SPE

SPE 23814

Kinetics of BaS04 Crystal Growth and Effect in Formation Damage R.M.S. Wat, * K.S. Sorbie, * A.C. Todd, * Ping Chen, and Ping Jiang, Heriot-Watt U.

'SPE Members

Copyright 1992, Society of Petroleum Engineers, Inc.

This paper was prepared for presentation at the SPE Inti. Symposium On formation Damage Control held in Lafayette, louisiana, February 26-27, 1992.

This paper was selected for presentation by an SPE Program Commillas following review of informatioil contained in an abstract submitted by the author(s). Contents of the paper as presented, have not been reviewed by the Society of Petroleum Engineers and are subJact to correction by the author(s). The material, as presented does not necessarily refleci any position of til? Society of ~etroleum Engineers! its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. PannlSSlOllto copy is restricted to an abstract of not more than 300 words. illustrations may not be copied. The abstract should contain conspicuous acknowledg­ment of where and by whom the paper Is presented. Wrne Ubrarian, SPE, P.O. Box 833836, Richardson, TX 75083-3838 U.S.A. Telex, 730989 SPEDAL.

ABSTRACT

In the North Sea, due to the extensive use of water injection for oil displacement and pressure maintenance, many reservoirs experience the problem of scale deposition when injection water starts to breakthrough. In most cases the scaled-up wells are caused by the formation of sulphate scales of Barium and Strontium. Due to their relative hardness and low solubility, there are limited processes available for their removal and the preventive measure such as the 'squeeze' inhibitor treatment has to be taken. It is therefore important to have a proper understanding of the kinetics of scale formation and its detrimental effect on formation damage under both inhibited and uninhibited environment.

In this paper, we will present results of BaS04 formation kinetics in both beaker tests and in highly reproducible sandpacks which simulates the flow in porous medium. The effect of scale depOSition on the dynamics of formation damage will also be discussed. In the studies of BaS04 crystal growth kinetics and formation damage, we have included both normal formation/injection brine mixture and the addition of scale inhibitor chemical. There are significant differences in the results of static (beaker tests) and dynamics (core flood) conditions, and between the normal and inhibited brine mix.

References and fi~ures at end of paper

INTRODUCTION

The formation of mineral scale associated with the production of hydrocarbon has been a concern in oilfield operation. Depending on the nature of the scale and the fluid composition, the deposition can take place within the reservoir which causes formation damage or in the production faci1itie~ where blockage can cause severe operational problems. The two main types of scale which are commonly found in the oilfield are carbonate and sulphate scales. Whilst the formation of carbonate scale is associated with the pressure and pH changes of the production fluid, the occurrence of sulphate scale is mainly due to the mixing of incompatible brines, ie. formation water and injection water. In the North Sea, the universal use of sea water injection as the primary oil recovery mechanism and for pressure maintenance means that problems with sulphate scale deposition, mainly barium and strontium, are likely to be present at some stage during the production life of the field.

Apart from its likely occurrence, the relative hardness and low. solubility of the sulphate scale means that very few remedial treatments are available for its removal. Processes like acidization which can successfully remove carbonate scale, ego CaC03, cannot effectively apply in this case. The common practice in the oilfield to overcome the problem of sulphate scale formation is by the preventive method of 'Squeeze' treatment in which scale inhibitor chemical is used to retard the kinetic growth of scale

429

2 Kinetics of BaS04 Crystal Growth and Effect in Fonnation Damage SPE 23814

crystal. Indeed most of the work reported to date [1,2,3,4] on the kinetic studies of BaS04 crystal growth are related to the screening of effective inhibitor chemicals. In this paper we shall present the results on the kinetic growth of BaS04 scale under the influence of added sand particles, seeded crystal and inhibitor chemical. Furthermore, results of in­situ scaling in highly reproducible sandpacks have been included to provide information on likely formation damage and comparison of crystal growth under static and dynamic flow environment.

EXPERIMENTAL

In the crystal growth kinetic experiments, a supersaturated solution was first prepared by mixing equimolar amount of BaCl2 and H2S04 solutions. By controlling the rate of mixing and temperature variation, ie. +/- 0.50 C, solutions of relative supersaturation of 5 could be preserved for a few hours. The growth experiments were then initiated by the rapid addition of either sand particles, seed crystals of BaS04, or both. The process of growth was monitored by measuring the solution conductivity with time which was then converted to the rate of change in scaling ion concentrations. The seed crystals were previously prepared by mixing O.OIM of BaCl2 and H2S04 solutions over a period of 24 hours before being ftltered and washed. The final suspension contained approximately 39 mg of seed crystals per 1 cm3 of suspension fluid. The sand particles used were clean, acid washed silica sand supplied by BDH, of GPR grade which had a mean particle size of 273 Ilm. The background conductivity due to the possible surface charge of the sand particles had been included in the calculations. The value was obtained by measuring a standard solution containing 10 g of sand and 100 cm3 of distilled water after it had been purged of dissolved carbon dioxide by bubbling through with nitrogen gas. In the case of inhibited crystal growth experiments, the procedures were similar except that 10 ppm of inhibitor was added to the original BaCl2 and H2S04 solutions. The inhibitor used was a commercially available phosphonate type called Diethlenetriaminepenta (methylene phosphonic acid) which contained 50% activity.

In the dynamic experiments, the in-situ scaling process was investigated using reconstituted sea water and formation brine in the sandpack assembly. The compositions of the two different brines are given in table 1. A ratio of 60% of formation water to 40% of sea water was used for the experiments. This

430

value was based on the scaling tendency model [5] whose prediction gave the highest degree of supersaturation with maximum amount of precipitation of BaS04 scale. Separate inlets were provided for the different brines and these were positioned so as to ensure that the mixing only took place within the specific section of the column. A schematic diagram of the in-situ scaling experiment is shown in figure {I}. Two different sandpacks, using the same batch of BDH silica sand as in the kinetic studies, were prepared for the inhibited and uninhibited scaling experiments. The inhibitor used was the sodium salt of the same phosphonate type which, unlike in the previous case, contained approximately 25% activity. The inhibitor chemical was added to the sea water stream and gave an an overall concentration of 50 ppm on mixing. The combined injection rate of the two different brines was 30 cm3/hr. This represented a residence time of approximately 55 minutes within the mixing section. Details of the sandpack assemblies and the injection sequence are listed in table {2}. The scaling process was monitored by analysing the effluent concentration of the scaling cations using Atomic Absorption Spectroscopy (AAS) and, where appropriate, the concentration of inhibitor chemical. At the end of each experiment, the sandpack was carefully dismantled and the spatial variation of the scale deposit was analysed using Scanning Electronic Microscopy (SEM). In order to provide direct comparison of the scaling mechanism, static jar tests using the identical sea water and formation brine mixture, with or without inhibitor, had been carried out.

RESUL TS AND DISCUSSION

1 . Crystal Growth Kinetics

The rate of crystal growth was monitored by measuring the solution conductivity. This was then converted to the concentration of the scaling ions remaining in solution using the following equations (6] :

where

Il = Ilo - Kcen

k =:; CIl /1000

(1)

(2)

A. = equivalent conductivity in ohm-1m2equiv-1

1..0 = equivalent conductivity at infinite dilution in ohm-1m2equiv-1

C = equivalent concentration in unit of

SPE 23814 R.M.S. Wat, K.S. Sorbie, A.C. Todd, P. Chen, P. Jiang 3

k = Kc =

equivalents per litre, 1O-3/C in m3 equiv-1

conductivity in ohm-1m-1 constant obtained from equation (1) with C and A. taken at 5x 10-4 mol/l

The accuracy of the above equations was confirmed by constructing a calibration curve using different supersaturated solutions of H2S04 and BaCh. The conductivity of the calibrated samples was measured and compared with the calculated values given by the above equations and good agreement was obtained as shown in figure {2}.

For both inhibited and uninhibited growth, the experimental details are summarised in table 3. In general, it is possible to prepare solutions with relative supersaturation of 8 before any premature precipitation starts due to instability[7]. In all the experiments discussed here, solutions of relative supersaturation of 5 were used and these were based on the barium sulphate solubility taken as 1.009 x 10-5 mollL[8]. In the case where the growth was initiated by the addition of seed crystals, the changes in solution conductivity over time are shown in figure 3. As expected, the rate of crystal growth is much retarded when inhibitor is present. Furthermore, results from both experiments exhibit the initial surge over the first 80 minutes which characterize the growth reaction from a metastable supersaturated solution and is generally attributed to the surface nucleation process[9]. It is then followed by a more gradual rate of growth which can be represented by a second order rate equation [4,7,9,11] in the form of:

dm rate of crystal growth = --' = ks(m - m a )2 (3)

dt

where m = molar concentration IIIo = equilibrium (solubility) value ks = crystal growth rate constant, a

function of the number of active growth sites on the added crystals

In figure 4, the conductivity profiles are converted to concentration and expressed as the integrated form of equation (3). From figure 4 the values of ks of experiment 54 and 57 are found to be 0.0IxlO-2 and 1.26xlO-2 respectively. This indicates that the rate of growth in an inhibited environment is about two orders of magnitude slower than when no scale inhibitor is used. The retardation of growth is generally attributed to the blocking of active growth

sites on the crystal surface by the inhibitor molecules [1].

For those experiments in which the crystal growth process was initiated by heterogeneous nucleation, ie. the addition of sand particles (expts. 30 & 53) or a mixture of sand and seed crystals (expts. 55 & 58), the rate of growth can no longer be adequately expressed by the equation (3). The results from these experiments, however, have been included and are shown in figures [5] and [6] in which comparison between the inhibited and uninhibited growth can be made. In the case where growth was initiated by sand particles only, there was no appreciable differences in the rate of growth. On the other hand, significant rate reduction was observed in the presence of inhibitor when the growth process was initiated by both sand and seed crystals. This further indicates that the inhibition mechanism was mainly due to the blocking of active sites on the crystal surface.

2. In-situ Scaling Studies

The pressure drop over the mixing zone of the sandpack was monitored throughout the experiment. No appreciable change was observed as shown in table [2]. This was mainly due to the original high column permeability and the total scale deposit was relatively insignificant when compared with the pore volume of the mixing section.

Apart from monitoring the pressure drop, effluent samples were collected during the experiments. They were analysed for the concentration of the two main scaling cations, ie. barium and strontium, whose effluent profiles are shown in figures 7 and 8 respectively. In these figures, the effluent profiles of both the inhibited and uninhibited experiments are shown. Also included in these figures are the [Ba++] and [Sy++] concentrations measured during the static jar tests. In the case of uninhibited scaling, experiment ES-1, the [Ba++] ion concentration at the outlet of the sandpack remains constant at approximately 3.5 ppm throughout the experiment.­This is consistently lower than that of the similar beaker test even though the brines used and their mixing ratio were identical. The increase in scaling tendency of BaS04 within the sandpack was likely due to the abundant heterogeneous growth sites which were available.

431

For the experiment ES-2 in which 50 ppm of inhibitor was present, the results show quite a different trend. In this case the [Ba++] ion concentration in the sandpack effluent, after it

4 Kinetics of BaS04 Crystal Growth and Effect in Formation Damage SPE 23814

initially drops to a minimum of 14.5 ppm, rises and remains steady at a much higher level than the beaker test results. This indicates that the rate of BaS04 precipitation within the sandpack is somewhat slower than that in static condition when inhibitor chemical is present. The initial surge of the precipitation rate, however, as indicated by the drop in [Ba++] concentration, is due to the non-equilibrium adsorptjon of the inhibitor chemical. When the sea water containing scale inhibitor was first injected into the sandpack, the advancing front of the inhibitor chemical was retarded due to adsorption. Meanwhile, both the sea water and formation water, with depleted inhibitor concentration, propagated down stream and resulted in extensive scale precipitation. The rate of precipitation gradually decreased as more free inhibitor molecules were available after the adsorption sites had been saturated. The delay of inhibitor breakthrough due to adsorption and the corresponding rise in BaS04 precipitation, as indicated by the drop in [Ba++] concentration, can be clearly seen in their breakthrough profiles which are shown in figure 9. Furthermore, apart from the initial nonequilibrium process, the inhibitor effluent concentration remains constant at approximately 40 ppm (80% injected value) throughout the experiment. The loss of material during this steady state is likely due to a combination of adsorption and bonding mechanisms of inhibitor molecules with the newly formed crystal surface.

The differences of SrS04 scale precipitation under dynamic and static conditions are less pronounced as shown in figure 8. Indeed the final [Sr++] concentrations in the sandpack effluent and beaker test samples agree well with each other for both the inhibited (expt. ES-2) and uninhibited (expt. ES-l) cases. The lack of sensitivity of strontium scale precipitation is mainly due to the fact that the supersaturation of SrS04 is about one order of magnitude smaller than that of the BaS04 [5] in the brine mixtures used for the experiments.

Other important information on the in-situ scaling mechanism is the morphology of the scale deposit. In figure 10, the SEM pictures of the uninhibited scale deposit (ES-l) at three different locations of the sandpack are shown. They are taken at 2.5 cm, 12.5 cm and 38.5 cm respectively from the point of mixing. A number of interesting observations can be seen from these and many other similar SEM pictures. Firstly and as expected, the amount of scale deposit is most abundant near the point of mixing and generally decreases with distance. Secondly, perhaps more importantly, the scaling mechanism is very 'localised' with mainly heterogeneous growth on sand grain surface. There is no strong evidence,

432

including careful examination of the SEM picture of the outlet filter and pressure monitoring, to suggest that homogeneous nucleation and subsequent particle blockage is the main contribution to the permeability impairment. The formation damage due to scale deposit is likely caused by the continuous growth of scale crystals which have remained stationary at the active growth sites. Finally, the differences in crystal morphology can be seen by comparing the pictures in figure 10. The scale crystal nearest to the point of mixing (figure lOa) exhibits a very 'healthy' state of growth whilst for those which are further away (figures lOb and 1Oc) both the size and growth have been somehow impaired. This is because near the point of mixing, the brines have the richest composition of the scaling ions. The precipitation process is fast and is continuously supplied with fresh brines. Further downstream, however, the composition of the brines has been significantly modified with the continuous depletion of the scaling ions. This means that under steady state condition as during the sandpack experiments, there is a transition zone of varying supersaturation along the sandpack column and this gives rise to the different scaling tendency.

CONCLUSIONS

In the current studies, we have presented results which are used to quantify the kinetics of BaS04 crystal growth under a controlled environment. The work also extends to cover the realistic system in which in-situ scaling takes place under dynamic flow conditions. Based on the results obtained from these studies, the following conclusions can be made.

(1) The rate of growth of BaS04 scale, after the initial surge, can be represented by a second order rate equation. However, this does not extend to the case when crystal growth is initiated by heterogeneous nucleation.

(2) When scale inhibitor is present, the rate of growth is significantly retarded and the rate constant, ks, is approximately two orders of magnitude smaller than the uninhibited case. The differences in rate will vary according to the fluid system and the experimental conditions.

(3) For the in-situ scaling experiments, the crystal growth process appears to be localised. The permeability decline is likely

SPE 23814 R.M.S. Wat, K.S. Sorbie, A.C. Todd, P. Chen, P. Jiang 5

caused by the continuous growth of crystals and not by particle transport and flow blockage.

(4) The scale precipitation process in porous medium is likely to be dominated by

'heterogeneous nucleation with the [Ba++] ion concentration in the sandpack effluents ~onsistently less than that of similar beaker tests. However, when the process is inhibited by the addition of scale inhibitor, the extent of inhibitor adsorption on the sand surface will significantly modify. the process. The effluent [Ba++] concentration in this case is higher than the beaker test which indicates less precipitation in-situ.

(5) The scale morphology varies considerably from the point of mixing. Under steady state, the scaling process within the porous medium is contributed by a range of supersaturated mixtures. The point where the incompatible brines first come into contact with each other has the highest supersaturation and with maximum amount of deposit. This is followed by a transition zone of rapidly depleted scaling ions.

ACKNOWLEDGEMENT

We would like to thank the member organisations funding the Heriot-Watt University Oilfield Scale Research Project which includes B.P., Chevron U.K. Ltd., Ciba-Geigy Additives, Marathon, Shell UK Exploration and Production, Statoil, Texaco, and M.T.D. Ltd/SERC (Grant no. GRIE63322).

REFERENCES

1. Nancollas, G. H., " Oilfield Scale - Physical Chemical Studies of Its Formation and Prevention", Chemicals in the Oil Industry 1985, 143-164.

2, Vetter,O.J., "An Evaluation of Scale Inhibitors", Journal of Petroleum Technology, p997, August 1972.

3. Boyle,M.J. and Mitchell, R.W., "Scale Inhibition Problems Associated With North Sea Oil Production ", SPE 8164, presented at the Offshore Europe 79 Conference, Aberdeen, Scotland, 3-7 September.

433

4. Liu, S. T. and Nancollas, G. H., " The crystal Growth and Dissolution of Barium Sulphate in the Presence of Additives", The Journal of Colloid and Interface Science, Vol 52, No.3, 9,1975. 583.

5. Yuan, M.D. and Todd, A.C., "Prediction of Sulphate Scaling Tendency in Oilfield Operations", paper SPE18484 presented at the SPE International Symposium on Oilfield Chemistry, Houston, Texas, 8-10 Feb 1989.

6. Moore, W. J.," Physical Chemistry", Lowe & Brydone Ltd, London 1968.

7. Nancollas, G. H. "Crystallisation of Barium Sulphate in Aqueous solution" Trans of Faraday 59,1963. 737.

8. VanDer Leeden, M. C. and Van Rosmalen, G. M., " The Influence of Various Phosphonates on the Growth Rate of Barium Sulphate Crystals in Suspension", Estudios Geol, 38, 1982, p279-287.

9. Leung, W, H. and Nancollas, G. H.,"A kinetic Study of the Seeded Growth of Barium Sulphate in the Presence of Additives", J. Inorg. Nucl. Chem. Vol 40, p1871.

10. Liu, S, T. and Nancollas, G. H., " The crystal Growth and Dissolution of Barium Sulphate in the Presence of Additives", The Journal of Colloid and Interface Science, Vol 52, No.3, 9,1975. 583.

11. Rubin, A. J. , "Aqueous - Environmental Chemistry of Metals", Ann Arbor Science Publishing Inc, Michigan 1974.223-224.

SPE 2381 4

Table 2 Summary of Sand pack Experiments

Sand pack Experiment ES-l ES-2 T->O ~~ }>O It>O

Table 1: Composition of Formation and Injection Sea Waters [Column Length (cm)

69.5 41.8 69 40 fPore Volume (ml) 45.47 26.32 48 29

Ions (mglL) Jo'ormatlon water inJection sea water

I Sand weIght (2) 205.50 199.76

Na :lY,40C 1 J,H9l ..:a 2,8OC 428

Mg :lot 1,368 K 37C 46l Sr 575

[Dead Volume (ml) 1.0 1.5

I PorosIty 0.37 0.3~ I Permeability (d) A->B B-> A->B B-> Initial 24.5 ~.nr 8.97 39.58

J:la 25 Final -25.5 29.31 .64 41.52 52,64 1~,/{)t

::iU4 2,96l [Floods Sequence: 1) tracer, U (1->0) 1) tracer, ~ (1->0

2) tracer, U (B->O) 2) tracer, (B->O) 3)FW& (1->0) 3)FW& (1->0)

sw (B->O) SW& Inhibitor (B->O) 4) tracer, U (B->O)

[Flowrate (ml/hr) 30 60 30 ResIdence T,me (min.) 52.9 26.5 58 Pore Vol. Injected

26.65 79.95 74

Table 3 Summary of Crystal Growth Kinetics Experiments

Expt. super- Initial 11 Final 11 Seed Sand pH ks(10- 2)

saturation cone. cone. Crystal (g)

oom oom (ml)

30 5 5.00 4.0

57 5 1.00 4.0 1.26

58 5 1.00 5.00 4.0

53 5 10.00 9.76 5.00 3.73

54 5 10.00 9.57 1.00 3.76 0.01

55 5 10.00 9.48 1.00 5.00 3.75

Ip, I~ manometer I Pd

25cm 15 cm FW 18ml/hr.

m -. I A ~ B D 0 ,

pump ,

~~~~ , , , sw , , 12mIlhr.

, sample , , collector ,

B section

inlet & Pb

'"'" --+_'"'01 inlet

Fig. 1-Apparatus layout for In-situ scaling experiments.

434

80

70

60 a y

~ 50 ::l

,:; ... 40 ] y

= 30 "0 I: ~ U

20 0 measured values -- calculated

10

O~~--r-~~r-~~--~-r--~-r--r-,-~~

0.00e+0 1.00e-5 2.00e-5 3.00e-5 4.00e-5 5.ooe-5 6.00e-5 7.00e-5

Concentration, mol/l

Fig. 2-Predlcted and measured conductivity of supersaturated solutions of H2 S0 4 and BaCI 2 •

40oooT-------------------------------------~

30000

20000

• IO ppm inhibitor (Expt. 54)

o uninhibited (Expt. 57)

150

Time (min)

200 250

Fig. 4-Rate of change of scaling Ion concentrations In Experiments 54 and 57.

300

~

;::; ~ a

I/)

&. Q

2 I: .S -; .. -= '" y I: ~

U

s ~ a

I: ~

.~ .. -= '" y I: ~

U

SEE 2381 4

7

6

5

4

3

• 2 10 ppm inhibitor (Expt.54)

0 uninhibited (Expt. 57)

1+-~--.-~--~~~~--~_r--~~--~~ o 50 100 150 200 250

Time (min)

Fig. 3-Conductlvlty profiles of Inhibited and uninhibited crystal growth experiments Initiated by adding seeded crystals.

300

6.5 ,-----------------------------------------..,

6.0

5.5

5.0

4.5

• IO ppm inhibitor (Expt.53)

o uninhibited (Expt.30)

4.0 +--~__,-~----r-~-_._-~-..._---,.-~-~---l o 50 100 150 200 250 300

Time ( min)

Fig. 5-Concentratlon profiles of scaling Ions remain In solution after the addition of sand particles.

7

6 ~ --'0 Ei 5 III .;. ~

6 4 c: .~ -; ... 3 .... c: .. u c: e

2 U

1 0 50 100

• 10 ppm inhibitor (Expt.55)

o uninhibited (Expt.58)

150 200

Time (min)

250 300

Fig. 6-Concentratlon profiles of scaling Ions remain In solution after the addition of seed crystals and sand particles.

3ro~--------------------------------------, Sr (34Oppm on mixing, no 12)

Sr (340ppm on mixing, with 50ppm 12)

340 Sr (beaker test, no 12)

Sr (beaker test,with 50ppm 12)

320

300

~.~.. • 300ppm "» Iii. .... ••• 41.. t ••••• I ____ 6 ___________________________________ jl ________ _

• 280

2W+-~--.-~~r-~_.--~_r--~_.~--._~__4

o 10 20 30 40 50 60 70

PV (mixing zone) Injected

Fig. B-Effiuent concentration proliles of (Sr + + I for Inlllblted and unlnhlblteclln.aftu scaling exparirnanta together with the reaulte !rem the parallel beaker testa.

i Q,

~ <II

=:I

150

125

100

75 •

50

25

0 0

o Ba (15Oppm on mixing, no 12)

• Ba (15Oppm on mixing, with 50ppm 12)

Ba (beaker test, no 12)

Ba (beaker test with 50ppm 12)

.. .............. • • • ••••• . / ---------------------------------------11-----------

J8ppm

·----·-0--0---0--···-0-····-0-·--0-:-·0···-····0·-- , 8.6ppm

10 20 30 40 50 60 70

PV (mixing zone) Injected

Fig. 7-Eflluent concentration prolilea of (Be + + I for inhibited and uninhibited In-sltu scaling exparimente together with the reaulta from the parallel beaker teate.

c: 1.0 .~ ~ -; ... • ooCJXJr::p:jJ

0 0 = 0.8 0 .. • cPcP u c: ~ e

u , .... 0.6 ~ 1:1 • .. = 00

IE .. 0.4 0 • 0 "CI .. .~ 0 '; 0.2 ~ II ,'.I"IUIIMh~ III • Ei • ... ml-e O~ c:

0.0 0 2 3 4 5 6 7

pore volume (mixing zone) injected

Fig. 9-Braakthrough profiles of Inhlbltor and [Be + + I for the Experiment E8-2.

A B

c

Fig. 10-SEM piclures 01 scale cryslal lormed al (a) 2.5 cm, (b) 12.5 cm, and (c) 38.5 cm Irom poinl 01 mixing In Experiment ES-l.

SPE 2381 4