GB2306465A - Scale inhibition using alkanolamine methylene phosphonate - Google Patents

Abstract

A method of inhibiting or preventing scale formation, eg barium scale, comprises adding a scale inhibiting amount of an agent consisting of a methylene phosphonate derivative of an alkanolamine. Preferably only a threshold amount, ie less than stoichiometric amount, of the agent is added, the threshold amount being sufficient to inhibit scale. The method may be used to prevent scale formation in an oil well where brine is injected into an aquifer to form a solution containing at least 200mg of alkaline metal per litre, and the agent is injected at the lower end of the aquifer. Preferably the scale inhibitor contains at least one compound of the formulae: Where R may be -H, -CH 3 , -C 2 H 5 , -OHCH 2 or R'. R' may be -OHCH 2 or [(CR 2 ) n NCR 2 PO 3 H 2 ] m CR 2 PO 3 H 2 .

Description

INHIBITION OF SCALE This invention relates to scale inhibition and in particular inhibition of the very severe scale formation which may occur for example in oil field operations, in which the water present in the rock formations (formation water) and/or the water which is discharged with the oil at the well head (produced water) contains high concentrations of alkaline earth metal and exhibits unusually high scale forming potential.

Scale formation in hard water systems usually involves the precipitation of calcium carbonate.

The widespread problems caused by scaling in industrial and domestic water systems are most commonly alleviated by the use of scale inhibitors. A large number of such inhibitors are known including complexants which sequester the calcium ions preventing precipitation. Rather than use complexants, which require stoichiometric proportions, it is usually economically attractive to use threshold agents which, even at very low concentrations, eg, in the range 10 to 100 parts per million prevent scale formation by modifying the form in which the solids precipitate.

"Threshold amount" as used herein means an amount of scale inhibitor which is sufficient to prevent or substantially inhibit scale formation, but which is substantially less than the stoichiometric amount which would be required to maintain the scale forming components in solution if the inhibitor were a chelant.

One class of threshold scale inhibitors which has proved particularly effective consists of the amino methylene phosphonates, compounds which comprise one, or more usually several, =NCH2PO3H2 groups, and their salts. Typical examples include amino tris(methylene phosphonic) acid and diethylenetriamine pentakis (methylene phosphonic) acid. In the oil industry the latter is the most commonly used member of the class and usually the most effective.

In addition to calcium carbonate certain other alkaline earth metal salts may cause scaling problems in certain situations. Among the most troublesome of these are the sulphates of strontium, magnesium and, particularly, barium. The most serious problems are typically encountered in the oil industry when oil is being recovered from formations containing very high concentrations of barium dissolved in the formation water. To assist recovery of oil the formation is often pressurised by injecting water, usually sea water, which contains relatively high concentrations of dissolved sulphate. This is referred to herein as "squeeze treatment".

The resulting scale formation causes blockages in the well interrupting the flow of oil and, in severe cases may even necessitate the drilling of a new well.

The normal method of alleviating this problem is to inject relatively high concentrations of an effective threshold scale inhibitor into the formation at the downhole end of the well. However, when the barium concentrations are very high this method has proved insufficient, even using prohibitively high dosages of scale inhibiting agents, because the barium precipitates the inhibitor preventing a sufficiently high concentration of the latter from being attained In the most severely affected formations it has even been necessary to try to remove sulphate from the injection water e.g. by reverse osmosis, which is a very expensive and not fully effective expedient. None of the threshold inhibitors which are normally most effective in controlling scaling has been found adequate to deal with this particular problem.

If the formation waters contain high levels of alkaline earth metal, particularly calcium, a further problem arises when the produced water nears the top of the well. Reduced pressure causes evolution of carbon dioxide and lowers the solubility of the calcium giving rise to severe calcium carbonate scaling. As in the case of squeeze treatment, where the alkaline earth metal content is very high the usual threshold scale inhibitors are insufficiently soluble to provide an effective concentration.

Surprisingly, we have now found that certain methylene phosphonate derivatives of alkanolamines are very significantly more effective at preventing scale in waters containing very high concentrations of barium and/or calcium than any of the other scale inhibitors which have been tested.

Alkanolamine methylene phosphonates are known to have some scale inhibiting activity, but have not been used commercially to any great extent because they tend to cyclise forming intermolecular cyclic phosphates containing structures such as:

which have been found ineffective as scale inhibitors. Hitherto their only significant application has been in highly alkaline systems which may tend to hydrolyse the ring structure. We believe, however, that it may be the presence of the cyclic material that renders the alkanolamine methylene phosphonates more compatible with water containing high concentrations of e.g.

barium.

According to one embodiment, our invention provides a method of treating water which has a tendency to deposit barium sulphate scale in order to inhibit scale formation which comprises adding a threshold amount of a scale inhibiting agent comprising a methylene phosphonate derivative of an alkanolamine.

The scale inhibiting agent typically contains at least one compound of the formulae

wherein each R is preferably hydrogen, but may also be a methyl, ethyl, hydroxymethyl, or R' group and R' may be a hydroxyethyl or preferably a [(CR2)nNCR2PO3H2]m CR2PO3H2 group where n is 2 or 3 and m is from 0 to 10, or salts of such compounds.

According to a second embodiment this invention provides a method of preventing scale formation in an oil well when an injection water comprising sea water is injected into an aquifer to form a produced water containing at least 200mg of dissolved alkaline earth metal per litre, which method comprises injecting into said aquifer at the lower end of said well a scale inhibiting amount of a scale inhibiting agent comprising at least one compound according to formula I and/or II.

According to a third embodiment the invention provides a method of inhibiting scale formation from a scale forming water system containing more than 200mg dissolved alkaline earth metal per litre and having a pH between 5 and 8.5, which comprises adding thereto from 5 to 1000 ppm of a scale inhibiting agent comprising at least one compound according to formula I and/or II.

According to a further embodiment our invention provides a method of inhibiting scale formation in oilfield produced water containing more than 200mg per litre alkaline earth metal which comprises adding to said produced water a scale inhibiting amount of a scale inhibiting agent comprising at least one compound according formula I and/or II.

The aforesaid compound of formula I or II preferably has from two to eight methylene phosphonate groups, preferably 2. We prefer that the compound is a derivative of ethanolamine or 2-hydroxy propylamine. The compound may be present as the free acid or preferably as an alkali metal (e.g. lithium, potassium or preferably sodium), ammonium, C, to C,0 primary, secondary or tertiary amine or quaternary ammonium, alkanolamine or alkaline earth metal (eg, calcium) salt.

The scale inllibitillg agent preferably comprises from 1 to 100% by eight (more preferably 5 to 90% e.g. 10 to 80% especially 20 to 70%) of at least one compound of formula II from 0 to 99% (more preferably 5 to 90% e.g. 10 to 80% especially 20 to 70%) by weight of a compound of formula I and optionally from 0 to 99% (preferably 1 to 80% e.g. 2 to 60% especially 5 to 50%) or one or more other scale inhibiting agents, such as acetodiphosphonic acid, aminotris (methylene phosphonic) acid, ethylenediamine tetrakis (methylene phosphonic) acid, diethylenetriamine pentakis (methylene phosphonic) acid, triethylenetetramine hexakis (methylene phosphonic) acid, tetraethylene pentamine heptakis (methylene phosphonic) acid, phosphono succinic acid, 2-phosphono-1, 2, 4 tricarboxy butane and l-phosphono-1, 2, 3, 4 tetracarboxy butane, salts of any of the above acids, alkali metal phosphates and condensed phosphates such as trisodium phosphate, tetra potassium pyrophosphate, sodium tripolyphosphate, sodium tetraphosphate and sodium hexametaphosphate, sodium ethylenidiamine tetracetate or sodium nitrilotriacetate.

In addition to the foregoing the scale inhibiting agent may optionally contain up to 95% based on the weight thereof of other water treatment agents including: surfactants, such as anionic surfactant (e.g. C,0-C20 alkyl benzene sulphonates, C,0-C20 olefin sulphonates, C10-C20 paraffin sulphonates, C,0-C20 soaps, C,0-C20 alkyl phenol sulphonates, sulphosuccinates, sulphosuccinamates, fatty ester sulphonates, C,0-C20 alkyl phenyl ether sulphates, C,0-C20 alkyl ethanolamide sulphates, C,0-C20 alpha sulpho fatty acid salts, C,0-C20 acyl sarcosinates, isethionates, C,0-C20 acyl taurides, C,0-C20 alkyl hydrogen phosphates), non-ionic surfactants (e.g.

ethoxylated and/or propoxylated C,0-C20 alcohols, ethoxylated and/or propoxylated C,0-C20 carboxylic acids, alkanolamides, amine oxides, and/or C,0-C20 acyl sorbitan and/or glyceryl ethoxylates), amphoteric surfactants (e.g. betaines, sulphobetaines, and/or quaternised imidazolines), and/or cationic surfactants (e.g. benzalkonium salts, C,0-C20 alkyl trimethyl ammonium salts, and/or C,0-C20 alkyl trimethyl phosphonium or C,0-C20 alkyl tris (hydroxymethyl) phosphonium salts); sequestrants; chelating agents, corrosion inhibitors; biocides (e.g. tetrakis (hydroxymethyl) phosphonium salts, isothiazolinone, biguanides, formaldehyde. glutaraldehyde); foam controlling agents such as silicone antifoams; oxygen scavengers such as hydrazines and/or hydroxylamines; pH controlling and/or buffering agents such as amines, borates, citrates and/or acetates; chromium salts; and/or other water treatment agents such as polymeric dispersants and coagulants including polymaleic, polyacrylic and polyvinylsulphonic acids and their copolymers and salts, starches and/or carboxy methyl cellulose, lignin sulphonates and/or molybdates; solvents such as water; and inert diluents.

Oxidising biocides and/or bleaches (e.g. chlorine, chlorine dioxide, sodium perborate) are usually chemically incompatible with the methylene phosphonate derivatives of alkanolamines and are preferably not present, although hydrogen peroxide may sometimes be included.

The scale inhibiting agent is usually maintained in the water to be treated at an effective threshold scale inhibiting concentration e.g. 5 to 1000 ppm more usually 10 to 800 ppm especially 20 to 700 ppm more especially 50 to 600 ppm. We particularly prefer concentrations in the range 100 to 500 ppm. In practice the scale inhibiting agent is normally injected through the well into alkaline earth metal containing rock formations, as a squeeze treatment, or added to produced water at or near the top of the well, usually at the well head..

The water system treated is typically one in which the total alkaline earth metal in solution, including calcium, barium, strontium and magnesium is greater than 200mg per litre more preferably greater than 500 mg per litre, usually greater than 800mg per litre, e.g. greater than 1000mg litre, especially greater than 1,100 mg per litre. The invention is typically most useful where the predominant scale forming alkaline earth metal present is barium. The water systems treated are usually around neutral pH or slightly acidic e.g. pH 5 to 8 especially 6.5 to 7.8. The above analyses are typically based on the total content of the produced water, including solids.

The compounds of formula I and II are typically prepared by reacting together formaldehyde, phosphorous acid and a hydroxy alkylamine or hydroxyalkyl alkylene polyamine, in the presence of a mineral acid catalyst.

The invention will be illustrated by the following examples: Tables 1 and 2 give the composition of water produced from two oil wells which exhibit very severe scaling problems. Table 3 gives a typical average composition of the formation water in the field from which the two wells produce.

Tests were conducted on three examples of the invention and on a number of commonly used commercial scale inhibitors for comparison. Example 1 was the sodium salt of N-(hydroxyethyl)-N, N', N' - tris (phosphonomethyl) ethylene diamine, Example 2 was the sodium salt of N-(2-hydroxypropyl) -N, N', N'-tris (phosphonomethyl) ethylenediamine and Example 3 was the sodium salt of 2-hydroxy-N, N-bis (phosphonomethyl) ethyl amine, each product was about 50% cyclised.

1. WATER COMPOSITIONS TABLE 1 1.1. Produced Water No 1 Ion Concentration (mega) Sodium 17,090 Potassium 767 Calcium 930 Magnesium 440 Barium 2 Strontium 16 Chloride 29,010 Sulphate 1,200 Bicarbonate 1,760 TABLE 2 1.1. Produced Water No 2 Ion Concentration (mg/l) Sodium 30,460 Potassium 675 Calcium 305 Magnesium 35 Barium 2,260 Strontium 95 Chloride 47,110 Sulphate 32 Bicarbonate 3,280 pH=7.5 to 7.7 TABLE 3 1.1. Formation Water Ion Concentration (mg/l) Sodium 25,990 Potassium 1,050 Calcium 320 Magnesium 43 Barium 765 Strontium 33 Total Iron 3.8 Chloride 40,440 Bicarbonate 2,920 Sulphate 13 2. COMPATIBILITY TESTS 2.1 Test Method The tests were carried out in a 250 ml glass beaker suspended in 1 litre, glass water bath.Stirring of the beaker contents was achieved by means of a magnetic follower. All tests were carried out at 820C and the inhibitor and brine solutions were preheated to this temperature prior to each test.

20gm of test inhibitor was introduced into the beaker and stirred. Test brine was then added, in stages, to achieve inhibitor concentrations from 90% to 10% at 10% intervals. For each inhibitor concentration, stirring was continued for 5 minutes and the mixture was then checked for haze by side illumination using a narrow beam torch.

The test was repeated using 2gm of inhibitor (to achieve 1% to 0.5% inhibitor concentration, at 0.1% intervals). The range 0 to 1% concentration was further studied by adding 200 ml of brine to the beaker and adding inhibitor to the brine to achieve inhibitor concentrations of 0.1% to 1.0% at 0.1% intervals.

Observations of haze (indicating precipitation) or clear solutions were recorded for each experiment.

Additional tests were conducted, for each inhibitor, where 200 to 500 ppm solutions were made and stored for 16 hours. At the end of this period, each mixture was checked for haze.

The results of these tests are summarised in Table 4.

3. FUNCTIONAL TESTS 3.1 Bottle Tests Equal volumes of test water and a standard synthetic sea water were mixed at 85"C with various concentrations of inhibitor. The mixtures were maintained at 85"C in an oven for 16 hours and then filtered. Any evidence of scale was counted as a failure.

3.2 Dynamic Tube Blocking Tests Two equal flows of test water and synthetic sea water respectively, were mixed together with inhibitor at a temperature of 85"C and circulated for 20 minutes or until a change of pressure indicated build up of scale. The latter counted as a failure.

TABLE 4 - COMPATIBILITY TESTS

Inhibitor FORMATION WATER PRODUCED WATER No 1 PRODUCED WATER No 2 Brine Inhibito 16 Hours Brine Inhibito 16 Hours Brine Inhibito 15 Hours r r r 100 - 10 - 1 - 0.1 - 200 500 100- 10 - 1 - 0.1 - 200 500 100- 10- 1- 0.1- 200 500 10% 1% 0.1% 1% ppm ppm 10% 1% 0.5% 1% ppm ppm 10% 1% 0.5% 1% ppm ppm Example 1 Y Y Y Y Y Y Y Y Y Y Y N Y Y Y Y Y N Example 2 Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Example 3 Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Polyacrylat Y Y Y Y Y Y Y Y N Y N N Y N N N N N e Polymaleate N N N N N N N N N N N N N N N N N N PVSA Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y DETPMP N N N N N N N N N N N N N N N N N N PVSA = Polyvinylsulphonate DETPMP = Diethylenetriamine pentamethylenephosphonate Y = Yes it is compatible N = No it is not compatible

TABLE 5 - BOTTLE TEST RESULTS INHIBITOR EFFECTIVE CONCENTRATION (PPM) Example 1 500 Example 2 450 Example 3 200 Polyacrylate > 1000 Polymaleate > 1000 PVSA > 1000 DETPMP > 1000 Conditions Temperature: 85"C Mixing Ratio: 50/50 TABLE 6 - DYNAMIC TUBE BLOCKING TESTS

INHIBITOR EFFECTIVE CONCENTRATION (PPM) Examplel 300 Example 2 350 Example 3 150 Polyacrylate > 1000 Polymaleate > 1000 PVSA > 1000 DETPMP > 1000 Conditions: Temperature: 85"C Mixing Ratio: 50/50 From the foregoing it is apparent that the alkanolamine phosphonates are compatible with the mixture and highly effective. Of the commonly used commercial scale inhibitors only polyvinyl sulphonate is compatible with the three brines at concentrations within the normal range of concentration of threshold agents used as oilfield scale inhibitors. None of the comparative products are effective.

Claims (5)

1. A method of treating water which has a tendency to deposit barium scale in order to inhibit scale formation which comprises adding a threshold amount of a scale inhibiting agent comprising a methylene phosphonate derivative of an alkanolamine.

2. A method according to claim 1 wherein said scale inhibiting agent contains at least one compound of the formulae:

wherein the R groups, which may be the same or different are each selected from hydrogen and methyl, ethyl, hydroxymethyl and any of the R' groups hereinafter defined and R' is hydroxyethyl or a [(CR2)nNCR2PO3H2]m CR2PO3H2 where n is 2 or 3 and m is from 0 to 10; and/or at least one salt of any such compound.

3. A method of preventing scale formation in an oil well when an injection water comprising sea water is injected into an aquifer to form a produced water containing at least 200 mg. of dissolved alkaline earth metal per litre which method comprises injecting into said aquifer at the lower end of said well a scale inhibiting amount of a scale inhibiting agent comprising at least one compound according to formula I and/or II as specified in claim 2.

4. A method of inhibiting scale formation from a scale forming water system containing more than 200 mg dissolved alkaline earth metal per litre and having a pH between 5 and

8.5, which comprises adding thereto from 5 to 1000ppm of a scale inhibiting agent comprising at least one compound of the formula I and/or II as specified in claim 2.

5. A method according to any foregoing claim wherein said scale inhibiting agent comprises: N-(hydroxyethyl)-N,N",N"-tris (phosphonomethyl) ethylenediamine; N-(2hydroxypropyl)-N,Nl,Nl-tris (phosphonomethyl) ethylenediamine; and/or 2-hydroxy-N,N-bis (phosphonomethyl) ethylamine or their alkali metal salts.