It is known to use various salts, and in particular salts of the formula H (CHCO2M) n P03 MI, where each M and M'is a cation and"n"is an even number, as scale and/or corrosion inhibitors in aqueous systems (see, for example, EP 0 491 391) It is also known to use various oil soluble surfactants to inhibit or prevent the deposition of asphaltenes or of waxes or to disperse previously formed deposits.

There is, however, a need for scale and/or corrosion inhibitors which are oil soluble, e. g. inhibitors for use in oil production and also for more effective oil soluble surfactants, as well as for surfactants that can perform dual function as scale inhibitors and as asphaltene dispersants.

A common problem in oil production arises when potentially scale forming aqueous inclusions occur in a production well which is wet with oil. It is often impossible to introduce conventional, water-soluble scale inhibitors into such systems without damaging water sensitive formations or inhibiting the flow of oil by introduction of aqueous solutions into the well. Oil-soluble scale inhibitors can be introduced in an oil phase, e. g. xylene, which does not damage the formation or interfere with the normal flow of oil. They are required to be able to pass from solution in the oil phase into aqueous inclusions by partitioning or hydrolysis so as to inhibit scale formation in the aqueous phase.

A major problem in oil production is the precipitation of asphaltenes or of waxes which, like mineral scales, can block the flow of oil through the formation or through pipelines and other well-head, storage or refinery equipment. There is a major requirement for compounds which can inhibit the precipitation of asphaltenes and/or waxes or which can disperse asphaltene or wax deposits.

Existing scale inhibitors are unable to meet these requirements.

We have discovered that novel esters of phosphono or phosphino acids, wherein at least one carboxyl group is esterified with a C6-25 alkyl, alkaryl or alkenyl group, are effective as oil-soluble scale inhibitors, and as wax or asphaltene inhibitors or dispersants Accordingly, the present invention in a first aspect provides the aforesaid novel esters. According to a second aspect the present invention provides a process for making said novel esters by refluxing the corresponding phosphono carboxylic acid with from 1 to n moles of a C6 20 alcohol in a solvent having a boiling point greater than that of water, where n is the number of carboxy groups, and removing water from the system.

According to a third aspect the present invention provides the use of said novel esters as oil-soluble corrosion and/or scale inhibitors and/or as wax or asphaltene inhibitors and/or dispersants. A small stoichiometric excess of the alcohol, e. g. up to 1.3n, can generally be tolerated. The reflux temperature is preferably 110 to 170 °C, e. g. 120 to 160°C.

Alternatively the novel esters may be prepared by telomerising an ester of an unsaturated carboxylic acid with a phosphite or hypophosphite telogen.

H (CHC02) n P03 M'

"n"is preferably 2 or more preferably 4, but may be higher, e. g. up to 12 or, less preferably, up to 100. We particularly prefer a mixture of compounds wherein n is 2 and 4 such as may be obtained by using in the above preparation an acid derived from the reaction product of a maleate salt with a phosphite salt at a pH above 5 as taught in EP 0 491 391.

Although not generally preferred the invention also provides compounds in which the phosphono or phosphino group is esterified with a C6 20 group, their use and their preparation by esterifying the aforesaid acids with more than n moles of alcohol, and using higher temperatures and/or dehydrating agents which permit the esterification of phosphonic acid groups, or by reacting a mixture of a phosphite or hypophosphite ester with fatty alkyl ester of an unsaturated carboxylic acid.

The invention further provides solutions of the aforesaid esters in oil, and especially in aromatic oil such as xylene or in mineral oils, and methods of inhibiting scaling and/or precipitation of asphaltene, or of dissolving or dispersing scale or asphaltene or wax deposits by injecting said solutions into oil wells, pipelines or formations. The aforesaid esters may also be used as gas hydrate inhibitors The phosphonocarboxylic acid may for example be phosphonoacetic acid or, preferably, phosphonosuccinic acid. Alternatively it may be 1 phosphono-2,3, 4-tricarboxy butane or preferably a telomer of the type described in EP 0 491 391 which may be obtained by reacting a water soluble salt of an unsaturated carboxylic acid such as maleic, acrylic, methacrylic, fumaric, crotonic, isocrotonic, citraconic, mesaconic, aconitic, itaconic, angelic, tiglic, cinammic, allocinammic, oleic or elaidic acid with a phosphite at a pH above 5, in the presence of free radicals.

Particularly preferred are compositions of the formula H (CHO2M) nPO3Ml where M is a C6-20 alkyl, alkaryl or alkenyl group and M'is preferably H or an alkali metal, ammonium or alkaline earth metal and n is a number from 1 to 20, and preferably an even number from 2 to 10. Compounds of the latter type may be prepared by reacting maleate with phosphite. Also included are phosphono polyacrylates and phosphono acrylate/maleate copolymers. Other monomers which may be used in addition with any of the above include vinyl phosphonic, vinylidine diphosphonic, vinyl sulphonic and styrene.

Apart from a phosphite the telogen may a hypophosphite or a phosphonophosphino compound such as the compounds X203PCHYCZ2PO2X2 wherein X is hydrogen or alkali metal ammonium or alkaline earth metal and Y and Z are each hydrogen, a P03X2, S03X or CO, X group or an alkyl or aryl moiety. The preparation of such compounds and of carboxylic acid telomers derived therefrom is described in EP 0 861 846.

We prefer that the alkyl groups in our esters comprise from 10 to 25 carbon atoms, e. g. 12 to 22.

The esters of our invention are preferably xylene-soluble, e. g. to the extent of at least 1 % by wt.

The invention is illustrated by the following examples: Example 1 An aqueous solution of phosphonosuccinic acid (50%) (0.252 mol) was mixed with 1 dodecanol (0.504 mol), 100 ml of xylene was also added plus a few drops of concentrated sulphuric acid.

The mixture was heated using a Dean and Stark apparatus until water began to distil out of the reaction. Refluxing began at 105°C and water distilled out over 5-6 hours, until the temperature finally reached 131 °C, at which point the reaction was cooled to give a homogeneous solution.

The xylene solvent was then stripped to give a sticky orange oil. H203P CO2H CH3 (CH2), lOH H203P CO2 (CH2), lCH3 Y HO2C Xylene CH3 (CH2) 1102C Example 2 A mixture of phosphonosuccinic acid and the telomer (n = 4, M and M' = H) in aqueous solution (48. 5%) (0.16 mol) was mixed with lauryl alcohol (0.32 mol) and xylene (80ml) with 0. 5ml of concentrated H, S04 also added. Again a Dean and Stark apparatus was used to remove water from the reaction. The reaction was heated to 144°C over 10 hours until the distillation of water had stopped. Cooling followed by removal of the xylene afforded a sticky oil. H, O, P CO, H CH3 (CH, ("OH H, O, P CO, (CH,)"CH, 0 > 0 HO2C m H Xylene CH3 (CH2)"02C m n average m = 1. 2 The products of Examples 1 and 2 were soluble in mineral oil and provided effective inhibitions of scaling, corrosion and asphaltene precipitation.

Example 3 The product of Example 2 was tested for its ability to disperse asphaltene deposits.

Test Methodology Dissolve 1.14g asphaltene into 50ml toluene-asphaltene stock solution.

Add 1 ml stock solution to 50 ml hexane and shake for 1 minute.

'Measure % transmittance at 640nm versus time.

Repeat above but add 100ppm product of Example 2 (previously prepared as a 1 % stock solution in toluene-only partially soluble) prior to shaking for one minute.

Results as follows Time (minutes) % Transmittance Blank ITC1310 0 4.0 3.9 5 8.5 4.0 10 15.4 4.1 15 17.8 4.3 As the asphaltene drops to the bottom the transmittance increases, i. e. the higher the transmittance the greater the amount of asphaltene that is still suspended.