ARTICLE pubs.acs.org/EF
Dual Kinetic Hydrate Inhibition and Scale Inhibition by Polyaspartamides Pei Cheng Chua, Mariann Sæbø, Anette Lunde, and Malcolm A. Kelland* Department of Mathematics and Natural Sciences, Faculty of Science and Technology, University of Stavanger, 4036 Stavanger, Norway ABSTRACT: Polyaspartamides are a recently developed class of kinetic hydrate inhibitors (KHIs) with good biodegradability, made from polysuccinimide (PSI). Because polyaspartates are well-known biodegradable scale inhibitors (SIs), which are also made from PSI, we have investigated the possibility of using PSI to make a single product that would function as both a KHI and a SI. We have synthesized an extended range of PSI derivatives and tested them as KHIs in high-pressure autoclaves with synthetic natural gas and as SIs for both calcium carbonate and barium inhibition in a high-pressure dynamic tube blocking rig. In comparison to earlier work, we have further improved the KHI performance of the polyaspartamide class by incorporating a high percentage of isopentyl groups into the structure without losing water solubility. We have also found that, for some polymers, good scale inhibition can be obtained for both types of scale without compromising the KHI performance when dosed at several thousand parts per million (ppm), a typical KHI dosage. The polyaspartamides have high cloud points and are compatible with brines containing calcium ions up to a concentration of at least 2000 ppm. Thus, use of certain polyaspartamides as KHIs for gas hydrate control should not require the extra use, where needed, of a specialized SI for topside carbonate or sulfate scale inhibition.
’ INTRODUCTION Kinetic hydrate inhibitors (KHIs) are now a well-known technology for preventing gas hydrate plugs in upstream oilfield operations.1 3 KHIs are water-soluble polymers, often with added synergists that improve their performance. KHIs delay the nucleation and usually also the crystal growth of gas hydrates. The nucleation delay time (induction time), which is the most critical factor for field operations, is dependent upon the subcooling (ΔT) in the system; the higher the subcooling, the lower the induction time. The absolute pressure is also an important factor.4 7 Several classes of polymers are now used in commercially available KHI formulations. These include (i) homo- and copolymers of the N-vinyl lactams, N-vinyl pyrrolidone (VP) and N-vinyl caprolactam (VCap),8 15 (ii) hyperbranched poly(ester amide)s,16 18 (iii) polypyroglutamates,19 21 and (iv) polyisopropylmethacrylamides.22,23 Recently, a new class of KHI polymer, polyaspartamides, was reported.24,25 These polymers are made by reacting polysuccinimide (PSI) with alkylamines (Figure 1). They show good biodegradability, high cloud points, and a performance close to that of a commercial 1:1 VCap/VP co-polymer. If any water is present in the reaction of PSI with akylamines, the basicity of the amines produces a significant amount of hydroxide ions in solution. When these ions react with PSI they give a polymer with carboxylate groups. The reaction is exploited commercially with metal hydroxides to give polyaspartate salts, which are scale inhibitors (SIs) with high biodegradability (Figure 1).26 31 We have investigated the possibility of using PSI to make a single product that would function as both a KHI and a SI. We have synthesized a range of new PSI derivatives, polyaspartates and polyaspartamides, and tested them as KHIs in a highpressure autoclave with synthetic natural gas (SNG) and as SIs r 2011 American Chemical Society
for both calcium carbonate and barium inhibition in a high-pressure dynamic tube blocking rig.
’ SYNTHESIS OF POLYASPARTAMIDES The general synthetic method for preparing the polyaspartamides was identical in all cases and has been described previously.32,33 An outline of the synthesis is given in Figure 1. Polymers made by reacting PSI with 100 mol % isobutylamine were not fully watersoluble at room temperature. The same was true for polymers made from isopentylamine if the mole percentage was above 80 90%. Finally, we made a polyaspartamide using PSI and 3-dimethylamino-1-propylamine. The pendant groups in this polymer are similar to those found in polymers containing the monomer dimethylaminoethylmethacrylate (DMAEMA), such as Gaffix VC-713, a N-vinyl caprolactam/N-vinyl pyrrolidone/DMAEMA terpolymer from International Specialty Products (ISP), a known KHI.34 DMAEMA-Based co-polymers without N-vinyl lactams have also been reported as KHIs.35 ’ KHI EQUIPMENT AND TEST METHOD KHI performance tests were carried out in the high-pressure autoclave apparatus shown in Figure 2 and discussed previously.1,36 The autoclave consists of a stainless-steel cell with a volume of 23 mL. The temperature was measured to an accuracy of (0.l °C, and the pressure was measured with an accuracy of (0.2 bar. The test method used was the “constant cooling” test method described in detail below. In all of the experiments, we used the same SNG mixture that gives structure II hydrates (Table 1). The aqueous phase was distilled (DI) water. No liquid hydrocarbon phases were used. At the onset of each Received: July 10, 2011 Revised: September 24, 2011 Published: October 04, 2011 5165
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Figure 2. Steel cell high-pressure test equipment.
Table 1. Composition of SNG component
mole percentage
methane
80.67
ethane
10.20
propane isobutane
4.90 1.53
n-butane
0.76
N2
0.10
CO2
1.84
Figure 1. General synthesis of polyaspartates or polyaspartamides from PSI. “constant cooling” experiment, the pressure was 78 bar. The equilibrium temperature at this pressure was calculated using Calsep’s PVTSim software to be 20.1 °C for DI water and 90 bar, which was approximately 0.3 °C lower than our laboratory experiments in a larger 200 mL titanium cell to determine the equilibrium temperature by standard slow hydrate dissociation (Figure 3).37,38 The cooling rate near the equilibrium temperature was 0.14 °C/h. From our experience, the equilibrium temperature could have been up to 0.2 °C lower if we had used a cooling rate of 0.05 °C/h, which would have given very good agreement with the predicted equilibrium temperature, but this makes the experiments considerably more time-consuming. For the rest of this paper, we will assume that the equilibrium temperature for our SNG water system at 78 bar is 20.1 °C. At the onset of each “precursor constant cooling” experiment, the pressure was 98 bar. The equilibrium temperature at this pressure was calculated using Calsep’s PVTSim software to be approximately 18.3 °C (Figure 3). The same initial procedure for the preparation of the KHI experiment and filling of the cell was followed in all high-pressure experiments in the steel cell: (1) The polymer to be tested was dissolved in DI water to the desired concentration, either 2500, 5000, or 10 000 parts per million
Figure 3. PT diagram showing the calculated equilibrium curve for the SNG in DI water. (ppm) (0.25, 0.5, or 1.0 wt %, respectively). (2) The magnet housing of the cell was filled with the aqueous solution containing the additive to be tested. For standard constant cooling tests, we used 8 mL of aqueous solution. The magnet housing was then mounted in the bottom end piece of the cell, which thereafter was attached to the steel tube and the cell holder. (3) The desired amount of the aqueous solution was filled in the cell (above the cell bottom) using a pipet; the top end piece was 5166
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Figure 4. Typical constant cooling test with 5000 ppm of a reference KHI.
mounted; and the cell was placed into the cooling bath (plastic cylinder). (4) The temperature of the cooling bath was adjusted to 1 3 °C above the hydrate equilibrium temperature at the pressure conditions to be used in the experiment. (5) After the cell was purged twice with the SNG, the cell was loaded with SNG to the desired pressure while stirring at 600 rpm. The constant cooling test method was carried out as follows. After the initial procedure given above, the cell was charged with SNG gas at 78 bar and 20.5 °C. The cell was stirred at 600 rpm while cooling at a constant rate to 2 °C over 18 h. As a result of being a closed system, the pressure drops as the temperature decreases. Because the rate of cooling was slow, we were able to determine the detectable start of hydrate formation as the first deviation from the constant decrease in the absolute pressure in the cell during cooling. This indicates that gas is being used to form gas hydrates. We also noted the temperature at which fast, catastrophic hydrate formation took place as the point when the pressure drop curve became almost vertical, i.e. when gas consumption for gas hydrate formation was very fast. This led to a hydrate plug forming in the cell and stopping the stirrer. A typical plot of pressure and temperature versus time is given in Figure 4. A total of five to six constant cooling experiments were carried out with each polymer at 0.5 wt % in the aqueous phase. The onset temperatures for first detectable hydrate formation (To) and the temperatures for catastrophic hydrate formation (Ta) are given. Hydrate formation is first detected at 528 min at To = 12.2 °C, as a pressure drop deviation from the pressure drop because of the temperature decrease. Fast hydrate formation occurs after 651 min at Ta = 10.5 °C.
’ SCALE INHIBITOR EQUIPMENT AND TEST PROCEDURE A schematic of the high-pressure dynamic tube blocking rig is shown in Figure 5, and a photograph of the full rig shown in Figure 6. This is an automated scale inhibitor dynamic test rig assembled and purchased from Scaled Solutions Ltd., U.K. The heart of the rig consists of three pumps, which can pump fluids up to 10 mL/min through a microbore coil of 316 steel. This coil is placed in a heated oven and is 3 m long with 1 mm internal diameter. The initiation and rate of scaling occurring in the coil is measured by recording the differential pressure across the coil. All data are collected on a personal computer (PC) using Labview 8.0 software. The rig is designed for temperatures between 20 and 200 °C
Figure 5. Schematic of the dynamic tube blocking equipment for scale inhibitor testing. and pressures up to 300 bar (ca. 4200 psi). All experiments described in this work were carried out at 100 °C and 80 bar. The equipment was set up to automatically carry out four stages of testing in each experimental run: (1) a blank test with no scale inhibitor, (2) a series of tests with scale inhibitor for 1 h each at decreasing concentrations, (3) a repeat test with the scale inhibitor starting at the previous concentration that led to rapid scale formation in the first series of scale inhibitor tests, and (4) a second blank test with no scale inhibitor. Pump 1 is used to inject scaling cations (brine 1); pump 2 is used to inject scaling anions (brine 2) as well as the coil-cleaning solutions; and pump 3 is used to inject scale inhibitor solution. The software can be automatically set to reduce the scale inhibitor concentration, which for our experiments was every 1 h. For example, the scale inhibitor concentrations might begin at 100 ppm and decrease to 50, 20, 10, 5, 2, and 1 ppm every 1 h until scale formation occurs. Rapid scale formation because of failure at any particular scale inhibitor concentration is taken as the point when the differential pressure increases to over 0.5 bar (7 psi). We call this the fail concentration (FIC) of the scale inhibitor and not the minimum inhibitor concentration (MIC). This is to avoid confusion with the operational use of the term MIC defined as the minimum inhibitor concentration, which prevents scale formation. If a 5167
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Figure 6. Equipment for dynamic tube blocking testing of scale inhibitors.
Figure 7. Example of logging data for a scale inhibitor test. chemical failed at the first and highest concentration injected, the test was repeated at higher concentrations to make sure that we determined the true fail concentration. If the concentration of inhibitor in the mixed brine was 100 ppm or more, the stock solution to be injected was made to be 3000 or 5000 ppm. This was performed to avoid injecting too much fluid from pump 3 and significantly changing the scaling potential, which otherwise would give erroneous scale inhibitor performance results. Between each stage, the scale in the coil, whether calcium carbonate or barium sulfate, was cleaned out using 5 wt % tetrasodium ethylenediamine tetraacetate (Na4EDTA) solution at pH 12 13 for 10 min at 9 mL/min flow rate and then with DI water for 10 min also at 9 mL/min flow rate. Aqueous acetic acid (5 wt %) was initially used to clean out carbonate scaling, but we found that this gave gradual corrosion and leaks within the microbore coil. A typical graph showing the four stages in an experiment is shown in Figure 7. This experiment was for calcium carbonate scaling, but similar graphs are also obtained with barium sulfate scaling. The data recorded
in the graph are the absolute pressure on either side of the scaling coil (absolute 1 and absolute 2), as well as the differential pressure (diff P) across the scaling coil. The differential pressure is usually about 1 psi at a flow rate of 10 mL/min without any scaling present. As seen from the graph, enough scale formed without any inhibitor after approximately 14 min for the differential pressure to rise above 7 psi. At this point, the coil is cleaned out with the Na4EDTA solution, causing a drop in differential pressure to 1 psi. Then, DI water is pumped for 10 min to clean out the Na4EDTA solution. At about 34 min into the experiment, the first scale inhibitor test is begun using the highest preset inhibitor concentration. At this point, pump 2 switches from cleaning solution to scaling brine 2, causing a momentary drop in differential pressure, as seen on the graph. Such momentary drops are seen at the end of each cleaning cycle before new scaling ions are pumped through. In this example, we injected a scale inhibitor at 50, 20, and 10 ppm for 1 h each. After 40 min at 10 ppm (i.e., 194 min on the logger), rapid scale formation occurred. After cleaning, the repeat scale inhibitor test is carried out but starting from 20 ppm at 214 min on the logger. After 34 min at 10 ppm (307 min on the logger), scale forms rapidly again. This shows that the reproducibility of the experiments is very good, which was true for all experiments in this study. The final stage of the experiment was a new blank test without added inhibitor. In this case, the second blank scaling time was 19 min. We have found that the time for scaling in the second blank test is normally a little longer than the first blank test, for both carbonate and sulfate scaling. This may be due to the time needed to flush out the DI water cleaning fluid in the system, which is not present before the first blank test. Before the first blank test, we normally flush the scaling brines, one at a time, to check for good flow in the system. For this study, we chose to use model fluids based on production from the Heidrun oilfield, Norway. The composition of aqueous produced fluids from this field is given in Table 2. We used formation water only to produce calcium carbonate scaling and a 50:50 volume mixture of formation water and synthetic seawater to produce barium sulfate 5168
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Table 2. Composition of Heidrun Formation Water, Seawater, and a 50:50 Mix of These Solutions
Table 4. Salts Used To Make Brines 1 and 2 for Barium Sulfate Scaling
Heidrun formation
seawater
50:50 mix of
ion
water (ppm)
(ppm)
formation and seawater (ppm)
+
Na+ Ca2+
19510 1020
10890 428
Mg2+
265
1368
+
K
545
Ba2+ Sr2+
ion
38.64 5.31 13.66
816.5
K+
502
KCl
460
502.5
Ba2+
142.5
0.51
285
0
142.5
Sr2+
BaCl2 3 2H2O
72.5
0.44
145
0
72.5
SO42
SrCl2 3 6H2O
1480
Na2SO4
0
0
2960
1480
Fe2+
25
FeCl2 3 4H2O
0.16
1000
0
500
component
brine 1 (g/L)
brine 2 (g/L)
49.59
49.59
39020
Ca2+
2040
7.48
Mg2+
CaCl2 3 2H2O
530 1090 570
MgCl2 3 6H2O KCl BaCl2 3 2H2O
4.43
K+ Ba2+ HCO3
290
NaCl
SrCl2 3 6H2O NaHCO3
0
25
FeCl2 3 4H2O
0.18
2.75
’ RESULTS KHI Test Results. Hydrate formation is known to be a stochastic process.2,3 Therefore, four to five experiments were carried out at any one set of conditions with each polymer in the high-pressure steel autoclave. This was to ensure that statistically significant results could be obtained when comparing To or Ta values between two polymers.39 We did not obtain good statistical performance differences (p value > 0.05) using t tests for the onset temperatures (To) for all polymers tested. Some polymers gave very similar To values. However, as will become apparent in the text below, we did obtain clear statistical significance in the results for some polymers, which allowed us to make conclusions regarding the structure performance relationship.
4.37
average To (°C)
average Ta (°C)
no additive
18.0
16.1
1:1 VP:VCap co-polymer
12.0
8.4
100% NaOH 50:50 iBuNH2/NaOH
17.9 15.3
16.4 15.1
polyaspartamide
0.88
scaling. For calcium carbonate scaling experiments at 100 °C and 80 bar and a total flow rate of 10 mL/min, we found that using a bicarbonate concentration of 500 ppm gave blank scaling times of over 1 h. Therefore, we doubled the bicarbonate concentration to 1000 ppm to give suitably short blank scaling times of about 10 15 min at the test conditions. The compositions of brines 1 and 2 to make calcium carbonate and barium sulfate scales are given in Tables 3 and 4, respectively. In all experiments, we injected a 50:50 mixture of brines 1 and 2. Usually 3 5 L of brines 1 and 2 were made up and degassed prior to use. Degassing was performed using a vacuum pump for 15 20 min. This had two effects: first, to prevent gas bubbles from forming in the solutions, because this could stop the pumps from injecting fluids, and second, to remove dissolved oxygen, which could oxidize iron(II) ions to iron(III) ions. When making brine 2 for carbonate scaling, we degassed before adding sodium bicarbonate to avoid disturbing the equilibrium with CO2 and carbonate ions.
1.92
Table 5. Average Onset (To) and Fast Hydrate Formation (Ta) Temperatures for Constant Cooling KHI Tests Using 5000 ppm of Varying Polyaspartamidesa
2.08 1.01
1000
Fe2+
35.04
NaCl
Na+
Sr2+
brine 2 (g/L)
CaCl2 3 2H2O MgCl2 3 6H2O
ppm
15200
brine 1 (g/L)
724 807
15200 724
Table 3. Salts Used To Make Brines 1 and 2 for Calcium Carbonate Scaling ion
Na
component
Ca2+ Mg2+
SO42 HCO3
ppm
75:25 iBuNH2/NaOH
12.2
11.9
75:25 iBuNH2/MeNH2
12.1
11.7
repeat with very dry DMF
12.0
11.7
using MeNH2 in ethanol
11.8
11.6
75:25 iBuNH2/ethanol
11.7
11.4
90:10 iBuNH2/DEA
12.2
11.0
90:10 iBuNH2/DMAPb 80:20 iBuNH2/MEA
12.5 12.7
11.1 12.0
80:20 iPeNH2/DEA
12.0
12.0
80:20 iPeNH2/DMAP
10.7
10.3
75:25 iPeNH2/DMAP
10.5
9.8
67:33 iPeNH2/NaOH
13.5
13.2
a
MEA, monoethanolamine; DEA, diethanolamine; DMAP, 3-dimethylaminopropylamine. b The 5000 ppm solution is slightly cloudy.
The constant cooling KHI test results are summarized in Table 5. Generally, the scattering in onset temperatures, To, was only 10 15% and about 10% for rapid hydrate formation temperatures (Ta). With no additive, hydrate formation is first detected at an average of 18.0 °C, which is only about 1.5 °C lower than the equilibrium temperature. For comparison purposes, a low-molecular-weight 1:1 N-vinyl pyrrolidone VP/VCap (sample kindly supplied as Luvicap 55W by BASF, Germany) was also tested. This gave an average onset temperature for hydrate formation of 12.0 °C (p value < 0.05 for comparison to tests with no additive), followed by a long slow growth period until rapid hydrate formation occurred at an average of 8.4 °C. In contrast, all of the polyaspartamides gave short slow growth periods. All of the synthesized polyaspartamides contained isopentyl or isobutyl groups because an earlier studied indicated that these alkyl groups gave polymers with the best KHI performance.24 In this earlier study, we varied the percentage of these isoalkyl groups, which are introduced by reacting PSI with the relevant isoalkylamine. The study had concluded that 75 80% isobutyl groups and 20 25% methyl groups gave a polyaspartamide with the best performance. Because polyaspartamides with 100% 5169
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Table 6. Dynamic Scale Inhibitor Test Results on a Carbonate Scalea polyaspartamide
a
first blank (min)
first scale (min) (ppm)
second scale (min) (ppm)
second blank (min)
100% NaOH
15
14 (1)
15 (1)
17
50:50 iBuNH2/NaOH
14
40 (5)
33 (5)
19
75:25 iBuNH2/NaOH
17
5 (10)
56 (20)
17
75:25 iBuNH2/MeNH2
9
17 (50)
18 (50)
16
repeat with dry DMF
12
20 (50)
17 (50)
17
67:33 iPeNH2/NaOH
9
25 (10)
30 (10)
17
90:10 iBuNH2/DEA
13
25 (25)
27 (25)
17
90:10 iBuNH2/DMAP 80:20 iPeNH2/DMAP
13 10
15 (25) 47 (100)
17 (25) 44 (100)
17 16
90:10 iPeNH2/DMAPb
8
59 (100)
57 (100)
18
DEA, diethanolamine; DMAP, 3-dimethylaminopropylamine. b The 2000 ppm solution is slightly cloudy.
Table 7. Dynamic Scale Inhibitor Test Results on a Sulfate Scalea
a
polyaspartamide in DMF
first blank (min)
first scale (min) (ppm)
second scale (min) (ppm)
second blank (min)
100% NaOH 50:50 iBuNH2/NaOH
9 10
40 (2) 20 (20)
39 (2) 19 (20)
15 11
75:25 iBuNH2/NaOH
8
29 (50)
22 (50)
12
75:25 iBuNH2/MeNH2
8
58 (250)
12
repeat with dry DMF
9
54 (250)
50 (250)
17
67:33 iPeNH2/NaOH
9
46 (20)
40 (20)
13
90:10 iBuNH2/DEA
12
55 (100)
57 (100)
13
90:10 iBuNH2/DMAPb
11
23 (100)
17 (100)
14
1 (100)
DEA, diethanolamine; DMAP, 3-dimethylaminopropylamine. b The 2000 ppm solution is slightly cloudy.
pendant isobutyl or a very high percentage of isopentyl groups were previously found to be water-insoluble, we also used several other more hydrophilic alkylamines in minor amounts to increase the percentage of the most active isoalkyl groups with losing water solubility. These included monethanolamine (MEA), diethanolamine (DEA), and 3-dimethylaminopropylamine (DMAP). Using these amines, we were able to raise the percentage of isoalkylamines to 80 90% with losing water solubility. For polyaspartamides with isobutyl groups, we found that we could not improve the KHI performance significantly compared to the polymer made with a 75:25 mixture of isobutylamine/methylamine. As Table 5 shows, the average To value for several polyaspartamides lies around 11.7 12.5 °C and, thus, the relative performance of these polymers was indistinguishable to any significant statistical level (p values > 0.05). However, two polymers with isopentyl groups did perform statistically significantly better than any of the isobutylated polymers (p values < 0.05). These were polymers with pendant groups made from 80:20 iPeNH2/DMAP and 75:25 iPeNH2/DMAP. The increased performance may be due to the increased size of the isopentyl group, which will give greater perturbation of the bulk water structure and stronger van der Waals interactions with hydrate cavities, both of which can hinder hydrate nucleation. Because the percentage of carboxylate groups is increased above about 25% in the polyaspartamide, there is a clear trend to a decrease in the KHI performance. For example, a polymer made from reacting PSI with a 50:50 mol ratio of isobutylamine/ sodium hydroxide, giving 50% carboxylate groups, gave an average To value of 15.3 °C. This is approximately 3 °C higher than the average To value for a polymer with 25% carboxylate groups and 75% isobutyl groups. A polymer with 33%
carboxylate groups and 67% isopentyl groups gave an average To value of 13.5 °C, also about 3.0 °C higher than the best isopentylated polymer. These observations are all statistically significant (p values < 0.05). Finally, sodium polyaspartate, which has only pendant carboxylate groups, gave a performance similar to no additive. This polymer has no pendant hydrophobic groups at all and will therefore have a negligible effect on the hydrate formation processes of nucleation and crystal growth. Scale Inhibitor Results. The scale inhibitor results in the dynamic tube blocking equipment are summarized in Tables 6 and 7 for calcium carbonate and barium sulfate scales, respectively. The general trend with both types of scaling is that the scale inhibition performance increases as the percentage of carboxylate groups increases. This is the opposite trend to that found for KHI performance. Thus, the fail concentration with a 100% polyaspartate made by reacting PSI with sodium hydroxide is as low as 1 ppm. When 50% of the carboxylate groups are replaced with isobutylamide groups, the fail concentration drops to 5 ppm. The fail concentration is even higher (10 ppm) for a polymer with 33% carboxylate and 67% isopentylamide groups. This may be due to two reasons. First, a slightly lower molar concentration for this polymer, as the isopentyl group, has a higher atomic mass than the isobutyl group. Second, the isopentyl groups may cause greater steric hindrance for the carboxylate groups to interact and inhibit the growth of scale particles. However, the most important result is that, even when using polyaspartamides made only from amines and PSI (i.e., without deliberately introducing carboxylate groups), they still function as scale inhibitors albeit at much higher concentrations than polyaspartates with 100% carboxylate groups. The highest fail concentrations were 100 ppm. This is a high concentration if the 5170
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Figure 8. Dynamic carbonate scale test using a polyaspartate/polyaspartamide made from a 50:50 iBuNH2/NaOH molar mixture in DMF.
ARTICLE
We have found that, for some polymers, good scale inhibition can be obtained for both types of scale without compromising the KHI performance when dosed at several thousand ppm, a typical KHI dosage. The polyaspartamides have high cloud points and are compatible with brines containing calcium ions up to a concentration of at least 2000 ppm. Thus, use of polyaspartamides as KHIs for gas hydrate control at up to about 7 8 °C subcooling should not require the extra use, where needed, of a specialized SI for topside carbonate or sulfate scale inhibition. It has been shown that many corrosion inhibitors are not very compatible with commercial KHI polymers.43,44 Polyaspartates have been shown to work as corrosion inhibitors.45 47 Thus, a high KHI dosage of a PSI derivative may also help prevent corrosion besides giving gas hydrate and scale protection. We are currently investigating this line of research.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: +47-51831823. Fax: +47-51831750. E-mail: malcolm.
[email protected].
’ REFERENCES
Figure 9. Dynamic sulfate scale test using a polyaspartamide made from a 75/25 iBuNH2/MeNH2 molar mixture in DMF.
polymer was to be used alone as a scale inhibitor, because there are a variety of carboxylated, phosphonated, or sulfonated scale inhibitors, which will perform at much lower concentrations.2,40 42 However, if the polyaspartamide is used primarily for kinetic hydrate inhibition, which usually requires a dosage of a few thousand ppm, this should be ample polymer to combat even severe carbonate or sulfate scaling potentials (Figures 8 and 9). We also did not encounter any slow pressure buildup in the coil because of inhibitor Ca2+ precipitation with any of the polyaspartamides with either carbonate or sulfate scaling brines. This indicates that the polyaspartamides are compatible with these brines at 100 °C. However, the maximum concentration of inhibitor was only 200 ppm. Therefore, we checked the calcium compatibility using polyaspartamide concentrations of up to 5000 ppm, which is a typical concentration used for kinetic hydrate inhibition. Again, we observed no precipitation in bottle tests with the brines used in this study at up to 100 °C.
’ CONCLUSION We have synthesized an extended range of PSI derivatives (polyaspartamides) and tested them as KHIs in high-pressure autoclaves with SNG and as SIs for both calcium carbonate and barium inhibition in a high-pressure dynamic tube blocking rig. In comparison to previously published work, we have further improved the KHI performance of the polyaspartamides class by incorporating a high percentage of isopentyl groups into the structure without losing water solubility.
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