GB2246347A

GB2246347A - Corrosion inhibition

Info

Publication number: GB2246347A
Application number: GB9016223A
Authority: GB
Inventors: William Neagle
Original assignee: Ciba Geigy AG
Current assignee: Novartis AG
Priority date: 1990-07-24
Filing date: 1990-07-24
Publication date: 1992-01-29
Also published as: GB9016223D0

Abstract

A method of inhibiting corrosion of a metal pipe, usually a ferrous metal pipe, used for the recovery or transport of oil comprises contacting the internal surface of the pipe with a hydroxyethercarboxylic acid or a salt or lactone thereof, for example a salt of formula R<x>OCH2CH(OH)CH2OCH2COONa where some of the groups R<x> denote a linear C12 alkyl group and the remainder denote a linear C14 alkyl group.

Description

Corrosion Inhibition This invention relates to corrosion inhibition and in particular to the inhibition of corrosion in metal pipelines used in an oilfield for the recovery or transportation of oil.

The harsh fluid environment encountered by oilfield pipelines is highly corrosive. In order to protect the metal, usually ferrous metal, pipelines against this environment, they have conventionally been treated with organic corrosion inhibitors. The inhibitors which are commonly used are long chain amines. It has been proposed, for example in US4197091, to use polymerised fatty acids as corrosion inhibitors to protect metal pipelines used for the transportation of petroleum products containing small amounts of Water, but carboxylic acids are not used in oilfields to protect metal pipelines from the corrosive mixture of oil and seawater which they encounter. The long chain amines conventionally used for that purpose are toxic.There is consequently a continuing need for a low toxicity replacement for such amines which will provide comparable or improved corrosion inhibition in the metal pipelines.

It has now been found that certain low molecular weight carboxylic acids which have previously been proposed, in EP-A-O 291 458, EP-A-0 302007 and EP-A-0 354 871, as corrosion inhibitors for use in functional fluids are effective low toxicity corrosion inhibitors for metal surfaces in contact with oil-seawater mixtures of the type encountered in oilfield pipelines. These carboxylic acids give good corrosion inhibition results at low addition levels.

Accordingly, the present invention provides in one aspect a method of inhibiting corrosion of a metal pipe used for the recovery or transport of oil which comprises contacting the internal surface of the pipe with (A) a carboxylic acid or salt of formula I

where X denotes a group of formula II or III

M denotes a hydrogen atom, a metal ion, an ammonium ion or a substituted ammonium ion, R denotes a hydroxyl group or, when X denotes a group of formula II, may alternatively denote - CH2OR10, when X denotes a group of formula II, R1 denotes a group of formula Rll(oCH2)d- IV and R2 denotes a hydrogen atom, when X denotes a group of formula m, R1 denotes a C4-C30 alkyl group, optionally interrupted by one, two or three oxygen atoms or substituted by one, two or three hydroxy groups, a C5-C12 cycloalkyl group, a C6-C1o aryl group optionally substituted by one, two or three C1-C12 alkyl groups or a C7-C13 aralkyl group optionally substituted by a hydroxyl group, and R2 denotes a hydrogen atom or a C1-C4 alkyl group, R3, R4 and R5 each independently denote hydrogen or methyl, R6 denotes a hydrogen atom, a C1-C4 alkyl group or -COOM, R7 denotes a hydrogen atom, a C1-C4 alkyl group, -CH2COOM or -CH2CH2COOM, R8 denotes a hydrogen atom, a C1-C4 r alkyl group or -COOM, provided that at least one group R8 denotes -COOM, R9 denotes a hydrogen atom, a C1-C4 alkyl group, -CH2COOM or -CH2CH2COOM, R10 denotes a C1-C4 alkyl group, R11 denotes a C4-C20 r alkyl group, optionally interrupted by one or more oxygen atoms, a C4-C20 alkenyl group, a C6-C1o aryl group, a C7-C15 alkaryl group, a C7-C15 aralkyl group or a Cs-C12 cycloalkyl group, a denotes zero or an integer of 1 to 6, denoting zero when R denotes -CH20R1 b denotes an integer of 1 to 6, c denotes zero or an integer of 1 to 20 d denotes zero or 1 and n denotes zero when R denotes -cH2OR10 or denotes 1 when R denotes hydroxyl; and/or (B) a lactone formed by cyclisation of a compound of formula I where R and M each denote a hydrogen atom.

In another aspect, the present invention provides the use of a compound (A) of formula I as hereinbefore defined and/or (B) a lactone thereof as hereinbefore defined as a corrosion inhibitor for a metal pipe used for the recovery or transport of oil.

It will be appreciated that lactones (B) can usually be formed by cyclisation of hydroxycarboxylic acids of formula I, i.e. compounds of formula I where M and R each denote a hydrogen atom, when X denotes a group of formula II, a denotes zero and b denotes 1, such lactones being of formula V

where R1, R2 and R5 are as hereinbefore defined.

It will also be apparent that lactones (B) can be formed by cyclisation of compounds of formula I where M and R each denote a hydrogen atom and X denotes a group of formula m, such lactones being of formula VI

where C, R1, R2, R6 and R7, R8 and R9 are as hereinbefore defined.

In accordance with the invention, the corrosion inhibitor used to treat the metal pipe may be a single compound (A) of formula I or a lactone (B) thereof; a mixture of one or more compounds (A) of formula I with one or more lactones (B); or a mixture of compounds (A) of formula I, or a mixture of lactones (B), in which a substituent, e.g. Roll, varies from one compound of formula I to another or in which a and/or b and/or c can be an average value representing a broad range of mixtures.

In formula I, where R denotes -CH2OR10, Rl may denote methyl, ethyl, propyl or butyl, methyl being especially preferred among these groups.

When X in formula I denotes a group of formula II, preferably R denotes hydroxyl or methoxymethyl, R2, R4 and R5 each denote hydrogen and R3 denotes hydrogen or methyl.

In formula II, a preferably denotes zero, 1,2,or 3, especially zero or 1; and b preferably denotes 1,2, or 3, especially 1.

When R11 in formula TV denotes C4-Q0 alkyl it may be, for example, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, or eicosyl; preferred alkyl groups R11 are those of straight chain.

When R11 denotes C4-C20 T alkyl interrupted by one or more oxygen atoms, it may be, for example, ethoxyethyl, ethoxybutyl, ethoxyoctyl, ethoxydecyl, ethoxyundecyl, ethoxydodecyl, ethoxytridecyl, ethoxyhexadecyl, ethoxyoctadecyl, diethoxyoctyl, diethoxynonyl, diethoxydecyl, diethoxyundecyl, diethoxydodecyl, diethoxytridecyl or diethoxylhexadecyl.

When R11 denotes C4-C20 aillienyl, it may be butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl, decenyl, undecenyl, dodecenyl, tridecenyl, tetradecenyl, pentadecenyl, hexadecennyl, heptadecenyl, octadecenyl, nonadecenyl or eicosenyl.

R'1 as C6-C10 aryl may be phenyl or naphthyl. As C7-C15 alkaryl, Rll may be, for example, tolyl, xylyl, ethylphenyl, butylphenyl, hexylphenyl, octylphenyl or nonylphenyl.

R11 as C7-C15 aralkyl may be, for example, benzyl, naphthylmethyl, alpha-methylbenzyl or alpha, alpha-dimethylbenzyl.

Preferably R11 denotes a C6-C16 straight chain alkyl group, more preferably a C8-C15 straight chain alkyl group and especially a C12-C14 straight chain alkyl group.

Preferred compounds of formula I where X denotes a group of formula II are those where R denotes a hydroxy group, R11 denotes straight chain C6-C16 alkyl, a denotes zero, 1,2 or 3, b denotes 1,2 or 3 and d denotes zero or 1.

More preferred such compounds are those where R11 denotes straight chain C5-C15 alkyl, a denotes zero or 1, b denotes 1,2 or 3 and d denotes zero or 1. Especially preferred such compounds are those where R11 denotes straight chain C12-C14 alkyl, a denotes zero, b denotes 1 and d denotes 1.

Other preferred compounds of formula I where X denotes a group of formula II are those where R denotes a methoxymethyl group, R11 denotes straight chain C6-C16 alkyl, a denotes zero, b denotes 1,2, or 3, d denotes zero or 1 and n denotes zero. More preferred such compounds are those where R11 denotes straight chain C8-Cls alkyl, a denotes zero, b denotes 1 or 2, d denotes 1 and n denotes zero. Especially preferred such compounds are those where R11 denotes straight chain C12-C14 alkyl, a denotes zero, b denotes 1, d denotes 1 and n denotes zero.

Preferred lactones of formula V, derived from hydroxycarboxylic acids of formula I where X denotes a group of formula II, are those where R11 denotes straight chain C6-C16 alkyl, more preferably C8-C15 alkyl, and d denotes zero or 1. Especially preferred such lactones are those where R11 denotes Cl2-Cl4 alkyl and d denotes 1.

Compounds of formula I where R denotes a hydroxyl group and X denotes a group of formula II and lactones derived therefrom may be prepared as described in US 4 713 487 or EP-A-O 302007. Compounds of formula I where R denotes -CH2OR1 and X denotes a group of formula II may be prepared as described in EP-A-O 354 871.

In compounds of formula I where X denotes a group of formula III and in lactones of formula VI derived therefrom, R1 as a straight or branched C4-C30 alkyl group may be e.g.

a straight or branched chain butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, eicosyl or triacontyl, preferably a straight chain C4-C20, especially C6-C15 alkyl group.

When R1 is a straight or branched C4-C30 alkyl group optionally interrupted by from one to three oxygen atoms, it may be e.g. a group having the formula C3H7OCH2,C4HgOCH2 C5R11OCH2, C6Hl30CH2, C7H15OCM2, C8H17OCH2, C9H19OCH2,C10H21OCH2, CllH23OCH2, C14H29OCH2 or C19H39OCH2; CH3OCH2CM2OCM2, C2HsOCH2CH20CH2, C3M7OCM2CH2O CH2, C4H9OCH2CH2OCH2, C5H11OCH2CH2OCH2, C6H13OCH2CH2OCH2, C7H15OCH2CH2OCH2, C8H170CH2CH20CH2, C9H19OCH2CH2OCH2, C10H21OCH2CH2OCH2 or C27H55OCH2CH2OCH2; CH3OCH2CH2OCH2CH2OCH2, C2H5OCH2CH2OCH2CH2OCH2, C3H7OCH2CH2OCH2CH2OCH2, C4H9OCH2CH2OCH2CH2OCH2, C5H1 10CH2CH20CH2CH20CH2, C10H21OCH2CH2OCH2CH2OCH2 or C25H51OCH2CH2OCH2CH2OCH2;; c2H5OCH2CH2, C3H70CH2CH2, C4HgOCH2CH2, C5HllOCH2CH2, C6H130CH2C112, C7H15OCH2CH2, C8H17OCH2CH2, C10H21OCH2CH2 or C28H57OCH2CH2.

When R1 is straight or branched chain C4-C30 alkyl substitued by one, two or three hydroxy groups,it may be e.g. a group having the formula HO (CM2)4; HO(CH2)6, HO(CH2)7, HO(CH2)8, HO(CH2)9 or HO(CH2)30;

C5-C12 cycloalkyl groups R1 include, for instance, cyclopentyl, cyclohexyl, cyclooctyl and cyclododecyl.

C6-C10 aryl groups R1 optionally substituted by 1 to 3 Cl-Cl2 alkyl groups include, e.g.

phenyl, naphthyl, tolyl, xylyl, p-dodecylphenyl and l-octylnaphthyl groups, preferably phenyl.

C7-C13 aralkyl groups R1 include benzyl, naphthylmethyl and 4-hydroxybenzyl, preferably benzyl.

When X denotes a group of formula m, R2 may denote a hydrogen atom or a methyl, ethyl, isopropyl or n-butyl group.

In preferred compounds of formula I where X denotes a group of formula III and in preferred lactones of formula VI, R denotes a hydroxyl group, R1 denotes a C4-C20 alkyl group optionally interrupted by one, or two or three oxygen atoms, more preferably a straight chain C6-C15 alkyl group optionally interrupted by one or two oxygen atoms, especially a straight chain C12-C14 alkyl group; R2 denotes a hydrogen atom; R6 denotes a carboxyl group; R7 denotes a hydrogen atom; R8 denotes a carboxyl group and R9 denotes a hydrogen atom.

In compounds of formula I where X denotes a group of formula II, and lactones of formula VI, c preferably denotes zero or an integer of 1 to 10, more preferably zero or an integer of 1 to 5.

The symbol M may denote, for example, a hydrogen atom; an alkali metal ion such as a sodium or potassium ion; ammonium; substituted ammonium, including cations obtainable by protonation of primary, secondary or tertiary amines, preferably alkylamines, hydroxyalkylamines or N-heterocycles, i.e. alkylammonium and hydroxyalkylammonium ions such as methylammonium, ethylammonium, n-propylammonium, n-octylammonium, n-dodecylammonium, 2-hydroxyethylammonium, 2-hydroxypropylammonium, 3-hydroxypropylammonium, dimethylammonium, diethylammonium, diisobutylammonium, bis(2-hydroxyethyl) ammonium, trimethylammonium, triethylammonium and tris(2-hydroxyethyl) ammonium, and N-heterocyle onium ions such as pyridinium, pyrrolidinium, morpholinium, imidazolinium and triazolinium, and quaternary ammonium ions such as trimethylbenzylammonium and triethylbenzylammonium ions.

Preferably M denotes hydrogen, an alkali metal ion, especially a sodium ion, or a hydroxyalkylammonium ion, especially a tris(2-hydroxyethyl) ammonium ion.

Compounds of formula I where X denotes a group of formula III and lactones of formula VI may be prepared as described in EP-A-0 291 458.

In accordance with the present invention, compounds (A) of formula I and/or lactones (B) derived therefrom, may be used together with optional additives such as antioxidants, metal deactivators, complexing agents, precipitation inhibitors, biocides, buffering agents, anti-foaming agents and other corrosion inhibitors, as described in EP-A-0 291 458 and EP-A-0 302007. Other optional additives include demulsifiers, surfactants, and viscosifiers. The compounds (A) and/or (B) may be dissolved or dispersed in a suitable carrier liquid such as water, alcohols, hydrocarbons or mixtures of two or more thereof.

The compounds (A) and/or (B), together with optional additives, may be used to protect metal pipes, usually ferrous metal pipes, in various oilfield locations. For example they may be used to protect downwell piping or pipelines used to transport oil, usually in admixture with seawater, away from the oilfield. Various conventional methods of contacting the internal surface of the pipe with the corrosion inhibitor (A) and/or (B) may be used, for example the 'constant contact' or 'film persistency' methods.

In effecting a downwell treatment, a slug of the corrosion inhibitor may be forced down into the well using a carrier liquid such as benzene, toluene, xylene, naphtha, fuel oil, diesel oil, heavy oils or preferably, for economic reasons, seawater. One technique used in downwell treatment is known as 'squeezing'; this technique involves injecting the corrosion inhibitor in a carrier liquid such as seawater under pressure into the oil-producing formation so that the corrosion inhibitor is adsorbed on formation strata and is subsequently gradually desorbed as oil is recovered from the formation and comes into contact with the metal pipe as it flows to the surface.

An internal surface of a metal pipe to be treated in accordance with the invention may be contacted with the compound (A) and/or (B) in admixture with petroleum oil-containing fluid, usually a mixture of oil and seawater, flowing through the pipe. Where the corrosion inhibitor (A) and/or (B) is to be constantly in contact with the pipe, it is generally added to the fluid being passed through the pipe in an amount of at least 5 ppm, preferably 10 to 200 ppm, especially 50 to 100ppm.

When a batch method is used to treat a pipe in accordance with the invention, the internal surface of the pipe is contacted with the compound (A) and/or (B), usually in admixture with a carrier liquid so that (A) and/or (B) preferably constitute 1 to 10% by weight of the mixture, for a time sufficient to form a durable film over the surface to be treated, usually at least 12 hours, and repeating the contact treatment when necessary to maintain the film.

The invention is illustrated by the following Examples, in which parts are by weight unless otherwise indicated. In these Examples, corrosion inhibition by various compounds (A) and (B) at various dosages is tested under conditions which simulate those encountered in oilfield usage.

Corrosion inhibitors used in the Examples are as follows: Inhibitor I is of formula RXOCH2CH(OH)CH2OCH2COONa, where some of the groups RX denote a linear C12 alkyl group and the remainder denote a linear C14 alkyl group, prepared by adding sodium hydroxide to the product of Example 21 of EP-A-O 302 007.

Inhibitor II is the free acid corresponding to Inhibitor I, i.e. the lactone prepared in Example 21 of EP-A-O 302007.

Inhibitor m is of formula

where RX is as in Inhibitor I, prepared as in Example 7 of EP-A-O 354 871.

Inhibitor IV is the sodium salt of Inhibitor III Inhibitor V is a salt of Inhibitor III with triethanolamine Inhibitor VI is a salt of formula I where X is a group of formula III; R is -OH; Rl is Cq H19; R2, R7 and R9 are each - H; R6 and R8 are each - COOM; and M is Na; prepared as described in Example 3 of EP-A-0291458.

Inhibitor VII is of formula

Inhibitor IX is a lactone acid obtained by hydrolysing the intermediate lactone ester of Example 10 of EP-A-0291458, i.e. the product obtained on removing excess starting materials.

Examples 1 - 18 A synthetic brine solution containing Na+ (29247 ppm), Ca2+(849 ppm), Mg2+(200 ppm), C1-(47375 ppm) and HCO3-(480 ppm) is prepared. In a control experiment, no inhibitor is added to the brine so as to give a blank value. Otherwise, one of the Inhibitors I to IX is added to the brine. The brine is then mixed with heavy oil distillate in a brine: distillate weight ratio of 3:1 to give a test fluid which simulates the fluid encountered by a metal pipe in oilfield usage. The test fluid is purged with carbon dioxide for 30 minutes and placed in a beverage bottle. A weighed steel coupon (l0mm x 100mm) is immersed in the fluid and the bottle is sealed. The bottle is tumbled in an oven at 660C or 930C for 72 hours, after which time the coupon is cleaned and reweighed.The percentage inhibition is calculated with reference to the blank value of the weight loss obtained when no inhibitor is present in the test fluid, according to the formula % Inhibition = b - li X 100 1b where lb is the weight loss for the blank and li is the weight loss with the inhibitor present.

The results for different dosages of Inhibitors I to IX at different temperatures are shown in the following table.

Ex. Inhibitor Dose (ppm) Temp. (OC) %Inhibition 1 I 100 660C 96 2 I 50 660C 95.8 3 I 5 660C 47.2 4 I 50 930C 97 5 H 100 660C 96 6 m loo 660C 63.8 7 IV 100 660C 70.7 8 IV 50 66"C 61.8 5 II 100 660C 96 6 m loo 660C 63.8 7 IV 100 660C 70.7 8 IV 50 66"C 61.8 9 V 100 660C 94.9 10 VI 100 660C 67.2 11 VI 50 660C 67.4 12 VII 100 660C 97.7 13 VII 50 930C 98.6 14 VII 100 930C 97.5 15 VIII 100 930C 97.5 16 VIII 50 930C 100 17 Ix 100 930C 97.5 18 Ix 50 930C 68

Claims

Claims 1. A method of inhibiting corrosion of a metal pipe used for the recovery or transport of oil which comprises contacting the intemal surface of the pipe with (A) a carboxylic acid or salt of formula I

where X denotes a group of formula II or m

M denotes a hydrogen atom, a metal ion, an ammonium ion or a substituted ammonium ion, R denotes a hydroxyl group or, when X denotes a group of formula H, may alternatively denote -CH2OR10, when X denotes a group of formula II, R1 denotes a group of formula Rll(oCH2)d- IV and R2 denotes a hydrogen atom, when X denotes a group of formula m, R1 denotes a C4- C30 alkyl group, optionally interrupted by one, two or three oxygen atoms or substituted by one, two or three hydroxy groups, a C5-C12 cycloalkyl group, a C6-C10 aryl group optionally substituted by one, two or three C1-C12 alkyl groups or a C7-C13 aralkyl group optionally substituted by a hydroxyl group, and R2 denotes a hydrogen atom or a C1-C4 alkyl group, R3, R4 and R5 each independently denote hydrogen or methyl, R6 denotes a hydrogen atom, a C1-C4 alkyl group or -COOM, R7 denotes a hydrogen atom a C1-C4 alkyl group, -CH2COOM or CH2CH2COOM, R8 denotes a hydrogen atom, a C1-C4 alkyl group or -COOM, provided that at least one group R8 denotes a carboxyl group, R9 denotes a hydrogen atom, a C1-C4 alkyl group, -CH2COOM or -CH2CH2COOM, R10 denotes a C1-C4 alkyl group, R11 denotes a C4-C20 alkyl group, optionally interrupted by one or more oxygen atoms, a C4-C20 alkenyl group, a C6-C10 aryl group, a C7-C15 alkaryl group, a C7-C15 aralkyl group or a C5-C12 cycloalkyl group, a denotes zero or an integer of 1 to 6, denoting zero when R denotes -CH20R10, b denotes an integer of 1 to 6, c denotes zero or an integer of 1 to 20, d denotes zero or 1, and n denotes zero when R denotes -CH20R10 or denotes 1 when R denotes hydroxyl; and/or (B) a lactone formed by cyclisation of a compound of formula I where R and M each denote a hydrogen atom.
2. A method according to claim 1 where X denotes a group of formula H, R denotes hydroxyl or methoxymethyl, R2, R4 and R5 each denote hydrogen and R3 denotes hydrogen or methyl.
3. A method according to claim 1 or 2, in which a denotes zero, 1,2 or 3.
4. A method according to claim 3, in which a denotes zero or 1.
5. A method according to any of the preceding claims, in which b denotes 1, 2 or 3.
6. A method according to any of the preceding claims, in which R11 denotes a C6-C16 straight chain alkyl group.
7. A method according to claim 6, in which R11 denotes a C12-C14 straight chain alkyl group.
8. A method according to claim 1 or 2, in which R denotes a hydroxyl group, R11 denotes straight chain C6-C16 alkyl, a denotes zero, 1, 2 or 3, b denotes 1, 2 or 3, d denotes zero or 1 and n denotes 1.
9. A method according to claim 8, in which R11 denotes C8-C15 alkyl and a denotes zero or 1.
10. A method according to claim 8, in which R11 denotes C12-C14 alkyl, a denotes zero, b denotes 1 and d denotes 1.
11. A method according to claim 1 or 2, in which R denotes a methoxymethyl group, R11 denotes straight chain C6-C16 alkyl, a denotes zero, b denotes 1, 2 or 3, d denotes zero or 1 and n denotes zero.
12. A method according to claim 11, in which R11 denotes Cg-Cls alkyl, b denotes 1 or 2 andddenotes 1.
13. A method according to claim 12, in which R11 denotes C12-C14 alkyl and b denotes 1.
14. A method according to claim 1 where X denotes a group of formula m, R denotes a hydroxyl group, Rl denotes a C4-C20 alkyl group optionally interrupted by one, two or three oxygen atoms, R2 denotes a hydrogen atom, R6 denotes a carboxyl group, R7 denotes a hydrogen atom, R8 denotes a carboxyl group and R9 denotes a hydrogen atom.
15. A method according to claim 14, in which Rl denotes a straight chain C6-C15 alkyl group optionally interrupted by one or two oxygen atoms.
16. A method according to claim 15, in which R1 denotes a straight chain C12-C14 alkyl group.
17. A method according to any of claims 1 and 14-16, in which c denotes zero or an integer of 1 to 10.
18. A method according to claim 17, in which c denotes zero or an integer of 1 to 5.
19. A method according to any of the preceding claims, in which M denotes a hydrogen atom, an alkali metal ion or a hydroxyalkylammonium ion.
20. A method according to any of the preceding claims in which the metal pipe is of a ferrous metal.
21. A method according to any of the preceding claims, in which the internal surface of the pipe is contacted with the compound (A) and/or (B) in admixture with petroleum oil-containing fluid flowing through the pipe.
22. A method according to any of claims 1 to 20, in which the internal surface of the pipe is contacted with the compound (A) and/or (B) for a time sufficient to form a durable film over said surface and repeating the contact treatment when necessary to maintain the film.
23. A method according to claim 1, in which the compound (A) or (B) is substantially as described in any of the foregoing Examples.
24. Use of a compound (A) and/or (B) as specified in any of claims 1 to 19 and 23 as a corrosion inhibitor for a metal pipe used for the recovery or transport of oil.