Article pubs.acs.org/EF
N,N‑Dimethylhydrazidoacrylamides. Part 2: High-Cloud-Point Kinetic Hydrate Inhibitor Copolymers with N- Vinylcaprolactam and Effect of pH on Performance Mohamed F. Mady†,‡ and Malcolm A. Kelland*,† †
Department of Mathematics and Natural Science, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ‡ Department of Green Chemistry, National Research Centre, Dokki, Cairo 12622, Egypt ABSTRACT: Poly(N,N-dimethylhydrazidoacrylamides)s (PDMHAMs) and a series of copolymers of (N,N-dimethylhydrazidoacrylamide) and (N-isopropylacrylamide) were recently investigated as kinetic hydrate inhibitors (KHIs). Poly(Nvinylcaprolactam) (PVCap) and related copolymers have been used as a class of commercial KHIs in the oil and gas industry to prevent plugging of pipelines with gas hydrates. In this study, we have synthesized and investigated the ability of copolymers of DMHAM monomer and VCap monomer to inhibit structure II gas hydrate formation in high-pressure rocking cells at approximately 75 bar. Various polymer molecular weights have been investigated at various pH conditions. It was found that the novel polymers have high cloud points in deionized (DI) water or brine solutions at high or low pH under pressure compared to polyVCap. A 1:2 copolymer of DMHAM/VCap gave the best KHI performance for this class of copolymer with a cloud point at 50 °C in DI water. Also, a 1:1 DMHAM/VCap copolymer gave excellent KHI performance, as well as giving no cloud point up to 100 °C in DI water or high-salinity solutions (3.6−15 wt % NaCl) at pH 5.0. The cloud point was found to be 67 °C at pH 12.0 for the 15 wt % NaCl solution, making it compatible for use in high-salinity water-based deep-water drilling fluids. It was also found that high pH improved the performance of the KHIs compared to otherwise identical tests at pH 5.0. This may be related to removing the structure I (SI)-forming and relatively more soluble acid gas CO2 from the system at high pH.
1. INTRODUCTION Natural gas hydrates are crystalline solids, in which gas molecules (guest molecules) are trapped inside hydrogen-bonded water cavities. Typical guest molecules include carbon dioxide and small hydrocarbons, such as methane, ethane, and propane. Lowdosage hydrate inhibitors (LDHIs) are now one of the significant methods of gas hydrate control technology.1−4 One category of LDHIs, kinetic hydrate inhibitors (KHIs), were first developed in the 1990s and are now used to prevent gas hydrate plugging of flow lines worldwide.1,3 KHI formulations contain a primary ingredient of a watersoluble polymer that delays the nucleation and usually the crystal growth of gas hydrates. The most critical factors for field operations using a KHI are the delay time to hydrate nucleation (induction time) and its dependence upon the subcooling at the field conditions, field fluids, and type of gas hydrate [structure I (SI) or structure II (SII)] that can potentially form.5 Alkylacrylamide polymers and their use in oil and gas applications have been widely investigated in recent research.5 Hydrophobically modified methacrylamide and acrylamide polymers are well-known KHIs. However, there are some limitations to the use of both N-isopropylmethacrylamide (IPMAM) and N-isopropylacrylamide (IPAM) polymer KHIs.6−8 One of these limitations is the cloud point (Tcl). It was reported that Tcl values (as well as deposition points) for both IPAM and IPMAM homopolymers are low [30−40 °C in deionized (DI) water], making it difficult to use in the field because of precipitation problems at high wellhead injection temperatures.9,10 © XXXX American Chemical Society
In a continuation of our efforts on acrylamide-based KHIs,11−13 we have also recently investigated variations on these polymers. We kept in mind that the hydrophobic groups in a KHI are considered a key structural feature for their KHI performance, as long as there are sufficient hydrophilic groups to keep the polymer water-soluble.14,15 Therefore, we designed and synthesized a poly(N,N-dimethylhydrazidoacrylamide) (polyDMHAM) homopolymer and also copolymerized the DMHAM monomer with IPAM (Figure 1). The hydrazine-based starting material used to make the side chains of the DMHAM polymers is 1,1-dimethylhydrazine, a cheap, commercial chemical that has been used in rocket fuels.
Figure 1. (From left to right) (A) Structures of poly(Nisopropylmethacrylamide) (polyIPMAM; R = CH3) and poly(Nisopropylacrylamide) (polyIPAM; R = H), (B) structure of poly(N,Ndimethyhydrazidoacrylamide) (polyDMHAM), and (C) structure of copolymers of N,N-dimethylhydrazidoacrylamide (DMHAM) and Nisopropylacrylamide (IPAM). Received: November 6, 2014 Revised: January 7, 2015
A
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diameter, 150 mm × 3), Tosoh Co. [two polymers gave high polydispersity index (PDI) values; attempts were made to obtain better molecular weight data using water and polyethylene glycol (PEG) molecular weight standards, but the results were not improved]. 2.3. Cloud Point (Tcl) Measurement. Cloud point measurement was carried out by a standard procedure: an aqueous solution of 1 wt % polymer was heated carefully at about 2 °C/min with constant stirring, making visual observations throughout. The cloud point temperature was determined by the first sign of haze in the solution. When cloud points were measured, pH was adjusted after making the polymer solution using dilute hydrochloric acid (for low pH solutions) and sodium hydroxide (for high pH solutions). 2.4. Synthesis of Novel Polymers. 2.4.1. PolyDMHAM Homopolymer (2). PolyDMHAMs were synthesized using typical radical polymerization procedures as described in our previous work. The synthesized polyDMHAMs are summarized in Table 1.14
KHIs have been designed primarily for multiphase transportation in production lines. Several products are commercially available and have been used in field applications since about 1995.16−22 In deep water drilling, the high pressure and low temperature can cause hydrate formation in the drilling fluid if there is an influx of natural gas. However, the issue of low cloud point (Tcl) and deposition point (Tdp) is one reason that has limited the use of KHIs in deep-water drilling operations. The mud circulates during drilling between a maximum and a minimum temperature. The maximum mud circulation temperature is often 60−80 °C, although in some drilling operations, it can be higher.23,24 Thus, if the cloud point of the KHI polymer in the mud is above the maximum circulation temperature, it will never go turbid and undergo a phase change but will always remain soluble in the aqueous phase of the mud. N-Vinylcaprolactam (VCap) polymers have been used commercially in KHI formulations in oil and gas applications since the very first field applications (Figure 2). Many different
Table 1. Molecular Weight of Novel Polymers According to Gel Permeation Chromatography polymer (molar ratio, %) polyDMHAM-I (100) polyDMHAM-II (100) polyDMHAM-III (100) polyVCap (100) DMHAM/VCap (1:1) DMHAM/VCap (1:1) DMHAM/VCap (1:4) DMHAM/VCap (1:9) DMHAM/VCap (2:1)
Figure 2. Poly(N-vinylcaprolactam) (PVCap).
copolymers of VCap have been investigated previously, but only a few hydrophilic monomers are known to be useful for incorporation into VCap copolymers without significant loss of KHI performance relative to poly(N-vinylcaprolactam) (PVCap) homopolymer. For example, VCap/N-vinylpyrrolidone (VP) copolymers still exhibit good KHI activity and are commercially available.1,3,5,25 A 1:1 VCap/N-vinyl-N-methylacetamide (VCap/VIMA) copolymer was shown to be a superior SII hydrate KHI to PVCap.26 VCap/VIMA-based KHI products were used in the field for several years, although they are currently not available because of the increased cost of the VIMA monomer.17,18,27 The above-mentioned facts encouraged us to continue our exploration of polyDMHAM, and we sought to find highperformance KHI polymers that have cloud and deposition points of at least 50 °C. Thus, we have carried out the radical copolymerization of DMHAM with VCap monomer to give copolymers with a variety of molecular weights. The new polymers were tested for their ability to prevent SII gas hydrate formation in high-pressure multi-cell rocker rig experiments.
initiator (AAPH) (%, w/w)
molecular weight (Mn)
PDI
2
900
7.53
4
1600
68.02
10
600
24.46
2 2
4092 5657
1.51 1.44
2
6080
1.34
2
7100
1.35
2
9900
1.43
2
3657
1.48
2.4.2. PolyVCap Homopolymer (4). To a Schlenk tube equipped with a Young’s tap and a magnetic stirring bar were added VCap (3, 0.7 g, 5.03 mmol) and aqueous isopropyl alcohol (10 mL). 2,2′-Azobis(2methylpropionamidine)dihydrochloride (AAPH, 2%, w/w) was added, and nitrogen was bubbled through the mixture for 10 min. The mixture was then heated to 60 °C with stirring for 12 h. After completion of the reaction (monitored by 1H NMR), the solution was allowed to cool to room temperature and volatiles were removed under vacuum. This gave an almost quantitative yield of the polyVCap product. 2.4.3. Synthesis of Copolymers of DMHAM and VCap (5). The example shown here is for a 1:1 molar ratio copolymer. To a Schlenk tube equipped with a Young’s tap and a magnetic stirring bar were placed DMHAM (1, 0.5 g, 4.38 mmol), VCap (3, 0.61 g, 4.38 mmol), and 2% (w/w) AAPH (22.2 mg, 2 mol % of total monomer) in aqueous isopropyl alcohol (10.0 mL), flushed with nitrogen for at least 10 min. Under the protection of nitrogen, the reaction mixture was allowed to heat stepwise from room temperature to 40 °C and up to 60 °C, at which time additional 2% (w/w) AAPH was added. The reaction mixture was kept at each temperature for about 16−18 h. After this time, the reaction medium became viscous and the stirring was stopped. The polymer solution was cooled to room temperature. The solvents were removed under vacuum to leave a white solid. The reaction was monitored throughout by 1H NMR spectroscopy to determine the percentage of conversion of the monomer. It was found that, using this method, the copolymer yield is at least 97% by NMR and that the ratios in the copolymers are therefore roughly the same as the ratio of monomers added. Five DMHAM/VCap copolymers with different molecular weights were synthesized by the above procedure. The molecular weights of new polymers are summarized in Table 1. The route for synthesis of all new polymers is shown in Figure 3.
2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were purchased from VWR, Nippon Chemical Industrial Co., Ltd., Tokyo Chemical Industry Co., Ltd., and Sigma-Aldrich. All solvents were used as purchased without further purification. Caution: N,N-Dimethylhydrazine is toxic and a carcinogen and can be adsorbed through the skin. Care must be taken, and adequate gloves must be worn, when using this chemical. 2.2. Characterization of KHI Polymers. Nuclear magnetic resonance (NMR) spectra were recorded on a 300 MHz Varian NMR spectrometer in deuterated dimethyl sulfoxide (DMSO-d6) and D2O using tetramethylsilane (TMS) as an internal standard, which was used to calculate the conversion of the monomers. The molecular weight and molecular weight distribution of the new polymers were determined by gel permeation chromatography, using polystyrene samples as molecular weight standards. Gel permeation chromatography was performed with a HLC 8220 chromatograph (Tosoh Co., Tokyo, Japan) equipped with TSK gel super HM-H H4000/H3000/H2000 (7.8 mm B
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Figure 3. Synthesis of homopolymer and copolymer KHIs. 2.5. High-Pressure Gas Hydrate Rocker Rig Equipment Test Methods. Kinetic hydrate inhibition tests described herein were carried out by a constant cooling (ramping) test method. Tests were conducted in five high-pressure 40 mL steel rocking cells each containing a steel ball, as shown in Figure 4. The equipment was manufactured by PSL
Table 2. Composition of SNG component
mol %
methane ethane propane isobutane n-butane N2 CO2
80.67 10.20 4.90 1.53 0.76 0.10 1.84
pumping and filling with SNG to 2 bar, and then the procedure was repeated. (3) The cell was pressurized to 75 bar and rocked at 20 rocks/ min at an angle of 40°. (4) The cells were cooled from 20.5 °C at a rate of 1 °C/h down to 2 °C. If rapid hydrate formation had not occurred during this time, as judged by a large fast pressure drop, the temperature was held at 2 °C until it occurred. (5) The pressure and temperature for each individual cell as well as the cooling bath were logged on a computer. A model graph of the data for 1:1 DMHAM/VCap polymer at pH 5.0 obtained on one run with all five cells under standard constant cooling KHI tests in the multi-cell rocker rig is presented in Figure 5. The experimental pressure drops during cooling because of the system being closed. The first deviation from the pressure drop, because of the temperature drop, is taken as the time for the first observed onset of hydrate formation, To. At some point after this initial pressure drop, rapid hydrate formation takes place. This is detected by a fast pressure drop in the cells. The temperature at which this happens is called Ta. In addition, exotherms are sometimes seen around the time of the fastest rate of hydrate formation, near Ta. For example, we see this in experiments with no KHI. However, the example given is for a reasonably powerful KHI, which also slows the growth of hydrate crystals sufficiently that the exotherm is not easily seen as the produced heat is dissipated by the cooling effect of the cell materials and surrounding water bath. Figures 6 and 7 show the determination of the onset temperature (To) and the temperature of fast hydrate formation (Ta) for one of a series of 8−10 tests using 2500 ppm of 1:1 DMHAM/VCap polymer at pH 5.0 and 12.0, respectively. The overall results show that the temperatures are homogeneous for all cells in the water bath and none of
Figure 4. Rocker rig showing the five steel cells in a cooling bath. Systemtechnikk, Germany. The gas composition used was a synthetic natural gas mixture (SNG) given in Table 2. This gas composition preferentially forms SII gas hydrate. Details of the KHI test procedure are given below. At the start of each constant cooling experiment, the pressure was approximately 75 bar. The equilibrium temperature (Teq) at this pressure was determined by standard laboratory dissociation experiments, warming at 0.025 °C/h for the last 3−4 °C.28,29 Five repeat equilibrium tests were carried out, which gave 20.2 ± 0.05 °C as Teq. This value agrees very well with a calculated Teq value of 20.5 °C using Calsep’s PVTSim software. The constant cooling KHI test procedure was as follows: (1) Each cell was filled with 20 mL of DI water or aqueous solution of a dissolved additive. (2) Air in the cells was removed with a combination of vacuum C
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Figure 5. Pressure and temperature logged from five cells during a standard constant cooling KHI test in the multi-cell rocker rig at pH 5.0.
Figure 6. Pressure and temperature curves versus time during a standard constant cooling test using 2500 ppm of 1:1 DMHAM/VCap polymer at pH 5.0.
temperature (LCST) of about 30−40 °C in DI water depending upon the polymerization conditions and the molecular weight.29 In continuation of our study of the cloud point for DMHAM polymers, we tested the cloud point of the new synthesized DMHMAM/VCap polymers in DI water and at high salinity in the range of 3.6−15 wt % NaCl and at low and high pH. High pH fluids are less practical for use in production flow lines, where
the cells contains any systematic errors that lead to consistently better or worse results.
3. RESULTS AND DISCUSSION 3.1. Study of the Cloud Point of the New KHI Polymers. We have previously shown that the DMHAM polymers exhibit no cloud point in DI water up to 100 °C.14 In addition, polyVCap is known to have a cloud point or lower critical solution D
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Figure 7. Pressure and temperature curves versus time during a standard constant cooling test using 2500 ppm of 1:1 DMHAM/VCap polymer at pH 12.0.
Table 3. Molecular Weights of the Polymers and Cloud Points (Tcl) at Varying Salinity and pH cloud point Tcl (°C) pH 5.0
pH 12.0
polymer (molar ratio, %)
Mn
DI H2O
3.6% NaCl
7.0% NaCl
15.0% NaCl
15.0% NaCl
polyDMHAM-I (100) polyDMHAM-II (100) polyDMHAM-III (100) polyVCap (100) DMHAM/VCap (1:1) DMHAM/VCap (1:2) DMHAM/VCap (1:4) DMHAM/VCap (1:9) DMHAM/VCap (2:1)
900 1600 600 4092 5657 6080 7100 9900 3657
>100 >100 >100 34 >100 50 40 34 >100
>100 >100 >100 25 >100 33 28
>100 >100 >100
>100 >100 >100
>100 >100 >100
>100
>100
67
>100
>100
>100
>100
injection valve, even though the produced fluids at the injection point were well above the polymer cloud point temperature.33 The polymer was shown to make its way along the flow line with a chromatographic distribution of polymer molecular weights along the line. However, in general, operators still prefer to use a KHI with a cloud point (or at least a deposition point) above the injection point temperature (usually the wellhead) to be fairly safe of avoiding fouling issues near the injector and the loss of the KHI effect in the flow line. The Tcl of 1:1 DMHAM/VCap copolymer was observed to remain at over 100 °C in DI water and high salinity, but it dropped to 67 °C at pH 12.0. Also, 1:2, 1:4, and 1:9 DMHAM/ VCap copolymers were determined to have Tcl values of 50, 40, and 34 °C, respectively, in DI water at pH 5.0. The above results show that polyDMHAM has much higher hydrophilicity properties because of dimethylhydrazinium ionic groups forming in acidic conditioms and, therefore, has increased aqueous solubility. The high Tcl values in DI water at high salinity and alkalinity indicate that DMHAM polymers will have much improved water solubility for injection compatibility and salinity compatibility compared to commercial VCap polymers. 3.2. High-Pressure KHI Experiments. High-pressure natural gas hydrate constant cooling rocking cell experiments were carried out using 2500 ppm of novel KHI in DI water from
there is the potential for carbonate scaling, but can be useful in drilling fluids. The results are summarized in Table 3. PolyDMHAM-I, polyDMHAM-II, and polyDMHAM-III (varying molecular weight homopolymers) exhibited no cloud point up to 100 °C in DI water at high salinity and low or high pH. Also, the cloud point of 2:1 molar ratio DMHAM/VCap copolymer is also over 100 °C at the same range of conditions. At pH 5.0, our in-house low-molecular-weight polyVCap gave a cloud point of 34 °C in DI water, which dropped to 25 °C in 3.6 wt % NaCl aqueous solution. pH plays an important role for produced fluids in contact with acid gases, such as CO2 and H2S. The acid gases decrease the pH, often to around a value of 4−5, in the aqueous phase and cause corrosion problems of carbon steels.30,31 Therefore, it should be noted that all of the Tcl values in this study were measured at typical produced water acidic conditions as well as a high pH for some polymers. A high pH of 11−12 is typical of water-based drilling fluids. It should also be noted that there have been observed problems with deposition of sticky polymer deposits in laboratory heated probe tests, even at temperatures below the cloud point of the polymer (Tylor and Jardine, Baker Hughes, Inc., private communication). Conversely, the low cloud point polymer PVCap has been successfully injected in one Middle East field without a buildup of polymer deposit around the E
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Table 4. Constant Cooling KHI Tests in the Multi-cell Rocker Riga pH 5.0
a
pH 12.0
polymer (molar ratio, %)
To(av) (°C)
Ta(av) (°C)
To(av) − Ta(av) (°C)
To(av) (°C)
Ta(av) (°C)
To(av) − Ta(av) (°C)
no polymer polyDMHAM-I (100) polyDMHAM-II (100) polyDMHAM-III (100) polyVCap (100) DMHAM/VCap (1:1) DMHAM/VCap (1:2) DMHAM/VCap (1:4) DMHAM/VCap (1:9) DMHAM/VCap (2:1)
17.9 12.8 13.5 13.6 10.6 10.7 9.1 9.9 10.6 11.1
17.7 10.9 10.7 12.0 9.1 8.6 8.1 8.7 9.4 8.5
0.2 1.9 2.8 1.6 1.5 2.1 1.0 1.2 1.2 2.6
17.5
17.2
0.3
12.1 12.4 8.1 9.0
10.8 11.3 7.3 8.1
1.3 1.1 0.8 0.9
All polymer concentrations are 2500 ppm.
75 bar and 20.5−2.0 °C over 18.5 h. Table 4 and Figure 8 summarize the average onset temperatures (To) and fast hydrate
formation temperatures (Ta). The To value is considered the most important of the two temperature parameters because this refers to the first detection of hydrate formation, after which crystal growth can potentially lead to hydrate plugging. The difference between the To and Ta values gives some indication of the ability of the additive to retard the gas hydrate crystal growth process. Experiments were carried out using polymer solutions at two initial pH values, 5.0 and 12.0. These values were obtained from measurements before the solutions were added to the rocker rig. The pH of these solutions could be affected when the cells are pressurized with the SNG because this mixture contains the acid gas CO2. The amount of dissolved CO2 under pressure and, thus, the equilibrium amount of carbonic acid will be higher under pressure compared to when the pressure is released after the experiments. We measured the pH of the tested solutions after release of the pressure. For the solutions with initial pH 5, we observed very similar pH after the experiment. Aqueous test solutions at an initial value of pH 12 became less basic after the high-pressure experiments because of dissolution of CO2 gas in the aqueous phase, forming bicarbonate ions from reaction of insitu-generated carbonic acid with the hydroxide ions.
Figure 8. Average values from 8 to 10 experiments for the onset temperature (To) and fast hydrate formation temperature (Ta) in constant cooling KHI experiments at 2500 ppm at pH 5.0.
Figure 9. To and Ta values from the standard constant cooling tests at pH 5.0. F
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Figure 10. To and Ta values from the standard constant cooling tests at pH 12.0.
1:2 DMHAM/VCap copolymer. The average To and Ta values for 8−10 experiments were 9.1 and 8.1 °C. Also, 1:4 DMHAM/ VCap copolymer gave a good performance compared to polyVCap, in which the average To and Ta values are 9.9 and 8.7 °C, respectively. The statistical p value for comparison of To values for the two polymers polyVCap and 1:4 DMHAM/VCap copolymer is 0.001, showing a statistically significant difference in the KHI performance. The 1:2 and 1:4 DMHAM/VCap copolymers have somewhat higher cloud points (Tcl) of 50 and 40 °C in DI water at pH 5.0 compared to polyVCap. However, the molar percentage of DMHAM in DMHAM/VCap copolymers needs to be a little higher before a large increase in the copolymer cloud point occurs, as discussed below. Increasing the ratio of VCap/ DMHAM monomers to 9:1 decreased the KHI performance further as well as the cloud point. Increasing the molar percentage of DMHAm monomer to 50% in 1:1 DMHAM/VCap copolymer at pH 5.0 raised the cloud point to over 100 °C, even in 15 wt % NaCl solution, and also gave a fairly good performance as a SII gas hydrate KHI (Figure 6). The average To and Ta values are 10.7 and 8.6 °C, respectively. This performance at pH 5.0 is very similar to our sample of polyVCap, which has much greater solubility issues in DI water or high-salinity brines. Furthermore, the performance of 1:1 DMHAM/VCap copolymer in an initially high pH 12.0 solution was improved in comparison to pH 5.0. The average value of the onset temperature (To) decreased to 9.0 °C, and the average rapid hydrate formation temperature (Ta) decreased to 8.1 °C, as outlined in Figure 7. This is probably related to the effect of removing CO2 from the system at high pH, as discussed earlier. The cloud point of 1:1 DMHAM/VCap copolymer was found to be 67 °C at pH 12.0 for the 15 wt % NaCl solution, making it compatible for use in high-salinity water-based deepwater drilling fluids.
For the evaluation of the performed tests, a comparison to no additive was carried out. The hydrate onset temperature from 8 to 10 experiments occurred at an average To of 17.9 °C, and the average Ta value is 17.7 °C. These values are typical from previous identical studies reported previously.13−15 In addition, we tested a solution of sodium hydroxide with initial pH 12, which will drop when the cells are pressurized with the SNG containing the acid gas CO2. The results given in Table 4 show that there is no significant difference in the To values, giving an average from 10 experiments of 17.5 °C. In comparison of the 10 To values for each polymer, the KHI performance difference between the three different lowmolecular-weight homopolymers, polyDMHAM-I, polyDMHAM-II, and polyDMHAM-III, is not statistically significant. This was concluded by determining the p value of over 0.05 in independent sample statistical t tests with equal variances for each polymer. When the p value is less than the predetermined significance level, which is often 0.05, this indicates that the observed result would be highly statistically significant.32 Figures 9 and 10 show the degree of scattering in the results from the standard constant cooling tests for all of the novel KHI polymers at pH 5.0 and 12.0, respectively. The results of standard constant cooling tests at pH 5.0 show that the average value of the onset temperature (To) for polyDMHAM-II and polyDMHAM-III homopolymers was very close (13.5−13.6 °C), with rapid hydrate formation temperature (Ta) in the range of 10.7−12.0 °C. However, at pH 12.0, the average values of onset temperatures (To) for the same polymers dropped to 12.1−12.4 °C and the rapid hydrate formation temperature (Ta) dropped to 10.8−11.3 °C. For polyVCap homopolymer, the average To and Ta values are 10.6 and 9.1 °C at pH 5.0. Again, the performance is improved at a higher pH of 12.0, in which the average To value drops to 8.1 °C and the average Ta value is now 7.3 °C. The reason for the improved performance at higher initial pH may be related to the removal of some or all of the CO2 acid gas under the alkaline conditions by forming bicarbonate or carbonate ions. CO2 is a SI hydrate former and has much higher solubility in water than small hydrocarbons. Therefore, removal of this acid gas will reduce the driving force and kinetics for hydrate formation. Removal of any weak protonation of the ketonic oxygen atom in the caprolactam ring may also occur at high pH, which may affect the KHI performance. The To and Ta values at pH 5.0 of different ratios of DMHAM/ VCap copolymers show that the best copolymer tested was the
4. CONCLUSION We have synthesized and investigated the ability of copolymers of DMHAM monomer and VCap monomer to inhibit SII gas hydrate formation in high-pressure rocking cells at approximately 75 bar. Various polymer molecular weights have been investigated at various pH conditions. It was found that the novel polymers have high cloud points in DI water or brine solutions at high or low pH under pressure compared to polyVCap. A 1:2 copolymer of DMHAM/VCap gave the best KHI performance for this class of copolymer with a cloud point at G
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50 °C in DI water. Also, a 1:1 DMHAM/VCap copolymer gave excellent KHI performance as well as giving no cloud point up to 100 °C in DI water or high-salinity solutions (3.6−15 wt % NaCl) at pH 5.0. The cloud point was found to be 67 °C at pH 12.0 for the 15 wt % NaCl solution, making it compatible for use in high-salinity water-based deep-water drilling fluids. It was also found that high pH improved the performance of the KHIs compared to otherwise identical tests at pH 5.0. This may be related to removing the SI-forming and relatively more soluble acid gas CO2 from the system at high pH. We are currently studying the KHI performance of DMHAM polymers on SI hydrate and polymers of N,N-dimethylhydrazidomethacrylamide (DMHMAM) monomer with methyl groups in the polyvinyl backbone.
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(21) MacDonald, A. W. R.; Petrie, M.; Wylde, J. J.; Chalmers, A. J.; Arjmandi, M. Proceedings of the SPE Gas Technology Symposium; Calgary, Alberta, Canada, May 15−17, 2006; SPE 99388. (22) Glenat, P.; Peytavy, J. L.; Holland-Jones, N.; Grainger, M. Proceedings of the 11th Abu Dhabi International Petroleum Exhibition and Conference; Abu Dhabi, United Arab Emirates, Oct 10−13, 2004; SPE 88751. (23) Kelland, M. A.; Iversen, J. E. Energy Fuels 2010, 24, 3003. (24) Kelland, M. A.; Monig, K.; Iversen, J. E.; Lekvam, K. Energy Fuels 2008, 22, 2405. (25) Kelland, M. A. A review of kinetic hydrate inhibitorsTailormade water-soluble polymers for oil and gas industry applications. In Advances in Materials Science Research; Wytherst, M. C., Ed.; Nova Science Publishers, Inc.: New York, 2011; Vol. 8, Chapter 5. (26) Colle, K. S.; Oelfke, R. H.; Kelland, M. A. U.S. Patent 5,874,660, Feb 23, 1999. (27) Talley, L. D.; Mitchell, G. F. Proceedings of the SPE Annual Technical Conference and Exhibition; New Orleans, LA, Sept 27−30, 1999; SPE 56770. (28) Gjertsen, L. H.; Fadnes, F. H. Ann. N. Y. Acad. Sci. 2000, 912, 722. (29) Tohidi, B.; Burgass, R. W.; Danesh, A.; Ostergaard, K. K.; Todd, A. C. Ann. N. Y. Acad. Sci. 2000, 912, 924. (30) DeWaard, C.; Williams, D. E. Ind. Finish. Surf. 1976, 28, 24. (31) Nesic, S. Corros. Sci. 2007, 49, 4308. (32) Myers, R. H.; Myers, S. L.; Walpole, R. E.; Ye, K. Probability and Statistics for Engineers and Scientists; Pearson Education International: Upper Saddle River, NJ, 2007. (33) Talley, L. ExxonMobil, private communication.
AUTHOR INFORMATION
Corresponding Author
*Telephone: +47-51831823. Fax: +47-51831750. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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DOI: 10.1021/ef502489d Energy Fuels XXXX, XXX, XXX−XXX
