ethylamine, diethylamine, n-propylamine, iso-propylamine, iso-butylamine, n-butylamine, t- butylamine, n-pentylamine, iso-pentylamine and cyclopentylamine, 3- (dimethylamino)propyl amine and 2-(dimethylamino)ethylamine. Not all the cyclic imide groups may be reacted with said alkylamines. In such cases, the remaining imide groups can be saponified (hydrolysed) to form carboxylic acid or carboxylate groups.

The polymers of the invention can be used together with a synergist (performance enhancing chemical). Examples of synergists include polymers and copolymers of N- vinylcaprolactam, N-vinyl-pyrrolidone and alkylated vinylpyrrolidones, alkyl- and dialkylacrylamide polymers and copolymers, hyperbranched polymers or dendrimers including polyesteramides, polymers and copolymers of maleic anhydride, which have been reacted with alkylamines to form imide or amide groups, polysaccharides and derivatives of such including sugars and starch, polyoxyalkylenediamines, small alcohols, small glycol ethers or ketones, amphilphilic molecules with molecular weight of less than 1000 Daltons, proteins, peptides or polyaminoacids.

Preferred small glycol ethers include n-butoxyethanol, iso-butoxyethanol, n- butoxypropanol or iso-butoxypropanol. These can be the solvent for the clathrate hydrate inhibitors of this invention.

Preferred amphiphilic molecules include quaternary ammonium surfactants.

The polymers of the invention may also be used as scale inhibitors and/or corrosion inhibitors. I.e. in some cases it may be unnecessary to use another molecule as scale or corrosion inhibitor if the clathrate hydrate inhibitors of this invention can do the job.

Synthesis of polymers

Example 1

9.7g (0.1 mole) of polysuccinimide (Mw = ca. 4000) and 40 ml of dimethylformamide were put into a 100 ml flask equipped with a stirrer. After the polymer had dissolved by warming the solution was cooled to 20 °C and 7.2g (0.12 mole) of isopropylamine was added dropwise at room temperature over a period of 30 minutes. After the addition, the compounds were stirred for a further 2 hours at room temperature, and the resulting reaction mixture was poured into excess diethyl ether to precipitate the ring-opened polymer. The polymer was washed with diethyl ether and dried in vacuo. Thus was obtained a kinetic hydrate inhibitor sample (1) that was soluble in distilled water at up to 95 ⁰ C.

Example 2

The procedure in Example 1 was repeated but instead of isopropylamine, 8.76g isobutylamine (0.12 mol) was used. The polymer was isolated as in Example 1 to give a kinetic hydrate inhibitor sample (2) that was soluble in water at up to 95 °C.

Example 3

2.Og (0.21 mol) of polysuccinimide was dissolved in N-ethyl pyrrolidone (NEP) (10 ml) by warming. To this solution was added isobutylamine (1.155g, 0.016mol) and 40 % methylamine in water (0.407g, 0.0052mol) and the solution stirred overnight at 60 °C. To this solution was added sufficient monobutyl glycol ether (BGE) to give a 15 wt.% solution of polymer in a mixture of BGE and NEP solvents. Thus was obtained a kinetic hydrate inhibitor sample (3). A 5 % solution of sample (3) in distilled water does not lead to polymer precipitation even when the solution is warmed to 95 °C.

Equipment and Test Procedure

To evaluate the performance of the hydrate inhibitors of this invention, the examples given herein use high pressure cells and methods of using them described in M. A. Kelland, T.M. Svartaas and L.A. Dybvik, Proc. SPE Annual Technical Conference/Production Operations and Engineering, 1994, pp 431-438.

A titanium cell was mounted in a plastic cylindrical cooling bath. The titanium cell consists of a titanium tube enclosed in a holder between two titanium end pieces. The

cell has an internal diameter of 20 mm, height of 100 mm and a wall thickness of 20 mm. 15 mm of the top piece and 13 mm of the bottom piece protrudes into the cell, and the total volume between the top and bottom piece is approximately 22 ml. The titanium cell is equipped with a stirrer mechanism. A stirrer blade is connected to a magnet housing in the bottom end piece via an axle. An external rotating magnetic field created by a laboratory stirrer bar drive is used to regulate the stirrer speed. The stirrer motor can be regulated to maintain a constant speed (independent of motor load) in the range 0 to 1700 rpm. The regulator/amplifier unit has output connections for both torque and rotation speed readings. The stirrer speed readings are calibrated using a stroboscope.

Temperature control of the cell is obtained by circulating water in the plastic cooling bath outside the titanium cell and through a cooler/heater unit connected to a temperature control unit. The cell system is equipped with two temperature sensors for the measurement of the temperature inside the cell (in the gas phase) and in the water bath. Pressure is measured with a pressure transducer through the inlet tubing connection in the top end piece of the cell. The temperature was measured to an accuracy of .+-.0.1 degree. C. and the pressure was measured to an accuracy of .+-.0.2 bar. All data were collected in a data logger.

The same procedure for preparation of the experiment and filling of the cell was followed in all experiments. All tests were performed on fresh 3.6wt.% NaCl solution and synthetic natural gas (SNG). Decane was added as a hydrocarbon phase in all experiments.

A description of the general test procedure is given here:

1 ) The inhibitor to be tested was dissolved or dispersed in 3.6wt.% NaCl solution to the desired concentration.

2) The magnet housing of the cell was filled with the aqueous solution containing the inhibitor to be tested. The magnet housing was then mounted in the bottom end piece of the cell, which was thereafter attached to the titanium tube and the cell holder.

3) The desired amount of the aqueous solution containing dissolved inhibitor and decane was placed in the cell (above the cell bottom) using a pipette, the top end piece was fitted, and the cell was placed in the cooling bath (plastic cylinder).

4) The temperature of the cooling bath was adjusted to 2-3 °C outside the hydrate region at the pressure conditions to be used in the experiment.

5) Prior to loading the cell with hydrocarbon gas, it was purged twice with the SNG used in the experimental hydrocarbon fluid.

6) The data logging was started, and the cell was loaded with the hydrocarbon fluid to the desired pressure while stirring at 700 rpm. Normally, the hydrocarbon fluid was SNG.

When the temperature and pressure in the cell had stabilised the experiment was started.

All nucleation experiments, called "kinetic inhibition" experiments, were conducted at constant temperature. Once the temperature and pressure had stabilised after loading of the cell the stirring was stopped. The closed cell was then cooled to the experimental temperature, resulting in a decrease in pressure. When the temperature and pressure again had stabilised, stirring at 700 rpm was started. The induction time, ti, for hydrate formation was measured from the time of start of stirring at the experimental temperature. The induction time is determined from the first sign of pressure drop after the start of stirring.

The procedures given herein for synthesis of polymers in solvents are only examples of the possible synthetic techniques, which can be used for the methods according to the invention.

Kinetic Inhibition Experiments

Examples given below are carried out using SNG, decane and 3.6 wt.% NaCl solution at 90 bar and 5.2-7.7 °C. The subcooling δT is 13.6-16.1 ⁰ C.

Several kinetic inhibition experiments were carried out with no additives at 6.8-7.7 °C. The induction time before rapid gas uptake took place was 0-20 minutes in all experiments. Tests with 15000ppm monobutyl glycol ether dissolved in the 3.6 wt.% NaCl solution at 6.8 °C also gave induction times of less than 20 minutes.

Test 1

5000ppm of Luvicap EG (polyvinylcaprolactam in ethylene glycol from BASF) was tested at 6.7 °C. The induction time was 259 minutes.

Test 2

5000ppm of Sample (1) was tested at 6.7 °C with added 15000ppm BGE. The induction time was 54 minutes.

Test 3 5000ppm of Sample (2) was tested at 6.7 °C with added 15000ppm BGE. The induction time was 736 minutes.

Test 4

33000ppm of Sample (3) was tested at 7.0 °C. This contains 5000ppm polymer. The induction time was 1602 minutes.

Test 5

Test 3 was repeated at 5.2 °C. The induction time was 264 minutes.

Tests 2-5 are all within the claims of the invention.