NACE-2014-3766

Scale Prediction for Iron, Zinc and Lead Sulphides and Its Relation to Scale Test Design

Cyril Okocha

Heriot-Watt University Institute of Petroleum Edinburgh EH14 4AS

Scotland U.K

Ken Sorbie Heriot-Watt University Institute of Petroleum Edinburgh EH14 4AS

Scotland U.K

ABSTRACT The accurate prediction and management of oilfield sulphide scales, such as iron sulphide, is an important issue in oil production. This has become particularly significant as high temperature high pressure (HTHP) fields are being brought into production, and the life of mature fields is being extended. In such systems, additional scales such zinc and lead sulphide (ZnS and PbS) are often being reported. This paper presents a detailed description of a sulphide modelling approach, leading to the prediction of saturation ratios (SRs) and masses of the formed sulphide scales, final solution compositions, final pH levels etc. The equilibrium equations for the sulphide system are presented and solved in a manner in which they are compared directly with the experimentally measured quantities. The actual Saturation Ratios (SRs) (e.g. SR = [Fe2+][S2-]/Ksp,FeS) are calculated for the various experiments and the prediction model is used directly to the design the details of the sulphide scaling experiment in the blank solutions. Some calculated sulphide examples are presented and some key predictions of the sulphide scaling model are tested experimentally for FeS, ZnS and PbS systems. The quantitative agreement between the predictions of the model and the experiments are very good. The resulting sulphide test methodology is thus well underpinned theoretically and it is then applied to evaluate some examples of proposed commercial sulphide inhibitors/dispersants. Key words: Sulphide scale prediction, Sulphide Scales, HTHP, FeS, PbS, ZnS, Inhibitor testing, sulfide scale.

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Paper No.

3766

2014 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 1440 South Creek Drive, Houston, Texas 77084.The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

INTRODUCTION Oil production decline due to mineral scale formation has been a persistent problem since the early days of the oil industry until the present day1, 2, 3. The reservoir fluids (water, usually a brine), gas and oil are usually in equilibrium at the time the reservoir is discovered. However, when a new well is drilled, completed and begins to flow the natural equilibrium is disturbed and this can lead to solids depositions including scale buildup along the production system. Typically, oilfield scales are formed from different processes including, (a) direct precipitation from the water that occurs naturally (e.g. CaCO3), and (b) by the mixing of two incompatible waters to form scale precipitates (e.g. BaSO4)

4. The ability to correctly predict the occurrence and severity of scales in producing oilfields is of great importance to the oil industry. Sulphide scale deposits have been reported where sour (H2S containing) reservoirs are being produced and/or in wells in deeper, higher temperature reservoirs5. Evaluation and effective management of mineral scale using chemical inhibitors and dispersants has been carried out for many years to assist flow assurance in these systems6. Iron sulphide scale has been the most commonly observed of the oilfield sulphide scales to date and this has posed a number of problems in field and storage facilities5. Commercial prediction codes have been developed which predict various oilfield scales, although not all of these codes can predict sulphides scales. However, these codes are not open and the precise formulation of the sulphide equations cannot be viewed by the user. In addition, it may not be possible or easy to simulate the exact process in which sulphide scaling experiments are performed. In this paper, we present a description of how simple experiments can be carried out to test inhibitors which will help to prevent or disperse sulphide scales. This procedure requires that FeS be produced (or ZnS or PbS) in blank (uninhibited) tests in a systematic manner at given saturation ratios (e.g. SR = [Fe2+][S2-] /Ksp,FeS). For iron sulphide formation tests in the laboratory, for example, two component solutions (A and B) are mixed in order to form sulphide scale as follows: Solution A containing the Fe2+ ions at a given concentration and at a given pH; and Solution B containing a certain concentration of Na2S as a source of sulphide ion (actually [H2S], [HS

-] and [S2-]). Solution B is quite alkaline since H2S is a very weak acid but NaOH is a strong alkali. On mixing of Solutions A and B, a precipitate or colloidal dispersion of FeS forms and the resulting solution has a certain final pH which is measured. The mixed solution (A + B) has some initial Saturation Ratio (SR) of FeS (or PbS or ZnS) which in the final equilibrium solution will be SR = 1 and a certain mass of sulphide scale will form. The final solution also has some final composition (of [Fe2+], [H2S], [HS

-] and [S2-]), pH etc. In this paper, the chemical equilibrium equations for FeS (ZnS, PbS) prediction are revisited, which are strongly coupled to the H2S and pH in solution. The equilibrium equations for the sulphide system are derived and solved in a manner in which can be compared directly with the experimentally measured quantities in Solutions A and B described above. The model shows how the actual Saturation Ratios (SRs) are calculated for the various experiments and the prediction model can be used directly to design the details of the sulphide scaling experiment in the blank solutions. Thus, when inhibitors are applied, it has been used in systems which are well characterised in terms of SR. A number of calculated examples are presented and some key predictions of the sulphide scaling model are tested experimentally. It is shown that very good agreement is found between the model predictions and the experiments for sulphide experiments producing FeS, ZnS and PbS. Some results on the inhibition efficiency (IE) of various sulphide scale inhibitors/dispersants are then presented.

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EXPERIMENTAL PROCEDURE Brine Preparation The experiments performed were carried out in an aqueous medium. No actual field brines were used. Instead, all brines were prepared in the laboratory from distilled water and mineral salts supplied by VWR international1. Brines were prepared in 1, 5, or 10 liter batches depending on the amount required for each experiment. The bottle tests were performed in an open system at room temperature and pressure. Hydrogen Sulphide Preparation The hydrogen sulphide used in this paper was introduced as aqueous H2S. This was achieved by dissolving sodium sulphide (Na2S) in distilled water or brine as the test conditions required, no H2S gas or liquid-H2S was used in these experiments. Brine preparation, nitrogen sparging and degassing involving H2S were carried out in the fume cupboard and the nitrogen glove box to limit exposure to the H2S. The brine was sparged before the Na2S was added, the system was sealed until mixing with high pH of the final brine kept the system from losing lots of H2S. Oxygen reduction for FeS formation During all the FeS experiments, the tests were performed as described above, but with an additional oxygen (O2) reduction regime. These near anaerobic tests were performed in a nitrogen glove box. A strict oxygen reduction regime was developed with the sole aim of keeping the amount of oxygen (O2) as low as possible between 0-20ppb. This procedure begins immediately after the salts are dissolved in water as stated above, the regime includes the following:

1. Using Fe stable salts such as Ammonium iron (II) sulphate (NH4)2SO4FeSO4.6H2O commonly known as Mohrs salt.

2. Degassing the brines to achieve 0-20ppb (checked using dissolved O2 test kits) 3. Nitrogen sparging of the brines for more than 1hr, finishing off by placing nitrogen blanket over

the solutions before sealing with the top. All experiments performed in nitrogen glove box had O2 free nitrogen gas flowing into the box creating an anaerobic environment apart from PbS and ZnS only experiments where O2 is not critical. In the course of these studies, O2 levels of between 0-10 ppb were usually achieved before brine mixing, and these levels were maintained throughout the tests because the tests were performed in the nitrogen glove box.

RESULTS The Sulphide Scaling Equations The Sulphide- Metal (Fe, Zn, Pb) System The sulphide scaling equations are bound up with the overall fate of H2S and metal cation (Fe

2+, Zn2+ and/or Pb2+) in an oilfield produced brine. The chemical equations for the sulphide-metal system (using iron as example) are as follows:

1 Trade name

3


2 32 ( ) 1

1

1 2 3

2

. (1)

aq

x xH S H HS K

x

x x x

HS H S

2 4

2

3

3 2 4

2 2

( ) 1 5 4

. (2)

. s sp

x xK

x

x x x

Fe S FeS K x x

5 4 7

2 2 6

(3)

. w

x x x

H O H OH K x x

2 6

(4)

x x

where the following notation for the concentrations of the seven (7) species in the system have been used: x1 =[H2S](aq), x2 = [H+], x3 = [HS-], x4 = [S

2-], x5 = [Fe2+], x6 = [OH

-] and x7 = [FeS](s) Note that there are 7 unknowns in the system at equilibrium but four (4) equilibrium equations (Equations 1 to 4 above), and hence 3 more equations are required. These are, as usual, the 2 mass balances (for S and Fe) and 1 charge balance equation as follows:

1 3 4 7

5 7

( ), (5)

( ),

S

Fe

Total Sulphur M X x x x x

Total Fe M X x x

2 3 4 5 6

(6)

arg ( ), - - 2 2 (7)Total Ch e M C x x x x x

Thus, Equations 1 7 define the sulphide metal system exactly in that there are 7 equations in 7 unknowns. For a set of known (measured or literature) equilibrium constants, and input total amounts of sulphur (XS), iron (XFe) and charge (C), the above 7 equations can be solved numerically to give the equilibrium state of the system i.e. the equilibrium values of the unknowns. These equations have been solved numerically and this sulphide model has been coded in visual basic for applications (VBA)2 within a spreadsheet. The equilibrium constants used in these sulphide scaling calculations have been taken from the literature. There is some uncertainty on the exact values of some of these constants but recommended values (for T = 20C) have been reported7 - 11 and the actual numbers used are summarized in Table 1.

2 Trade name

4


Table 1

Equilibrium constants (T = 20oC) for the sulphide system in Equations. 1 7 (and Equations 8 12)

Base Case Numerical Solution of Sulphide Equations Suppose a given mixture of sulphide (from Na2S) and iron (as Fe

2+) in two solutions are mixed together. This would give a specific mixture composition in which the total sulphur (XS) would be known (in whichever form - [H2S], [HS

-] and [S2-]) along with the total iron (XFe) and the total charge (C). The equations of the full sulphide model can be solved using equations (Equations 1 7 above) to obtain the composition of the final equilibrium solution (i.e.). As an example: suppose the composition of the initial solution was given by defining the initial aqueous, x10 = [H2S](aq) = 0.001M (34.1 ppm) and x50 = [Fe

2+](aq) = 0.03M (1675.5 ppm) and these were present in distilled water ([H+] = [OH-] = 1E-07 M). Solving the equation set for the sulphide model will result in the following numerical solution: x2 = [H+] = 6.14691E-06 (pH = 5.21) and x5 = [Fe

2+](aq) = 0.0299982M (1675.4ppm). By back substitution, we can then obtain x1 =[H2S](aq)= 0.9956E-04M, x3 = [HS-]= 2.64333E-06, x4 = [S2-]= 4.30025E-18, x6 = [OH

-] = 1.62683E-09 and x7 = [FeS](s)= 1.751E-06M (equivalent to ~0.154mg of FeS per litre of solution). At equilibrium, the Saturation Ratio of FeS, SR = [Fe2+][S2-]/Ksp,FeS = 1, but before the precipitation SR = 4.072E+11. This is an exact numerical solution for the 7 equilibrium equations and several issues should be noted for this equilibrium composition. The final pH = 5.21 and is on the acidic side and would be expected since H2S in water is weakly acidic. A tiny amount of the iron is missing ([Fe2+] goes from 1675.5ppm to 1675.4ppm) and the missing Fe turns up as the ~0.15mg of FeS; although a very small amount of FeS forms, it is at a very high Saturation Ratio (SR) of ~4.07E11 this is hugely higher than the SR seen for BaSO4 indicating that the FeS scale is occurring at more severe SR levels.

Eq. Constant

Reaction

Recommended value Mol/L

Range reported

References

1K 2 H S H HS

9.632x10-8 1.333x10-8

9.632x10-8 [12] [9] [7]

2K 2 HS H S

~1.00x10-17 1.148x10-12

1.00x10-19 [13] [14] [10]

1spK 2 2

( ) sFe S FeS

1.29x10-19 1.36xx10-17 1.29x10-19

[15] [16] [17]

2spK 2 2

( ) sZn S ZnS

2.03x10-25 2.03x10-25 3.00x10-23

[18] [19] [20]

3spK 2 2

( ) sPb S PbS

3.80x10-28 4.0 x10-28

[21] [22]

wK 2 H O H HO

1.00x10-14 1.00x10-13

1.00 x10-14 [23] [24] [25]

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Given the very high SR levels, it is difficult to predict what levels of sulphide scale inhibitors (SI) that may be required to disperse this small amount of FeS. This is the function of the inhibitor tests which are described and carried out later part of this paper. However, solving the above equations as they stand does not model how the actual sulphide inhibition experiments i.e. by mixing 2 solutions (A and B), as described above are performed. The procedure used to design and analyze these experiments is given in the following section. The Fe Solution (Solution A) + the Na2S System (Solution B) As noted above, the actual experimental procedure for forming FeS (or other sulphides) in our inhibitor testing is not to set the [H2S] and [Fe

2+] levels in distilled water. Hence, the sulphide prediction described above does not reflect what is carried out in the actual laboratory experiments. Instead, to predict what happens in our experiments, it should be recognized that the process takes two solutions A and B and mixes them together; these solutions are defined as follows: Solution A - containing the Fe2+ ions at a given concentration, x50 = [Fe2+](aq), and at a given pH; and Solution B - containing a certain concentration of Na2S as a source of sulphide ion (actually [H2S], [HS-] and [S2-]). Solution A requires no calculation and it is specified completely by simply inputting x50 = [Fe2+](aq) and pH (and therefore [H+] =10-pH). It is not yet clear which pH to choose for Solution A and this will be explained below. However, for the calculation presented below, suppose we take a Solution A of composition, x50 = [Fe2+](aq) = 10ppm at pH = 6.6 (i.e. initial x2 = [H

+] = 10-6.6 = 2.5119E-07M , x6 = [OH-] = 3.9811E-8M). This solution A composition will be used for illustration below. Note that there is no sulphur (S) in Solution A the only species with non-zero concentrations are the Fe2+, H+ and OH- ions. On the other hand, Solution B is the source of sulphur which is added as the salt Na2S (i.e. as S

2-). However, this sulphide ion (S2-) will re-speciate in water to give an alkali solution, governed by Equations 1, 2 and 4 in the sulphide system above (no Fe equations); that is, the Na2S solution is described solely by the following 5 equations when mass and charge balance equations are added (note that Equations. 1, 2 and 4 above are duplicated here for completeness as Equations. 8, 9 and 10, respectively):

2

2 32 ( ) 1

1

1 2 3

Summary of the Solution B ( ) System

. (8)

aq

Na S

x xH S H HS K

x

x x x

HS

2 2 42

3

3 2 4

2 2 6

. (9)

. w

x xH S K

x

x x x

H O H OH K x x

2 6

1 3 4

(10)

Mass Balances (S and Fe)

( ), S

x x

Total Sulphur M X x x x

2 3 4 6

(11)

Charge Balance

arg ( ), - - 2 (12)Total Ch e M C x x x x

6


Therefore to predict the pH of Solution B, the above system of 5 equations, Equations 8 - 12 (equilibria + mass + charge balance equations) must be solved. As an example solving the Solution B equations, suppose the initial input sulphide ion concentration is [S] = 20ppm (giving [S2-] = 6.2375E-04 M) in neutral brine or distilled water, i.e. x2 = [H

+] = 1.0E-07M. The converged numerical solution of this set of equations is x2 = [H

+] = 1.6029E-11 (pH = 10.795), x1 = [H2S](aq)= 1.0379E-07M, x3 = [HS-]=

6.2365E-04, x4 = [S2-]= 3.8907E-10 and x6 = [OH

-] = 6.2386E-04. Note that the sulphide does indeed greatly re-speciate with nearly all of the S ending up as the [HS-] ion with only tiny amounts of [H2S](aq) and [S

2-] being formed. The process also leads to a fairly alkaline solution being formed (pH = 10.795) since the higher negative charge ultimately appears as OH- ions, as expected since the sulphide ion is a weak base. The blank solution for a sulphide inhibition test is made by adding together Solutions A and B. Here, we take for examples the actual Solution A and B compositions discussed above; Solution A was simply specified as being 10ppm [Fe2+] at pH 6.6, whereas Solution B was specified as being [S] = 20ppm and a speciation calculation (solving Equations. 8 12) had to be carried out as shown above. Before reacting, the initial composition of this A+B mixture is given by the average composition as shown below for the 2 example solutions above. In the example here, a 50:50 mix of Solutions A and B is taken and the average initial composition is as given below. The mixture has effectively 10ppm sulphide + 5ppm iron (half the values in the individual solutions in a 50:50 mix). However, this solution is not at equilibrium, it simply provides us with the total masses of S and Fe and the effective charge of the changing ions (ions that do not change such as Na+ and Cl- are neglected; the total charge in an actual solution is of course zero). These are then used in the full sulphide model (Equations. 1 -7 above) to predict the final equilibrium composition of the fluid which in this case shown below. Several points can be noted from this result: firstly, the pH of the final solution is quite alkaline (pH = 10.35); because there is an excess of S, virtually all of the iron is consumed (forms FeS) the final [Fe2+] is tiny, as is [H2S]. The FeS readily appears because the solubility product of FeS is so low ( =1.29x10-19) the Saturation Ratio (SR) = ~6.29x1010. The solution is greatly oversaturated despite the very low initial concentrations of iron (5ppm) and sulphide (10ppm) in the initial mix.

Figure 1: Preconverged equations solutions before equilibrium

7


Figure 2: Converged equations at equilibrium Experimental Confirmation of Sulphide Prediction Models Predicted vs. Experimental Na2S (Solution B) pH Values In this section, results are presented which test the accuracy of the sulphide prediction model discussed above. Firstly, the model was tested on the simple solution of Na2S (in brine or distilled water) where no FeS forms (Solution B). This is described by the restricted set of equations, Equations 8 12, with values of K1, K2 and Kw as in Table 1, as discussed above. Figure 3, and Figure 4, show the measured pH for a series of Na2S solutions in distilled water compared with the direct predictions from the equilibrium model (Equations 8 12). The same data is presented in these two figures but Figure 5 shows the sulphide concentration axis with a log scale so that the lower concentration pH results are clear. The agreement between predicted and experimental pH values for concentrations above 1ppm is excellent. This is promising since it shows the model is sufficiently quantitatively accurate for experimental test design purposes.

Back Substitution to obtain

all species ..

Converged solution z1, z2

z1 = x2 pH

= [H+]= 4.49739E-11 M 10.35

z2 = x5

[Fe2+]= 2.61041E-09 M 1.46E-04 ppm

Back substitute to obtain ..

x4 ={S2-] 4.94175E-11 M Total S (M)

3.1188E-04

x3=[HS-] 2.22250E-04 M

x1=[H2S] 1.03773E-07 M

x6=[OH-] 2.22351E-04 M

Total Fe (M)

x7 =[FeS] 8.95229E-05 M 8.9526E-05

Mass FeS

per Litre = 7.87032E+00 mg

Sp_FeS = 1.00000E+00 after pptn.

6.21286E+10 before pptn..

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Figure 3: Comparison of the measured (solid line) and predicted (points) pH values for a range of Na2S solutions

([S2-] = 0 250ppm)

Figure 4: Comparison of the measured (solid line) and predicted (points) pH values for a range of Na2S solutions

using a log scale for ([S2-]). Predicted vs. Experimental pH Values for the Full Solution A+B Experiments In this subsection results are presented of how accurately the model can predict the final pH of the Solution A + B experiments.

6

7

8

9

10

11

12

13

0 50 100 150 200 250 300

pH

[S2-] concentration in ppm

Experimental vs Predicted pH for Na2S solutions

6

7

8

9

10

11

12

13

0.1 1 10 100 1000

pH

[S2] concentration in ppm

Experimental vs Predicted pH for Na2S solutions

9


Table 2 and Figure 4 show the model predicted and experimentally measured pH values of the FeS system produced by mixing Solutions A and B for a number of solutions with varying compositions.

Figure 5 shows plots of experimental FeS solution final pH results vs. the model predicted pH results. The predicted pH results are found to match the experimental measurements within experimental error. Note that the pH was measured in duplicate and the reproducibility is very good.

Figure 6 and Figure 7 show the plots of predicted mass of FeS precipitate and the experimental FeS precipitate collected and measured after mixing solution A+B. Again, these results indicate that the model is accurate in predicting sulphide SR, precipitated mass and final pH of sulphide solutions.

Figure 5: Comparison of measured and predicted pH values for a range of FeS solutions [Fe2+

] = 1, 5 and 10 ppm.

7

7.5

8

8.5

9

9.5

10

10.5

11

0 5 10

[Fe] concentration in ppm

NaS 1

Exp_NaS 1

NaS 5

Exp_NaS 5

NaS 10

Exp_NaS 10

pH

Predicted vs Experimental pH for FeS Solutions

10


Figure 6: Comparison of measured and predicted mass for a range of FeS solutions [Fe2+

] = 10 and 20ppm.

Figure 7: Comparison of measured and predicted mass for a fixed [S2-] = 50ppm for a range of FeS solutions [Fe

2+] =

10, 20 and 50ppm

0

2

4

6

8

10

12

14

16

18

10 20

Mas

s (m

g) /

Litr

e (L

)


NaS 20

NaS 20 Exp.

Experimental vs Predicted FeS mass (Mg/L)

0

10

20

30

40

50

60

10 20 50

Mas

s (m

g/li

tre

) (L)

NaS 50

NaS 50 Exp


Experimental vs Predicted FeS mass (Mg/L)

11


Table 2

Results of FeS pH measurements with a number of solutions with varying compositions

Solution A Solution B 50:50 A+B

[Fe2+] (ppm)

pH [Na2S] or [S2-

] (ppm) Predicted [S2-] pH

Experimental [S2-] pH

Predicted FeS pH

Experimental FeS pH

1ppm 6.70 1ppm 9.50 9.21 8.93 8.43

1ppm 6.70 5ppm 10.20 10.44 9.84 9.59

1ppm 6.70 10ppm 10.50 10.80 10.17 10.20

5ppm 6.41 1ppm 9.5 9.21 8.31 8.18

5ppm 6.41 5ppm 10.20 10.44 9.51 9.35

5ppm 6.41 10ppm 10.50 10.80 10.05 9.98

10ppm 6.13 1ppm 9.50 9.21 8.10 8.11

10ppm 6.13 5ppm 10.20 10.44 8.46 8.22

10ppm 6.13 10ppm 10.50 10.80 9.82 9.56

Figure 8 shows the plots of predicted and experimental PbS solution final pH compared with the experimental pH results. Again, Figure 8 shows close agreement between the predicted and experimental final pH. Some minor differences are observed for the predicted and experimental final pH for 1ppm Pb2+ and 1ppm [S2-] but this gap reduces as the Pb2+ concentration increases. The pH results are found to match within experimental error. Note that the pH was measured in duplicate and the reproducibility is very good. Finally, plots of predicted precipitate and experimental PbS precipitate collected and measured after the solutions A + B are mixed, are shown in Figure 9 and the results is a close match to the predicted values.

Figure 8: Comparison of measured and predicted pH values for a range of PbS solutions; [Pb2+

] = 1, 5 and 10ppm for

values of [S2-] = 1, 5 and 10ppm

9.1

8.51

6.54

8.42

7.82

6.14

5.5

6.5

7.5

8.5

9.5

10.5

1 5 10

pH

[Pb] concentration in ppm

NaS 1

NaS 5

NaS 10

NaS 1 Exp

NaS 5 Exp

NaS 10 Exp

Predicted vs Experimental pH for PbS Solutions

12


Figure 9: Comparison of measured and predicted mass for fixed [S2-

] = 50ppm for a range of PbS solutions; [Pb2+

] =

10, 20 and 50ppm

CONCLUSIONS

A metal/sulphide model has been developed from the equilibrium equations obtained from the literature. The sulphide model is produced by solving 7 equilibrium equations involved in sulphide (FeS, ZnS, PbS) scale formation. The equations include, [H2S], [HS

-], [H+], [FeS], and the corresponding mass and charge balance equations. The numerical solution for the 7 equilibrium equations was carried out using the Newton-Rhapson numerical method3. The sulphide model is capable of predicting accurately H2S, and FeS/PbS Saturation Ratio (SR). Excellent agreement has been observed between the results from the sulphide prediction model, particularly the final solution pH and the precipitated mass of FeS/PbS, and laboratory experiments. The sulphide prediction model has been applied mainly to the FeS system in this work. However, this is the most difficult sulphide to validate experimentally because of the various possible oxidation states of iron (Fe II and Fe III). The ZnS and PbS systems do not have this oxidation concern and, since these scales also have a much lower Ksp than FeS, the model has been shown to be easily applied to these scales as in the case of PbS described in the paper. The main objective of the modeling work has been to develop a reliable and analyzable experimental test for sulphides (FeS, Zn S and PbS) which can then used in the evaluation of sulphide inhibitors/dispersants. Such test results are presented in

Figure 10 Figure 11 which show static efficiency bottle test results performed using this sulphide model, the pH of the Fe2+/Pb2+/Zn2+ was set to achieve a combined final pH of ~7.0. The scale inhibitors were assessed against 20mg/L of Zn and 20mg/L Pb with excess H2S of 50mg/L in mild brine (Seawater).

3 Numerical analysis method

0

5

10

15

20

25

30

35

10 20 50

Mas

s (m

g)/L

itre

(L)

[Pb] concentration in ppm

Predicted Mass

Experimental Mass

Predicted vs Experimental PbS mass (Mg/L)

13


Four (4) commercial potential sulphide inhibitors/dispersants (labeled A1, A2, P and S) were tested for their efficiency in preventing ZnS and PbS. The scale inhibitors tested indicate ZnS efficiency % as follows A2 > A1 > P > S and PbS efficiency % A1 > A2 > P >S the results are done in duplicates and are reproducible. Based on conditions in the field the minimum inhibitor concentration (MIC) may vary for different inhibitors. The sulphide model methodology described here is a robust methodology for designing the sulphide test condition base case scaling scenarios, and testing effectiveness of sulphide inhibitors/dispersants. The model procedure presents supporting sulphide scaling calculations which help to design the precise test conditions, and quantify the resulting Saturation Ratios (SR). The model can be designed to carry out experiments based on using the sulphide prediction model to design how the experiment is carried out in order to achieve the correct target conditions of T and pH, and hence the correct SR of FeS/ZnS/PbS in the test mix. The process gives a systematic way of carrying out the sulphide bottle tests. For FeS, the experimental method is able to keep O2 at an extremely low level ( 20 ppb) although this is not important for ZnS and PbS. The range of tests on the samples used include visual inspection, [M2+] assay using ICP, particle size analysis and, in some case, centrifugation and re-testing but these details will be discussed in a future paper

Figure 10: ZnS Inhibition Efficiency at 90C for 4 sulphide SIs S, P, A1 and A2; [SI] = 10, 50 and 100ppm; [Zn2+] =

20ppm

0

20

40

60

80

100

10ppm 50ppm 100ppm

S

P

A1

A2

SI concentration (mg/L)

ZnS Scale Inhbitors at 90C [Zn]= 20mg/L

ZnIn

hib

tion

n Efficie

ncy %

14


Figure 11: PbS Inhibition Efficiency at 90C for 4 sulphide SIs S, P, A1 and A2; [SI] = 10, 50 and 100ppm; [Pb2+] = 20ppm

ACKNOWLEDGEMENTS

The authors will like to thank the sponsors of the FAST4 JIP at Heriot Watt University: Baker Hughes, BP, BG group, BWA Water Additives, Champion Technologies, Chevron, Clariant Oil Services, ConocoPhillips, Equion, Halliburton, MI Swaco, MWV, Nalco, Petrobras, Petronas, PTT, REP, Shell, Statoil and Talisman, ThermPhos and Total

REFERENCES 1. Miles, L. A New Concept in Scale-inhibitor Formation Squeeze Treatments, in SPE Rocky

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0

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10ppm 50ppm 100ppm

S

P

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A2

SI concentration (mg/L)

PbS Scale Inhbitors at 90C [Pb]= 20mg/L

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ibtio

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Efficien

cy %

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