WO1988006143A1

WO1988006143A1 - Anti-scaling compounds

Info

Publication number: WO1988006143A1
Application number: PCT/GB1988/000084
Authority: WO
Inventors: Frank Blake Williamson
Original assignee: Donmarn Limited
Priority date: 1987-02-11
Filing date: 1988-02-11
Publication date: 1988-08-25
Also published as: GB8703128D0

Abstract

Anti-scaling derivatives of raffinose in which one or more of the hydroxyl groups thereof have been converted to sulphate half ester groups or such groups in salt form.

Description

Anti-Scaling Compounds

This invention relates to compounds which may be used as anti-scaling and peptizing agents. The invention also relates to processes for the preparation of the compounds, to anti- scaling compositions containing them, and to the use thereof. Scaling causes widespread problems in both industrial and domestic environments and is the result of undesired deposition of insoluble inorganic materials such as calcium carbonate and other metal salts in, for example, pipes and apparatus. A very large number of compounds have been proposed for use to hinder or prevent the formation of scale or to aid in its removal.

The formation of scale is particularly troublesome in the oil industry and especially in the recovery of oil from off- shore oil fields. Thus, oil-bearing rock formations often additionally contain significant quantities of water, so-called "formation water". The formation water usually contains potentially scale-forming materials such as metal ions capable of forming insoluble compounds under appropriate conditions. In order to increase the amount of oil recovered from off-share oilfields it is usual to pump seawater into the rock formation.

When the seawater meets and intermingles with the formation water this very often produces conditions ripe for scale formation by precipitation of insoluble compounds. For example, some formation waters contain relatively large quantities of barium, strontium and/or calcium ions while seawater contains relatively large quantities of sulphate ions with the result that when the seawater mixes with such formation waters, insoluble sulphates and carbonates (deriving from carbon dioxide dissolved in formation fluids) precipitate and scale build up occurs. The scale is formed not only within the porous media, so hindering the flow of oil to the recovery well, but also in downstream pipes and other apparatus. In some cases, scale build-up is so rapid and severe that, without preventative measures being taken, almost complete occlusion of pipes and other apparatus may occur within days or weeks.

Moreover, the formation waters often contain naturally occuring radioisotopes e.g. from leaching from the rock formation, and these radioactive materials may be incorporated into the deposited scale. Although the radioactivity of the scale is generally of a low specific activity any subsequent removal of the scale and disposal of scale encrusted pipes and apparatus has to be carefully controlled and expensive and/or inconvenient precautions taken to prevent .contamination of workers and the environment.

As indicated above, a large number of compounds have been hitherto proposed for inhibiting or preventing scale formation in industrial and domestic situations. For example, it has been proposed to use various polysaccharide derivatives for inhibition of calcium carbonate scales. Thus, US 4561982 describes a method for inhibiting scale in water and aqueous systems comprising adding to the water or aqueous system an oxidised polysaccharide in an amount of from 1-20 parts per million wherein the oxidised polysaccharide contains at least 10$ (referred to the number of monosaccharide residues in the polysaccharide) of carboxyl group-bearing units of the formula:

(where R is H, (CH_2n)-COON or COOM; M is H, an alkali metal ion or an ammonium group, and n is an integer of from 1 to 3), said oxidised polysaccharide being obtained by the oxidative cleavage of a polysaccharide containing 6-membered monosaccharide rings having vicinal hydroxyl groups. US 4585560 describes a method of inhibiting the formation of calcium carbonate-containing deposits on a surface by applying a composition comprising an anti-calcification effective amount of a polysaccharide-containing fraction substantially free of protein components, isolated from a calcium carbonate-containing tissue obtained from a calcium carbonate forming organism. The desired fraction can be isolated from various calcium carbonate-containing tissues including algae coccoliths and the like, and comprises a polysaccharide- containing soluble matrix containing phosphate and sulphate groups. US 4603006 describes a method of inhibiting the formation of CaCO₃- containing deposits on a surface by applying a composition comprising an anti-calcification - effective amount of a mono- or disaccharide polymer of at least 500 MW of formula Poly (XnYm) where m/n varies from 1 to 10; X and Y are substituted furanosic or pyranosic mono- or disaccharide residues; and the polymer is formed by αor β glycosidic linkages between adjacent monomers. Amongst the large number of possible substituents for X and Y there is mentioned sulphate group-containing residues.

Different oilfields have different degrees of acidity.

This is primarily influenced by temperature and the partial pressure of carbon dioxide and hydrogen sulphide and dissolved solids. Because of changes in temperature, pressure and gas concentration (particularly carbon dioxide) as the oil flows from the reservoir, through the production pipes and through the process equipment downstream of the wellhead, the pH of the water in the system increases. Hence any anti-scaling agent designed to protect different parts of the production system must remain fully functional over the varying pH range. In addition the chemical composition of the formation waters varies from reservoir to reservoir and stratum to stratum. High concentrations of salts usually decrease the effectiveness of scale inhibitors, and particular metal cations such as calcium and iron can reduce the effectiveness of scale inhibitors. However, the presence of some cations such as strontium, which may co-crystallize with barium to form a mixed barium/strontium sulphate, sometimes results in increased effectiveness of particular inhibitors. Similarly the presence of sodium ions can reduce the thermodynamic drive to scale formation by virtue of their ability to form soluble complexes with sulphate anions.

In the case of seawater flood operations the concentration of dissolved ions changes with time and conditions during the recovery process. In particular, in the later stages the sulphate anion concentration increases. This tends to reduce the activity of any inhibitor present.

None of the hitherto commercially available anti-scaling agents are wholly effective in preventing the build-up of scale derived from oil-rock formation water, especially where the scale predominantly contains insoluble barium sulphate, and when the formation water is of low pH, high salts content and/or contains significant concentrations of calcium, magnesium, and/or iron. life have found that anti-scaling agents for particular application in oil recovery should preferably exhibit the following characteristics: have high solubility in water; form only water-soluble complexes with dissolved ions present in the formation and recovery equipment; should not crystallise nor self-associate in water; and/or should include strong acidic groups that do not complex strongly with any of the metal ions present in the recovery system. We have now discovered novel polysaccharide derivatives which are particularly effective in inhibiting the production of scales such as formation water scales, especially but not exclusively heavy metal, e.g. barium, sulphate scales.

Thus, in one aspect the invention provides anti-scaling derivatives of raffinose in which one or more of the hydroxyl groups thereof have been converted to sulphate half ester groups, or preferably, such groups in salt form. In preferred compounds according to the invention, generally an average of about six of the hydroxyl groups are converted to sulphate half ester groups or salt forms thereof,

Raffinose is a trisaccharide having D-galactose , D- glucose and D-fructose residues and has the formula:

It will be appreciated that the raffinose molecule contains three primary hydroxyl groups and eight secondary hydroxyl groups. In the anti-scaling compounds of the invention, one or more of the hydroxyl groups have been converted to sulphate half ester groups or, preferably, such groups in salt form e.g. of formula -OSO₃M (where M is hydrogen or a metallic or non-metallic cation preferably an alkali metal cation, or an ammonium ion).

In another aspect, therefore, the anti-scaling compounds of the invention have the formula:

(wherein R¹ to R¹,¹ which may be the same or different, are independently hydroxyl groups or groups of formula -OSO₃ M, with the proviso that at least one of R¹ to R¹¹ is a group of formula -

OSO₃M).

In a further aspect, the anti-scaling compounds according to the invention are characterised in that they, have been prepared by reacting raffinose with a sulphating agent serving to convert one or more of the hydroxyl groups of the raffinose to sulphate half ester groups or such groups in salt form. Prsferably the raffinose is reacted in such a way as to convert an average of about 6 of the hydroxyl groups thereof to sulphate half ester groups or salt forms thereof. Thus, in a still further aspect, the invention provides a process for the preparation of a compound having anti-scaling activity which comprises reacting raffinose with a sulphating agent substantially to convert one or more of the hydroxyl groups of the raffinose to sulphate half ester groups or salt forms thereof.

The compounds of the invention are useful principally, but not exclusively, far the inhibition of build-up of sulphatecontaining scales, in particular barium sulphate-containing scales. These sulphate-containing scales are practically very difficult to cope with both with respect to inhibition of their formation and because of their chemical stability and insolubility ones formed. The compounds of the invention have been especially developed to inhibit the formation of barium sulphate scales. They remain effective in a wide range of chemical and physical environmental conditions.

In the process of the invention, the reaction with the sulphating agent is preferably effected under anhydrous conditions. In this respect, the raffinose is conveniently treated prior to the sulphation reaction by heating under an inert atmosphere, e.g. under vacuum or while passing hot, dry nitrogen over the raffinose. In this way substantially all traces of moisture can be removed from the raffinose prior to reaction with the sulphating agent. The sulphation reaction is advantageously carried out in a non-aqueous solvent such as, for example, dimethylsulphoxide, dimethylformamide, dichloromethane, chloroform, tetrahydrofuran, carbon disulphide, pyridine, and mixtures thereof.

The most preferred sulphating agent for use in the process of the invention is chlorosulphonic acid. Other sulphating agents may be used however, such as, for example, concentrated sulphuric acid (oleum), sulphur trioxide and derivatives thereof such as dimethyl pyrosulphate, trimethylaminosulphonic acid and a sulphur trioxide/pyridine complex preferably prepared by reacting anhydrous pyridine with gaseous SO₃. The sulphating agent will generally be used in an amount appropriate to introduce the desired number of sulphate half ester groups into the raffinose molecule. we have found that particularly effective anti-scaling compounds are produced when about 6 to 7 moles of chlorosulphonic acid are used per mole of raffinose.

The sulphonation reaction is conveniently effected at a temperature of from about -20°C up to the bailing paint of the reaction mixture employed, preferably while vigourously stirring the reaction mixture. When using chlorosulphonic acid as the sulphonating agent and pyridine as solvent we have found it preferable to carry out the reaction at about 50°C. When the desired degree of sulphation of the raffinose has been effected, the reaction mixture can be worked-up by careful addition of water to react with any excess sulphating agent. If pyridine has been used as the solvent and chlorosulphonic acid as the sulphating agent, it may be convenient to add just enough water to the reaction mixture to permit precipitation of pyridine sulphate and pyridine chloride complexes at reduced temperature. The solvent may be recovered (for reuse, if desired) by distillation e.g. under a reduced pressure. If desired, the compounds may be converted to salt form by reacting with a suitable base or by work-up with an appropriate alkaline solution. Preferred salts of the compounds include the alkali metal salts, e.g. with sodium or potassium cations, and quaternary ammonium salts e.g. with ammonium or guanidinium cations. Of course it will be appreciated that not all of the half ester groups may be in salt form. In general, compounds for use according to the invention as anti-scaling agents will be in water-soluble farm e.g. as water-soluble salts, and will have the ability to adsorb on the surfaces of the so called "seed" crystals that will be present according to a Freundlich isotherm. The exponent value of the isotherm is related to the anti-crystallization activity of the compound.

The anti-scaling compounds according to the invention may be isolated from the reaction mixture by any suitable purification technique e.g. using molecular exclusion chromatography such as with Sephadex G15 or G25 (available from Pharmacia).

While not wishing to be bound by theoretical considerations, it is believed that the anti-scaling compounds of the invention are effective because of their ability to interfere with the scale growth process. Scale is formed by essentially a crystal growth process and is therefore initiated by growth at a nucleation site. In the discussion which follows we will consider the formation of a barium sulphate scale, but the invention is not limited to anti-scaling compounds far only such scales. Thus, the invention is generally applicable to all anti-scaling requirements, but particularly those involving barium, strontium and calcium-containing scales, which as indicated above may include radioactive minerals e.g. comprising radionucleides formed as a result of the natural decay of uranium-238 and thorium-232.

In the aqueous environment from which a barium sulphate scale is formed, each of the positively charged ions and the negatively charged ions is surrounded by a relatively ordered arrangement of water molecules. This arises from the dipolar nature of the water molecule; the "negative end" of the water molecules being attracted towards the cations, and the "positive end" of the water molecules being attracted towards the anions. In solution, the water molecules which surround each of the ions tend to keep the ions apart and therefore tend to prevent their coming together in crystal growth. It is believed that crystal growth requires the formation of dehydrated ions, that is ions which have been stripped of their associated water molecules and which can therefore approach ions of opposite charge at the crystal growth site.

We understand that the compounds according to the invention produce their anti-scaling activity in one or more of several ways. Firstly, we postulate that the compounds of the invention bind at the crystal growth site and thus "poison" the growth process. Secondly, we believe that the compounds act to prevent ions approaching the crystal growth site by forming an "ordered water structure" at the growth site i.e. an arrangement of water molecules at the growth site which effectively forms a barrier against the incoming depositing ions. Additionally, we consider that the compounds of the invention have the ability to bind to the dehydrated cations and anions themselves and thus prevent the ions from taking part in the crystal growth process. The specific activity of the compounds to act as anti-scaling compounds in different situations appears to be due to the degree of sulphation and the stereochemistry of the compound concerned. Thus, by modifying the compounds it is possible to tailor the compound to be most effective in the particular scaling problem encountered. For example, one compound according to the invention may have high activity in preventing barium sulphate scale, while another may be more active against calcium carbonate scale. The compounds of the invention may be further modified by introducing the following functional groups, by appropriate techniques, into the molecular structure of the anti-scaling compounds: carboxylate, sulphonate, sulphamate, phosphate, phosphonate and phosphamate. The number of functional groups attached to each anti-scaling molecule and the particular selection of functional group(s) depends on the particular application envisaged. The activity of the compounds may also be modified by polymerisation of the starting raffinose or the raffinose derivative. Polymerisation may be effected in a number of ways including those involving free radical and dehydration mechanisms. Indeed a combined polymerisation, carboxylation and sulphation may be effected by reacting with hot sulphuric acid. The molecular randomness introduced by this reaction broadens the activity of the compounds to include inhibiton of the formation and occurrence of other crystal nucleating and scale-forming minerals, including carbonates.

Thus, in an oil recovery process, the selection of the compound of the invention for any particular intended use may, for example, depend upon the reservoir rock being treated, the temperature and pressure of the reservoir, the connate waters and pH of the produced fluids.

As indicated above, the compounds of the invention have particular utility in the prevention of scale deposition from oil field formation waters. Since the formation waters from different oil fields will contain different scale-forming components and different proportions thereof, the anti-scaling compounds according to the invention may, if desired, be used with other scale inhibiting or preventing compounds, such as conventional scale inhibiting compounds which are involved in inhibition of so-called heterogeneous nuclei. The compounds may be added to the seawater to be pumped into the oil-bearing formation where it is desired to prevent scale in the formation itself. The compounds may also be introduced at specific points of the ail recovery apparatus e.g. at points immediately upstream of places particularly susceptible to scaling problems. Thus, the scale inhibitors of this invention may be applied by a "trickle" application, either at the surface or in the well; a batch appliction, either at the surface or in the .well; or by a "squeeze" application in the well. Trickle application involves continuously feeding a small quantity of the scale inhibitor at a surface treatment location (i.e. at a treatment facility above the reservoir) or at the bottom of the well. The quantity of scale inhibitor to be used depends on the amount of potential scale build-up, which in turn is, for example, dependent on flow rates, temperature, pH and pressure in the recovery system. For example, an appropriate quantity of scale inhibitor may be continuously added to injection water that is used to .pressurize the reservoir; the inhibitor may be admixed with produced fluids during the treatment of fluids on a platform or at the wall site; and so on. The scale inhibitor may also be continuously injected into the fluids at the bottom of the well, or at some point between the bottom of the well and the surface.

Squeeze application generally relies upon physical adsorption of the inhibitor onto the reservoir rock or precipitation of the inhibitor in the matrix of the reservoir rock. Precipitation is sometimes preferred in order to increase retention and treatment life. Precipitation may occur by regulating the cations that come into contact with the inhibitor, e.g. by introducing a brine after or before the inhibitor is introduced into the matrix, or by pH control by subsequently injecting material having a pH lower than the previously injected inhibitor. However, precipitation may cause adverse permeability reductions in certain environments and in those cases it may not be the most convenient method of squeeze application.

It is preferred that squeeze treatment with the inhibitors of this invention is effected by physical adsorption,

The scale inhibitor may be chemically modified by placing functional groups onto the inhibitor which cause the inhibitor to adhere to the reservoir rock. Examples of functional groups which may be used for this purpose are as indicated above. Alternatively, the anti-scaling compounds may be chemically linked to, far example, a high molecular weight polymer, e.g. a polyacrylate, cellulose or chitin. The modified product should be capable of being slowly desorbed off of the reservoir rock as the well is worked. Such modifications should not substantially substract from the scale inhibiting properties. In such an application, it is desirable to provide enough inhibitor to satisfy the adsorption sites an the reservoir rack contacted by the produced fluids. The volume of formation to be contacted by the scale inhibitor will depend upon inter alia relations between adsorptive capacity, percent reversible desorption, desired protection time, desorption characteristics, production rate, produced fluid volume, and economic considerations. Generally speaking, the inhibitors can provide effective protection at a concentration level of about 1 to about 100 ppm. Application of the scale inhibitor into the formation is preferably carried out by first cleaning the conduit that will be in contact with the inhibitor, e.g. the tubing and/or casing, and then stimulating the formation, e.g. by fracturing or chemically treating, to restore productivity. Thereafter, the scale inhibitor is squeezed into the matrix of the formation. A preflush, e.g. of water or water containing a viscosity enhancing agent can be injected prior to the scale inhibitor. Also water or water containing a viscosity enhancing agent can intermittently be injected during the injection of the scale inhibitor. After all of the scale inhibitor is injected, an overflush of water or water containing a viscosity improving agent is injected to displace the scale inhibitor out into the formation. The well is then shut off for sufficient time to permit the inhibitor to be adsorbed onto the reservoir rock, about 12 to about 36 hours is generally desirable. Thereafter, the well is returned to production. The well should be monitored to analyse the amount of scale inhibitor being produced; once the rate of produced scale inhibitor drops below a desired amount, the well is shut off and retreated. Retreatments may occur as often as 3, 6 or 9 months. Batch application of the inhibitor can occur on the surface by admixing the inhibitor with produced fluids during the processing of the fluids. Alternatively, a given quantity of inhibitor can be pumped down the casing or tubing at regular intervals. Such treatments are usually short lived and more expensive than other application techniques. As indicated above, the scale inhibitors of this invention are particularly useful as barium sulfate scale inhibitors. In some circumstances, their effectiveness may be reduced in the presence of calcium carbonate. Thus, it may be preferred that a calcium carbonate scale inhibitor be admixed with the inhibitors of this invention. Examples of calcium scale inhibitors include sodium tripolyphosphate; sodium hexametaphosphate; oxidised polysaccharides; polymeric carboxylic acids; polyphosphoric acids; esters of orthophosphoric acids; phosphonic acids; amino alkylene phosphonic acids; bis (phosphonomethylene) aminomethylene carboxylic acid; aminomethylenephosphonic acid; 1-hydroxyethylidene-1,1-diphosphonic acid; copolymers of maleic anhydride acid and allyl acetate; Flocon Antiscalant 247 Chemical (a liquid synthetic polymer marketed by Pfizer Limited, Ramsgate Road, Sandwich, Kent, England); Flocon Antiscalant 280 and 285 (a maleic acid copolymer marketed by Pfizer Limited); Calnox ML-1559 Chemical (a blend of organic phosphonate and a low molecular weight polymer, marketed by Baker Oil Treating, Kirkhill Place, Dyce, Aberdeen, Scotland); Falquest 1400 Chemical (a pentasodium salt of aminotrimethylenephosphonic acid, marketed by Fallek Chemical Company, 2125 Center Avenue, Fort Lee, New Jersey 07024, USA); Falquest 1320 Chemical (aminotrimethylenephosphonic acid), marketed by Fallek Chemical Company); Falquest 1500 (1-hydroxyethylidene-1,1-diphosphonic acid, marketed by Fallek Chemical Company); Falquest 1652 (the ammonium salt of hexamethylenediaminetetra(methylenephosphonic acid), marketed by Fallek Chemical Company); Falquest 1656 (the sodium salt of hexamethylenediaminetetra(methylenephosphonic acid), marketed by Fallek Chemical Company); Falquest 1746 (the sodium salt of ethylenediaminetetra(methylenephosphonic acid), marketed by Fallek Chemical Company); Falquest 1860 (diethylenetriaminepenta(methylenephosphonic acid), marketed by Fallek Chemical Company); Falquest 1866 (sodium salt of diethylenetriaminepenta(methylenephosphonic acid), marketed by Fallek Chemical Company); Sodium Glucoheptonates (the sodium salt of 2, 3, 4, 5, 6, 7-hexahydroxy-1-heptanoic acid), marketed by Fallek Chemical Company; Calnox ML-1559 Inhibitor (a blend of organic phosphonate and polyacrylate homopolymers); Nalfloc 1285 (a blend of copolymer acrylates marketed by Nalco UK Limited); Corexit 8389 (a blend of phosphonates marketed by Esso Chemical Ltd., Blackness Industrial Estate, Aberdeen, Scotland); Servo CS 511 (neutralized phosphoric acid and polycarboxylic acid marketed by Servo UK, 115 Crombie Road, Aberdeen, Scotland); Servo CS 515 (neutralized polycarboxylic acid); and Monsanto 8417 (a phosphonate marketed by Monsanto Chemicals, St. Louis, USA).

The formation waters are often present in the oil fields at very high temperatures e.g. up to 150°C or more, and under considerable pressures. The compounds for use directly in the formation should therefore be stable at such temperatures and pressures. Clearly the stability of compounds intended for use in downstream applications where the temperatures and pressures are not so extreme, is not so critical.

In situations where the thermodynamic drive is overwhelmingly towards the formation of insoluble material the compounds of the invention are also particularly useful in that they alter the morphology and surface charge on any barium sulphate formed such that it is non-adhesive and remains as a stable colloidal suspension in the water phase, i.e. the barium sulphate particles are peptized. In this aspect the compounds of the invention have wide industrial application. Situations with high concentrations of dissolved carbon dioxide are particularly prone to the formation of both insoluble carbonates and sulphates, which are effectively nucleated by crystalline carbonates. As an additional precaution against scaling in such situations the incorporation of an appropriate acid, e.g. phosphorus acid H₃ PO₃, which we have discovered has itself scale dissolving properties, will by reducing the pH of the water increase the solubility of potentially scale forming minerals. Because the compounds of this invention may be strong acids and will remain ionic and active at low pH values, they have the unique advantage of being able to continue to function under such conditions.

In tests which have been carried out, e.g. using the seeded mechanistic tests of Nancollas et al. (see Leung W.H. and Nancollas G.H., J. Crystal Growth (1978), 44, pp. 163-167, and

Gardnar G.L. and Nancollas G.H., J. Phys. Chem. (1983), 87 , pp.

4699-4703), compounds according to the invention have exhibited a far superior inhibiting activity on barium sulphate crystal growth in comparison with commercially available inhibitors. Thus, the activity has been confirmed at various barium to sulphate ratios and pH values, in the presence of various added salts; at various total dissolved solids contents, and in the presence of oil. Methods for the detection of soluble barium have included conductivity, complexometric titration, atomic absorption spectrometry and radiometric detection of 133 Ba. The following results are typical and were obtained at pH 3.6; 25°C; at Ba: SO₄ ^2- of 1:1; and 30-fold supersaturation. Dose rates are expressed in parts per million and mineral growth rates as a percentage of the uninhibited rate. The comparative known inhibitors are commercial anti-barite preparations currently in use. TABLE I

Inhibitor Doss Primary "surge" Secondary

(ppm) rate (%) "steady state" growth {%)

Compound of

Invention 12.5 complete inhibition complete inhibition

6.3 0.0005

Preparation A 10.0 10 56

" B 30.0 100 75

" C 12.5 0 53

" D 30.0 61 30

It has been found that compounds of the invention exhibit good activity even at low pH values, and high concentrations of sulphate ions.

We have also studied compounds according to the invention in non-seeded jar tests, including those conducted at pH 2, which utilised the time dependent hydrolysis of sulphamic acid to generate sulphate ions in the presence of existing barium ions in pre-prepared brine. Tests were carried out in sealed ampoules at 60°C. The more effective the inhibitor, then the higher the concentration of sulphate that is tolerated before the onset of precipitation. Thus, the length of time to the onset of barium sulphate precipitation can be taken as a measure of barium sulphats inhibition. The results of these tests again show our inhibitors to be superior to all other inhibitors which we have tested. Thus, in tests at pH 2-3, temperature 61-62°C, with a barium ion concentration of 1000 ppm and sulphate ion concentration (maximum) 3000 ppm, brine 2.8% by weight, the following results were obtained: Table II

Inhibitor Dose (ppm) Time to haze point

(minutes)

- None - - 75 Compound of Invention 30 210

Compound of Invention* 25 180

Comparative Compound E 100 120

^" " F * 30 85

^" " G* 25 110 * in additional presence of 1000 ppm ionic calcium.

The majority of commercial inhibitors which we have tested show no activity under these test conditions.

We have further used a naphelometric method to study activity at high barium ion concentrations. Our studies indicate that any barium sulphate crystals forming remained in aqueous colloidal suspension for long periods of time.

The invention is illustrated by the following non- limiting Examples (all temperatures are in °C):- Example 1. Raffinose hexahydrate (103.4g) was heated gradually in order to avoid melting to 100° and maintained in a stream of dry nitrogen at this temperature for 3 hours. The anhydrous raffinose was dissolved in 1.6 litres anhydrous pyridine in a three-necked glass reaction vessel, fitted with a stirrer, a condenser with moisture guard tube, and a dropping funnel. A dry nitrogen atmosphere was maintained in the reaction vessel. The temperature was raised to 50°C and chlorosulphonic acid (114 ml) in dichloromethane (540ml) was added dropwise over one hour. A further 12ml of chlorosulphonic acid was added over the next hour with vigourous stirring, after which time the stirring was stopped and the reaction mixture allowed to cool to 20° and subsequently stand for 12 hours. After cooling to about 2°, sulphate was removed as crystalline pyridinium sulphate. A resinous reaction product is present in the vessel. Crystals of pyridine sulphate and pyridine chloride were filtered from the supernatant pyridine solution, following which pyridine was recovered by vacuum distillation. The distillation residue was combined with the resinous reaction product and the whole dissolved in 1 litre of water and neutralised to pH 7 with 6 M sodium hydroxide solution.

The product was dissolved in water and passed through a column of Sephadex G25 using water as eluant. The carbohydrate concentration of eluate fractions was monitored using a colorimetric assay. As many as 11 peaks were resolved. The desired material was that corresponding to the largest carbohydrate peak.

The product was recovered from the appropriate combined fractions by evaporation under reduced pressure as a glassy solid.

The product exhibited the following peaks when subjected to proton noise decoupled 13C n.m.r. spectroscopy (90,56 MHz), all peaks in ppm with reference to external TMS standard; 103.1 102.1 (98.1) 96.4 (90.9) 80.0 79.1 70.476.0 74.6 74.3 72.472.271.469.8 68.3 67.6 66.6 66.2 65.2 (64.1) (44.2) (17.2) (2.2).

(The figures in parenthesis are believed to be due to impurities).

When subjected to IR spectroscopy (multiple specular reflectance), the following peaks (intar alia) ware observed:

611, 710, 840, 958, 1019, 1143, 1230, 1300, 1466, 1638 and 1723 cm^-1.

Exampla 2.

Anhydrous raffinose (2 g, prepared as described in Example 1) was dissolved in 75 ml anhydrous pyridine and 3.9 g of a pyridine/sulphur trioxide complex was added over a period of thirty minutes at ambient temperature. The mixture was stirred for 150 minutes. Pyridine was removed under vacuum and water (30 ml) was added to the residue. The aqueous solution was neutralised to pH 7 with aqueous sodium hydroxide (6 M) . The product is worked-up and isolated as described in Example 1. Example 3.

Trimethylaminosulphonic acid (3.36 g) was stirred in dimethylformamide (30 ml) at 0°. Before all the sulphating agent had dissolved anhydrous raffinose (2.03 g, prspared as described in Example 1) was added and stirring continued for 24 hours. The solvent was then evaporated off under reduced pressure, whereafter water was added and evaporation repeated. Water was then further added and aqueous barium hydroxide added to pH 7. The precipitate was centrifuged off and the aqueous solution was allowed to stand overnight. The solution was neutralised to pH 7 with barium hydroxide solution and the precipitate was centrifuged off. The supernatant was evaporated to a clear oil.

The product is isolated using the procedure of Example 1.

Example 4.

Trimethylaminosulphonic acid (1.74 g) was stirred in dimethylformamide (15 ml) at room temperature. Before all the sulphating agent had dissolved anhydrous raffinose (1.05 g, prepared as described in Example 1) in dimethylformamide was added.

The solution was initially heated to 70°C, but the temperature was lowered after 15 min. to 40° for 3 hours, and then heated to 80° for 5 mins. and 75° for 14 hours. After this period a pale yellow solution had been produced and this was evaporated to dryness.

The product is worked-up and isolated as decribed in Example 1. Example 5. Anhydrous raffinose (2.0 g, prepared as described in

Example 1 ) in dimethylformamide (30 ml) was stirred and pyridine/sulphur trioxide complex (3.8 g) in dimethylformamide (25 ml) was added dropwise over 1.5 hours at room temperature. The solution was stirred for a further 3 hours, and then evaporated to dryness. 30 ml of water were added to the resinous product and the aqueous solution was neutralised with barium hydroxide to pH 7. The precipitate was centrifuged off and the clear supernatant is evaporated to a yellow resinous product which is further processed as described in Example 1. EXAMPLE A

A typical application in the ail production industry is by the use of dawnhole trickle application at a dose rate 25 ppm into the produced water. This affords protection against barium/strontium sulphate scale formation in the tubulars and the topsides process equipment.

Claims

CLAIMS:

1. Anti-scaling derivatives of raffinose in which one or more of the hydroxyl groups thereof have been converted to sulphate half ester groups or such groups in salt form.

2. A raffinose derivative according to claim 1 wherein an average of about six of the hydroxyl groups thereof have been converted to sulphate half ester groups or salt forms thereof.

3. A raffinose derivative according to either of claims 1 and 2 in the form of a polymer.

4. A raffinose derivative according to any preceding claim wherein one or more carboxylate, sulphonate, sulphamate, phosphate, phosphonate or phosphamate groups have been introduced into the molecular structure.

5. Compounds of general formula:

(wherein R¹ to R¹¹ , which may be the same or different, are independently hydroxyl groups or groups of formula -OSO₃M, wherein M is hydrogen or a metallic or non-metallic cation, with the proviso that at least one of R¹ to R¹¹ is a group of formula

-OSO₃M).

6. A compound according to any preceding claim in the form of a salt with an alkali metal cation or ammonium anion.

7. A process for the preparation of a compound having anti-scaling activity which comprises reacting raffinose or a polymer thereof with a sulphating agent substantially to convert one or more of the hydroxyl groups of the raffinose to sulphate half ester groups or salt forms thereof.

8. A process according to claim 7 wherein reaction of the raffinose or polymer thereof with the sulphating agent is effected under anhydrous conditions.

9. A process according to either of claims 7 and 8 wherein the raffinose or polymer thereof is treated, prior to reacting with the sulphating agent, by heating under an inert atmosphere to remove moisture from the raffinose.

10. A process according to any one of claims 7 to 9 wherein the sulphating agent comprises chlorosulphonic acid and about 5 to 7 moles or chlorosulphonic acid are used per mole of raffinose.

11. A process according to any one of claims 7 to 9 wherein the sulphating agent comprises hot sulphuric acid.

12. A process according to any one of claims 7 to 11 wherein the product of the reaction with the sulphating agent is polymerised.

13. A process according to any one of claims 7 to 12 wherein the raffinose or derivative thereof is further modified by introducing one or more carboxylate, sulphonate, sulphamate, phosphate, phosphonate or phosphamate groups into the molecular structure.

14. Use of a compound according to any one of claims 1 to 6 in the inhibition or prevention of scale formation.

15. Use of a compound prepared by a process according to any one of claims 7 to 13 in the inhibition or prevention of scale formation.

16. Use according to either of claims 14 and 15 in the inhibition or prevention of scale formation in oil recovery.