Thermal stability of amine methyl phosphonate scale inhibitors

Journal of Petroleum Science and Engineering 43 (2004) 259 – 270 www.elsevier.com/locate/petrol

Thermal stability of amine methyl phosphonate scale inhibitors S.J. Dyer, C.E. Anderson, G.M. Graham* Flow Assurance Scale Team, Department of Petroleum Engineering, Heriot Watt University, Edinburgh EH14 4AS, UK Received 15 August 2002; accepted 13 February 2004

Abstract The performances of five different amine methyl phosphonate-based scale inhibitors have been tested against sulphate and carbonate scale after thermal ageing at 160 jC, to determine whether they can be applied in HP/HT reservoirs. The successful application of phosphonate scale inhibitors in these reservoirs would result in significant cost savings when compared with some of the polymeric species that are currently used. These tests have indicated that after thermal ageing at an initial pH of pH 5 and pH 2 all five inhibitors were still able to prevent carbonate scale in dynamic tests. However, the performances of some of the phosphonate species against sulphate scale were reduced by thermal ageing. The performances of HMDP, HMTPTP and NTP against sulphate scale were also reduced after thermal ageing at 190 jC and pH 5. Ion exchange chromatography has shown that the reduction in inhibitor performance against sulphate scale corresponded to the structural breakdown of the phosphonate molecules. Potential breakdown products of the phosphonate species have been determined theoretically. The performances of commercial samples of these potential breakdown products have been tested against both carbonate and sulphate scale to determine experimentally whether they could be the products formed during thermal ageing. It is suggested that the thermal stabilities of the phosphonate species are determined by their molecular structure, with the species containing hexyl linkages between the amine groups having less steric strain and hence greater thermal stability than the species which only have shorter ethyl linkages between amine groups. D 2004 Published by Elsevier B.V. Keywords: Phosphonate; Thermal stability; Scale; Structure

1. Introduction It is important to determine whether scale inhibitor species currently applied in conventional reservoirs will have any application in the more severe HP/HT conditions. In particular, thermal stability and brine * Corresponding author. Fax: +44-1506-410445. E-mail address: [email protected] (G.M. Graham). 0920-4105/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.petrol.2004.02.018

compatibility issues are likely to be controlling factors in the selection and application of inhibitor chemicals in these harsh conditions (Collins, 1995; Gru¨ner, 1996; Jordan et al., 1996; Graham et al., 1997a,b, 1998; Jasinski, 1997; Brockmann et al., 1998; Dyer et al., 1999). Previous work has shown that phosphonate-based inhibitor species may not be applicable due to poor thermal stability (Jonasson et al., 1996; Jordan et al., 1996; Graham et al., 1997a,b), although some studies have indicated less dramatic

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results for certain phosphonates (Gru¨ ner, 1996; Brockmann et al., 1998). Most investigations into thermal stability have been conducted using polyphosphino carboxylic acid (PPCA), polyvinyl sulphonate (PVS) and VS-Co (sulphonated co-polymer) scale inhibitors, as initial tests suggested that both diethylenetriamine penta(methylenephosphonic acid) (DETPMP) and 1,3-dipropyl 2ethyltetraamine hexa(methylenephosphonic acid) (DETHMP) species had poor thermal stability (Graham et al., 1997b). However, some work investigated the thermal stability at 160 jC of a range of phosphonate-based species including HMDP (hexamethylenediamine tetramethylene phosphonic acid), NTP (nitrilotris methylene phosphonic acid) and DETPMP and compared them with VS-Co, PVS and PPCA species (Anderson, 2002). From these tests it was observed that after thermal ageing at 160 jC and the supplied pH, all of the inhibitor species tested were effective against carbonate scale in dynamic tests, but NTP and DETPMP were no longer effective against barium sulphate scale in static tests. However, the third phosphonate species (HMDP) was still effective against sulphate scale, which was unexpected. The relatively low MIC’s of HMDP coupled with its thermal stability at temperatures up to at least 160 jC, suggest that this inhibitor has potential for deployment in HP/HT reservoirs. The thermal stability of HMDP is postulated to be due to either (i) no chemical degradation of HMDP under these conditions or (ii) the breakdown products of HMDP being able to ‘‘fit’’ into the barium sulphate lattice. The ability to lattice match is understood to be a criterion for scale inhibition (Black et al., 1991; Benton et al., 1993; Graham, 1994). This paper describes the further examination of the thermal stability of phosphonate inhibitor species, particularly HMDP, to determine their potential application in HP/HT reservoirs. Five phosphonate inhibitor species have been tested: DETPMP (diethylene – triamine penta(methylene phosphonic acid)), HMDP (hexamethylenediamine tetra-methylene phosphonic acid), NTP (nitrilotris (methylene) tri phosphonic acid), HMTPMP (bis (hexamethylene) triaminepentakis (methylene phosphonic acid)) and PEHOMP (pentaethylene hexamineoctakis (methylene phosphonic acid)). Each inhibitor species was thermally aged at 160 jC at initial pH values of both pH 2 and pH 5 and

then tested for performance against carbonate and sulphate scale.

2. Experimental procedure Thermal ageing was carried out at 160 jC on five phosphonate inhibitor species, DETPMP, HMDP, NTP, HMTPMP and PEHOMP. The chemical structures of the inhibitor species used in this thermal stability study can be found in Fig. 1. These phosphonate species were selected for various reasons. DETPMP was selected as it is regularly used in the industry as a scale inhibitor and initial tests with NTP and HMDP showed promising results for their thermal stability. HMTPMP was selected because, like HMDP, it has hexyl linkages between the amine groups and likewise PEHOMP was selected as it has ethyl groups linking its amine groups, similar to DETPMP. 2.1. Thermal ageing Thermal ageing of the inhibitor species was performed in Teflon lined stainless steel hydrothermal bombs as follows. The inhibitor species were dissolved in sulphate free sea water (SFSW) at a concentration of 5% active. The Teflon cups were weighed, 30 ml of each test solution was placed in the cups and the cups were re-weighed. The Teflon cups were then placed in the hydrothermal bombs and sealed tightly. The bombs were placed in an oven for 5 days at 160 jC. Two sets of inhibitor samples were thermally aged, one with the pH adjusted to pH 5 and the other to pH 2 prior to ageing. These test pH’s were selected as pH 5 is a typical downhole pH and pH 2 is an extreme downhole pH which could be experienced. The pH was adjusted using NaOH(aq) or HCl(aq) before adding the 5% inhibitor solution to the Teflon cups. The initial solution pH of HMDP was pH 5.97, DETPMP was pH 0.51 and NTP was 0.78. Following the thermal ageing process, the ‘‘bombs’’ were allowed to cool to room temperature, made up to weight with distilled water and the performances of the thermally aged solutions were compared to their respective stock solutions in dynamic carbonate inhibition efficiency and static and dynamic barium sulphate inhibition efficiency tests.

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Fig. 1. Chemical structures of the phosphonate scale inhibitor species used in this study.

2.2. Dynamic carbonate inhibition efficiency tests 2.2.1. Brine preparation The dynamic tests were undertaken using a North Sea Formation Water (North Sea FW 1) containing 560 ppm of bicarbonate. Two brines were prepared, one containing the scaling cations (Brine 1) and the other to contain the bicarbonate ions (Brine 2). The sodium ions were split between the two brines. The brines were filtered through a 0.45 Am filter and then the bicarbonate was added to Brine 2. The pH of Brine 1 was adjusted to pH 6.5, whilst the pH of Brine 2 was not adjusted, but it was measured to be pH 7.5. The bicarbonate was added to Brine 2 daily, so as to prevent changes in the brine composition with time. Table 1 gives the brine compositions used

in these tests. The scale inhibitor was added to Brine 2. The brines were pumped into the dynamic tube blocking rig at 5 ml/min/brine so as to give a mixed brine composition equivalent to the North Sea FW 1. 2.2.2. Conditions The dynamic carbonate inhibition efficiency tests were conducted at 95 jC and 1500 psi (102 bar). 2.2.3. Tests A test (blank) scale, with no scale inhibitor present, was run at the beginning of each day to ensure that scaling occurred in a repeatable manner. In the blank run the differential pressure increased to >1 psi in <25 min. The minimum inhibitor concentration (MIC) was

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Table 1 Composition of North Sea formation water and sea water used in this study Composition (ppm) Ion

Sodium Calcium Magnesium Potassium Barium Strontium Bicarbonate Sulphate

Dynamic carbonate inhibition efficiency tests

Static barium sulphate inhibition efficiency tests

Dynamic sulphate inhibition efficiency tests

North Sea FW 1

Brine 1

Brine 2

Forties FW

Sea Water

North Sea FW 2

Sea Water

25,210 2600 585 345 0 135 560 0

25,210 5200 690 1170 0 270 0 0

25,210 0 0 0 0 0 1120 0

29,370 2809 504 372 252 574 0 0

10,890 428 1368 460 0 0 0 2960

31,275 2000 739 654 269 771 0 0

10,877 428 1368 460 0 0 0 2960

Brines 1 and 2 were mixed 50:50 to give the composition of the North Sea FW 1 in dynamic tube blocking tests.

then determined for each scale inhibitor species by decreasing the inhibitor concentration in the brine mix every 30 min until scaling occurred (>1 psi in <30 min). The MIC is defined as falling between the lowest inhibitor concentration that prevented scale and the highest concentration that failed to prevent scale. The MIC was initially determined for each stock (non-thermally aged) inhibitor species and then the MIC determination was repeated for the thermally aged samples. The determination of both the stock and the thermally aged MIC was repeated for each inhibitor. 2.3. Dynamic barium sulphate inhibition efficiency tests The test procedure for these tests was similar to the dynamic carbonate tests, with the exceptions detailed below. 2.3.1. Brine preparation The dynamic tests were undertaken using a 50:50 mix of Forties type FW/SW. Table 1 gives the brine compositions used in these tests. The brines were filtered through a 0.45 Am filter and degassed under vacuum, then the pH was adjusted to pH 5.5 for both brines. The scale inhibitor was added to the brine containing the scaling anions. 2.3.2. Tests A blank scale, with no scale inhibitor present, was run at the beginning of each day to ensure that

scaling occurred in a repeatable manner (differential pressure increase >1 psi in <30 min). The tests on the stock and thermally aged samples were carried out as for the dynamic carbonate inhibition efficiency tests. 2.4. Static barium sulphate inhibition efficiency tests Static barium sulphate inhibition efficiency tests were conducted using all the inhibitor species that had been thermally aged at both pH 5 and pH 2. The brine system used in the tests was a 50:50 mix of North Sea Formation Water 2 (FW)/Sea Water (SW). This represented a moderate sulphate scaling brine. The composition of the brines used in these tests can be found in Table 1. The required concentrations of scale inhibitor were dissolved in the sea water. The sea water/inhibitor mix and formation water were filtered (0.45 Am) and split into 100 ml aliquots. 2 ml of buffer (the pH was controlled to pH 5.4) was added to each 100 ml of SW/inhibitor mix and all the bottles were heated in a water bath to 95 jC. Once at temperature 100 ml of formation water was mixed with the 100 ml SW/inhibitor solution. The tests were sampled after 2 h by removing 1 ml of the particular test supernatant from the test bottle and adding it to 9 ml of stabilising solution (PVS/KCl solution pH>8). The samples were then analysed by ICP for barium concentration. The barium sulphate inhibition efficiencies were calculated by comparing the initial barium concentration (t=0 h) with the final barium concentration (t=2 h).

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2.5. Ion exchange chromatography The stock and thermally aged phosphonate samples were diluted to between 0.5 and 10 ppm active with distilled water. The ion chromatography system was set up with 5 ml vials of sample placed in the autosampler. Analysis of the samples was carried out in triplicate. 2.5.1. Instrument set-up The Dionex ion chromatography was set up as follows: Eluent 1¼Pure water; Eluent 2 ¼ HNO3 ; 200 mM; Flow rate ¼ 1 ml=min

Gradient method Time

%A

%B

Initial 0 6 21

99 99 99 20

1 1 1 80

Inject Relay 1 Inject Inject

Reagent 1: 4% Ammonium persulphate; Flow rate: 0.5 ml/min (total=1.5 ml/min). Reagent 2: Solution containing conc. sulphuric acid, sodium dodecyl sulphate, sodium molybdate and ascorbic acid in 1 l pure water; Flow rate: 0.5 ml/min (total=2 ml/min). UV Detector 800 nm Column: IonPac AS11-HS s/n 00112, IonPac AG11HC s/n 00107. Loop Size: 5 ml concentrated onto AG11-HC. Heating Coil: Reagent 1 added and heated to 100 jC.

3. Results and discussion 3.1. Dynamic carbonate inhibition efficiency tests Table 2 presents the MIC’s for the stock and thermally aged samples of the five phosphonate species when the initial pH was adjusted to both pH 5 and pH 2. Fig. 2 gives an example of a differential pressure versus time plot for the stock and thermally

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Table 2 Minimum inhibitor concentration (ppm) determined in carbonate and sulphate dynamic inhibition efficiency tests on stock and thermally aged samples Scale inhibitor

Stock pH 2

Thermally aged pH 2

Stock pH 5

Thermally aged pH 5

Carbonate tests NTP HMDP DETPMP PEHOMP HMTPMP

0 – 0.5 0 – 0.5 0 – 0.5 0.5 – 1 0 – 0.5

0 – 0.5 0 – 0.5 0 – 0.5 0.5 – 1 0 – 0.5

0 – 0.5 0 – 0.5 0 – 0.5 0.5 – 1 0 – 0.5

0 – 0.5 0 – 0.5 0 – 0.5 0.5 – 1 0 – 0.5

Sulphate tests NTP HMDP DETPMP PEHOMP HMTPMP

40 – 50 30 – 40 10 – 20 10 – 20 5 – 10

>100 >100 >100 >100 10 – 20

10 – 15 5 – 10 5 – 10 5 – 10 10 – 20

15 – 20 5 – 10 40 – 50 50 – 60 10 – 20

aged solutions of HMDP at pH 2. From Table 2 it can be seen that the MIC of the samples thermally aged at both pH 5 and pH 2 were the same as those of the stock samples for all inhibitor species. The MIC’s of the inhibitor species were as follows: HMDP, NTP, DETPMP and HMTPMP 0 – 0.5 ppm active and PEHOMP 0.5 –1 ppm active. 3.2. Static barium sulphate inhibition efficiency tests Figs. 3 and 4 present the barium sulphate inhibition efficiency results for the stock and thermally aged inhibitor solutions initially at pH 5 and pH 2, respectively. From Fig. 3 it can be seen that the inhibition performance of NTP was significantly reduced by thermal ageing, as the inhibition efficiency decreased from f62% for the stock solution to f27% for the thermally aged solution after 2 h residence time, at an inhibitor concentration of 35 ppm. In contrast, Fig. 3 also indicates that when tested at a concentration of 25 ppm, no reduction in performance was observed for HMDP after 2 h residence time. As with HMDP the performance of HMTPMP was not affected by thermal ageing at an initial pH of 5, but the performances of DETPMP and PEHOMP were significantly reduced after thermal ageing under these conditions. Fig. 4 indicates that the inhibition efficiency of stock NTP decreased from f45% after 2 h residence time at an inhibitor concentration of 35 ppm, to <10%

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Fig. 2. Differential pressure versus time plot for the stock and thermally aged (160 jC) solutions of HMDP at pH 2.

after thermal ageing at pH 2. This figure also indicates that the inhibition efficiency of the other four phosphonate species decreased after thermal ageing. It can be seen that the inhibition efficiencies of stock HMDP, DETPMP, PEHOMP and HMTPMP were between 70% and 95% after 2 h residence time, at an inhibitor concentration of 25 ppm. However, after thermal ageing at an initial pH of 2 the inhibition

efficiency of all of these phosphonates decreased to <10%. 3.3. Dynamic barium sulphate inhibition efficiency tests The dynamic carbonate and static barium sulphate inhibition efficiency tests detailed above gave differ-

Fig. 3. Barium sulphate inhibition efficiency of stock and thermally aged (160 jC, initial pH 5) phosphonate inhibitor species. Efficiency tests were conducted at 95 jC, 50:50 North Sea FW/SW, pH=5.4, residence time 2 h, [inhibitor]=25 ppm, [NTP]=35 ppm.

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Fig. 4. Barium sulphate inhibition efficiency of stock and thermally aged (160 jC, initial pH 2) phosphonate inhibitor species. Efficiency tests were conducted at 95 jC, 50:50 North Sea FW/SW, pH=5.4, residence time 2 h, [inhibitor]=25 ppm, [NTP]=35 ppm.

ent performance results after thermal ageing for the different inhibitor species. It was not known whether these different results were as a result of the different test techniques used (the residence time in the dynamic tests was of the order of a few seconds as opposed to 2 h in the static tests) or whether it was due to the different types of scale being inhibited. Dynamic sulphate inhibition efficiency tests were therefore carried out to determine whether the differing results were due to test technique or scale type. The MIC’s determined for each of the five stock and thermally aged scale inhibitors can be found in Table 2. The MIC’s were measured after thermal ageing at initial pH values of both pH 2 and pH 5. From Table 2 it can be seen that the stock and thermally aged samples of HMDP initially at pH 5 both had MIC’s of 5 – 10 ppm and the MIC of HMTPMP remained unchanged after thermal ageing at 10 –20 ppm. However, thermal ageing increased the MIC of both DETPMP and PEHOMP from 5 – 10 ppm for the stock solution to 40– 50 and 50– 60 ppm, respectively. The MIC of NTP was also marginally increased after thermal ageing from 10– 15 ppm for the stock to 15– 20 ppm. The results from the tests conducted on the samples initially at pH 2 indicated that after thermal ageing the performances of all five scale inhibitors had declined.

The results from the dynamic sulphate inhibition efficiency tests were similar to those obtained for the static sulphate inhibition efficiency tests in that the performances of HMDP and HMTPMP were unchanged after thermal ageing at pH 5, whilst the performances of NTP, DETPMP and PEHOMP all declined after thermal ageing at pH 5. These sulphate test results varied from the carbonate inhibition efficiency tests, which indicated no change in performance after thermal ageing for any of the five phosphonate species. The sulphate test results indicate that degradation of all the scale inhibitor species was occurring when the initial pH was adjusted to pH 2, but degradation of only NTP, DETPMP and PEHOMP was occurring when the initial pH was 5. However, the carbonate test results indicate that even though degraded after thermal ageing the products from the degradation of the phosphonate species were still able to inhibit the more easily inhibited carbonate scale, despite failing to prevent the sulphate scale. 3.4. Ion exchange chromatography Ion exchange chromatography was carried out on the stock and the thermally aged phosphonate scale inhibitor species to observe any changes in structure with thermal ageing.

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Fig. 5. Ion exchange chromatography of stock (NHA) and thermally aged (HA) HMDP. pH was initially pH 5 and thermal ageing was at 160 jC.

Figs. 5 and 6 present the chromatograms for stock and thermally aged HMDP at initial pH values of 5 and 2, respectively. From Fig. 5 it can be seen that the stock and thermally aged samples gave similar results and both had large peaks at about 9.5 min retention time. However, the samples at pH 2 (Fig. 6) indicated that after thermal ageing this large peak had disappeared and the peak size of the lower retention time peaks had increased. This indicated that thermal ageing at pH 2 had destroyed the higher molecular weight species and created smaller molecular weight species, whereas when the initial pH was 5 there was no reduction in average molecular size. These results indicate that the higher retention time molecule (the

HMDP molecule itself) inhibited sulphate scale and its degradation after thermal ageing at pH 2 meant that the sample was no longer able to inhibit sulphate scale. The results also indicate that the lower retention time molecules were able to inhibit carbonate scale. The ion exchange chromatography results for HMTPMP were similar in that the molecule was observed to have been degraded after thermal ageing at pH 2, but not at pH 5. Figs. 7 and 8 present the chromatograms for stock and thermally aged DETPMP at initial pH values of 5 and 2, respectively. From these figures it can be seen that after thermal ageing at both pH values the large peak at about 11.5 min retention time was signifi-

Fig. 6. Ion exchange chromatography of stock (NHA) and thermally aged (HA) HMDP. pH was initially pH 2 and thermal ageing was at 160 jC.

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Fig. 7. Ion exchange chromatography of stock (NHA) and thermally aged (HA) DETPMP. pH was initially pH 5 and thermal ageing was at 160 jC.

cantly reduced or disappeared and the lower retention time peaks increased in size. This again is in agreement with the sulphate inhibition efficiency test results in that it indicates that the higher molecular weight species (the DETPMP molecule itself) inhibited the sulphate scale, but the degradation of that molecule meant that the scale could not be inhibited. However, as with HMDP the lower molecular weight degradation products were able to inhibit carbonate scale. The ion exchange chromatography results for NTP and PEHOMP were found to be similar to those of DETPMP and indicated that thermal ageing at both pH 2 and 5 destroyed the NTP and PEHOMP molecules.

3.5. Potential degradation products The work carried out during this project was not intended to determine the exact chemical structure of the thermal ageing degradation products, only whether smaller products still capable of inhibiting scale formation were formed after exposure to the potential high downhole temperatures. However, as the degradation products of the thermally aged phosphonate scale inhibitor species inhibited carbonate scale an attempt was made to try and determine the structure of the potential degradation products. Ortho phosphoric acid, methyl phosphonic acid and amino methyl phosphonic acid were all theoretically considered to be potential

Fig. 8. Ion exchange chromatography of stock (NHA) and thermally aged (HA) DETPMP. pH was initially pH 2 and thermal ageing was at 160 jC.

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degradation products. This is because methyl phosphonic acid and amino methyl phosphonic acid have been identified as degradation products of NTP, as under hydrothermal conditions (125 –300 jC) direct and metal ion catalyzed decarboxylation and CN bond cleavage have been observed (Nowack and Stone, 2000). Furthermore, the CP bond has been shown to be susceptible to hydrolysis (Jonasson et al., 1996). The structures of the potential degradation products are given in Fig. 9. These three species were tested in dynamic tests against both carbonate and sulphate scale, in the same way that the phosphonate scale inhibitor species had been tested. The results from these tests indicated that ortho phosphoric acid had an MIC of <0.5 ppm active against carbonate scale, but the MIC’s of methyl and amino methyl phosphonic acid were much higher at 30 – 40 and 20 – 30 ppm active, respectively. Against sulphate scale the three potential degradation products all had MIC’s >100 ppm active. These results suggest that ortho phosphoric acid may be responsible for inhibiting the carbonate scale in the thermally aged phosphonate species. However, it is not able to inhibit the sulphate scale and therefore another product, which does inhibit sulphate scale, must be present in the thermally aged samples. Ion exchange chromatography of the three potential degradation products indicates that they all have similar peaks to those found in the degraded phosphonate species. However, the peaks were of similar retention times and therefore it was not possible to resolve which peaks in a thermally degraded sample could be due to a given theoretical degradation product. Owing to the close proximity of the peaks there was insufficient separation of the different fractions to analyse them separately to determine the composition of each fraction. Furthermore, the salinity and acidity of the samples were considered to be too high and the degradation products were too similar, for analyses by MS and 31P NMR.

3.6. Thermal ageing at 190 jC The performances of HMDP and HMTPMP were tested further in dynamic carbonate and static and dynamic sulphate tests after thermal ageing at 190 jC and pH 5. This was to determine whether they had potential for application in higher temperature reservoirs. The results from the tests indicated that the performances of these two inhibitor species were unchanged against carbonate scale after thermal ageing at 190 jC. However, in the static sulphate inhibition efficiency tests the performance of HMTPMP decreased significantly after thermal ageing, whilst the performance of HMDP declined partially. In the dynamic sulphate inhibition efficiency tests the performance of HMTPMP declined, but the MIC of HMDP remained the same. Ion exchange chromatography tests showed that the main peak for HMTPMP was significantly reduced after thermal ageing at 190 jC (pH 5) and the main peak for HMDP was partially reduced. 3.7. Effect of structure on thermal stability The chemical structures of HMDP and HMTPMP both contain –(CH2)6 – linkages between the amine groupings (see Fig. 1) and these linkages are expected to impart improved stability, whereas DETPMP and PEHOMP have – (CH 2 ) 2 – linkages which are expected to increase the steric ‘‘strain’’, due to repulsion between the phosphonate groups and thereby reduce their thermal stability. NTP has only one amine group and therefore will have the greatest repulsion between the phosphonate groups. The results from these tests indicate that the thermal stability of HMTPMP was similar to that of HMDP and both of these species were more thermally stable than DETPMP, PEHOMP and NTP, as expected from their structures.

Fig. 9. Molecular structures of the potential degradation products of the phosphonate scale inhibitor species used in this study.

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4. Conclusions

References

The five phosphonate species tested were able to inhibit carbonate scale after thermal ageing at 160 jC (initial pH=5 and 2). HMDP, NTP and HMTPMP also inhibited carbonate scale after thermal ageing at 190 jC (pH 5). After thermal ageing at 160 jC (pH 5), HMDP and HMTPMP inhibited sulphate scale, but the performances of NTP, DETPMP and PEHOMP were reduced. However, after thermal ageing at 190 jC (initial pH 5) the performance of HMTPMP was significantly reduced against sulphate scale, but the performance of HMDP was only partially decreased. Reductions in performance against sulphate scale corresponded to the degradation of the inhibitor species, as observed by ion exchange chromatography. The hexyl chains in HMDP and HMTPMP may increase their stabilities, as they increase the distance between the phosphonate groups thereby reducing the steric strain. DETPMP and PEHOMP only have ethyl chains, which reduce the distances between the phosphonate groups and may increase the strain within the molecules. Some phosphonate species, such as HMDP and HMTPMP, may have potential for application as scale inhibitors in HP/HT reservoirs as they are thermally stable at temperatures in excess of 160 jC. The use of phosphonate scale inhibitor species in HP/HT reservoirs could have significant cost implications for scale treatment. However, it should be noted that although the thermal degradation products of the phosphonate species may inhibit carbonate scale, they could also potentially lead to the formation of calcium phosphate precipitate.

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Acknowledgements The authors would like to thank the following companies for funding the work of the Heriot-Watt University Flow Assurance Scale Team: Amerada Hess, Baker Petrolite, BioLab, BP, Chevron, Dyno, Enterprise, ENITechnologie, ExxonMobil, Marathon, Nalco/Exxon, Norsk Hydro, Petrobras, Philips, Saudi Aramco, Shell, Texaco, TotalFinaElf and TR Oil Services.

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Nowack, B., Stone, A.T., 2000. Degradation of nitrilotris(methylenephosphonic acid) and related (amino)phosphonate chelating agents in the presence of manganese and molecular oxygen. Environ. Sci. Technol. 34, 4759 – 4765.