Journal of Petroleum Science and Engineering 74 (2010) 132–137

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Journal of Petroleum Science and Engineering

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On modelling gas hydrate inhibition by salts and organic inhibitors

Rahim Masoudi ⁎,1, Bahman TohidiInstitute of Petroleum Engineering, Heriot-Watt University, Edinburgh, EH14 4AS, U.K.

⁎ Corresponding author. Tel.: +60 323312918; fax: +E-mail address: rahim_masoudi@petronas.com.my (

1 Present address: PETRONAS, PMU, KLCC, Kuala Lum

0920-4105/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.petrol.2010.08.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 March 2010Accepted 18 August 2010

Keywords:gas hydrateequation of statesaltorganic inhibitorinhibition effectthermodynamic model

We present the application of a recently developed thermodynamic model to obtain better understanding ofgas hydrate inhibition effects of combinations of salts (e.g., NaCl, KCl, CaCl2) and hydrate organic inhibitors(e.g., mono ethylene glycol and methanol) used for gas hydrate prevention scenarios in the gas and oilindustry. In the first section of this work, the effect of salt-organic inhibitor interactions on hydrate inhibitioneffects has been studied. For this purpose, two possible approaches for modelling the hydrate inhibitioneffect of a mixture of salts and organic inhibitors have been investigated. They are: (1) considering thesummation of separate effects of salt and organic inhibitors and ignoring the effect of interaction betweeninhibitors, and (2) considering the effect of the mixture of salt and organic inhibitor and taking into accountthe effect of interaction between these. The study shows that the hydrate formation inhibiting the effect ofthe approach 1 surpasses the effect calculated from approach 2, especially at high concentrations ofinhibitors. That means predictive hydrate numerical models and empirical correlations should precisely takeinto account the interaction between electrolytes and organic inhibitors in order to accurately predicthydrate inhibition effects.In the second section of this work, maximum gas hydrate suppression temperature locus for systemscontaining salts and organic inhibitors has been investigated. The aim of this section was to obtain aguideline for efficient application of combinations of salts and organic inhibitors in order to operate in thehydrate and salt free regions. It has been concluded that high concentrations of organic inhibitors arepreferred to salts as far as hydrate prevention is concerned.

60 323313306.R. Masoudi).pur, 50088, Malaysia.

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

In petroleum exploration and production operations, gas hydrates,or clathrate hydrate, pose a serious economic and safety concern dueto potential blockage of wells, flowlines and other process facilities.Considerable effort has therefore been put into the development ofmethods and tools, capable of predicting hydrate phase boundaries fordifferent fluids at various temperature and pressure conditions.

The risk of clathrate hydrate formation can be reduced by a numberof methods, including operating outside the hydrate stability region,removing/reducing free water, and/or adding thermodynamic inhibi-tors (e.g., electrolytes and/or organic inhibitors). Generally organicinhibitors (or salts and/or organic inhibitors in the case of drillingfluids)are added when the salinity of formation water is not adequate forhydrate prevention; however, this can increase the risk of another flowassurance problem, i.e., salt precipitation or “salting-out” (Amy et al.,2002; Matthews et al., 2002; Masoudi et al., 2004a).

Accurate knowledge of the most effective combinations of salts andorganic inhibitors could be crucial to the success of some of flowassurance scenarios. It is not uncommon to find errors of the order of

4.5–5.6 K in predicting hydrate dissociation temperatures in systemscontaining salts and methanol (Matthews et al., 2002). The origin ofsuch significant errors in the existingmodel predictions could bemainlyattributed to the fact that most of the available models do not take intoaccount the interactions between electrolytes and organic inhibitors. Inorder to overcome this drawback, these parameters should be preciselytaken into account in predictive thermodynamic models.

In this communication, the HWHYD thermodynamic model whichwas recently extended to systems containing salts and/or organicinhibitors (Masoudi et al., 2004b,c, 2005) has been utilised to predictthe hydrate inhibition effects of salts and organic inhibitors. First, theeffect of salt-organic inhibitor interactions on gas hydrate inhibitionhas been investigated and described. Next, the model was employedto examine the maximum inhibition locus in aqueous solutions ofsalts and organic inhibitors.

2. Thermodynamic model

The Heriot-Watt Hydrate (HWHYD) model uses the Valderramamodification of the Patel and Teja equation of state (VPT EoS) for fugacitycalculations in all fluid phases (Valderrama, 1990). Non-density depen-dent (NDD) mixing rules are used to model polar–non-polar and polar–polar interaction (Avlonitis et al., 1994). This combination has proven tobe a strong tool in modelling systems with polar as well as non-polar

Fig. 2. Experimental (Masoudi et al., 2004c) and predicted methane hydratedissociation conditions in the presence of KCl and ethylene glycol.

133R. Masoudi, B. Tohidi / Journal of Petroleum Science and Engineering 74 (2010) 132–137

components (Avlonitis et al., 1994). Salts are considered as entity pseudo-components in amodifiedVPTEoSbydefining their critical properties andacentric factors (Masoudi et al., 2004b,c, and d). The hydrate phase ismodelled by using the solid solution theory of van der Waals andPlatteeuw (1959), as implemented by Parrish and Prausnitz (1972). TheKihara model for spherical molecules is applied to calculate the potentialfunctions for compounds forming the hydrate phase (Kihara, 1953). Adetailed description of the numerical modelling is given elsewhere(Tohidi et al., 1995; Masoudi et al., 2004b). The HWHYD thermodynamicmodel has proven reliable in predicting the hydrate free zone of variousfluid systems (Tohidi et al., 1995, 1996, 1997; Østergaard et al., 1998;Masoudi et al., 2004b,c). Figs. 1 and 2 show examples of the reliability ofthepredictions fromtheHWHYDthermodynamicmodel compared to theexperimental data for themethane hydrate dissociation conditions of theNaCl/EG and KCl/EG mixtures.

The VPT EoS has the following form:

P =RTv−b

− a Tð Þv2 + b + cð Þv−bc

ð1Þ

where a=acα(Tr). For pure component the four parameters of Eq. (1)are obtained as follows:

ac = Ωac

R2T2C

PCð2Þ

b = ΩbRTCPC

ð3Þ

c = ΩcRTCPC

ð4Þ

α Trð Þ = 1 + m 1−T0:5r

� �� �2 ð5Þ

where P is the pressure, T is the temperature, v is the molar volume,and R is the universal gas constant. The subscripts c and r denotecritical and reduced properties, respectively. The coefficients Ωa, Ωb

and Ωc, and m are given by the following equations:

Ωac= 0:66121−0:76105ZC ð6Þ

Ωb = 0:02207 + 0:20868ZC ð7Þ

Ωc = 0:57765−1:87080ZC ð8Þ

m = 0:46283 + 3:5823ωZC + 8:19417 ωZCð Þ2 ð9Þ

Fig. 1. Experimental (C1-Distilled water data: Deaton and Frost, 1946, McLeod andCampbell, 1961, Jhaveri and Robinson, 1965; EG/NaCl data: Masoudi et al., 2004c) andpredicted methane hydrate dissociation conditions in the presence of NaCl andethylene glycol.

where ω and ZC are the acentric factor and critical compressibilityfactor, respectively.

Avlonitis et al. (1994) relaxed the correlation of α(Tr) of VPT EoSfor water and methanol and regressed a more specific correlation fortemperatures up to critical point:

α Trð Þ = 1 + m 1−TΨr

� �� �2 ð10Þ

Methanol: m=0.76757, Ψ=0.67933Water: m=0.72318, Ψ=0.52084In the HWHYD model, the NDD mixing rules are applied to

describe mixing in the attraction parameter (a), as follows:

a = aC + aA ð11Þ

aC = ∑i∑jxixj aiaj

� �0:51−kij

� �ð12Þ

aA = ∑px2p ∑

ixiapilpi ð13Þ

api =ffiffiffiffiffiffiffiffiffiapai

p ð14Þ

where p stands for polar component and lpi is the binary interactionparameter between the polar component and the other components,which is a function of temperature, calculated by the following

Fig. 3. Water fugacity change in aqueous solutions of NaCl and EG at 273.15 K and10 MPa.

Fig. 4. Hydrate suppression temperature (Delta T) in aqueous solutions of NaCl and EGat 10 MPa.

Fig. 6.Water fugacity change in aqueous solutions of KCl and EG at 273.15 K and 5 MPa.

134 R. Masoudi, B. Tohidi / Journal of Petroleum Science and Engineering 74 (2010) 132–137

expression:

lpi = l0pi–l1pi T−T0ð Þ ð15Þ

where lpi0 and lpi

1 are binary interaction parameters and T0 is the icepoint in K.

Using the above EoS and associated mixing rules the fugacity ofeach component in all fluid phases can be calculated from:

lnφi =1RT

∫∞V

∂P∂ni

� �T ;V ;nj≠i

−RTV

" #dV− ln Z for i = 1;2;…;N ð16Þ

fi = xiφiP ð17Þ

where P is thepressure,V is the volume,N is the number of components,and Z is the compressibility factor of the system, fi, xi and φi are thefugacity, mole fraction and fugacity coefficient of component i,respectively.

3. Salt-organic inhibitor interaction influence on hydrate inhibition

As explained earlier, one of the main shortcomings of the availablethermodynamic hydrate models and empirical correlations in theopen literature is that they are not sufficiently accurate in predictinggas hydrate inhibition effect of combinations of salts and organicinhibitors. One of the possible reasons, except the probable short-coming of the available approaches themselves, is that they don'tusually take into account the effect of interaction between salts andorganic inhibitors present in the system.

Fig. 5. Hydrate suppression temperature (Delta T) in aqueous solutions of NaCl andMeOH at 5 MPa.

In this section, the HWHYD thermodynamic model is applied toexamine the effect of salt-organic inhibitor interaction on hydrateinhibition. For this purpose, two possible approaches for modellingthe hydrate inhibition effect of a mixture of salts and organicinhibitors are investigated. They are: (1) considering the summationof separate effects of inhibitors and ignoring the effect of interactionbetween inhibitors, and (2) considering the effect of a mixture of saltand organic inhibitor and taking into account the effect of interactionbetween these.

The simplest possible assumption to model hydrate phaseboundaries in mixed inhibitor solutions is that the combined effectof salt and organic inhibitor on the water activity/fugacity is the sumof their separate effects. As Nasrifar et al. (1998) showed, summationof the separate activities can be assumed if the interaction betweenelectrolyte and organic inhibitor is negligible.

The HWHYD model was employed to predict the fugacity changesand hydrate suppression temperature in the presence of three salts;NaCl, KCl, and CaCl2, and two organic inhibitors, ethylene glycol andmethanol. The predictions were carried out for both possible cases;considering the summation of separate effects, and the combined effectof mixtures (i.e., considering the salt-organic inhibitor interaction).

Fig. 3 shows the water fugacity changes in the system at 273.15 Kat 10 MPa due to the presence of various concentrations of NaCl andethylene glycol. The figure presents both two possible approaches;summation of separate effects and effect of mixtures. There is a cleardifference between two examined approaches, especially at highconcentrations of salts and organic inhibitors. The correspondinghydrate suppression temperature for the same system and conditionswas predicted and presented in Fig. 4. The same calculation has beendone for NaCl/MeOH system. The results for hydrate suppression

Fig. 7.Hydrate suppression temperature (Delta T) in aqueous solutions of KCl and EG at5 MPa.

Fig. 8. Water fugacity change in aqueous solutions of CaCl2 and EG at 273.15 K and5 MPa.

Fig. 10. Hydrate suppression temperature (HST) in the presence of saturated NaCl andEG aqueous solutions at different pressures.

135R. Masoudi, B. Tohidi / Journal of Petroleum Science and Engineering 74 (2010) 132–137

temperature of aqueous solutions containing various concentrationsof NaCl and MeOH at 5 MPa have been presented in Fig. 5. As can beseen from the above figures, there is not a conclusive trend for hydratesuppression temperature in these cases. In order to reach aconclusion, therefore, the above calculations were repeated forvarious combinations of KCl–EG and CaCl2–EG. The results arepresented in Figs. 6 and 7 for water fugacity changes, and Figs. 8and 9 for hydrate suppression temperature. It should be noted that theconcentrations of salts and ethylene glycol presented in the abovefigures are in the solution. Clearly, for the calculations of thesummation of separate effects, the corresponding concentration ofsalt is calculated in organic inhibitor free basis and the concentrationof organic inhibitor is also calculated on a salt free basis.

As can be seen from the above figures, water fugacity change hasthe same trend as hydrate suppression temperature for both studiedapproaches. In addition, it can be concluded that the effect ofsummation of the separate effects is larger than the effect of mixedinhibitors, especially at higher concentrations. This conclusion is incontrast with the conclusion of Jager et al. (2002), who proposed thatthe combined effect of mixed inhibitors surpasses the sum of theseparate effects. It should be noted that the latter conclusion was onlybased on the investigation carried out on NaCl–methanol systems. Inaddition, the results show that, similar to water fugacity changes, thedifference between the two approaches is more significant for CaCl2system in comparison with KCl and NaCl systems, due to the fact thatCaCl2 is a 1:2 salt.

Considering the above, therefore, theoretical hydrate models andempirical correlations should properly take into account the interactionof all species in the aqueous solutions including salt-organic inhibitor

Fig. 9. Hydrate suppression temperature (Delta T) in aqueous solutions of CaCl2 and EG at5 MPa.

interactions, in order to accurately predict the inhibition effect of mixedinhibitors.

4. Gas hydrate maximum inhibition locus in aqueous solutions ofsalts and organic inhibitors

In this section, the HWHYDmodel has been utilised to examine thegas hydrate maximum inhibition locus in aqueous solutions of saltsand organic inhibitors. The aim is to gain a better understanding ofoptimal application of combinations of salts and organic inhibitors inorder to operate outside hydrate and/or salt regions.

The previously developed thermodynamic model is capable ofpredicting salt precipitation in aqueous electrolyte solutions with/without organic inhibitors (Masoudi et al., 2004a). In addition, as it isdetailed in our recent publications (Masoudi et al., 2004b,c), thehigher the concentration of salts and/or organic inhibitors in thesystem, the higher the hydrate preventive characteristic of the fluid inquestion.

Therefore, for any given organic inhibitor concentration, thesaturated salt concentration (i.e., the maximum possible amount ofsalts in aqueous organic inhibitor solution before salt precipitates)gives the maximum inhibition effect for gas hydrate. Since there aredifferent saturated concentrations of salts and organic inhibitors inthe aqueous solutions, inhibition effects will have different values.Consequently, investigation on the inhibition effect locus in thesecases could be very important in finding the best and the most

Fig. 11. Hydrate suppression temperature in the presence of saturated KCl and EGaqueous solutions at different pressures.

Fig. 12. Hydrate suppression temperature in the presence of saturated CaCl2 and EGaqueous solutions at different pressures.

136 R. Masoudi, B. Tohidi / Journal of Petroleum Science and Engineering 74 (2010) 132–137

effective saturated concentration of salts and organic inhibitors. Infact, the latter investigation should help to distinguish the borderbetween salt precipitations and hydrate formation events in thesystem. Furthermore, it is necessary to take into account the effect ofpressure and temperature on the maximum inhibition locus.

In order to investigate the above issues, the trend of the maximumhydrate inhibition locus with the aim of finding the most effectivecombination of salts and organic inhibitors is described. Firstly,saturated concentrations of salts and organic inhibitors have beenpredicted using the recently developed thermodynamic model.Secondly, the hydrate inhibition effects for these concentrationshave been calculated using the same model.

Fig. 10 presents the solubility curve and corresponding hydrateinhibition effect in aqueous solution of NaCl and EG. The effect ofpressure on the maximum inhibition locus is also depicted in thisfigure. This calculation has been repeated for aqueous solutions of KCl/EG and CaCl2/EG, as shown in Figs. 11 and 12. The temperature effecton the maximum inhibition locus and solubility curve in the aqueoussolutions of NaCl/EG is presented in Fig. 13.

In summary, with increasing temperature, as saturated concentra-tions increase, the maximum hydrate suppression temperature locusincreases to higher values. Moreover, increasing the pressure ofsystem shifts the maximum inhibition locus to higher values.Furthermore, with increasing concentration of organic inhibitor, thesystem has more ability to prevent gas hydrate formation, especiallyin the presence of NaCl and KCl in comparison with CaCl2. So the

Fig. 13. Hydrate suppression temperature in the presence of saturated NaCl and EGaqueous solutions at different temperatures.

following order of inhibition effect in the case of the combination ofsalts (S) and organic inhibitors (OI) can be concluded:

←Inhibition effect

Saturated OI N Saturated S2 + OI1 N Saturated S1 + OI2 N Saturated S

where the concentration of OI1NOI2 and also concentration of S1NS2.The results, however, are in contrast with the conclusions of Qiu

and Guo (2002), which showed a higher inhibition effect for NaCl incomparison with NaCl+EG.

5. Conclusions

The previously developed thermodynamic model has been appliedto obtain better understanding of the effects of various salts andorganic inhibitors on hydrate stability zone in petroleum explorationand production.

The above model was used to develop guidelines for maximumhydrate inhibition effect of salts and organic inhibitors without anysalt formation problem. It was discovered that higher concentrationsof organic inhibitors rather than salts is better for hydrate prevention.

The effect of salt-organic inhibitor interactions on hydrateinhibition has been investigated. It was found that the reliability ofthe hydrate model in the presence of thermodynamic inhibitors isstrongly related to salt-organic inhibitor interactions. That meanspredictive models should take into account the interaction betweenelectrolytes and organic inhibitors in order to accurately predicthydrate inhibition effects.

NomenclatureaA Contribution to the attractive term using asymmetric

mixing rulesaC Contribution to the attractive term using classical mixing

rulesac EoS attractive term at critical pointa, b, c Parameters of EoSfi The fugacity of component i in the fluid phaseskij The classical binary interaction parameterlpi, lpi0 , lpi1 The binary interaction parameter between the polar and the

other componentsm Parameter of the EoSN Number of componentsP PressureR Universal gas constantT TemperatureTC, PC, VC, ZC Critical propertiesV Volumexi Mole fraction of component iZ Compressibility factor

Greek symbolsα Temperature dependent functionφ Fugacity coefficientΩ Parameter in the VPT EoSω Acentric factorΨ Power parameter in VPT EoS

Acknowledgements

This study was funded through a Joint Industry Project at Heriot-Watt University, Institute of Petroleum Engineering, sponsored byABB, Petrobras, Shell, TOTAL and the DTI, which is gratefullyacknowledged.

137R. Masoudi, B. Tohidi / Journal of Petroleum Science and Engineering 74 (2010) 132–137

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