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CORRELATIONS Use of Boiling Point Elevation Data of Aqueous Solutions for Estimating Hydrate Stability Zone Amir H. Mohammadi and Dominique Richon* Centre Energe´ tique et Proce´ de´ s, Ecole Nationale Supe´ rieure des Mines de Paris, CEP/TEP, 35 Rue Saint Honore´ , 77305 Fontainebleau, France
Limited information is available for estimating the hydrate safety margin and controlling gas hydrate formation along pipelines and production facilities. In this work, the possibility of predicting the hydrate safety margin from the normal boiling point of aqueous solution is investigated by developing a predictive method that uses normal boiling point elevation data of aqueous solution in the presence of wide ranges of salt concentrations for determination of hydrate formation region. The developed method considers only the changes in normal boiling point elevation with respect to the normal boiling point of pure water for estimating hydrate stability zone, and therefore there is no need to have the analysis of the solution. As measurement of normal boiling point elevation for the aqueous phase is easier and more accurate than measurement of the hydrate dissociation point, such a method can reduce experimental costs and efforts. Independent data (not used in developing the correlation) are used to examine the reliability of this tool. It is shown that the predictions of this method are in acceptable agreement with the independent experimental data, demonstrating its reliability as a predictive technique. 1. Introduction A major challenge in deep-water field development is to ensure unimpeded flow of hydrocarbons to the host platform or processing facilities. The pipelines normally convey a cocktail of multiphase fluids, including formation water with various concentrations of salts and hydrocarbons and may therefore be prone to hydrate formation, which potentially can block pipelines and subsea transfer lines and lead to serious economic, operational, and safety problems. Considerable efforts are being made into the development of methods and tools capable of predicting hydrate phase behavior for related fluids. Furthermore, laboratory experiments are often carried out generally to improve and to validate the predictive models. The issue of hydrate phase boundary determination has received much attention over the years.1 Today, comprehensive thermodynamic models, mostly based on equations of state and statistical thermodynamics, are available for hydrate equilibrium predictions. Lately, “a changing hydrate paradigm from apprehension to avoidance to risk management” has been reported.2 Systematic ways of hydrate controlling along the pipeline and/or downstream to examine the degree of inhibition are very limited. Previously some investigators correlated the degree of hydrate inhibition in terms of the amount and nature of the inhibitor present in the system.1 Hydrate controlling systems can be produced based on properties of the aqueous phase.3 The possibility of predicting hydrate inhibition effects of salts from normal boiling point elevation data of the aqueous phase can have a real practical use, as measuring normal boiling point elevation of aqueous phase is easier than measuring hydrate * To whom correspondence should be addressed. Tel.: +(33) 1 64 69 49 65. Fax: +(33) 1 64 69 49 68. E-mail:
[email protected].
suppression temperature (Difference between hydrate dissociation temperature in the presence of salt aqueous solution and hydrate dissociation temperature in the presence of distilled water at a given pressure). In this work, the predictions of a comprehensive thermodynamic model4 for hydrate suppression temperatures and literature data5 for normal boiling point elevation of aqueous phase due to the presence of salt are used for developing a simple correlation for estimating hydrate stability zone of petroleum fluids in the presence of salt aqueous solution. Independent data (not used in developing the correlation) are then used to examine the reliability of this method. The predictions of this equation are in acceptable agreement with the independent experimental data, demonstrating the reliability of the predictive technique developed in this study. 2. Construction of the Correlation To develop a correlation between hydrate inhibition effects of salts and normal boiling point elevation data of aqueous solutions, we have used the results coming from a well-proven thermodynamic model, the HWHYD model4 for hydrate suppression temperatures, and literature data5 for normal boiling point elevation of aqueous phase due to the presence of salt. This model has proved to be a strong tool in modeling systems with ionic/polar as well as nonpolar compounds.4 The reasons for using the computed data rather than real experimental data are the following: 1. The amount of experimental hydrate suppression temperature is limited. 2. Because of the limited experimental data, any error could easily result in an unreliable correlation. 3. Experimental data can be used for validation of the correlation.
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Figure 1. Collapse of the data shown in Table 1 nearly onto one line, suggesting a linear relationship between hydrate suppression temperature and normal boiling point elevation for normal atmospheric pressure with a slope of 2.65: O, NaCl; 4, KCl; ], CaCl2; ×, KBr; +, NaBr; b, HCOONa; 2, HCOOK; 0, Na2SO4; [, MgCl2; 9, BaCl2.
Figure 2. Hydrate phase boundary of methane in the presence of NaCl aqueous solutions. Experimental data: 4, 3 wt % NaCl;7 O, 10 wt % NaCl;8 ×, 20 wt % NaCl.8 Bold solid curves, predictions of the new correlation; solid curves, predictions of the HWHYD model.4 Deviations at high concentrations can be attributed to unreliability of experimental data.
Table 1. Maximum Concentration of Salt in the Aqueous Phase System Used in Developing This Predictive Method salt
maximum wt %
no. of points
NaCl KCl CaCl2 NaBr KBr HCOONa HCOOK BaCl2 MgCl2 Na2SO4
23 13 20 34 32 24 30 16 5 5
12 7 10 17 16 12 15 9 5 5
As shown in Table 1, the normal boiling point elevation for normal atmospheric pressure and hydrate suppression temperature data for different aqueous solutions with wide ranges of salt concentrations were used in developing the correlation. These typical salts are normally present in the formation water or in drilling fluids. All these aqueous systems were assumed to be in contact with methane, the hydrate suppression temperatures for these systems were calculated at 20 MPa from the model,4 and the effect of gas composition (and therefore hydrate structure) and the pressure of the system on hydrate suppression temperature were ignored for engineering purposes.1 The degree of hydrate inhibition was then plotted versus the normal boiling point elevation data for various salt systems as shown in Figure 1. As can be seen, all the data collapse nearly onto one line, suggesting a linear relationship between hydrate suppression temperatures and normal boiling point elevation data with a slope of 2.65. The resulting correlation, which relates the hydrate dissociation temperature of the petroleum fluid system to the normal boiling point elevation of aqueous phase (with respect to distilled water), is then given by the following expression:
T ) T0 - 2.65∆Tb
(1)
where T is hydrate dissociation temperature (K), ∆Tb stands for the normal boiling point elevation of the aqueous solution for normal atmospheric pressure with respect to pure water (K), and T0 represents the hydrate dissociation temperature of the same fluid system in the presence of distilled water (K). As can be seen, the developed correlation is simple and enables fast calculation of the hydrate-free zone of various fluid systems, in the presence of salt aqueous solution. The only prerequisite
Figure 3. Experimental7 and predicted hydrate dissociation conditions of methane in the presence of aqueous solutions composed of various concentrations of NaCl and KCl. Experimental data:7 4, 3 wt % NaCl + 3 wt % KCl; 0, 5 wt % NaCl + 5 wt % KCl; +, 5 wt % NaCl + 10 wt % KCl; ], 5 wt % NaCl + 15 wt % KCl; ×, 10 wt % NaCl + 12 wt % KCl; O, 15 wt % NaCl + 8 wt % KCl. Bold solid curves, predictions of the new correlation; solid curves, predictions of the HWHYD model.4
to use of the correlation is the hydrate dissociation temperature of the same fluid system in the presence of distilled water (T0), which can be calculated using an appropriate correlation such as the general correlation reported by Østergaard et al.6 3. Results and Discussion The results predicted by the newly developed correlation are compared with some experimental data reported in the literature. In all of these comparisons, the values of ∆Tb are calculated from ref 5 and the values of T0 are calculated using the HWHYD thermodynamic model.4 Figure 2 shows a comparison between the predictions of this method, the HWHYD model4 and experimental data for hydrate dissociation conditions of methane in the presence of various concentrations of NaCl aqueous solutions. As shown in the figure, the predictions of the newly developed correlation and the HWHYD model4 are in acceptable agreement with the independent data. However, the results of these predictive methods show some deviations at high concentrations of NaCl.
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petroleum fluid systems in the presence of salt aqueous solutions was investigated. A simple correlation was developed to predict the hydrate stability zone as a function of normal boiling point elevation of aqueous systems due to the presence of salt(s). Acceptable agreement was achieved between the results of this method and independent experimental data in the literature. Nomenclature
Figure 4. Experimental7 and predicted hydrate dissociation conditions of methane in the presence of aqueous solutions composed of various concentrations of NaCl and CaCl2. Experimental data:7 4, 3 wt % NaCl + 3 wt % CaCl2; O, 6 wt % NaCl + 3 wt % CaCl2; ], 3 wt % NaCl + 10 wt % CaCl2; ×, 10 wt % NaCl + 3 wt % CaCl2; +, 10 wt % NaCl + 6 wt % CaCl2; 0, 6 wt % NaCl + 10 wt % CaCl2. Bold solid curves, predictions of the new correlation; solid curves, predictions of the HWHYD model.4
These deviations can be attributed to the unreliability of experimental data, as logarithm of hydrate dissociation pressure (P) versus temperature of the system is approximately linear and therefore any deviation of experimental data from the linear behavior indicates unreliability of the data. Limited experimental data for hydrate dissociation conditions of gases in the presence of mixed salt aqueous solutions have been reported in the literature. To examine the capability of this method for predicting hydrate inhibition characteristics for systems containing mixed salts, the predictions of this method and the HWHYD model4 are compared with experimental data for hydrate dissociation conditions of methane in the presence of NaCl + KCl and NaCl + CaCl2 aqueous solutions. The results are presented in Figures 3 and 4, respectively. As can be observed, acceptable agreement was achieved, demonstrating the capability of the method developed in this work to estimate hydrate inhibition effects of salts from normal boiling point elevation data of aqueous solutions. The use of this correlation is not recommended at very high concentrations of salts, as the data up to intermediate concentrations (Table 1) have been used for developing this method and high concentration of salt may lead to salt precipitation, which may affect calculation of hydrate phase boundary. Furthermore, the capability of the method can be further investigated by generating normal boiling point and hydrate data for more/mixed salt aqueous solutions. 4. Conclusions The possibility of using normal boiling point elevation data of aqueous solutions to estimate the hydrate stability zone of
P ) hydrate dissociation pressure (MPa) T ) hydrate dissociation temperature in the presence of saline water (K) T0 ) hydrate dissociation temperature in the presence of distilled water (K) ∆Tb ) normal boiling point elevation of the aqueous solution (i.e., at normal atmospheric pressure) with respect to pure water (K) ∆THyd ) hydrate suppression temperature (K): difference between hydrate dissociation temperature in the presence of salt aqueous solution and hydrate dissociation temperature in the presence of distilled water at a given pressure″ Literature Cited (1) Sloan, E. D. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker: New York, 1998. (2) Sloan, E. D. A changing hydrate paradigmsfrom apprehension to avoidance to risk management. Fluid Phase Equilib. 2005, 228-229, 6774. (3) Najibi, H.; Mohammadi, A. H.; Tohidi, B. Estimating the Hydrate Safety Margin in the Presence of Salt and/or Organic Inhibitor Using Freezing Point Depression Data of Aqueous Solutions. Ind. Eng. Chem. Res. 2006, 45, 4441-4446. (4) Heriot-Watt University Hydrate model: http://www.pet.hw.ac.uk/ research/hydrate/. Also: Østergaard, K. K.; Masoudi, R.; Tohidi, B.; Danesh, A.; Todd, A. C. A general correlation for predicting the suppression of hydrate dissociation temperature in the presence of thermodynamic inhibitors. J. Pet. Sci. Eng. 2005, 48 (1-2), 70-80. (5) Washburn, E. W. International Critical Tables of Numerical Data, Physics, Chemistry and Technology, 1-7; National Research Council: 19261930. Also: Mohammadi, A. H. Personal communication, 2006. (6) Østergaard, K. K.; Tohidi, B.; Danesh, A.; Todd, A. C.; Burgass, R. W. A General Correlation for Predicting the Hydrate-Free Zone of Reservoir Fluids. SPE Prod. Facil. 2000, 15 (4), 228-233. (7) Dholabhai, P. D.; Englezos, P.; Kalogerakis, N.; Bishnoi, P. R. Equilibrium Conditions for Methane Hydrate Formation in Aqueous Mixed Electrolyte Solutions. Can. J. Chem. Eng. 1991, 69, 800-805. (8) Kobayashi, R.; Withrow, H. J.; Williams, G. B.; Katz, D. L. Gas Hydrate Formation with Brine and Ethanol Solutions. Proceedings 30th Annual ConVention of Natural Gasoline Association of America; 1951; pp 27-31.
ReceiVed for reView May 15, 2006 ReVised manuscript receiVed October 28, 2006 Accepted November 14, 2006 IE060596K
