SPE 81573

Control of Scaling Tendencies in an Effluent Water Injection Plant in West Kuwait Oil
Fields: Laboratory Study
M. Salman , A.Al-Hashem and J.Carew, Kuwait Institute for Scientific research

Copyright 2003, Society of Petroleum Engineers Inc.

calculate the chemical equilibria in a defined system1. A
This paper was prepared for presentation at the SPE 13th Middle East Oil Show & Conference chemistry model is then developed which can be used in
to be held in Bahrain 5-8 April 2003.
various process analysis stages to define the likely quantities
This paper was selected for presentation by an SPE Program Committee following review of
information contained in an abstract submitted by the author(s). Contents of the paper, as
of precipitate to be found in a specific production and/or
presented, have not been reviewed by the Society of Petroleum Engineers and are subject to injection system at several different conditions of temperature
correction by the author(s). The material, as presented, does not necessarily reflect any
position of the Society of Petroleum Engineers, its officers, or members. Papers presented at and pressure.
SPE meetings are subject to publication review by Editorial Committees of the Society of
Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper
Scaling is usually associated with precipitation. It is
for commercial purposes without the written consent of the Society of Petroleum Engineers is recognized that precipitation does not necessarily lead to
prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300
words; illustrations may not be copied. The abstract must contain conspicuous scaling, but scaling is often thought to result from
acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O.
Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.
precipitation, followed by adhering of the precipitates to
surrounding surfaces. Scaling that occurs at ambient pressures
and static conditions may not necessarily occur at elevated
Abstract pressure and dynamic flows and vice versa2-3.
Scale inhibitors screening and evaluation study was conducted In this study, several scale inhibitors were evaluated
to minimize the possibility of scaling tendencies when effluent for effluent water produced in the Western zone of Kuwaiti oil
water is injected into the reservoir of West Kuwait oil fields. fields commingled with two different formation waters. The
Static autoclave jar test and dynamic scaling rig were used to water ratios chosen were based on the computer
screen and evaluate six commercially available scale compatibility study.
inhibitors. Five different effluent and formation waters were
tested using the static jar test and dynamic scaling test rig Experimental Procedure
under reservoir and surface conditions. The results of this scale Water Analysis
inhibitor screening and evaluation study were judged upon Analyses of the five waters used in this work are shown in
efficiency to inhibit calcium carbonate (CaCO3) and barium Table 1. All these waters were produced synthetically using
sulphate (BaSO4) scales. The two most effective scale AnalaR1 grade reagents. The waters were adjusted to the
inhibitors to reduce both CaCO3 and BaSO4 scales were correct pH values using small quantities of mineral acid or
selected with optimum dosages. The first effective inhibitor alkali. All four waters analyses used in this study are shown in
was an aqueous solution of organic phosphate with an Table 1. The total dissolved solids (TDS) of the synthetic
optimum dose of 15 ppm to control CaCO3 and 20 ppm to waters is about ± 2% of the actual corresponding waters. The
control BaSO4. The second effective inhibitor was a effluent to formation water ratios for each of the water
combination of organic amine and organic phosphates with an combinations under study were chosen to reflect the worst
optimum dose of 20 ppm to control both CaCO3 and BaSO4. case conditions with respect to scale formation as predicted
from a computer compatibility prediction study.
Introduction Scale Inhibitors
Thousands of cubic meters of effluent water are being Six scale inhibitors were selected and tested in this study.
produced in the southern and western zones of the Kuwaiti oil Table 2 shows the six scale inhibitors and their chemistries as
fields, and this amount is expected to rise in the future1. obtained from the vendors data sheets.
Because of environmental regulations, these huge amounts of Static Autoclave Jar Tests
effluent waters have to be either disposed of or treated both Static Autoclave jar tests were performed prior to the dynamic
now and in the near future. Two options for dealing with this testing in order to select an appropriate dose concentration for
effluent water include (i) reinjection in some reservoir each of the inhibitors under study. Each inhibitor was tested at
formations for pressure maintenance and (ii) disposal into 15 ppm concentration as recommended by the chemical
selected disposal zone to be decided by the oil producing suppliers and this represents the concentration of the products
company in the country. as received. The tests were carried out at a reservoir
A computer scaling program is used to perform the
compatibility study and then to determine the worse case
1
condition for scaling. The program utilises advanced AnalaR is a trade name for BDH Chemicals Company
thermodynamic properties for approximately 2000 species to Ltd., U.K.
2 SPE 81573

temperature of 180°F (82°C). The tests were conducted for 3 Case 2. Total effluent water disposal injection into the
days, after which the solutions were filtered through pre- reservoir of formation water 1 at 23.8 MPa and 75°C. The
weighed 0.45 micron pore sized filter membranes, dried at mixing ratios were 100% total effluent and 70:30% total
140°F (60°C) and re-weighed. effluent: formation water 1.
Examination of Filter Membranes Case3. Total Effluent water commingled at surface with
Selected membranes from the jar tests were examined by the aquifer water at 2.6 MPa at 65°C. The mixing ratios were
scanning electron microscopy/energy dispersive spectrometry 60:40% and 50:50% total effluent: Aquifer.
(SEM/EDS) in order to identify the scale species present.
Dynamic Scaling Tests Results and Discussion
Figure 1 is a schematic diagram of the dynamic scaling Static Autoclave Jar Test Results
apparatus used in this study. Synthetic formation and injection Table 3 shows the neat waters laboratory scaling test results
waters were stored in reservoirs 1 and 2. The waters were fed for the three test cases. The total scale deposited results show
to a positive displacement twin head pump where they were that these simulated water jar tests gave results very close to
pressurised to the reservoir pressure (30.3 MPa) prior to the computer scaling prediction results. The scale solids
entering the large bore heating coils 1 and 2 to equilibrate at delected were significant in quantity and the different types of
reservoir temperature (82°C). The two waters were scales present were visually distinguishable.
commingled across a mixing cell, at the ratios under Examination of Selected Membranes from Static
evaluation, prior to entering the pre-weighed capillary coil 3. Autoclave Jar Tests
This coil was used to collect any adherent scale that may have A number of 0.45 micron filters, used to determine the
formed under the test conditions. The effluent from coil 3 quantity of deposited scale, were selected for examination by
passed out of the heating zone and flowed to atmospheric scanning electron microscopy (SEM) and energy dispersive
pressure through a second coil (coil 4), to simulate conditions spectroscopy (EDS) in order to identify the types of scales
in the production system (49°C, 1.034 MPa). The effluent formed and their morphology. The results of these membrane
from coil 4 was passed through a 0.45 micron pore sized examination for Case 1 and Case 3 are given below:
membrane filters in order to collect any mobile scale that may Case 1 Membrane. Examination of a membrane surface
have formed. showed that a large part of the scale particles were composed
From these studies, it was possible to predict the of calcium carbonate (CaCO3) with lesser iron sulfide (black
composite scaling situation across the system and find which colour). Figure 2a and 2b show SEM/EDS scans of the
(if any) scale inhibitor prevented the formation of scale in the membrane surface. This membrane was dipped into a
system. Scale may be deposited as an even film throughout the container holding dilute (10% volume) HCl. It was observed
capillary coil, or as a narrow plug within the coil. When scale that the solids/scale effervesced vigorously, giving off gas
is deposited as a narrow plug this will result in a rapid increase bubbles (CO2). This was consistent with the scale being
in the differential pressure but a low deposited weight. An largely composed of CaCO3. The same effect was observed
even coating will manifest itself as a smaller differential with a Case 2 membrane.
pressure with a larger deposited weight. Case 3 Membrane. Examination of a membrane surface
Coils were degreased with chlorinated solvent then showed that a base matrix of very fine grained anhedral iron
acid washed with citric acid. The coils were washed with sulfide (black colour) occurred with rare crystals, both
water then with methanol and hot air dried. The coils were anhedral (irregular) and euhedral (see Figure 3a and 3b)
weighed before installation in the equipment. After each test crystals of BaSO4 scale. This membrane did not effervesce
run, the coils were removed and washed internally with when dipped into dilute HCl acid, indicating that no CaCO3
acetone then dried with hot air. The coils were re-weighed and scale was present, while EDS scans of individual scale
the amount of scale deposition determined4. particles/crystals showed that no calcium or strontium scale
The performance of the scale inhibitors in the mixture particles were present.
of formation and injection waters will be evaluated based on: Dynamic Scaling Test Results
Differential pressure increase in the high pressure coil 3 The dynamic scaling neat waters test results are all given in
(injection well and bottom hole production well simulation). Table 4. Note that the total scale values were very close to
Mass of scale build up in the high pressure coil 3. those in the static jar tests.
Mass of scale deposition in the low pressure coil 4 Case 1. In this case, there was more adherent scale (within
(simulation of production well tubing and topsides the high pressure capillary coil, (number 3) than mobile scale
crude facilities). (collected on the membrane filter) with only a small amount of
Mass of scale collected on the filter membranes. adherent scale found in the low pressure capillary coil
Water Commingling/Mixing Cases (number 4).
The following cases for dealing with the total effluent Case 2. In this case, there actually was slightly more
water and aquifer water were as follows: mobile scale than adherent scale, with only a small amount of
Case 1. Total effluent water reinjection into the formation adherent scale found in the low pressure capillary coil
of Effluent water 1 at reservoir conditions of 26.2 MPa and (number 4).
90°C. The mixing ratios were 100% total effluent and 70: Case 3. This 50:50 total effluent:aquifer water mixing ratio
30% total effluent: Effluent water 1. case yielded <1 mg/L adherent scale in the high pressure
capillary coil, and about 19 mg/L mobile scale collected on the
SPE 85173 3

filter membrane. A small amount of adherent scale had Conclusions and Recommendations
collected in the low pressure capillary coil. The results of this scale inhibitor screening and evaluation
Static Autoclave Jar Tests - Scale Inhibitor study were judged upon efficiency to inhibit two scale types,
Screening Results i.e. test cases 1 and 2 yielded calcium carbonate (CaCO3)
The list of selected scale inhibitor products is given in Table 2. scale, and test cases 3 which yielded barium sulphate
All six gave the effective recommended dose rate for their scale (BaSO4).
inhibitor chemical to be between 5 to 25 ppm. Following Results clearly showed that scale inhibitor C was
consultation, it was decided that a sensible inhibitor dose rate much higher in its effectiveness than the other active
for screening purposes would be 15 ppm and this would be inhibitors, for all the test cases, for both CaCO3 scale and
expected to be close to the typical optimum field dose rate, for BaSO4 scale.
these calcium-tolerant inhibitors. The scale inhibitor screening An optimum dose concentration of this inhibitor may
results are discussed below. give improved inhibition of BaSO4 scale, while retaining its
Case 1. For the 100% total Effluent cases (simulating an excellent effectiveness in inhibiting CaCO3 scale.
injection well near – wellbore flooding front), the inhibitor The selection of the second most effective scale scale
ranking was as follows: inhibitor was much less clear cut. For effectiveness against
CaCO3 scale alone, inhibitor A was always a very close
C> A= B> F>> D>> E second to inhibitor C but this product, was commonly only the
fourth most effective product for BaSO4 scale. The B and D
The most effective inhibitor, at 15 ppm dose rate, was scale inhibitors basically had the same effectiveness against
inhibitor C (see Table 5) which gave over 99% inhibition at CaCO3 scale, but the latter was almost always more effective
both test conditions. This was closely followed by scale at inhibiting BaSO4 scale. Therefore, scale inhibitor D is
inhibitor A, which also gave over 99% inhibition at both test recommended as the second most effective inhibitor, in this
conditions and scale inhibitor B was third most effective. The study, but optimisation may well show it has better
only product to prove obviously unsuitable was inhibitor E performance at a dose rate other than 15 ppm.
while inhibitor D came fifth in the ranking order. It should be When selecting any oil field chemical, a number of
noted that this case dealt with calcium carbonate (CaCO3) additional factors must be taken into account, such as cost,
scaling alone (no iron present). consistency of supply, chemical reliability, local service base,
For the 70% total Effluent: 30% Effluent Water 1 and environmental regulations regarding the use of such
mixing ratio case (see Table 6), the scale inhibitor ranking was chemicals. The relative price rankings for the products used
as follows: are as follows:

C> D>A>B>F>E D > C > E > A > F > B.

The ranking was very similar except that the D

inhibitor was now equal second best with inhibitor C. This Acknowledgement
may be due to its high calcium tolerance. The best product, The authors would like to thank Kuwait Institute for Scientific
inhibitor C still gave over 98% inhibition of CaCO3 scale, for Research (KISR) for permission to publish this work and Oil
both test conditions. Plus Limited for supervising this study.
Case 2. As with the previous test case, this test deals with
inhibition of CaCO3 scale (see Tables 7 and 8). For the 100% References
total Effluent cases (simulating injection well near-wellbore 1. Al-Hashem,A. Al-Sayegh,A. and Carew,J., “Consultancy
flooding fronts) the scale inhibitor ranking was as follows: Services for Laboratory Studies for Effluent – Disposal –
Table 7 : C> A> D> F> B> E West Kuwait”, Study No. 6: Scale Inhibitor Screening
Table 8 : C> A=B> D> F> E and Evaluation, Kuwait Institute for Scientiic Research
(KISR) Final Report, KISR 4809, (1995).
Again the top two inhibitors were C and A with C
giving ~ 98% inhibition for both test cases. Inhibitors D and B 2. Shen,J.. and Cosby,C.C., “In Sight into Strontium
exhibited the same effectiveness in inhibiting CaCO3 scale. and Calcium Sulphate Scaling Mechanism in Wet
Producers” J. Pet. Tech. (1983):pp.1299-1308.
Case 3. The scale inhibitor screening results are shown in
Tables 9 and 10. For both mixing ratios the scale inhibitor 3. Fulford,R.S. “Effects of Brine Concentration and
ranking was as follows: Pressure Drop on Gypsum Scaling in Oil Wells,” J.
Table 9 : C>D>A=B>F>E Pet. Tech. (1986): pp.559-564.

In terms of inhibition efficiency for BaSO4 scale, 4. Al-Hashem,A., Al-Sayegh,A. and Carew,J., in
inhibitor C again proved most effective (>70% inhibition “Evaluation of Scale Inhibitors for a Seawater
BaSO4) with inhibitor D, again, second best. Injection System in North Kuwait”.
CORROSION/98, paper No. 74, (Houston,
Table 10 : C> B=D >A > F> E TX:NACE International, 1998).
4 SPE 81573

TABLE 1: WATER ANALYSIS USED IN SCALING STUDY

COMMINGLED
EFFLUENT AQUIFER: TOTAL
PARAMETER EFFLUENT FORMATION AQUIFER FORMATION 1 EFFLUENT WATER
WATER 1 WATER 1 WATER (95:5) (50:50)
Na 72500 54700 68100 68896 68339
Ca 20000 13500 17000 19019 17606
Mg 3360 1800 2860 3198 2961
K 3000 2020 2750 1851 2780
Sr 563 380 500 535 511
Ba 1.7 6.0 8.0 1.6 6.1
Fe 0.2 14.0 24.7 0.2 17.4
B ND 34.0 46.0 ND ND
Cl 158855 111700 146000 150948 147484
Br ND 1138 1384 ND ND
SO4 1800 259 315 1770 752
CO3 NIL NIL NIL NIL NIL
HCO3 268 59 71.0 72.2 71.6
OH NIL NIL
NH3-N 75.0 56.3
CO2 93 50
H2S 94 0.13
TDS 99,419.9 72,454 91,288.7 93,5800.8 92,220.5
pH @25°C 6.48 6.29 4.50 6.46 5.48
SG @ 20°C 1.1660 1.1248 1.1600 1.1640 1.1636

*Note: All values in mg/L other than pH and SG.

ND = Not Determined

TABLE 2: THE CHEMIST RIES OF THE SIX SCALE INHIBITORS

SCALE CHEMICAL COMPOSITION* RELATIVE **
INHIBITORS PRICING ($/Drum)

A Partially Neutralized in (12%) Methanol Phosphonate 650

B Sulphonated Copolymer 720

C An aqueous solution of organic phosphates 480

D Organic amine/organic phosphate combination 450

E A solution of phosphonate 520

F Polycarboxylic Acid (60 - 80%) 680

*Based on manufacturer data sheet.

** Pricing will vary depending on quantity and method of shipment.
SPE 85173 3

TABLE 3: LABORATORY SCALING TEST RESULTS FOR NEAT WATERS

TEST CASE TEST CONDITIONS WORST CASE VISUAL DESCRIPTION OF SCALE

DEPOSIT ON MEMBRANE
1 3800 psia 100% Pale whitish-grey coloured fine grained
90°C Total Effluent scale deposit. No distinct crystals or iron
scales
2 3400 psia 70% Total Effluent: 30% Formation Dark grey-black base scale with a grey-
75°C Water 1 white fine grained scale above. Rare
needle-like crystals seen.
3 380 psia 40% Total Effluent: 60% Aquifer Dark black fine precipitated scale with rare
65°C clear crystals/needles above

TABLE 4: RESULTS OF THE DYNAMIC SCALING TESTS - NEAT WATERS

TEST TEST MIXING RATIO MOBILE HIGH LOW TOTAL

CASE CONDITION SCALE PRESSURE PRESSURE SCALE
COIL SCALE COIL SCALE
1 3810 psia 100% Total Effluent 31.81 44.84 0.04 76.69
2 3450 psia 100% Total Effluent 26.69 28.31 0.12 55.12
75°C
3 380 psia 50% Total Effluent: 50% 19.01 0.81 0.15 19.97
90°C Aquifer

Notes: All results are the average of two dynamic scaling tests.
The average volume of total water passed through the scaling rig was 25.6 litres, at a rate of 3 litres/hour.

TABLE 5: RESULTS OF THE STATIC AUTOCLAVE JAR TESTS FOR SCALE INHIBITOR SCREENING
TEST CASE 1: 100% TOTAL EFFLUENT WATER AT 3800 PSIA AND 90°C

SCALE INHIBITOR DOSE RATE SCALE DEPOSITED %

(PPM) INHIBITION
No inhibitor (I) NIL 73.52 -
No inhibitor (ii) NIL 77.04 -
A 15 0.744 99.01
B 15 0.788 98.95
C 15 0.669 99.11
D 15 4.86 93.54
E 15 23.02 69.42
F 15 0.853 98.87

Note: The blank runs average scale amount was 75.28 mg/L.
6 SPE 81573

TABLE 6: RESULTS OF THE STATIC AUTOCLAVE JAR TESTS FOR SCALE INHIBITOR SCREENING
TEST CASE 1: 70% TOTAL EFFLUENT WATER :30% EFFLUENT 1 WATER AT 3800 PSIA AND 90°C

SCALE DOSE RATE SCALE DEPOSITED %

INHIBITOR (PPM) INHIBITION
No inhibitor (I)* NIL 38.93 -
No inhibitor (ii)* NIL 37.28 -
A 15 1.01 97.35
B 15 1.38 96.38
C 15 0.68 98.22
D 15 0.84 97.80
E 15 3.66 90.40
F 15 2.93 92.31
∗ The blank runs average scale amount was 38.11 mg/L.

TABLE 7: RESULTS OF THE STATIC AUTOCLAVE JAR TESTS FOR SCALE INHIBITOR SCREENING
TEST CASE 2: 100% TOTAL EFFLUENT WATER AT 3450 PSIA AND 75°C

SCALE DOSE RATE SCALE DEPOSITED %

INHIBITOR (PPM) INHIBITION
No inhibitor (I)* NIL 57.78 -
No inhibitor (ii)* NIL 59.04
A 15 1.57 97.31
B 15 2.72 95.34
C 15 1.36 97.67
D 15 2.09 96.42
E 15 4.68 91.99
F 15 2.41 95.87
∗ The blank runs average scale amount was 58.41 mg/L.

TABLE 8: RESULTS OF THE STATIC AUTOCLAVE JAR TESTS FOR SCALE INHIBITOR SCREENING
TEST CASE 2: 70% TOTAL EFFLUENT WATER :30% FORMATION WATER 1 AT 3450 PSIA AND 75°C

SCALE DOSE RATE SCALE DEPOSITED %

INHIBITOR (PPM) INHIBITION
No inhibitor (I)* NIL 32.58 -
No inhibitor (ii)* NIL 30.82 -
A 15 0.82 97.41
B 15 0.83 97.38
C 15 0.49 98.45
D 15 0.89 97.19
E 15 2.54 91.99
F 15 1.47 95.36
∗ The blank runs average scale amount was 31.7 mg/L.
SPE 85173 3

TABLE 9: RESULTS OF THE STATIC AUTOCLAVE JAR TESTS FOR SCALE INHIBITOR SCREENING
TEST CASE 3: 40% TOTAL EFFLUENT WATER: 60% AQUIFER WATER AT 380 PSIA AND 65°C

SCALE DOSE RATE SCALE DEPOSITED %

INHIBITOR (PPM) INHIBITION
No inhibitor (I)* NIL 3.28 -
No inhibitor (ii)* NIL 3.56 -
A 15 1.42 58.48
B 15 1.45 57.60
C 15 0.94 72.52
D 15 1.02 70.18
E 15 2.87 16.08
F 15 1.68 50.88
∗ The blank runs average scale amount was 3.42 mg/L.

TABLE 10: RESULTS OF THE STATIC AUTOCLAVE JAR TESTS FOR SCALE INHIBITOR SCREENING
TEST CASE 3: 50% TOTAL EFFLUENT WATER: 50% AQUIFER WATER AT 380 PSIA AND 65°C

SCALE DOSE RATE SCALE DEPOSITED %

INHIBITOR (PPM) INHIBITION

No inhibitor (I)* NIL 2.98 -

No inhibitor (ii)* NIL 3.41 -
A 15 1.13 64.69
B 15 0.98 69.38
C 15 0.774 75.81
D 15 1.00 68.75
E 15 2.26 29.38
F 15 1.72 46.25

∗ The blank runs average scale amount was 3.20 mg/L.

8 SPE 81573

Figure 1 Schematic of Dynamic Scaling Apparatus

SPE 85173 3

Figure 2 (a) SEM Microphotograph showing general view of Case 1 membrane area
Surface revealing a fine grained matrix of calcium carbonate and lesser iron
10 SPE 81573

Figure 3 (a) SEM microphotograph showing general view of Case 3 membrane area
surface revealing an excellent example of a euhedral cruciform barium sulfate
cyrstal scale “rosette” sitting on a bed of fine iron sulfide and their (b) EDS
analysis.