GB2585343A - Aqueous acidic compositions comprising a corrosion inhibitor - Google Patents

Abstract

An aqueous acid composition, e.g. for use in a matrix acidizing treatment, comprising an acid; a polymeric acid corrosion inhibitor; and an acid corrosion inhibitor intensifier comprising iodide ions and metal ions. The metal ions may be one or more of copper, silver, bismuth or antimony, and the molar ratio of metal ions to iodide ions is 1:X where X is equal or greater than 2. The polymer preferably includes nitrogen. Also shown is a method of reducing corrosion using the formulation and a method of treating a subterranean formation.

Description

Aqueous Acidic Compositions Comprising A Corrosion Inhibitor

Background

There is described aqueous acidic compositions comprising a polymeric acid corrosion inhibitor (ACT).

Corrosion inhibitor compositions arc frequently used to protect metal surfaces exposed to acidic aqueous solutions. Notably, ferrous metals such as carbon steel or various alloy steels undergo significant generalised and localised corrosion when in contact with aqueous acids and, particularly, under elevated temperature conditions. Examples of fields in which such contact is common include industrial acid cleaning, metal pickling operations, and during various different operations in oil and gas exploration and production.

In the oil and gas industry, exposure to acidic solutions may arise as a result of various operations such as (i) subterranean matrix acidizing and acid fracturing treatments, (ii) oilfield production phase systems, wherein steel and alloy surfaces are exposed to acidic fluids associated with co-produced carbon dioxide and/or hydrogen sulphide and (iii) during CO2. pumping services in which steel and alloy surfaces are exposed to acidic fluids associated with injected carbon dioxide.

The technique of matrix acidizing involves exposure of wellbore steel to an injected acid. This operation may be performed with coiled tubing. which is run into the wellbore and then used to convey the acid to the region where the acid is to penetrate the rock formation. During such acid injection, the internal surface of the coiled tubing and a section of the wellbore casing is exposed to the acid. in the subsequent flowback phase, the steel casing in the wellbore and the exterior surface of the coiled tubing can be exposed to so-called 'unspent' acid flowing back with formation fluids to the surface.

Steel may be protected against corrosion by contacting the steel with an organic corrosion inhibitor that adsorbs to the metal surface. Adsorbed inhibitor(s) may influence the rate of corrosion by one or more of several mechanisms: (i) by forming a physical barrier film which restricts the diffusion of species to/from the metal surface, (ii) by blocking anodic and/or cathodic reaction sites directly, (iii) by interacting with corrosion reaction intermediates adsorbed on the surface and (iv) by influencing the electrical double layer that forms at the metal/solution interface.

Non-polymeric corrosion inhibitors predominate in commercial corrosion inhibitor formulations used for domthole operations and in associated research and development activities. Notwithstanding, it has been variously proposed and demonstrated over the last 50 years that polymeric corrosion inhibitors can show greater efficiency than their equivalent monomers. Notably, polymeric corrosion inhibitors can show effective inhibition when used at significantly lower dosages relative to non-polymeric corrosion inhibitors. Thus, because they can be used in lower dosages, polymeric corrosion inhibitors can provide a more environmentally friendly and cost-effective alternative to non-polymeric or monomeric corrosion inhibitors.

US patents 5002673, 5130034, 5209859 disclose corrosion inhibitor formulations that comprise metal salt mixtures. The function of the metal provided by the metal salt is to complex with the quaternary ammonium compound(s) and form a protective deposit on metal tubulars and equipment. Antimony (SbC13) is the most preferred metal salt.

It is well known to use a corrosion inhibitor in combination with a corrosion inhibitor intensifier. An intensifier is a compound capable of enhancing the performance of a selected acid corrosion inhibitor. Corrosion inhibitor intensifiers are commonly used in environments in which the effectiveness of the corrosion inhibitor is reduced, such as, for example, at elevated temperatures.

Iodide is a known intensifier for both monomeric and polymeric corrosion inhibitors. Iodide's high selectivity as compared with anions of common mineral acids, e.g. hydrochloric acid, means that it is preferentially adsorbed to the surface of contacted metals, providing a minor inhibiting effect in its own right, and additionally providing a more preferential surface for the adsorption of corrosion inhibitor molecules. This process may not necessarily be sequential as there may also be significant co-adsorption of the iodide ions and inhibitor molecules.

US5697443 describes use of matrix acidizing fluids containing acid component(s), a corrosion inhibitor comprising a quaternary ammonium compound and a corrosion inhibitor intensifier consisting of a source of iodide ions and a source of formic acid.

Cuprous iodide is known for use as a corrosion inhibitor intensifier. In addition to iodide, the presence of copper ions provides an additional cathodic inhibition effect.

US2010/0056405 Al describes use of self-diverting acid formulations containing a viscoclastic surfactant with cuprous iodide as a corrosion inhibitor intensifier.

US2011/0100630A I and W02011/053585A2 describe an aqueous acidic fluid comprising water, acid, corrosion inhibitor and (i) a copper salt and (ii) an alkali metal iodide (NaT or KT) to generate copper iodide in-situ wherein the copper salt and the alkali metal iodide are present in approximately a 1:1 stoichiomctric ratio of copper to iodine.

SPE 142675 (2011: Baker Hughes, Shell) -describes corrosion inhibition tests on 13Cr steels, 2205 Duplex and Inconel using an inhibitor intensifier in which cuprous iodide is formed in-situ.

US3773465 details matrix acidizing fluid compositions containing acid component(s), an acid corrosion inhibitor consisting of an acetylcnic alcohol and/or mixtures of organic nitrogen compounds and acetylenic alcohols, together with dissolved cuprous iodide, Cul, as a corrosion inhibitor intensifier. US3773465 indicates an optimum weight ratio of Cu' to 1-of 1:2 [equating to a molar ratio of 1:1]. US3773465 describes that the copper and iodide ions may be provided through soluble precursor salts (e.g. CuCI + KI) for in-sits/ formation of cuprous iodide. It notes that where such soluble salts are used, the degree of corrosion inhibition observed is poorer as compared with using dissolved cuprous iodide but that by providing a 5-15% molar excess of the salt (KI) supplying the iodide the degree of inhibition is improved. No improvement was observed using 15% molar excess Ki as compared with 5% molar excess KI.

SPE 23634 (SPE Advanced Technology Series, Vol. 2, No.1) (1994: Pctrobras) describes experimentation in which propargyl alcohol and alcohol-amine based inhibitors are used with copper iodide intensifiers and a 5%-15% molar excess of iodide.

SPE 23634 recognises the well-known problem of galvanic corrosion caused by heterogeneous electro-deposition of copper metal forming micro-cathodes which results in accelerated corrosion of the remainder of the ferrous surface, typically in the form of pitting.

Similarly, NACE paper 214 (Corrosion 96) by Kuznetsov and Andreev, which reviews mixed inhibitors and aspects of synergism in corrosion inhibition, describes that precipitated metals do not form a continuous coating but a "skeleton" deposit which leads to galvanic corrosion.

Summary

The inventors have investigated and quantified the inhibition efficiency of monomeric inhibitors (with or without an intensifier) and polymeric inhibitors as a function of time.

The inventors have identified that the inhibition efficiency profile of polymeric inhibitors differs from monomeric inhibitors. In particular it has been realised that monomeric inhibitors are fast acting, i.e. they reach their maximum corrosion inhibition potential rapidly but that subsequently, their efficiency may decrease over time. On the other hand, polymeric inhibitors are relatively slow acting but maintain a higher efficiency over time -i.c., they provide a more persistent inhibition performance. For example, during contact with an acid for a period of three hours, under dynamic flow conditions, when a monomeric acid corrosion inhibitor (ACI) is present the rate of corrosion is observed to increase over the period and thus most of the weight loss occurs in the latter part of the period. Conversely, when a polymeric inhibitor is present, most of the weight loss occurs in a period immediately following initial contact with the acid, before the polymeric ACI has reached its full efficiency.

The inventors have discovered that, surprisingly, a combination of a polymeric inhibitor with an intensifier as per the present disclosure provides faster acting corrosion inhibition as seen with a monomeric inhibitor but, in addition, the high inhibition efficiency is maintained over time. Moreover, the combination significantly reduces any potential for galvanic corrosion.

The inventors have discovered that by using a polymeric acid corrosion inhibitor (ACI) in combination with certain metal iodide intensifiers, there is a surprising improvement in inhibition performance over the use of either alone. As such and according to a first aspect of the disclosure there is provided an aqueous acidic formulation comprising: an acid; a polymeric acid corrosion inhibitor; and an acid corrosion inhibitor intensifier comprising: iodide ions and metal ions, the metal ions being one or more of copper, silver, bismuth and antimony.

The formulation of the disclosure provides substantially improved corrosion inhibition over a polymeric inhibitor alone or a monomeric inhibitor with or without an intensifier. The improved inhibition performance provided by the claimed composition is attributed to a synergistic effect between the polymer and the metal ion.

The inventors have observed that the polymeric ACI acts to regulate the deposition of the metal ion(s) providing a relatively uniform distribution of reduced metal(s) across the treating surface as compared with the relatively heterogeneous deposition pattern observed when no inhibitor is used. This regulation of the extent and distribution of reduced metal is believed to be achieved by a balance between a rapid cathodic inhibition process, and a slower anodic inhibition process plus some degree of co-adsorption. Consequently, the reduced metal is able to provide cathodic corrosion inhibition across the surface to be treated thus contributing to the effectiveness of the corrosion inhibition composition, whilst the formation of heterogeneously electrodeposited metal areas large enough to act as micro-cathodes is largely avoided or at least significantly reduced.

Surprisingly, it is also observed that the polymeric ACT acts to regulate the amount of dissolved metal that deposits on the treating surface. For example, with certain steels such as N80 casing carbon-steel, a polymeric ACT appears to reduce the amount of copper that is deposited on the steel surface, as compared with when the same surface is contacted with Cul alone, whereas with other steels, such as HS80 coiled tubing low carbon steel, comparatively less copper is deposited (as compared with N80 steel) when contacted with CuI alone and the polymeric ACT acts to increase the amount of copper deposited on the HS80 surface. Thus, the polymeric ACT acts to regulate the electrodeposition of copper on the treating surfaces and, in both cases, such regulation results in a significant improvement in the efficiency of corrosion inhibition without significant galvanic corrosion effects.

Further the use of the combination of copper ions and iodide ions with a polymeric ACT has been shown to increase the amount of polymeric ACI that is adsorbed onto the treating surface as compared with when the polymeric ACT is used alone or with metal salts that do not readily electrodeposit on the treating surface.

Notwithstanding the discovery that the use of a polymeric ACT provides greater resistance to galvanic corrosion, thus allowing greater amounts of metal containing intensifier to be used, the inventors have also discovered that it is highly favourable that the composition comprises a molar ratio of metal ions:iodide ions of 1:X where X> 2.

In many instances it is more preferable that X is much greater, e.g. X > 5. X may be between 5 and 100; X may possibly be as high as 1000. Although not wishing to be bound to a particular theory, it is believed that effectiveness of this novel ratio is because a relatively large concentration of iodide ions promotes adsorption of the polymeric ACI onto the treating surface, whilst only a relatively low concentration of electrodepositing metal ions is required to provide the optimum or near optimum degree of corrosion inhibition protection.

This ratio is believed to have independent inventive merit and thus according to a further aspect of the disclosure there is provided an aqueous acidic formulation comprising: an acid; a polymeric acid corrosion inhibitor; and an acid corrosion inhibitor intensifier comprising: iodide ions and metal ions wherein the molar ratio of metal ions to iodide ions is I: X, where Xis equal or greater than two.

The composition of the aqueous acid formulations described above may be selected, e.g. through selecting the concentration of polymeric ACT and/or intensifier(s), such that the open circuit potential (OCP) (i.c. the equilibrium potential assumed by the treating surface in the absence of electrical connections to the metal) of the system equilibrates (e.g. drops) to within +15%, and more favourably to within ±3%, of a baseline-OCP for the same treating surface in contact with the acid alone. Such an equilibrium position of the OCP indicates that the combined polymeric AC1 and electrodepositing metal salt intensifier system can be selected to exhibit so-called "mixed" inhibition which minimises the potential for subsequent galvanic corrosion.

The polymeric acid corrosion inhibitor, which is soluble in the acid, may comprise nitrogen, i.e. it is a N-comprising polymer. The polymeric acid corrosion inhibitor may comprise nitrogen(s) that carry a permanent cationic charge and/or nitrogen(s) that become protonatcd in the acid solution. Such polymers can form complexes with the electrodepositable metal ion(s).

The polymeric acid corrosion inhibitor(s) may incorporate nitrogen comprising functional groups on each and every repeat unit or only on some of the repeat units. Such functional groups may comprise primary, secondary or tertiary amine groups and/or heterocyclic nitrogen(s) and/or quaternary ammonium nitrogen(s). Thus, the polymeric acid corrosion inhibitor(s) may comprise repeat units containing amine groups, heterocyclic nitrogen groups and/or quaternary ammonium nitrogen groups. The polymeric acid corrosion inhibitor(s) may include a homo-polymer, wherein each repeat unit may comprise at least one nitrogen atom. Alternatively, or in addition, the polymeric acid corrosion inhibitor(s) may be comprised of a co-polymer comprising two or more different constituent repeat units wherein at least one of the copolymers, and possibly both include nitrogen.

The polymer may comprise, three or more constituent repeat units as these polymers display to a greater extent the advantages of a polymer over the respective monomer. When greater than three repeat units are present, the polymer may be linear or branched. The acid corrosion inhibitor formulation may include a mixture of polymeric and non-polymeric constituents.

Favourably, the polymeric acid corrosion inhibitor comprises a polymer having a backbone comprising a continuous chain of carbon atoms, and/or a continuous chain of carbon and oxygen atoms and/or a continuous chain of carbon and nitrogen atoms. The backbone of the polymer may be aliphatic.

The polymeric AC1 may include a homo-polymer. The polymeric acid corrosion inhibitors may include one or more of the following homo-polymers: polyallylamine, poly(vinylamine) hydrochloride, poly(1-lysine) hydrobromide, polydiallyldimethylammonium chloride, polydnylpyridine (PVPy), polyvinylquinoline (PVQ), polyvinylpvrrolidone (PVP), polyvinylcaprolactam (PVC), chitosan, aminated cellulose, aminated starch, polyethylenimine (PEI) and polypropylenimine (PPI).

The polymeric AC1 may include a copolymer. A number of the repeat units which constitute several of the homo-polymeric acid corrosion inhibitors given in the last paragraph can be present in co-polymeric acid corrosion inhibitors such as those given in the following list: poly(al lylam ne)-graft-poly(N-i sopropylac rd am i de), poly(ethe ram ine), poly(ethylene glycol)-block-polyethylenimine (PEG-PET), poly(vinylpyridine-co-styrenc), poly(vinylpyridinc-co-butyl methacrylate), poly(vinylpyrrolidone-co-vinylace tate), poly(vinylpyrrolidone-eo-styrene) and poly(vinylpyrrolidonc-co-dimethylaminocthyl methacrylate).

The polymeric acid corrosion inhibitor may comprise more than one of the above-mentioned polymers.

The polymeric acid corrosion inhibitor may comprise one or more polymers modified with alkyl groups and/or aromatic groups and/or alkyl-aromatic groups and/or alkyl amino groups. Through modification it is possible to tailor the polymeric acid corrosion inhibitor to provide greater protection depending on the material that provides the treating surface. For example, for the treatment of certain materials, e.g. duplex stainless steel, polymers modified in this way show increased adsorption of both polymer and copper to the surface, polymers modified to a greater degree showing a higher amount of adsorption. The degree of modification is also preferably selected to ensure solubility of the polymeric acid corrosion inhibitor in the acidic solution. Modifiers that include cyclic groups are preferred, and those with multiple cyclic groups more so.

The modified polymeric acid corrosion inhibitor may comprise one or more of the polymers given in the following list: PET modified with heptyl, PET modified with benzyl, PET modified with methyl naphthyl, PVPy modified with benzyl, PVPy modified with methyl naphthyl, PVPy modified with both benzyl and propylaminc, PVPy modified with both methyl naphthyl and propylaminc, chitosan modified with benzyl, chitosan modified with methyl naphthyl. Such modifications introduce alkyl and/or alkyl aromatic groups which enhance and strengthen interactions with the treating surface to achieve a higher inhibition efficiency. The same modifications may create nitrogen(s) that carry a permanent cationic charge (quatemisation). The degree of modification may be optimised to maximise inhibition efficiency and ensure solubility of the polymeric acid corrosion inhibitor in the acidic solution.

Further, the polymeric acid corrosion inhibitor may comprise one or more polymers or copolymers modified with polymerisable groups such as acetylenic alcohol groups, for example PVPy modified with Oct-1 -yn-3-ol. Optionally, PVPy may be modified with pyrrolidonc groups, c.g. PVPy modified with 2-pyrrolidone.

The polymeric acid corrosion inhibitors may comprise polymer(s) modified with pyridyl, quinolyl, hydrogenated pyridyl and/or hydrogenated quinolyl groups. For example, PVPy modified with (1,2,34-tetrahydroquinolin-l-y1) propane.

The polymeric acid corrosion inhibitor may comprise a dual modified polymer, namely a polymer modified with two different modifiers. One of the modifiers may be selected in order to increase the solubility of the polymeric acid corrosion inhibitor. For example the polymeric acid corrosion inhibitor may be modified with first modifier that is a relatively insoluble such as benzyl or napthylmethyl and also a second modifier that is relative soluble in comparison such as propylaminc. In this way it may be possible to increase the overall degree of modification of the first, relatively insoluble modifier whilst maintaining or increasing solubility of the polymeric acid corrosion inhibitor within the aqueous acid.

The polymeric acid corrosion inhibitor may be present in the aqueous acidic formulation at a concentration below its critical overlap concentration. This ensures that presence of the polymeric acid corrosion inhibitor does not significantly increase the viscosity of the acidic solution into which it is mixed. In addition, for some applications, for example acidizing applications, the use of low molecular weight polymeric acid corrosion inhibitors present at a dosage or concentration well below their critical overlap concentration in the acidizing fluid is likely to minimise any potential for incomplete removal of the polymers during the acid flow back stage. The critical overlap concentration (often termed c*) is the lowest concentration of a polymer in a solvent at which the hydrodynamic volume of the polymer chains will experience overlap. At concentrations of polymer below c* (i.e. at concentrations of polymer in the "dilute" regime), the specific viscosity of the polymer solution increases linearly with polymer concentration (the specific viscosity, lisp, is given by the relationship (i -ris)/(is) where q is the viscosity of the polymer solution and is is the viscosity of the solvent). Relative to the dilute regime, at concentrations of polymer above c* (i.e. in the "semi-dilute" regime), the slope of the relationship between lisp, and polymer concentration is greater leading to a more significant effect of the polymer on the viscosity of the aqueous acidic solution. The critical overlap concentration for a specific polymer will depend upon the solvent properties in which it is dissolved. The parameter c* can be straightforwardly determined through empirical experimentation and log-log plots of isp versus concentration.

Favourably the polymeric acid corrosion inhibitor is present within the aqueous acidic 20 formulation in an amount between 0.005wr/0 -Iwt% inclusive. More favourably the polymeric acid corrosion inhibitor is present within the aqueous acidic formulation in an amount between 0.01wt% and 0.05wt.% inclusive.

The polymeric acid corrosion inhibitor may have a weight average molecular weight less than 100000 g/mol, and more preferably less than 20000 g/mol. The average molecular weight of a polymer is related to its intrinsic viscosity via the Mark-Houwink equation [ii = KM where 'K' and 'a' are the Mark-Houwink constants and 'M' is molecular weight. Further, for typical random coil polymers, the critical overlap concentration, c*, is related to the intrinsic viscosity rii by c* 4/[i] (G. Robinson et at Carbohydrate Research v107 (1982) P17-32). As such, for many polymers with molecular weights of 100000 g/mol or below, c* will be higher than 2yyt%. The latter concentration is much higher than the typical concentration of a polymeric acid corrosion inhibitor that will be present in the aqueous acidic formulation for the purposes described in the introduction.

Further, polymers with weight average molecular weight below 100000 g/mol are preferred because, and although not wishing to be bound to any theory, the inventors speculate that, for at least acidizing applications, relatively small polymer molecules are not expected to cause any formation damage during and after acidizing flowback as compared to higher molecular weight polymers.

The iodide ions of the intensifier component(s) of the aqueous acidic formulation are favourably provided through an iodide salt, for example via the addition of one or more of: alkali metal iodide salt, ammonium iodide salt and substituted (alkylated) ammonium iodide salt. Most or all of the iodide ion concentration may be provided from iodide salt(s) other than copper iodide, silver iodide, bismuth iodide or antimony iodide.

The formulation may comprise a metal salt as the source of the metal ion intensifier.

Example metal salts include one or more of a metal halide, a metal oxide or a metal oxyanion salt. The metal oxy-anion salt may be comprised from one or more of the groups: carboxylate, sulphate, carbonate or phosphate. Example metal salts include: copper(I) iodide, copper(I) chloride and copper(II) chloride. The corrosion inhibitor intensifier may include mixtures of salts which are sources of both iodide and metal ions, e.g. mixtures of potassium iodide and copper(I) chloride. The metal comprising salt and the iodide comprising salt may be added at the same time or separately.

Copper, silver, bismuth and antimony are each able to electro-deposit on ferrous surfaces and/or co-adsorb with iodide and polymeric acid corrosion inhibitor(s) and thus are suitable metal ions for the purpose. Nevertheless, copper, silver, bismuth are more preferred, and copper is presently considered to be the most preferred because of its ability to form various complexes with nitrogen containing polymer ACIs.

Where silver oxide is used, the aqueous acidic formulation may further comprise a reducing agent to promote electro-deposition of the metal ions onto the metal surface to be protected. Examples of possible reducing agents arc sodium borohydride and/or ascorbic acid.

The acid is typically a mineral acid or a mixture of mineral acids. Examples include hydrochloric acid and hydrofluoric acid and a mixture thereof. Depending on the application, organic acids may be used e.g. in a mixture.

The aqueous acidic formulation may be used for inhibiting corrosion of ferrous surfaces that are contacted with an aqueous acid. Examples include low carbon steels used to fabricate coiled tubing (e.g. HS80TM, HS90TM and HS110TM supplied by Tenaris), typical carbon steels used to fabricate oilwell casing (e.g. N80, L80) or corrosion resistant alloys such as duplex stainless steels. Contact between the aqueous acidic formulation and one or more of these surfaces may occur at elevated temperatures e.g. above 150°C.

The aqueous acidic formulations herein described may be used to treat subterranean formations penetrated by a wellbore, for example oil and gas wells, while minimizing/preventing corrosion to metal, and in particular, minimizing/preventing corrosion of ferrous metal surfaces within the wellbore that the aqueous acidic fluid contacts. The aqueous acidic formulations may be, for example: a matrix acidizing fluid in which water may be intentionally added to an inorganic and/or organic acid for reservoir stimulation purposes; an acid fracturing fluid; acidic waters produced from hydrocarbon reservoirs (wherein the acidity may be due to co-produced acid gases (CO2, 1-12S) and/or naturally occurring carboxylic acids); or acidic waters generated during CO2 injection operations, e.g. for enhanced oil recovery or carbon sequestration purposes.

in an associated method, the polymeric acid corrosion inhibitor and intensifier are added to the aqueous acid and the resulting formulation is then pumped into the wellbore.

The use of a low molecular weight polymer as a polymeric acid corrosion inhibitor in an acid formulation at a concentration below its critical overlap concentration is also thought to be independently inventive and thus according to a further aspect of the disclosure there is provided a method of treating a subterranean formation penetrated by a wellbore comprising flowing an aqueous acidic formulation through a wellbore into the subterranean formation, wherein the aqueous acidic formation comprises an acid; and a polymeric acid corrosion inhibitor(s); wherein the polymeric acid corrosion inhibitor(s) is present at a concentration below its critical overlap concentration; and the polymeric acid corrosion inhibitor has an average molecular weight of equal to or less than 100,000 g/mol.

Brief Description of the Drawings

Figure 1 is a graph comparing rotating (1000rpm) cylinder electrode (RCE) tests of N80 steel samples exposed to aqueous acidic formulations (at 80°C) including either monomeric and polymeric corrosion inhibitors with either a CuT + KT inhibitor intensifier package comprising I OmM iodide concentration or a copper-free intensifier package (.1(1) comprising 10mM iodide; the parameter 1/Rp is proportional to corrosion rate.

Figure 2 is a graph showing RCE tests (1000rpm) of N80 steel samples exposed (80°C) to aqueous acidic formulations comprising a polymeric corrosion inhibitor with an intensifier package comprising either iodide alone or iodide with copper.

Figure 3 is a graph derived from RCE tests (80°C) showing how the open circuit potential of the surface of N80 changes over time when in contact with an aqueous acidic formulation comprising a polymeric corrosion inhibitor with corrosion intensifiers having differing ratios of copper to iodide but all with constant iodide concentration (10mM); the molar ratio of copper:iodide ranges from 9:10 to 1:10.

Figure 4A is an SEM image of an N80 steel sample surface following exposure to an aqueous acidic formulation including a Cul -T ICI intensifier package without a polymer acid corrosion inhibitor.

Figure 4B is an SEM image of an N80 steel sample surface following exposure to an aqueous acidic formulation including a polymeric inhibitor and a + KI intensifier.

Figure 5 is a graph showing 3 hour cumulative weight-loss measurements of HS80 samples exposed to aqueous acidic formulations (80°C) comprising a polymeric acid corrosion inhibitor being either PET unmodified or PET variously modified with heptyl, naphthylmethyl or benzyl groups, each tested with 10mM KI (no copper) and a range of CuI + KI intensifier packages comprising increasing concentrations of Cu but all with a constant iodide concentration ( OmM); results in the absence of polymeric acid corrosion inhibitor are given for comparison.

Figure 6 is a graph showing 3 hour cumulative weight-loss of HS80 samples exposed (80°C) to aqueous acidic formulations including a naphthylmethyl modified PEI polymer acid corrosion inhibitor present at various concentrations in the range 0.01wt% to 0.2wt% and the intensifier package comprising 1mM CuT and 9mM KT (molar ratio copper: iodide = 1:10).

Figure 7 is a graph showing cumulative (1/RE,) RCE results of HS80 exposed for three hours to various aqueous acid formulations at 80°C under varying dynamic conditions (1000 to 6000 rpm RCE). The formulations contain 0.2wt% polymeric acid corrosion inhibitor PEI or PEI-NM each in the presence of 10mM iodide (KI) or 1mM CuI + 9mM KT.

Figure 8 is a graph showing cumulative (1/Rn) RCE (2000 rpm) results (80°C) of N80 samples exposed for 3 hours, comparing the efficiency of naphthylmethyl modified PET and unmodified PEI polymeric acid corrosion inhibitors in the presence of 10mINI ([Cu] = 0) and CuI + KI mixtures with increasing Cu content and constant iodide (10mM).

Figure 9 is a graph showing cumulative weight loss (3 hours, 80°C) of duplex stainless steel 2205 samples in the presence of various aqueous acidic formulations that compare the efficiency of naphthylmethyl modified PET and unmodified PET polymer acid corrosion inhibitors in the presence of 5mm CuT + 25mM KT intensifier and as a function of the polymeric acid corrosion inhibitor concentration.

Figure 10A is a chart derived from X-ray photoelectron spectroscopy (XPS) analysis indicating the degree of copper and iodide adsorption on duplex stainless steel (DSS) 2205 samples exposed to aqueous acidic formulations containing 0.2wt% PEI modified with naphthylmethyl and unmodified PEI polymeric acid corrosion inhibitors in the presence of 1mM CuT + 9mM KI and the control experiment is 1mM CuI + 9mM KT in the absence of polymeric acid corrosion inhibitor.

Figure 10B is a chart derived from XPS analysis that indicates of the degree of copper and polymer adsorption on DSS 2205 samples exposed to aqueous acidic formulations 10 including modified PEI (PEI-NM) or unmodified PEI polymeric acid corrosion inhibitors in the presence of a 1mM CuT + 9mM KT corrosion inhibition intensifier.

Figure 11 is a chart showing cumulative weight loss (3 hours, 80°C) of HS80 samples (two different batches #6266 and #5065) exposed to aqueous acidic formulations each containing 10mM iodide, various concentrations of Cu (0.1-1mM Cul) and 0.2wt% polyvinylpyrrolidone (PVP) polymeric acid corrosion inhibitor. The figure also shows data for equivalent formulations in the absence of the polymeric acid corrosion inhibitor.

Figure 12 is a chart showing cumulative weight loss (3 hours, 80°C) of HS80 samples exposed to aqueous acidic formulations comprising various concentrations (0.01- 0.2wt%) of PVP or polyvinylcaprolactam (PVCL) with an intensifier package comprising 0.2mM Cul + 9.8mM K1 (molar ratio copper:iodide = 1:50). Some control data for formulations which do not contain copper is also given.

Figure 13A is a graph showing cumulative weight loss (3 hours, 80°C) of HS80 samples exposed to aqueous acidic formulations comprising polyvinylpyridine (PVPy) polymeric acid corrosion inhibitors either unmodified or modified to varying degrees with benzyl groups. Each of the polymeric acid corrosion inhibitors was tested in the presence of 10mM iodide (10mM K1, 1mM Cul + 9mM K1 and 5mM Cul + 5mM K1).

Figure 13B is a graph showing cumulative weight loss (3 hours, 80°C) of HS80 samples exposed to aqueous acidic formulations comprising polyvinylpyridine (PVPy) polymeric acid corrosion inhibitors either unmodified or modified to varying degrees with naphthylmethyl groups. Each of the polymeric acid corrosion inhibitors was tested in the presence of intensifier packages comprising one of 10mM K1, 1mM Cul + 9mM K1 and 5mM Cul + 5mM K1 Figure 14A is a chart showing cumulative weight loss (3 hours, 80°C) of HS80 samples exposed to aqueous acid formulations including 0.1wt% PVPy polymeric acid corrosion inhibitors unmodified and modified with benzyl or naphthylmethyl groups as a function of the degree of modification of the PVPy and in the presence of 1mM Cd + 9mNl KI (molar ratio iodide/copper = 10).

Figure 14B is a chart showing cumulative (1/Rn) RCE (1000rpm) results of N80 samples exposed for three hours under dynamic flow conditions to aqueous acidic formulations including 0.025wt% PVPy polymeric acid corrosion inhibitors either unmodified and modified with benzyl or naphthylmethyl groups in the presence of lmN1 Cul + 9mM K1 (molar ratio copper:iodide = 1:10) shown as a function of the degree of modification of the PVPy.

Figure 15A is a chart derived from XPS analysis showing the degree of copper and iodide adsorption on HS80 samples exposed to aqueous acidic solutions including PVPy either unmodified or modified (7. I mol%) with naphthylmethyl groups and unmodified PEI each in the presence of 1mM Cul and 9mM K1; the results for a control sample (no polymeric acid corrosion inhibitor) are shown for comparison.

Figure 15B is a chart derived from XPS analysis showing the degree of polymer and iodide adsorption on HS80 samples exposed to aqueous acidic solutions including PVPy either unmodified or modified (7.1mol%) with naphthylmethyl groups and unmodified PEI each in the presence of 1mM Cud and 9mM Figure 16 is chart showing cumulative weight loss (3 hours, 80°C) of HS80 samples exposed to various aqueous acidic solutions comprising various chitosan polymeric acid corrosion inhibitors either unmodified or modified methylnaphthyl groups as a function of the concentration of polymeric acid corrosion inhibitor and in the presence of I mNI Cul and 9mM 1(1 (molar ratio copper:iodide = 1:10).

Figure 17 is a chart showing cumulative weight loss (3 hours, 80°C) of HS80 samples exposed to various acidic aqueous solutions comprising different polymeric acid corrosion inhibitors, namely PVP, polyallylaminc, poly(diallyldimethylammonium chloride) (poly(DADMAC)) and PVPy, each present at 0.2wt% and tested with an corrosion inhibition intensifier comprising 10mM KI ([Cu] = 0) and several CuI+KI mixtures (each containing I OrnM iodide) with increasing copper concentrations from 0.2mM to 5mM; control data (no polymeric ACI) also shown for comparison.

Figure 18 is a chart showing cumulative weight loss (3 hours, 80°C) of HS80 samples exposed to various aqueous acidic solutions each comprising 0.2wt% polyvinylpyrrolidone (PVP) and an intensifier package being either 10mM KI or Cuif+KI mixtures (containing lOmM iodide) or silver oxide with various reducing agents.

Figure 19 is a chart showing cumulative weight loss (3 hours, 80°C) of HS80 samples exposed to various aqueous acidic solutions comprising 0.2wt% naphthylmethylmodified PEI (PE1-PO-NM) and an with an intensifier package being either 10mM 1(1 or BiI3+KI mixtures (all containing 10mM iodide) with increasing bismuth content (0.01 to 0.1 1mM).

Figure 20 is a chart showing cumulative weight loss (3 hours, 80°C) of HS80 samples exposed to various aqueous acidic solutions comprising 0.1wt% PVPy with an intensifier package comprising either 10mM Kl, 1mM Cul + 9mM KI, 1mM CuCl + 10mM KI or 1mM CuC12 + 10mM KI; the control (10mM KI without polymeric ACI) is shown for comparison.

Figure 21 illustrates the use of coiled tubing in a matrix acidizing job, with a portion of the wellbore shown enlarged.

The below illustrates the performance of a broad range of different polymeric acid corrosion inhibitors used in conjunction with various corrosion inhibitor intensifiers. Nevertheless, these examples should not be considered restrictive.

Unless otherwise stated, all RCE (dynamic) and static weight loss experiments were carried out by exposing the metal samples to 4 mol/L HC1. The static experiments were carried out using a stirred reactor.

Synthesis of Poly (4-vinylpvridine) modified with benzyl Poly (4-vinylpvridine) weight average molecular weight 60,000 g/mol, supplied by Sigma-Aldrich (1g) and benzyl chloride (weight used dependent on the degree of modification required) were combined in ethanol (30m1) and heated at reflux for 19hr. The solution was cooled, the solvent removed, and the residue partitioned between water (-35m1) and ethyl acetate (-10m1). The aqueous phase was removed, extracted with ethyl acetate and freeze dried to give the modified polymer which was used without further purification.

Molecular weights of example benzyl modified PVPN, polymers are given in the table below.

% of Benzyl Modification M.wt (calculated) (g/mol) 96172 76542 12 68708 Synthesis of Poly (4-vinylpvridine) modified with methylnapthvl Poly (4-vinylpyridine) (as above) (1g) and 1-(chloromethyOnaphtha1ene Ow dependant on degree of modification required) were combined in ethanol (30m1) and heated at reflux for 19hr. The solution was cooled, and the solvent removed. Water and ethyl acetate were added to the residue, the liquor decanted, and the residue dissolved in ethanol and the solvent removed. The glassy solid was triturated with ethyl acetate and dried. The solid was washed with ethyl acetate via soxhlet extraction, dried, washed with diethyl ether and dried to give the modified polymer which was used without further purification. Molecular weights of example methylnapthyl modified PVPy polymers are given in the table below.

°A of Methylnapthyl Modification M it (calculated) (g/mol) 83232 67095 Synthesis of PEI modified with methvinaphthyl Two stage synthesis process: Stage 1: PEI (weight average molecular weight 2000g/mot supplied by Sigma Aldrich) (2g, 50% in water) was dissolved in water (20m1) and cooled to 5°C, propylene oxide ( I.3g) added and the mixture stirred for I.5hr and at ambient temperature for 19hr. The residual propylene oxide was removed by heating to 50°C and purging with nitrogen for 2hr. High vacuum was applied for 10mins and the resultant aqueous solution freeze dried to give PEI modified with 2-hydroxypropyl groups, 2.23g which was used without further purification.

Stage 2: PEI modified with 2-hydroxypropyl groups (997mg) was dissolved in acetonitrile and 1-(chloromethypnaphthalene (wt dependant on degree of modification required) added and the mixture heated at reflux for 24hr. The solvent was removed and the residue partitioned between water and ethyl acetate. The aqueous phase was removed, extracted with ethyl acetate and freeze dried to give the PEI modified with methylnaphthyl which was used without further purification.

Synthesis of PEI modified with benzvl PEI modified with 2-hydroxypropyl groups (330mg) was dissolved in water and benzyl chloride (wt dependant on degree of modification required) added and heated at 50°C for 29hr. The solution was extracted with ethyl acetate. The aqueous phase was purged with nitrogen and freeze dried to give PEI modified with benzyl which was used without further purification.

Figure 1 compares 1/Rn (reciprocal of the resistance to corrosion as determined by linear polarisation resistance (LPR) and as such directly proportional to instantaneous corrosion rate) of N80 steel as a function of exposure time (80°C) under dynamic (1000rpm RCE) conditions to an acid formulation over time in the presence of a monomeric acid corrosion inhibitor (naphthylmethyl quinolinium chloride (NMQC1)) and a polymeric acid corrosion inhibitor in the presence of 10mM iodide (KI) or 1mM CuT 9mM KI. Figure 2 compares 1/Rp of N80 RCEs (2000rpm) as a function of time for acid formulations comprising 0.2%-vt% of the polymeric acid corrosion inhibitor PET modified with naphthylmethyl groups (PE1-PO-NM)) with intensifier packages containing 10mM iodide, one without Cu (10mM KI) and one with a molar ratio copper:iodide = 1: I 0 (1mM CuT + 9mM KI).

Figure 1 evidences that with the combination of a monomeric inhibitor (NMQC1) with an intensifier package comprising K1 only, full inhibition efficiency is reached rapidly (within 60 mins) but this efficiency degrades with time. With addition of Cud (intensifier package 1mM Cud plus 9mM KI), the corrosion inhibition efficiency is increased but the trend showing that this efficiency degrades over time persists as for the copper-free system.

With a combination of a polymeric acid corrosion inhibitor with an intensifier package comprising KI only, development of the full inhibition efficiency occurs slowly as evidenced by the downward slope in figure 2. A combination of the polymeric acid corrosion inhibitor with the CuT+KT intensifier package provides a system which is fast acting and shows a significantly higher inhibition efficiency. With a Cu:T molar ratio of 1:10, substantially full inhibition efficiency is reached within 60 mins and this full efficiency is substantially maintained over the three hours (Fig 2) The presence of the copper acts to reduce the initially high corrosion rate that occurs when using the polymeric ACI with 10mM KT. As a result, the total corrosion inhibition effect compared with the use of the polymeric ACT without a copper comprising intensifier package is significant.

it will be noticed comparing Figure 1 with Figure 2 that the actual efficiency of the monomeric ACI and polymeric AC1 with K1 inhibitor are similar after 60 mins, i.e. both systems achieving 1/R1, 0.03 cm'-/ohm.

Figure 1 further illustrates that even with dosages of polymeric ACI as low as 0.0I wt%, in the presence of the Cui intensifier, a lower overall corrosion rate (represented as the area under the respective lines) is achieved compared with using a much higher dosage (0.14wt%) of the monomeric AC1.

With reference to Figure 3, fast acting copper ion deposition for the system comprising InM Cui + 9mM KI results in a sharp increase in the OCP initially and the slower acting polymeric ACI causes a subsequent slow decrease in the OCP. it is thought that the copper and polymer co-adsorb onto the treating surface such to provide a system that approaches ideal 'mixed' inhibition for which the corrosion rate is at a minimum. As the concentration of Cu within the intensifier package is increased, the polymeric ACI cannot compete leading to a higher OCP "peak" and an OCP that remains higher over the three-hour exposure period. In this example, the use of an intensifier package having a molar ratio copper:iodide of 3:10 (3mM CuI + 7mM KI) results in an OCP which is within 15% of the baseline OCP measured for the uninhibited acid (4 mon HCI) within 3 hours, and when the molar ratio copper:iodide is 1:10 (1mM Cul + 9mIM K1) an OCP within 3% of the baseline OCP is achieved within two hours..

A comparison of Figures 4A &4B illustrates the improved surface distribution of copper on a surface of N80 test material exposed to an acidic formulation comprising 4 mol/L hydrochloric acid and a combination of 0.2wt% PVPy-NM (naphthylmethyl modified) polymeric acid corrosion inhibitor with an intensifier package comprising 1mM Cud + 9mM Ki, compared with the same acidic formulation but without the polymeric acid corrosion inhibitor. In each case the N80 surface was contacted with the acidic formulation at room temperature for 24 hours. The lighter areas represent areas with high copper concentration; the dark areas have far less or no copper. With 1 mm CuI + 9mM 1(1 alone (Fig 4A), it can be seen that the copper has deposited very unevenly across the surface such that it is highly concentrated in particular areas leaving other areas with a dearth. Within the scant copper areas that sit adjacent to areas with high copper concentration, pitting, as indicated by the superimposed circles has occurred. With the combination of polymeric inhibitor and 1mA1 Cul + 9mM K1 intensifier, as shown in Fig 4B, there is a more even (less patchy) distribution of the copper across the surface as evidenced by the fewer white spots and fewer dark areas. Figures 4A and 4B suggest that the polymeric ACT acts to regulate both the quantity and distribution of copper deposited on the metal surface.

Figure 5 illustrates cumulative weight loss of HS80 steel samples exposed to 4 mol/L HC1 for 3 hours in the presence of variously modified forms of PEI polymeric acid corrosion inhibitor, and intensifier packages each comprising 10mM iodide but with varying concentration of copper ions such as to vary the molar ratio of copper to iodide. Where no polymeric ACT is present, the presence of low levels of copper has a beneficial effect on corrosion inhibition (relative to 10mM KT) but increased concentrations of copper, i.e. above 0.5 -1.0 mM, arc detrimental. This increased corrosion rate is attributed to a galvanic effect, due to excessive and heterogeneous deposition of copper on the treated surface. In the presence of the polymeric ACIs, a dramatic improvement in corrosion inhibition is observed with copper ion concentrations below lmM providing a corrosion rate of about 0.001-0.005 lb/ft'-over three hours. This benefit is maintained up to at least concentrations of 5mM Cu suggesting no galvanic corrosion is occurring even in the presence of such high concentrations of copper. This is evidence that the presence of the polymer ACI regulates copper cleetro-deposition leading to significantly lower corrosion rates and a lower sensitivity to 'overdosing' of copper (cathodic inhibitor). Also, referring to the Figure 5 data pertaining to the intensifier package I mM CuT + 9mM KT, it is clear that modification of the PEI improves inhibition performance, the lowest rate being achieved by modification with naphthylmethyl groups.

Figure 6 illustrates the cumulative weight loss of HS80 samples exposed to acid formulations comprising varying concentrations (0.01 -0.2 wt%) of modified PET (PEI-PO-NM) in the presence of an intensifier package comprising I mTVII CuI + 9mM KT. The inclusion of only 0.01 wt% (100ppm) to 0.05 wt% (500ppm) polymeric AC1 is enough to provide sufficient corrosion inhibition for many downhole applications. With the inclusion of 0.2 wt% polymeric ACT, the 3 hour cumulative weight loss was approximately 0.001 lb/W.

Figure 7 illustrates that more complex behavior is observed under dynamic flow conditions. Figure 7 compares cumulative I/Rp (3 hours) as a function of rotation speed for HS80 RCE test samples exposed to acidic formulations comprising different polymeric ACIs present at 0.2 wt% all with a 10mM concentration of iodide. For unmodified PEI, it is evident that the presence of Cu provides significant reduction of corrosion rate. PEI modified with napthyl methyl with or without a copper intensifier shows still better corrosion inhibition although the presence of Cu has only a relatively subtle affect.

Figure 8 illustrates cumulative ( I/Rp) RCE (2000 RPM) tests (three hours) of N80 samples in the presence of the unmodified and modified PET inhibitors (0.2 wt%) with intensifier packages comprising Cu! and IQ, where there is a constant concentration of iodide (10mM) and varying amounts of copper. It can be seen that the modified PEI provides improves corrosion inhibition relative to the unmodified PEI.

Figure 9 illustrates results of cumulative weight loss (3 hours, 80°C) showing the effect of an unmodified PEI ACT compared with a naphthylmethyl modifed PET both in the presence of a 5m1M Cul + 25mM 1(1 intensifier package on corrosion inhibition of duplex stainless steel (DSS) 2205 as a function of polymeric ACI concentration. It can be seen that the modified polymer provides improved corrosion inhibition. It is believed that the more noble surface of DSS results in a lower driving force for copper ion electro-deposition relative to HS80 or N80 and as such a greater overall concentration of iodide in the formulation is beneficial. Again, the results suggest the inclusion of as little as 0.01 wt% (100 ppm) polymeric ACI provides a significant increase in inhibition efficiency.

Figures 10A and I OB are results of X-ray photoelectron spectroscopy (XPS) analysis of the surfaces of the DSS 2205 samples used in the 0.2 wt% polymer tests shown in Figure 9. It can be seen that the presence of a polymeric ACI increases the adsorption of copper and iodide to the surface of DSS 2205 compared to when no polymeric ACI is present, and that PEI modified with naphthylmethyl leads to increased adsorption of polymer (but not of copper or iodide) relative to unmodified PEI. This contrasts with that observed for HS80 (see figure 15) in which the presence of PET decreases the amount of copper adsorbed onto the surface. These data suggest that the polymeric ACI has a regulating effect; acting to increase the adsorption of copper ions onto more noble surfaces like DSS 2205 -and which may facilitate initial corrosion protection through fast acting copper deposition.but slowing and/or homogenizing the distribution of copper onto less noble surfaces such as HS80 (and in which Cut is more strongly drawn) each to retard galvanic corrosion. It should be noted that the 'polymer peaks' results are given by analysis of nitrogen is peaks with correction for the fact that unmodified and modified polymers have different nitrogen contents.

Figure II illustrates, through cumulative weight loss experiments, the corrosion inhibition effect of a different polymeric inhibitor, polyvinylpyrrolidone (PVP), with CuI + KI intensifier packages on HS80 as a function of the copper concentration. All experiments used constant iodide concentration, 10mM. The experiments evidence the benefit of a combination of PVP with Cul + KI intensifier over the KI intensifier alone. When the copper:iodide molar ratio is 1:20, the weight loss is reduced by a factor z10 (no polymer) and by a factor z5 when PVP is present, relative to the copper free systems (10mM K1).

Figure 12 illustrates, the corrosion inhibition efficiency of PVP and PVCL ACTS with a 0.2mM CuT + 9.8 mM KT intensifier package on HS80 as a function of concentration of polymeric ACT. It can be seen that significant corrosion inhibition effects can be obtained with the addition of as little as OUT wt% (100ppm) polymeric ACI. It is also notable that significantly enhanced corrosion inhibition is given by using a Cu:I molar ratio of 1:50, relative to copper-free systems.

Figure 13A illustrates that with an intensifier package of 10mM KT ([Cu]-OmM) or, more preferably, I mlg CuT + 9mM KT, excellent corrosion inhibition is given by increased modification of PVPy with benzyl groups. At any given Cu concentration in the range 0-5mM (all tests at constant iodide 10mM), we observe that an increase in the degree of benzyl modification reduces corrosion rate.

Figure 13B shows there is a similar trend with increased modification of PVPy with naphthylmethyl groups.

This is further evidenced by the plots of Figures I 4A & 14B that illustrate cumulative weight loss of HSSO (Figure 14A) and cumulative ( I /Rp) (RCE 1000 rpm) of N80 (Figure 14B) in the presence of a 1mM CuI and 9mM KI intensifier package with PVPy polymer inhibitors modified to different degrees with either benzyl or naphthylmethyl groups. An AC1 comprising a polymer modified to a greater degree appears in general to improve corrosion inhibition efficiency, but the degree of the effect is dependent on the metal type. in addition, for both metal types, for a given degree of modification in the range up to 25 mo194), naphthylmethyl groups improve inhibition efficiency to a greater extent than benzyl groups.

Figures 15A & 15B are derived from X-ray Photoelectron Spectroscopy (XPS) analysis of the surface of HS80 samples comparing species adsorbed following exposure to acidic fluids comprising a linM Cul & 9mM KI intensifier package with various polymeric ACIs. For HS80, the presence of a polymeric ACI reduces the degree to which copper is adsorbed to the surface; the degree of polymer adsorption follows order: PEI > PVPy-NM > PVPy; and the area of the copper and iodide XPS peaks increases with increased polymer adsorption, suggesting co-adsorption of the polymer and copper and iodide.

Other Polymeric ACIs Figure 16 compares cumulative weight loss experiments of HS80 test samples with chitosan ACTs in the presence of an intensifier comprising 1mM CuT + 9mM KT. Naphthylmethyl modification reduces the weight loss relative to unmodified chitosan in the presence of I mM CuT and 9mM KI.

Figure 17 illustrates cumulative weight loss of HS80 steel exposed in the presence of various polymeric ACIs, with intensifier packages containing 10mM iodide and increasing concentrations of copper (added as CuT). The performance of poly(DADMAC) & poly(allylamine) are similar to non-modified PVPy. By comparison, superior performance is given by Polyvinylpyrrolidone (PVP) and non-modified PEI. As shown earlier, PVP shows good performance with 02mM (38 ppm) Cut + 9.8mM K1 (the Cu dosage in this intensifier package is 12.7 ppm).

Ag Oxide Intensifier Figure 18 compares cumulative weight loss results (3 hour, 80°C) for several HS80 samples contacted with test fluids that include a 0.2wt% PVP (10000 g/mol) AC1. The control contains no intensifiers (WL = 0.0625 lb/ft2). The KI / CuI mixtures shows the beneficial effect of increasing copper concentration (added as Cut) at constant iodide (10mM). The acidic fluids (4mol/L HCI) comprising silver oxide show the beneficial effect of using a combination of silver oxide and reducing agent, in the presence of 10mM KI, to reduce the corrosion rate relative to lOmmM KI alone. The concentrations of silver oxide and sodium borohydride can be reduced to 1.15mM and 0.4mM, respectively (WL = 0.0045 lb/ft2). However, this system is not effective in the absence of iodide (W L = 0.0665 lb/ft-) and it is only weakly effective in the absence of a reducing agent (WL = 0.0353 lb/ft'-).

Bismuth Intensifier Figure 19 illustrates that in combination with 0.2wt% PE1-PO-NM polymeric acid corrosion inhibitor and 10mM iodide, low concentrations of Bit; (0.01-0.05mM) reduce the corrosion rate relative to 10mM K1 alone. The fluids containing 0.01-0.05mM BiL have molar ratios bismuth to iodide in the range 1:200 to 1:1000.

CuCI & CuC12 Intensifier The chart of figure 20 illustrates cumulative weight loss experiments of HS80 which evidence that in combination with a 0.1wt% PVPy polymeric acid inhibitor Cut, CuCI & CuC12 copper salts are effective (at 1mM Cu) in combination with a total iodide concentration 10mM. This demonstrates that each of copper(I) iodide, copper(I)chloride and copper(TT) chloride provide effective copper sources for the acid corrosion intensifier.

Matrix Acidizing in the process of matrix acidizing, a producing formation is treated with acid to stimulate production. The process involves exposure of wellbore steel(s) to the acid. This operation may be performed with coiled tubing, which is run into the wellbore and then used to convey the matrix acidizing fluid down the tubing to the region where it enters the rock formation. When the matrix acidizing fluid injection period comes to an end, the wellbore casing and the exterior of the coiled tubing can be further exposed to overflush fluids and so-called "unspent" acid flowing back with formation fluids. Protection of the tubing and wellbore steel(s) can be achieved through adding to the acid before contact with the steel(s), a polymeric acid corrosion inhibitor and a corrosion inhibitor intensifier package comprising iodide salt(s) and metal salt(s).

Fig 21 schematically illustrates positioning of coiled tubing for a matrix acidizing operation. Flow from the wellbore is halted by closing valve(s) at the wellhead. Coiled tubing 12 is drawn off from a reel 14 and taken over a guide 16 which turns the tubing to descend vertically into a wellbore. The tubing 12 is lowered into the well through well control equipment 18 until the downhole end of the tubing reaches the perforations 22 which give access to the formation outside the well casing. The well control equipment 18 includes one or more valves able to prevent flow from the wellbore.

As shown by the enlarged view Fig 21A, the casing 24 lines the wellbore and is surrounded by cement 26 in a conventional manner. The perforations 22 extend through the casing and cement into the surrounding geological formation. The casing 24 is made of steel.

An inlet 20 to the coiled tubing is provided on the reel axis. in order to twat the formation, an acid, e.g. hydrochloric acid, comprising formulation is pumped into the coiled tubing 12 through its axial inlet as diagrammatically indicated by arrow 32 and down the coiled tubing 12 in the direction of arrow 34 to the formation which is to be treated. During this main pumping stage, the acid formulation comprises 0.01wt% -lwt% of a polymeric corrosion inhibitor having a weight average molecular weight equal or less than 100,000 g/mol together with an intensifier comprising Cu salt and K1 in which the molar ratio of copper to iodide ions is 1:X where X is greater than 2.

The selection of the polymer used as the ACI, its degree ofmodification and the modifier chosen will depend on the type(s) of steel that will be contacted by the acid formulation during the operation. The concentration of polymeric corrosion inhibitor and concentration of intensifier chosen can be varied depending, for example, on the temperature within the wellbore.

The polymeric acid corrosion inhibitor is added the acid so as to be present at a concentration below its critical overlap concentration (c*). c* will depend upon the polymer used, including its degree of modification, and the concentration and temperature of the acidic fluid. c* for any particular acid formulation can be determined straightforwardly through experimentation by identifying changes in the concentration-dependence of the specific viscosity of the formulations as a function of polymer concentration through the dilute and semi-dilute regimes.

It will be appreciated that the formulation described above could be of use in other operations in which metal is contacted with aqueous acids, such as for example those described in the background section of this application.

Claims (36)

1. Claims 1. An aqueous acidic formulation comprising: an acid; a polymeric acid corrosion inhibitor; and an acid corrosion inhibitor intensifier comprising iodide ions and metal ions, the metal ions comprising one or more of copper, silver, bismuth and antimony.
2. 2. An aqueous acidic formulation according to claim 1 wherein the polymeric acid corrosion inhibitor comprises a polymer, the polymer including nitrogen.
3. 3. An aqueous acidic formulation according to claim 1 or 2 wherein the polymeric acid corrosion inhibitor comprises a polymer having three or more constituent repeat units.
4. 4. An aqueous acidic formulation according to any previous claim wherein the polymer of the polymeric acid corrosion inhibitor has a molecular weight less than 100000 g/mol.
5. 5. An aqueous acidic formulation according to any previous claim wherein the polymeric acid corrosion inhibitor is present at a concentration below its critical overlap concentration.
6. 6. An aqueous acidic formulation according to claim 5 wherein the polymeric acid corrosion inhibitor is present at a concentration of less than 1 wt%.
7. 7. An aqueous acidic fommlation according to claim 6 wherein the polymeric acid corrosion inhibitor is present at a concentration of between 0.01 wt% -0.05 wt% inclusive.
8. 8. An aqueous acidic formulation according to any previous claim wherein the formulation includes an iodide salt as a source of the iodide ions.
9. 9. An aqueous acidic formulation according to claim 8 wherein the iodide salt comprises at least one of an alkali metal iodide salt, an ammonium iodide salt or a substitutcd (alkylatcd) ammonium iodide salt.
10. 10. An aqueous acidic formulation according to any previous claim comprising at least one of a metal halide, a metal oxide or a metal oxy-anion salt; as the source of the metal ions.
11. 11. An aqueous acidic formulation according to claim 10 wherein the metal oxy-anion salt comprises one or more from the any of the groups: carboxylate, sulphate, carbonate or phosphate.
12. 12. An aqueous acidic formulation according to any previous claim wherein the metal ions comprise copper ions.
13. 13. An aqueous acidic composition according to any previous claim wherein the molar ratio of metal ions to iodide ions is I:X, where X is greater or equal to two.
14. 14. An aqueous acidic formulation according to claim 13 wherein the molar ratio of the metal ions to iodide ions is between 1:5 and 1:100 inclusive.
15. 15. An aqueous acidic formulation according to any previous claim wherein the polymeric acid corrosion inhibitor includes a polymer having a backbone comprising a continuous chain of carbon atoms, carbon and oxygen atoms or carbon and nitrogen 25 atoms.
16. 16. An aqueous acidic fommlation according to any previous claim wherein the polymeric acid corrosion inhibitor comprises one or more from the list of polyallylam ine, poly(vinyl am ine) hydrochloride, poly(' -lysine) hydrobrom ide, polydiallyldimethylammonium chloride, polyvinylpyridine (PVPy), polyvinylquinoline (PVQ), polyvinylpyrrolidonc (PVP), polyvinylcaprolactam (PVC), chitosan, aminated cellulose, aminated starch, polyethylenimine (PEI) and polypropylenimine (PPI).
17. 17. An aqueous acidic formulation according to any previous claim wherein the acid corrosion inhibitor intensifier includes silver oxide and a reducing agent to reduce the silver oxide to provide the metal ions.
18. 18. An aqueous acidic formulation according to claim 17 wherein the reducing agent is sodium borohydride and/or ascorbic acid.
19. 19. An aqueous acidic formulation according to any previous claim wherein the polymeric acid corrosion inhibitor includes a polymer modified with one or more alkyl groups, aromatic groups, alkyl-aromatic groups, alkyl amino groups, pyrrolidone groups or acetylenic alcohol groups.
20. 20. An aqueous acidic formulation according to any previous claim wherein the polymeric acid corrosion inhibitor comprises one or more polymers from the list of PEI modified with heptyl, PEI modified with benzyl, PEI modified with methyl naphthyl, PVPy modified with benzyl, PVPy modified with methyl naphthyl, PVPy modified with both benzyl and propylamine, PVPy modified with both methyl naphthyl and propylamine, Chitosan modified with benzyl, Chitosan modified with methyl naphthyl. PVPy modified with Oct-1 -yn-3-ol, PVPy modified with 2-pyrrolidone or PVPy modified with (1,2,3,4-tetrahydroquinolin-l-y1) propane, poly(allylaminc)-graftpoly(N-isopropylacrylamide), poly(etheramine), poly(ethylene glycol)-blockpolyethylenimine (PEG-PE1), poly(vinylpyridine-co-styrene), poly(vinylpyridine-cobutyl methacrylate) poly(vinylpyrrolidone-co-vinylacetate), poly(vinylpyrrolidone-costyrene) and poly(vinylpyrrol idone-co-dimethylam inoethyl methacrylate).
21. 21. An aqueous acidic formulation according to any previous claim wherein the polymeric acid corrosion inhibitor includes a polymer modified with one or more from the list: pyridyl, quinolyl, hydrogenated pyridyl and hydrogenated quinolvl groups.
22. 22. An aqueous acidic formulation according to any previous claim comprising a complex comprised from the combination of the metal ions with a polymer of the polymeric acid corrosion inhibitor.
23. 23. An aqueous acidic formulation according to any previous claim wherein the composition of the aqueous acid formulation is selected, when contacted with a treating surface to provide an open circuit potential (OCP) that drops to within +15% of the baseline-OCP for the treating surface in contact with the uninhibited acid.
24. 24. An aqueous acidic formulation according to claim 23 wherein the composition of the aqueous acid formulation is selected to provide an open circuit potential that drops to within +3% of baseline-OCP for the treating surface in contact with the uninhibited acid.
25. 25. A fluid for treatment of a subterranean formation comprising the aqueous acidic formulation of any previous claim.
26. 26. A fluid for treatment of a subterranean formation within a borehole, the fluid comprising the aqueous acidic formulation of any previous claim.
27. 27. A formulation for reducing corrosion of a metal surface that is contacted with an acidic fluid, the formulation comprising: a polymeric acid corrosion inhibitor; and an acid corrosion inhibitor intensifier comprising iodide ions and metal ions, the metal ions being one or more of copper, silver, bismuth or antimony
28. 28. A method of reducing corrosion of a metal surface that is contacted with an acidic fluid, the method comprising providing the acidic fluid with: a polymeric acid corrosion inhibitor; and an acid corrosion inhibitor intensifier comprising: iodide ions and metal ions, the metal ions being one or more of copper, silver, bismuth or antimony.
29. 29. A method according to claim 28 wherein the metal surface is within a wellbore.
30. 30. A method of treating a subterranean formation penetrated by a wellbore comprising flowing the acidic formulation of any claim 1-24 through a wellbore into the subterranean formation
31. 31. A method of treating a subterranean formation penetrated by a wellbore comprising flowing an aqueous acidic formulation through a wellbore into the subterranean formation, wherein the aqueous acidic formation comprises an acid; and a polymeric acid corrosion inhibitor, wherein the polymeric acid corrosion inhibitor has a molecular weight less than 100,000 g/mol and is present at a concentration below its critical overlap concentration.
32. 32. A method according to claim 31 wherein the polymeric acid corrosion inhibitor is present at a concentration of 0.01 wt% -1 wt%.
33. 33. A method according to claim 31 or 32 wherein the aqueous acidic formulation comprises an acid corrosion inhibitor intensifier. 15
34. 34. An aqueous acidic formulation comprising: an acid; a polymeric acid corrosion inhibitor; and an acid corrosion inhibitor intensifier comprising: iodide ions and metal ions, 20 wherein the molar ratio of metal ions to iodide ions is 1:X, where X is equal or greater them 2
35. 35. An aqueous acidic composition according to claim 34 wherein the molar ratio of the metal ions to iodide ions is between 1:5 and 1:100 inclusive. 25
36. 36. A formulation for reducing corrosion of a metal surface that is contacted with an acidic fluid, the formulation comprising: a polymeric acid corrosion inhibitor; and an acid corrosion inhibitor intensifier comprising: iodide ions and metal ions, wherein the molar ratio of metal ions to iodide ions is I: X, where X is equal or greater than 2.