Novel Benchtop Wheel Loop for Low Dosage Gas Hydrate Inhibitor

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Novel Benchtop Wheel Loop for Low Dosage Gas Hydrate Inhibitor Screening: Comparison to Rocking Cells for a Series of Antiagglomerants Malcolm A. Kelland,*,† Anders Grinrød,‡ and Erik G. Dirdal† †

Department of Mathematics and Natural Sciences, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ‡ M-I Swaco (Schlumberger), Koppholen 23, 4313 Sandnes, Norway ABSTRACT: We have designed and built a novel high-pressure wheel loop (volume 200 mL), which allows for easy observation of the fluids inside the whole of the loop. The wheel is mounted horizontally in a cooling bath and is small enough for use as a benchtop device for screening low dosage hydrate inhibitors. The wheel could also be used for other flow assurance issues such as wax inhibition studies. The wheel does not rotate but moves with a Euler motion to cause movement of the fluids around the loop. To increase agitation and reduce the stochastic nature in the hydrate formation process, a few small steel balls were added to the wheel loop. We report here results using this wheel for a series of quaternary ammonium surfactant antiagglomerants (AAs) tested at about 40 MPa at varying AA concentrations and aqueous salinity. The results are compared to tests reported earlier using the same surfactants but in high-pressure sapphire rocking cells under similar test conditions. The two different types of equipment gave the same ranking of AAs but in general we found that the wheel tests required a higher concentration of AA to achieve the same good AA effect at the same test conditions, especially in saline solutions.

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INTRODUCTION The potential for gas hydrate plugging of pipelines is a notorious concern for the oil and gas industry, and the concern only gets greater as operators find oil and gas in deeper and colder waters. Under conditions of high pressure and low temperatures, gas hydrate can form and agglomerate into plugs, jeopardizing production and hindering the use of process equipment.1,2 Therefore, a variety of methods have been developed to inhibit gas hydrate formation, one of them being to utilize low-dosage hydrate inhibitors (LDHIs), which consist of kinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs).3 LDHIs are relatively expensive oilfield chemicals, and it is therefore important to determine effective concentrations in laboratory apparatuses. Test apparatuses and methods are numerous, and the majority comprise various equipment using oxolane (tetrahydrofuran, THF) hydrate, rocker rigs, autoclaves, differential scanning calorimeters, pipe wheels, and flow loops of various sizes.4 Prior methods for assessing hydrate inhibitors performance concentration tend to suffer from not being repeatable in addition to be inconsistent. Thus, there are always possibilities for new hydrate inhibition test methods and apparatuses. A stationary benchtop wheel for LDHI testing has been described previously.5 The fluids in this wheel can be moved by placing a magnet in the wheel. A second magnet is placed under the wheel and moved around the wheel so that the magnet in the wheel moves, pushing the liquids in front of it. It is a similar movement to pigging a pipeline. © 2014 American Chemical Society

To avoid any moving parts in the wheel we wondered if instead the wheel could be rotated thereby causing flow of the fluids around the wheel. We tested this and found that irrespective of the percentage fluid volume we did not get good flow and mixing of the fluids. This was also the case with the addition of steel balls to the wheel loop. Therefore, we turned to another method of moving the wheel which is the Euler disk mechanical motion.6 The Euler disk motion is a way of tipping the wheel at constantly different positions around a central axis. It is close to the motion of a spinning coin that has almost come to rest. This motion causes the fluids to move around and slush about in the loop. This new prototype benchtop wheel has been used for testing low dosage hydrate inhibitors (LDHIs) although it could be used for a wider variety of experiments such as hydrate plugging tendencies in under-inhibited thermodynamic hydrate inhibitor systems, or wax gelling and deposition studies. The wheel is submerged in a water bath. Unique features of the wheel include its small size enabling it to fit easily on a standard laboratory bench, the acrylic top allowing one to view any part of the flow channel in the wheel loop, and the mode of moving the liquid in the apparatus by the Euler motion. Hence, no Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: June 19, 2014 Accepted: October 21, 2014 Published: November 3, 2014 252

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pumps or internal equipment for propelling the liquid are needed LDHIs are divided into two main subcategories, kinetic hydrate inhibitors (KHIs), and antiagglomerants (AAs). Of the two subcategories it seemed more appropriate to study AAs in the wheel loop since good visual data is imperative for checking the performance of AAs, while for KHIs monitoring pressure changes are usually sufficient for screening purposes. AAs can be studied in PVT cells as long as visual observations are available (e.g., through sapphire windows), and the stirrer blade (and its speed) do not significantly affect the morphology of the hydrate crystals. However, the ability to observe any potential hydrate deposition in the whole wheel loop, both top and bottom, would seem to be an advantage over stirred PVT cells. A wheel that is not fully flat, but is designed with low points to simulate more closely a real flow line, would be an even greater advantage but we are considering this for a second generation wheel. We chose to use a series of monotail quaternary tri-nbutylammonium bromide surfactants for this study (Figure 1).7

Table 1. Quaternary Ammonium Salts Used in This Study quaternary ammonium salt

mole purity

octyltributylammonium bromide (OBAB) decyltributylammonium bromide (DBAB) dodecyltributylammonium bromide (DDBAB) tetradecyltributylammonium bromide (TDBAB) hexadecyltributylammonium bromide (HDBAB) octadecyltributylammonium bromide (ODBAB) cocotributylammonium bromide (CBAB)

> > > > > > >

0.99 0.99 0.99 0.99 0.99 0.99 0.99

which is partially soluble in both the aqueous phase and hydrocarbon phase. A synthetic natural gas was used in all experiments. The composition is given in Table 2. This gas preferentially forms a Structure II hydrate as the most thermodynamically stable gas hydrate phase.16 Table 2. Synthetic Natural Gas (SNG) component

mole fraction

methane ethane propane CO2 methylpropane butane N2

0.804 0.103 0.05 0.0182 0.0165 0.0072 0.0011

Several liquid hydrocarbon phases were used. European white spirit from Europris was used as the hydrocarbon phase in most experiments. It consists of a blend of mostly aliphatic hydrocarbons and some aromatic hydrocarbons, as judged by nuclear magnetic resonance (NMR) spectroscopic analysis. Other hydrocarbon phases were crude oils and proprietary condensates donated by Statoil, Total, and Shell oil companies.

Figure 1. Structure of alkyltributylammonium bromide surfactants.

Although they have high toxicity and limited biodegradation, all commercial AAs are based around salts of quaternary surfactants.2,8−15 This series of surfactants we chose was previously investigated by us in a multirocking cell apparatus with sapphire cells.16 The study in the new wheel gave us an excellent opportunity to compare results with tests run in the rocking cells. The use of rocking cells has overtaken autoclave testing as the most common method of screening AAs in the laboratory. The motion of fluids in rocking cells is obviously not ideal compared to a real pipeline, and the size of the equipment of rocking cells is generally fairly small. It was hoped that the wheel loop would give a more accurate representation of the flow of fluids in pipelines.

Figure 2. Schematic of the whole apparatus setup, from left to right: gas, gas booster/distributer, water heater/cooler, the table top wheel in the water bath as seen from above, the two electronic boxes, computer and monitor with the software interface.

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EXPERIMENTAL METHODS The linear alkyltributylammonium bromide surfactants used varied in aliphatic tail length from octyl (OBAB, eight carbon tail) to octadecyl (ODBAB, 18 carbon tail). They were synthesized in our laboratory using typical quaternization procedures as described previously.16 This could be performed by refluxing alkyl bromides with either primary amines and base or alkylating tertiary amines in a polar solvent. The synthesized AAs were all recrystallized to give > 99 % purity according to 1 H NMR. They are listed in Table 1. CBAB contains a mixture of chain lengths ranging from C8− C18 but mostly in the range C12−C14. All of the AA samples were water-soluble up to a mole fraction equivalent to at least 0.005 mass fraction (5000 ppm) and did not form emulsions with the liquid hydrocarbons used. The exception was ODBAB,

The test equipment is shown in Figures 2 and 3. After several adjustments to optimize the test equipment the experimental method for KHI testing was established as follows: • The required amount of liquids (33 mL aqueous phase and 67 mL liquid hydrocarbons) was added to the wheel. This is half the total volume of the wheel, which is 200 mL. • The wheel was then pressurized with the synthetic natural gas to 4 MPa. At this pressure and water cut of 33 % the equilibrium temperature was determined to be approximately 11.5 °C by dissociation experiments, warming by 0.1 °C every 15 min. • The swirling “Euler” motion was initialized. 253

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Figure 5. A section of the wheel with massive hydrate formation, resulting in a plug. Rank E.

(D) significant hydrate deposits some of which are gradually eroded into a coarse hydrate slurry (E) hard plug of hydrate that remains through the test period Once an experiment was finished the data recording was terminated and the water bath heated to 25 °C for 2h to fully melt any formed hydrates. The movement of the wheel was terminated and depressurized back to atmospheric pressure. The fluids in the wheel were removed and the wheel washed thoroughly three times with distilled water and hydrocarbon liquids. Some gas is consumed into the liquid phases during the first 10 to 15 min of an experiment. Since the wheel was a closed system, a further pressure drop was observed due to the constant cooling. A deviation from this constant pressure drop, however, indicated gas consumption from hydrate formation. Sometimes, if very rapid hydrate nucleation occurred, an exothermic spike for the temperature in the wheel was observable on the graphs. This can be seen in Figure 7 at

Figure 3. Benchtop wheel submerged in the water bath.

• Recording of both the temperature of the water heater/ cooler and the internal temperature in the wheel, in addition to the pressure in the wheel, using the homemade software was started. • The water heater/cooler (error margin ± 0.1 °C) was set to the desired test temperature (normally 20 °C) and cooling began at the desired rate (normally 7.2 °C/h) down to 0.0 °C. The wheel was held at this temperature for at least 8 h. A temperature of 0.0 °C was chosen to give maximum subcooling without the potential interference of ice formation. • Visual inspection of the wheel was taken at regular intervals until no further pressure drop was observed. The AA performance was ranked by visual inspection from A to E (A being best and E being worst). Examples of ranks A and E are shown in Figures 4 and 5.

Figure 6. A typical graph of an entire experimental run: 0.002 mass fraction (2000 ppm) TDBAB in decane which gave an AA performance rank B.

about 101 min. However, hydrate nucleation might have been initiated earlier but went undetected. In Figure 6 no exothermic spike is seen. This is due to rapid agglomeration and plugging of the wheel loop due to the poor AA effect of the chemical. Rapid plugging can lead to a greater amount of free water being trapped in the hydrates which slows down the rate of hydrate growth sufficiently that no exothermic spike is observed.

Figure 4. A section of the wheel showing the hydrate slurry formed, showing a good AA result. Rank A.

The rankings from A to E based on visual observations are defined as follows and are the same as used in rocking cell tests: (A) fine, dispersed hydrates without any deposits (B) no plugging/deposit observed but larger, coarser hydrate particles (C) some hydrate deposits, but some fluids/hydrates still moving

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RESULTS AND DISCUSSION To increase the mixing further within the wheel we placed small barriers in the wheel loop as well as five small steel balls. Without the steel balls and using the ramping method 254

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Figure 7. Experimental run for 0.001 mass fraction (1000 ppm) TDBAB in decane which gave an AA performance rank E.

described earlier we obtained hydrate onset temperatures (To) in the range of 1 to 2 °C (standard deviation approximately 0.2 °C) with an average To of 1.4 °C using deionized water and 33% water cut (% volume of water in the total liquids volume) with white spirit. When the quaternary ammonium AAs were added we often did not observe hydrates down to the minimum test temperature of 0 °C owing to the additional weak KHI effect of the AAs. However, the addition of the steel balls significantly raised the onset temperature (To) for hydrate formation with deionized water and 33% water cut to 4.2 °C. (This is still well below the equilibrium temperature of approximately 11.5 °C. The addition of the steel balls also reduced the spread of To values that is due to the stochastic nature of the hydrate formation process. The balls were also small enough (2 mm diameter) that they did not appear to affect the hydrate particle size nor the agglomeration process. Clay particles were also added to see if they would promote hydrate formation at higher temperatures. The average To value was found to be 2.3 °C, which did indication promotion of hydrate formation but only about 1 °C higher than using deionized water and white spirit alone. The rotation of the axel causing the Euler motion was also varied from 18 to 38 rpm. Within this range we did not observe any significant difference in the To values. Therefore, we kept to 18 rpm in all future experiments. Having decided on a system using this rotation speed, 33% water cut, white spirit and 5 steel balls, we then began testing the quaternary ammonium AAs. Tests on each AA were repeated to check for consistency. In all cases To values were in a narrow range of 0 to 2 °C which was helpful for ranking the performance of AAs since the subcooling is one of several parameters that is known to affect AA performance. The AAs were examined in both deionized water and 1.5 wt % NaCl solution. The results are given in Table 2 and Figures 8 and 9 and compared to results found in the RCS20 rocking cell equipment previously reported.16 The results given are the minimum concentration of AA required to achieve a ranking A or B in the wheel loop, that is, no sign of any hydrate deposits throughout the test period. No experiments in distilled water were carried out for OBAB, DBAB, and ODBAB. This is because a concentration of 0.0015 mass fraction (15000 ppm) in 1.5 wt % NaCl either gave no pass (rank C or worse) or only just gave a pass. It is known that single-tailed quaternary surfactant AAs generally perform better in saline solutions; therefore, we assumed these three AAs would only show worse results in deionized water. This is also validated by the results on DDBAB, TDBAB, HDBAB, and CBAB in which the

Figure 8. MCAA values for the benchtop wheel and the RCS20 rocking cells (1000 ppm = 0.001 mass fraction). Columns from front to back are for 1.5 wt % NaCl brine in the wheel, 1.5 wt % brine in the rocking cells, deionized water in the wheel, and deionized water in the rocking cells.

Figure 9. MCAA values in the benchtop wheel for AAs in 1.5 wt % (15000 ppm) NaCl solution (columns at front) and DI water (back columns); 1000 ppm = 0.001 mass fraction.

concentration of AA needed for good performance was always higher in deionized water. The results in Table 3 and Figures 8 and 9 show that the benchtop wheel results for the same series of AAs had the same performance trend as the results from the rocking cells, both for 1.5 wt % NaCl solution and deionized water. Thus, for this series of AAs, the benchtop wheel appears to be a complementary method for ranking the performance of AA that is consistent with rocking cells. However, the minimum concentration of AA (MCAA) needed for good performance in the wheel was surprisingly much higher than tests in rocking cells, particularly those tests carried out in saline solution. For example, for DDBAB (tail length 12 carbon atoms), only a mass fraction of 0.00025 (250 ppm) based on the water phase was needed to prevent agglomeration and deposition of hydrates in the rocking cells. In the wheel a minimum of 0.003 mass fraction (3000 ppm) was needed. This and other similar results show that the wheel is a more conservative method of testing AAs, particularly in brines. In fresh water the MMCAs in the wheel and rocking cells are fairly similar. As stated at the end of the introductory section, the flow in the wheel is less turbulent than in the rocking cells such that 255

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Table 3. AA Concentrations as Mass Fraction (with ppm Values in Brackets) Needed for Both the Wheel Apparatus and the Rocking Cells To Achieve a Pass (MCAA Values) wheel AA OBAB DBAB DDBAB TDBAB HDBAB ODBAB CBAB

rocking cells

DI water

1.5 wt % NaCl

0.004 (4000) 0.005 (5000) 0.015 (15000)

no pass no pass 0.003 (3000) 0.002 (2000) 0.002 (2000) 0.015 (15000) 0.004 (4000)

0.005 (5000)

1.5 wt % NaCl

7 wt % NaCl

DDBAB TDBAB

0.004 0.005

0.003 0.002

0.001 0.0005

0.003 (3000) 0.005 (5000) 0.01 (10000)

no pass no pass (0.00025) 250 0.001 (1000) 0.001 (1000) 0.01 (10000) 0.0005 (500)

0.004 (4000)

Table 5. Different Antiagglomerants and the Effect the Different White Spirits Had on the Concentrations in Mass Fraction (with ppm Values in Brackets) of the Antiagglomerants To Get a “pass” Result, i.e., Ranking A or B

Table 4. MCAA Mass Fraction Values (with ppm Values in Brackets) for DDBAB and TDBAB in Various Aqueous Solutions DI water

1.5 wt % NaCl

ance of some AAs, but this was outside the scope of this project.17 The final experiments in this study were conducted using an alternative white spirit (Kemetyl) to see if this made any difference to the MCAA values with DDBAB, TDBAB, and HDBAB using deionized water. The results with the standard white spirit (Europris) and the alternate white spirit (Kemetyl) are summarized in Table 5. The results indicate that the AA

hydrate crystals are less likely to be crushed and dispersed in the wheel system. This added “AA effect” in the rocking cells can lead to less AA being needed compared to the wheel. The flow in the wheel is also more similar to laminar, the steel balls having little effect on fluid turbulence or even the crushing of hydrate crystals. Thus, a lower mass transport rate of the AA to the hydrate crystal surface may also be present in the wheel compared to more turbulent rocking cells with relatively larger steel balls. We also investigated DDBAB and TDBAB at a higher brine concentration of 7.0 wt % NaCl. The results are summarized in Table 4 and Figure 10 and compared to results with 1.5% NaCl

AA

distilled water

AA

white spirit Europris

white spirit Kemetyl

DDBAB TDBAB HDBAB

0.004 (4000) 0.005 (5000) 0.015 (15000)

0.005 (5000) 0.005 (5000) no pass even at 0.015 (15000)

performance is worse for DDBAB and HDBAB in the Kemetyl white spirit, since higher MCA values are required for a rank A or B. 1H NMR spectroscopy indicates that the aromatic contents of the two white spirits are equal (ca. 25 %), and no other chemicals besides hydrocarbons have been added to the fluids. Therefore, we do not know the reason for the difference in AA performance between the two white spirits, but it does emphasize that is important to consider the effect of the liquid hydrocarbon phase when testing AAs.

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CONCLUSION We have built and established a new wheel loop equipment for testing AAs. A series of quaternary ammonium surfactant AAs were tested at about 40 MPa at varying concentrations and aqueous salinity. The results were compared to tests reported earlier using the same surfactants but in high-pressure sapphire rocking cells under similar test conditions. The two different types of equipment gave the same ranking of AAs, but in general we found that the wheel tests required a higher concentration of AA to achieve the same good AA effect at the same test conditions, especially in saline solutions. Different hydrocarbon fluids could also affect the AA performance.

Figure 10. MCAA values for DDBAB and TDBAB in various aqueous solutions. 1000 ppm = 0.001 mass fraction.

and deionized water. The general trend is that less AA is needed as the brine concentration increases. Another observation is that the ranking also changes depending on the brine concentration. DDBAB gave the lowest MCAA values with deionized water and 1.5% NaCl but not with 7.0 wt % NaCl. TDBAB gave the lowest MCAA of 0.0005 mass fraction (500 ppm) at this high salt concentration. Therefore, when ranking AAs it is necessary to always determine the performance of AAs in the same brine concentration that is found in the field. The distribution of ions, particularly monovalent and divalent cations, can also affect the perform-

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +47 5183 1823. Fax +47 5183 1750. E-mail: malcolm. [email protected]. Notes

The authors declare no competing financial interest. 256

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REFERENCES

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