LABORATORY EVALUATION OF KINETIC HYDRATE INHIBITORS: A NEW PROCEDURE FOR IMPROVING THE REPRODUCIBILITY OF MEASUREMENTS

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1 Proceedings of the th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July -10, LABORATORY EVALUATION OF KINETIC HYDRATE INHIBITORS: A NEW PROCEDURE FOR IMPROVING THE REPRODUCIBILITY OF MEASUREMENTS Christophe Duchateau and Christophe Dicharry Laboratoire des Fluides Complexes, Université de Pau UMR 5150, Centre National de la Recherche Scientifique BP 1155, 401 Pau Cedex FRANCE Jean-Louis Peytavy and Philippe Glénat Total, Centre Scientifique et Technique Jean Feger Avenue Larribau, 4018 Pau FRANCE Tong-Eak Pou and Manuel Hidalgo Arkema-Ceca, Centre de Recherche Rhône-Alpes Rue Henri Moissan, BP, 4 Pierre-Bénite Cedex FRANCE ABSTRACT Evaluation of KHIs in laboratory high pressure cells generally leads to scattered results. In this work we propose a new procedure that consists in characterizing the efficiency of KHI at second hydrate formation. This procedure limits the stochastic character of hydrate formation by using the persistence of precursory hydrate structures in water that previously experienced hydrate formation and decomposition. It is shown that the presence of these precursors strongly increases the reproducibility of measurements as compared to systems containing fresh water and allows unambiguous discrimination between blank (uninhibited system) and KHI tests. Keywords: gas hydrates, kinetic inhibitors, laboratory high pressure cells, reproducibility NOMENCLATURE FSD Full Scale Deflection G Gas phase H Hydrate phase HWHYD Heriot-Watt HYDrate predictive model KHI Kinetic Hydrate Inhibitor L w Liquid aqueous phase P cell Experimental Cell Pressure [MPa] T cell Experimental Cell Temperature [ C] T d Temperature applied to decompose hydrate [ C] T eq Three phase (L w -H-G) Equilibrium Temperature [ C] T form Hydrate Formation Temperature [ C] THI Thermodynamic Hydrate Inhibitor t ind Induction Time (period of time between the entrance of the system in the stability domain and the first indication of hydrate formation) [min] T init Temperature at the beginning of an experiment [ C] T targ Targeted Temperature (in the hydrate stability domain) [ C] T Subcooling (difference between T eq and T cell at P cell ) [ C] T max Maximal Subcooling: subcooling above which hydrate formation is instantaneous [ C] Corresponding author: Phone: Fax christophe.dicharry@univ-pau.fr

2 INTRODUCTION Gas hydrates can form at pressures and temperatures commonly encountered in oil and gas production systems [1]. They can plug these systems or eventually break the equipments. Hydrate prevention is commonly based on the injection of thermodynamic hydrate inhibitors (THIs) to shift hydrate stability region towards higher pressures and lower temperatures. As the search for oil and gas goes into colder and/or deeper regions, the quantity of THI required for protecting the installations from plugging increases and can represent prohibitive expenses [2]. The use of low dosage additives is a good way to reduce hydrate preventing costs [, 4]. This class of molecules can be separated into two categories. The first one contains antiagglomerants which allow the formation of transportable hydrate slurry. The second one is constituted by kinetic hydrate inhibitors (KHIs) which delay hydrate nucleation and/or crystal growth. Only KHIs have been considered in this paper. Before being used on production fields, KHIs efficiency has to be evaluated in laboratory [5]. Generally oil companies use flow loops to simulate the field conditions []. Because these equipments are rather expensive to run, KHIs providers prefer using laboratory high pressure cells. However this more convenient and less expensive way for testing KHIs generally yields scattered results, due to the stochastic nature of hydrate nucleation step [7]. Improving the reproducibility of high pressure cell tests is necessary to increase both the confidence in the evaluation of KHIs efficiency and to develop better KHIs. In this paper, we describe an experimental procedure allowing good reproducibility of KHIs efficiency evaluated with laboratory high pressure cells. Our procedure is based on the use of hydrate precursors remaining in a water phase that previously experienced a hydrate formation/decomposition cycle. This procedure is derived from the protocol that has been implemented by Total for 15 years in their flow loop [8]. EXPERIMENTAL APPARATUSES Chemicals The feed gas was a mixture of 8 mol% methane and 2 mol% propane supplied by Linde. This binary gas mixture is predicted to form structure II hydrates. The oil phase was a condensate from the Frigg field (North Sea). The aqueous phase was deionized water produced by a Millipore Milli-Q 185 E system. KHIs were poly(n-vinylpyrrolidone) (PVP-K0) purchased from Fluka, a commercial KHI referred to as KHI A, and an experimental KHI referred to as KHI B designed by Arkema/Ceca. Material The high pressure apparatus (Figure 1) consisted of four identical stirred cells from Parr Instrument Company, with an inner diameter of mm, a volume of 100 cm and a maximum working pressure of 20 MPa. Each cell contained an extractable glass liner in which the fluids were placed for the experiments. Thermal link between cell wall and glass liner was ensured by silicon oil (Huber SilOil M ). The cells were immersed in a temperature-controlled insulated bath agitated with two impellers to provide homogeneous temperature control during tests. Bath temperature was regulated by two refrigerating/heating circulators (Julabo F2-HE and Huber ICO45). Cells and bath temperatures were measured using PT-100 sensors with an accuracy of ±0.2 C. Cell pressures were measured with 25 MPa full scale transducers with a precision of 0.% FSD. The gas mixture was supplied to each cell by external stainless steel reservoirs. Figure 1 High pressure cells apparatus

3 A gas booster pump (Haskel AGT 2/152) was used to pressurize the reservoirs. Cells and bath temperatures, mixing speed, intensity of current supplied to maintain a constant stirrer speed and cell pressures were continuously monitored by a personal computer. Procedures Before being placed within the cells, each glass liner was filled with 12 cm of deionized water and 28 cm of condensate. In the case of experiments with inhibitors, the KHIs were dissolved in water at the required concentration. The cells were pressurized at 1 MPa at T init = 1.5 C and the fluids were stirred at 550 rpm for at least 1 hour to saturate the condensate with gas. When the cell pressure reached a steady-state condition, gas pressure was readjusted at 1.0 ± 0.1 MPa. The gas inlet valves between the cells and the reservoirs were then closed, and all experiments were done under isochoric conditions. In a typical hydrate formation experiment, T cell was decreased from T init (out of the hydrate stability domain) to a targeted temperature, T targ within the hydrate stability domain. For all experiments, the time required to reach T targ was fixed at 45 ± 5 minutes. Hydrate formation was detected by a sharp increase in T cell. To decompose hydrates, the temperature of the system was increased from T targ to a decomposition temperature T d above the three-phase (L w -H-G) equilibrium temperature T eq (at P cell ) at the heating rate of 0.4 C/min. Stirring speed was maintained at 550 rpm during the experiments. However, the agitation was stopped 0 min after the detection of hydrate formation, in order to control the amount of hydrates formed in the cells. Heriot-Watt predictive model (HWHYD) was used to calculate T eq at the measured cell pressure []. RESULTS AND DISCUSSION First hydrate formation The purpose of these first formation experiments was to illustrate the well-known fact that for systems with fresh water (water that had never experienced hydrate formation/decomposition), the stochastic character of hydrate nucleation leads to scattered results [10], and, above all, leads to the impossibility to discriminate between blank (uninhibited system) and KHI tests results. In Figure 2 are reported the maximal subcooling T max defined as (T eq T form ) where T form is the temperature at which hydrate formation was detected in the presence or not of KHI. In these experiments, T targ was fixed at 2 C. It is shown that T max could vary from more than 5 C between two cells; which corresponds to a relative dispersion of about 40 % (calculated as (T form,max T form,min )/T form,mean ). Maximal subcooling ( C) 'A' [1 wt%] 'A' [2 wt%] 'B' [2 wt%] PVP [0.7wt%] No inhibitor Figure 2 Maximal subcooling at first hydrate formation The induction times, t ind measured for 1 and 2 wt% of KHI A in water at a subcooling of 11.0 ± 0. C are reported in Figure. The induction times measured for these systems with fresh water ranged from 50 to 1450 min, which corresponds to a dispersion of % (relative to the median value). 11,4 11, ,8 'A' [1 wt%] 10, 'A' [2 wt%] Figure Induction times measured at a subcooling of 11.0 ± 0. C and at first hydrate formation for KHI A As expected, the stochastic character of hydrate nucleation led to poorly reproducible results. Under these conditions, the comparison of KHIs efficiency is not possible, and the discrimination between blank and KHI tests results is unavailing. Second hydrate formation The effect of a preliminary hydrate formation/decomposition cycle on the reproducibility of T max and t ind measured on a

4 second hydrate formation was investigated. The aim of these experiments was to reduce the stochastic character of hydrate nucleation, thanks to the residual hydrate structures left in the system after the preliminary hydrate formation/decomposition cycle [8, 11-1]. For the preliminary hydrate formation, the parameters were the same as for the first formation (temperature was decreased from T init = 1.5 C to T targ = 2 C within about 45 min). In each cell, thirty minutes after the detection of hydrate formation, the stirrer was stopped to limit hydrate formation. The aim of this step was to obtain quite the same amount of hydrates formed in all the cells before proceeding to the decomposition stage. Thirty minutes after the hydrate formation in the last cell, the stirrers were restarted (at 550 rpm) and the temperature was increased up to T d = 2.5 C at the heating rate of 0.4 C/min to dissociate hydrates. The system was maintained for 2 hours at this temperature which corresponds, according to the HWHYD model, to a superheating of 10.2 C at the experimental cell pressure. Experimental procedure was then exactly the same as for the first formation cycle. As it is shown in Figure 4, T max measured at second hydrate formation were not very different than those measured at first hydrate formation and their dispersion was roughly the same. This result suggests that neither hydrates nor water hydrate pre-structures (clusters) remained in the water phase at the end of the decomposition step. As a consequence, the systems behaved as if they have never experienced hydrate formation/decomposition. 18 First formation After 2 hours at 2.5 C After 2 hours at 1.5 C To preserve hydrate precursors in water, we softened the conditions imposed for hydrate decomposition. To this aim, hydrate decomposition was carried out at T d = 1.5 C for two hours which corresponds to a superheating of 0. C. At the end of this decomposition stage, P cell was recovered within the uncertainty of the pressure transducer, suggesting that no hydrates remained in the system prior to the second hydrate formation experiment. Under these conditions, T max measured at second hydrate formation not only decreased but their dispersion was also strongly reduced (Figure 4). The presence of precursory hydrate structures in the water phase provided templates for faster hydrate formation and therefore reduced the stochastic character of hydrate nucleation. The test conditions for KHI are harder and thus the differences between systems inhibited with efficient KHI and uninhibited systems are easier to discern. Note that the nature and proportion of the active anti-hydrate components in KHIs A and B being unknown parameters, no conclusion on the relative efficiency of KHIs A, B and PVP can be drawn from Figure 4. The induction times at second hydrate formation were measured at different subcoolings to determine the curve of efficiency of inhibited and uninhibited systems (Figure 5) 'A' [2 wt%] 'B' [2 wt%] 'A' [1 wt%] No inhibitor (8.8 ; 20) (7.0 ; 2000) Maximal subcooling ( C) 'A' [1 wt%] 'A' [2 wt%] 'B' [2 wt%] PVP [0.7wt%] No inhibitor Figure 4 Maximal subcoolings measured at first and second hydrate formations (after two hours at T d of 2.5 C and 1.5 C) Figure 5 Curves of efficiency: subcooling versus induction time It can be seen that both t ind and the dispersion of t ind (error bars in Figure 5) were strongly reduced when compared to the first hydrate formation (Figure ). For the system inhibited with 2 wt% of KHI A, the median value of t ind measured at a subcooling of about 11 C decreased from 700 min at first hydrate formation (Figure ) to 40 min at

5 second hydrate formation. The dispersion of t ind relative to the median value was lower than 40 % for all experiments and generally lower than 20 %. This should be compared to the % of relative dispersion measured at the first hydrate formation (Figure ). However one can observe that the dispersion of t ind increased as the subcooling decreases. Subcooling is usually considered as the driving force for hydrate formation [14]: the lower the subcooling, the lower the driving force, and thus the higher the dispersion of induction times. As expected, inhibited systems exhibited higher protection against hydrate formation as compared to the uninhibited ones, and increasing KHI concentration improved hydrate prevention. These observations show that the protocol is discriminating with regard to these parameters (presence and concentration of KHI). As the hydrate decomposition conditions seemed to have a great influence on the second formation experiments, their effect on T max was studied by varying T d between 1.5 and 2.5 C. In Figure are reported T max as a function of T d for uninhibited and inhibited systems. The values of T max were almost constant as long as the superheating was lower than 2.5 C (i.e., T d 21.5 C). For higher superheating, both the value and the dispersion of T max increased. These results suggest that though good reproducibility of measurements requires the presence of precursory hydrate structures in the system, the precise amount of these materials does not change significantly the value of subcooling above which immediate hydrate formation occurs, at least to a certain extent. The influence of hydrate decomposition conditions on t ind measured at different subcoolings for uninhibited and inhibited systems has been investigated too. The results are shown in figures 7 and 8, respectively h at 20.5 C 2h at 1.5 C 2h at 21.5 C Figure 7 Effect of hydrate decomposition conditions on the curve of efficiency of the uninhibited system Figure 7 shows that if increasing T d from 1.5 to 21.5 C did not modify T max for the uninhibited system, it shifted the measured induction times towards higher values at lower subcoolings; the lower the subcooling, the higher the shift for t ind. However, one can observe that the values of t ind were near or within the error bars at a given subcooling. Moreover, these error bars had rather the same width showing that the increase of the decomposition temperature did not change significantly the dispersion of induction times. The same behavior was observed for the system inhibited with 2 wt% of KHI B (Figure 8). 18 'B' [2 wt%] PVP [0.7 wt%] No inhibitor 11 2h at 1.5 C Maximal subcooling ( C) h at 20.5 C 2h at 21.5 C 2h at 1.5 C 2h at 21.5 C 2h at 2.5 C 2h at 25.5 C 2h at 2.5 C First formation Dissociation conditions Figure Effect of hydrate decomposition conditions on the maximal subcooling Figure 8 Effect of hydrate decomposition conditions on the curve of efficiency of the [2 wt%] KHI B inhibited system

6 These results seem to support that some variations in the amount of precursory hydrate structures left in the system after hydrate decomposition do not have a major impact on the results of KHI evaluation. However, further experiments with systems containing other KHIs should be made to confirm or not this point. CONCLUSION Hydrate nucleation in systems containing fresh water leads to very scattered results. Persistence of hydrate pre-structures in water after a preliminary hydrate formation/decomposition cycle can be used to outstandingly improve the reproducibility of measurements for uninhibited and inhibited systems. The superheating applied to decompose hydrates after the first hydrate formation is a key parameter. Too higher superheating leads to the destruction of too many hydrate precursory structures, and thus the measurements are poorly reproducible, just as they are with fresh water. However, our results suggest that a critical superheating exists below which the reproducibility of measurements remains acceptable. Although this new procedure for laboratory evaluation of KHIs efficiency seems promising, further experiments are necessary to know, in particular if the critical superheating is KHIdependent. AKNOWLEDGEMENTS Authors wish to thank Total and Arkema/Ceca for financial support, Arkema/Ceca for KHI furniture and all the members of the KHI working group for fruitful discussions. REFERENCES [1] Sloan ED. Clathrate hydrates of natural gases. Second edition. New York: Marcel Dekker Inc., 18. [2] Sloan ED. A changing hydrate paradigm from apprehension to avoidance to risk management. Fluid Phase Equilibria 2005;228-22:7-74. [] Phillips NJ, Kelland MA. The application of surfactants in preventing gas hydrate formation. In: Industrial application of surfactants IV, Proceedings of the 4 th Congress on Industrial Application of Surfactants, Cambridge, 18. [4] Varma-Nair M, Costello CA, Colle KS, King HE. Thermal analysis of polymer-water interactions and their relation to gas hydrate inhibition. Journal of Applied Polymer Science 2007;10: [5] Leporcher EM, Fourest JM, Labes-Carrier C, Lompre M. Multiphase transportation: a kinetic inhibitor replaces methanol to prevent hydrates in a 12-inc. pipeline. In: Proceedings of the European Petroleum Conference, The Hague, 18. [] Peytavy JL, Glénat P, Bourg P. Kinetic Hydrate Inhibitors - Sensitivity towards Pressure and Corrosion Inhibitors, paper IPTC 112. In: Proceedings of the International Petroleum Technology Conference, Dubai, 4- December [7] Kashchiev D, Firoozabadi A. Nucleation of gas hydrates. Journal of Crystal Growth 2002;24(- 4): [8] Peytavy JL, Glénat P, Bourg P. Qualification of Low Dose Hydrate Inhibitors (LDHIs): field cases studies demonstrate the good reproducibility of the results obtained from flow loops. In: Proceedings of the th International Conference on Gas Hydrates, Vancouver, -10 July [] Avlonitis D, Danesh A, Todd AC, Baxter T. The formation of hydrates in oil-water systems. In: Multi-phase Flow, Proceedings of the 4 th International Conference,Bedford, 18. [10] Natarajan V, Bishnoi PR, Kalogerakis N. Induction phenomena in gas hydrate nucleation. Chemical Engineering Science 14;4(1): [11] Song KY, Feneyrou G, Fleyfel F, Martin R, Lievois J, Kobayashi R. Solubility measurements of methane and ethane in water at and near hydrate conditions. Fluid Phase Equilibria 17;128: [12] Sloan ED, Subramanian S, Matthews PN, Lederhos JP, Khokhar AA. Quantifying hydrate formation and kinetic inhibition. Industrial and Engineering Chemistry Research 18;7(8): [1] Makogon YF, Holditch SA, Makogon TY. Kinetics and morphology of secondary gas hydrates experimental results. In: Proceedings of the Fifth International Conference on Gas Hydrates, Trondheim, [14] Arjmandi M, Tohidi B, Danesh A, Todd AC. Is subcooling the right driving force for testing low-dosage hydrate inhibitors?. Chemical Engineering Science 2005;0: