ABSTRACTthe formation of gas hydrates and maintain pipeline integrityfor various gas fields while greatly lowering total costs of off-shore gas field production.

INTRODUCTION

Clathrate hydrates are ice-like solids formed in the presence ofwater, a hydrate guest molecule, e.g., methane, ethane,propane or carbon dioxide (CO2), and the right combination oftemperature and pressure1. The crystal structure of hydrateconnects water molecules and the host molecule through hydro-gen bonding to form a 3D network that enclathrates apolarguest molecules in cages. The hydrate structure is stabilized bythe presence of a weak van der Waals interaction between theguest molecules and the water molecules. There are threeknown gas hydrate structures, namely, structure I (sI), structureII (sII) and structure H (sH), Fig. 1. The most abundant hydratestructures are the two cubic forms, which are hydrate sI and sII.The third structure, sH, is rarely seen outside the laboratory2.

Hydrate formation is favored at a lower temperature andhigher pressure3. The temperature of the hydrate formation in-creases with increasing pressure; sometimes hydrates can format ambient conditions. In fact, they can form at temperatureshigher than 35 °C if the pressure reaches above 1,000 bar1.Figure 2 illustrates the typical three-phase thermodynamic dia-gram of natural gas hydrates. It shows the hydrate zone, hydraterisk zone and hydrate free zone. Each zone’s characterization

The formation of gas hydrates is a major nuisance to the oiland gas industry, causing pipelines to plug, gas flow disrup-tions and safety issues. This phenomenon occurs in natural gaswhen the temperature and pressure of the flowing gas falls be-low the hydrate equilibrium region, leading to the formationof ice-like gas hydrates. Chemical inhibitors are commonlyused to mitigate hydrate formation through injection in thenatural gas pipelines system. Kinetic hydrate inhibitors (KHIs),a type of this chemical inhibitor, have received attention recentlydue to their environmental tolerance and low operating cost.Several KHIs have been developed and used in several gas fieldsaround the world; however, KHIs for gas fields rich in hydrogensulfide (H2S) and carbon dioxide (CO2) that are flowing underhigh sub-cooling temperature need to be developed.

In this study, we present a rational design and an experimen-tal approach to develop novel KHIs for gas fields with highsub-cooling temperatures, with structure I (sI) gas hydratespresent and with a high concentration of H2S and CO2. Thepresent methodology consists of carrying out performance test-ing, computational molecular modeling and physicochemicalcharacterization of polymeric materials. The performances ofcommercial KHIs, ready-made polymers and new synthesizedpolymers were evaluated using a rocking cell (RC-5) instrument.Hydrogen bonding between the polymers and water surrogateswas measured using attenuated total reflectance Fourier trans-form infrared spectroscopy (ATR-FTIR), and the results revealeda direct correlation between inhibition performance and hy-drogen bond strength. Additionally, the presence of an amidegroup in the polymeric inhibitors was found to be essential fordelaying hydrate formation, because it disrupts the local or-ganization of water clusters around guest gas molecules.

Computational molecular modeling using a density functionaltheory (DFT) method was also carried out to calculate hydro-gen bonding interaction energies between polymeric inhibitorsand water, and to link the molecular characteristics to actualperformance. Based on this approach, we were able to developa novel class of polymeric materials and KHI formulations thatprovides protection against gas hydrates at and beyond sub-cooling temperature levels of 4 °C and 5.6 °C with simulatednatural gas compositions. These KHIs could effectively prevent

Kinetic Hydrate Inhibitors for NaturalGas Fields: Rational Design andExperimental Development

Authors: Rashed M. Al-Eisa, Dr. Mohamed S. Elanany, Dr. Khalid A. Al-Majnouni, Ali A. Al-Jabran, Hassan A. Al-Ajwad, Dr. Ibrahim A. Abba and Dr. Abdullah R. Al-Malki

Fig. 1. Structures of the gas hydrates sI, sII and sH, presented along with the cagesused to build the structures and common guest molecules enclathrated into thehydrates2.

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inhibitors, such as methanol and monoethylene glycol, areused to alter the thermodynamic conditions, i.e., pressure andtemperature, at which the hydrates are formed. For thermody-namic inhibitors to be effective, they have to be injected at upto 60% of the weight of water, which leads to annual costs of$500 million for the oil and gas industry to prevent hydrateformation with treatments like methanol injection. These injec-tions are also inconvenient and hazardous due to their chemi-cal toxicity6.

The alternative approach, and the focus of this article,meets the industry’s requirements for kinetic hydrate inhibi-tion1-3. This approach can lead to substantial cost savings because of the small amount of the inhibitor required for injec-tion — < 1% of the weight of water. It can help to maintain effective pumping and storage capabilities, and can lead to thedesign of smaller production facilities. The kinetic control ap-proach, using what are known as low dose hydrate inhibitors,features either kinetic hydrate inhibitors (KHIs), which delaythe nucleation and crystal growth of hydrates, or anti-agglom-erants (AAs), which do not stop the nucleation, but insteadprevent the agglomeration. In both cases, hydrates can form ifenough time is allowed because the hydrate’s formation re-mains thermodynamically feasible5.

Two types of KHIs have been developed and commercial-ized in recent years. The first one is a water-soluble polymericmaterial that has a tendency to form hydrogen bonding withthe water, thereby preventing hydrate formation1. Typical ex-amples of polymeric KHIs are based on homopolymers orcopolymers of cyclic amide (lactam), such as polyvinyl capro-lactam and polyvinylpyrrolidone, or of other amides, such aspolyethyloxazoline and N-methyl-N-vinylacetamide. The sec-ond type is a non-polymeric material, such as tetrapentylam-monium bromide and butoxy-ethanol. Even though the latterperforms poorly as a KHI because of their weak interactionwith the hydrate surface, they are very effective as synergists1.

The field of KHIs is not a new field. The first patent forKHIs dates back to 1994. The discovery of the first and secondgenerations of KHIs was purely based on trial and error. Thisapproach is no longer followed, and a fundamental under-standing of the inhibition mechanism and polymer propertiesis essential to designing the next-generation KHIs. The mecha-nism of KHI action is believed to be two steps: The disturbingof a water molecule’s local organization, and the KHI’s strongadsorption to the hydrate surface6. The KHI adsorbs to the hy-drate clusters or surfaces, as schematically shown in Fig. 4, todelay the nucleation or the crystal growth for a period of time,which must be longer than the residence time of free water inthe mixture. The adsorption occurs through hydrogen bondingand weak van der Waals interactions. The adsorbed polymerchanges the diffusive-sorptional exchange at the inhibitor-water interface, which ideally results in decreasing the crystalgrowth rate and coagulation. For polymeric KHIs to be effec-tive, they must be very water soluble, and they should not hydrolyze to any insoluble compounds in the system. They

depends on the stability of the hydrate nuclei. The hydratezone is the most interesting and the one of immediate concernbecause in this region of the thermodynamic diagram, the hy-drates are thermodynamically stable and could form instanta-neously4.

Hydrates are a nuisance to the oil and gas industry becausethey deposit on pipe walls and accumulate as large plugs, Fig.3, blocking pipelines and eventually stopping production. Theprocess of removing hydrates after their formation in subseaproduction/transmission lines can be very time-consuming andcostly, while also posing safety concerns. Not only does hy-drate formation impede subsea production, but it can alsocause substantial losses to onshore production facilities andequipment. Prevention of hydrate formation is a must to main-tain high capacity gas production safely5.

Several methods are used to prevent hydrate formation, in-cluding dehydration, chemical inhibitors, and pressure andtemperature control. These methods are not suitable for allprocess schemes, as in the case of gas production from off-shore wells. Hydrate inhibition based on chemical inhibitors isconsidered to be the most efficient. These chemicals are cate-gorized as thermodynamic or kinetic inhibitors. Thermodynamic

Fig. 2. Three-phase equilibrium curve for typical natural gas hydrates4.

Fig. 3. Hydrate plug formation in a subsea hydrocarbon pipeline; picture fromPetrobras (Brazil).

SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2015

formation, the target KHI should work at sub-cooling temper-atures of 5 °C to 10.5 °C. The sub-cooling temperature is thedifference between the operating temperature and the three-phase equilibrium temperature at a defined pressure, as illus-trated by the blue line labeled 2 in Fig. 5.

Figure 6 shows the RC-5 used to test the formation of natu-ral gas clathrate hydrate from the synthetic gas. The RC-5 consists of five Hastelloy cells capable of operating under high-pressure (maximum 200 bar) and sour gas conditions. TheHastelloy cells are immersed in one temperature controlledbath containing ethylene glycol and water. The RC-5 enablesformation of natural gas clathrate hydrate under simulated op-erating conditions to test KHI effectiveness. Data acquisition iscompleted with software to enable measurement of the pres-sure and temperature with time for hydrate formation in eachof the Hastelloy cells.

should have a strong adsorption affinity to the hydrate surface,and they must be compatible with other added chemicals inthe pipelines, such as corrosion inhibitors6.

Designing new KHIs is necessary today to overcome issueslike specific gas stream compositions, changing productionconditions, compatibility with corrosion inhibitors and espe-cially the high severity of conditions in natural gas fields,namely a high concentration of hydrogen sulfide (H2S) andCO2, and a high sub-cooling temperature. In this article, wefollowed an experimental approach to develop next-generationKHI materials for such high severity conditions. Our approachbegan with an evaluation of commercial and new synthesizedKHIs in the rocking cell (RC-5) instrument. We then con-ducted physical and chemical characterizations of the poly-mers, using attenuated total reflectance Fourier transforminfrared (ATR-FTIR) spectroscopy, to understand the interac-tions between the polymers and the water. Finally, we modeledthe interactions between the polymers and water molecules todevelop the structure property relationships needed to createnext-generation KHIs.

EXPERIMENTAL

Rocking Cell (RC-5)

During winter months, operating conditions are suitable forhydrate formation, which raises possible safety and opera-tional concerns. Figure 5 shows the natural gas clathrate three-phase (hydrate-liquid-vapor) equilibrium line for the syntheticnatural gas, with high concentrations of H2S and CO2, whichwas used in this study. This synthetic gas forms sI gas hy-drates. When operating at temperatures lower and pressureshigher than those of the curving three-phase equilibrium line,natural gas clathrates are thermodynamically stable.

To protect gas pipelines and the wellhead against hydrate

Fig. 4. Schematic presentation of kinetic hydrate inhibition through the KHIpolymers’ adsorption to the hydrate surface through the hydrogen bondinginteraction with the methane hydrate cages.

Fig. 5. Synthetic natural gas clathrate three-phase (hydrate-liquid-vapor) equilibriumline. In blue, the intersection of lines 2 and 3 illustrates the sub-cooling temperatureof 10 °C where the natural gas clathrate hydrates are stable.

Fig. 6. RC-5 equipment for evaluating polymers as KHI at simulated operatingconditions.

In a typical run, each of the Hastelloy cells is charged with10 ml of aqueous polymers (3 wt%) and pressurized with syn-thetic gas to a range of 142 bar to 147 bar at 21 °C. After thereactant composition is fixed, the RC-5 temperature is pro-grammed to change in three stages, Table 1. Each of the tem-peratures is selected to fall in a stable natural gas clathratehydrate region. The three-phase equilibrium point, calculatedwith the HYSYS program, was 19 °C at 150 bar. For screeningpurposes, the new synthesized polymers were investigated at apressure of 70 bar and with similar sub-cooling temperaturesof 4 °C, 5.6 °C and 10.5 °C.

ATR-FTIR Characterization

A Varian 660-IR Spectrometer with a Specac Golden GateATR was used for the ATR-FTIR analysis. Spectra comprisedan average of 128 scans measured from 4,000 cm-1 to 700 cm-1

with a resolution of 4 cm-1. All analyses were performed atambient temperature. A 0.1 g dry sample of polymer was ana-lyzed first in the ATR-FTIR. Then 10 µl of heavy water (D2O)was added to the polymer sample and the spectra wererecorded. The reason for using D2O instead of ordinary wateris because the latter’s bending peak would obscure the regionof the amide peak in the ATR-FTIR spectra.

Computational Modeling

Geometry optimizations and energy calculations for water andthe KHI monomer structures polyethyloxazoline (PEOx) andpolyacrylamide-co-acrylic acid (PAC-AA) were performed us-ing a density functional theory (DFT) based method and theDMol3 program with the Materials Studio software packageMS 6.1, based on a generalized gradient approximation levelusing the Hamprecht, Cohen, Tozer and Handy exchange andcorrelation functional7, 8. The double numerical basis set withpolarization functions was used in all calculations carried outin this study. A water molecule was placed initially at a similardistance (~1.70 A

o ) from the KHI monomer carbonyl groups.

Geometry optimization and energy calculations were carriedout in the vacuum, vac, and in the aqueous media, aq, using aconductor-like screening model (COSMO)9. The low level oftheory, i.e., semi-empirical method, was sometimes used to re-fine the initial geometry of a structure before starting higherlevel DFT calculations, to ensure a global minimum wasreached. The interaction energy, EInt, is calculated as EInt =E(mono-H2O) − [Emono + EH2O], where EInt, Emono and EH2O refers

to the calculated interaction energy, total energy of the KHImonomer and total energy of water, respectively.

RESULTS AND DISCUSSION

Performance of Commercial KHI and Ready-Made PolymericInhibitors

Figure 7 shows the performance results of PAC-AA1 andPEOx, along with brine water, i.e., the control, and a commer-cial KHI. These polymers were chosen for investigation be-cause they are relatively good kinetic inhibitors of clathratehydrates and have high solubility in water; still, both polymersunderperformed compared to the commercial KHI. The PEOxperformed better than the PAC-AA as a KHI, delaying the hydrate nucleation at a sub-cooling temperature of 4 °C fornearly 6,000 minutes. After nucleation, however, hydrategrowth was very rapid (see the slope). PAC-AA did not im-prove on the induction time to nucleation, and a low rate ofhydrate growth was observed, where the curve for PAC-AAreaches a steady-state after the curve. At a sub-cooling temper-ature of 12 °C, a large amount of hydrate was formed in thepresence of PAC-AA — even more than in the brine water.This clearly indicated that at this temperature, PAC-AA pro-moted formation of natural gas clathrate hydrates. Both PAC-AA and PEOx have amide groups. In PEOx, the amide group

1 The PAC-AA used was switched from the acid form to the sodium salt form.

Fig. 7. Performance evaluation of commercial KHIs, including PEOx and PAC-AA.

Stage Start Temp (°C) Average Ramp (°C/min) Sub-cooling (°C/min) Duration (min)

S-1 21 0.1 5 7,000

S-2 14 0.1 7 7,000 to 8,500

S-3 12 0.08 12 8,500 to 11,000

aq vac aq

q

Table 1. Temperature program to form natural gas clathrate hydrate in the RC-5

is part of the polymer chain, whereas in the PAC-AA group,the amide group is one of the pendant groups of the copoly-mer, as can be observed in Fig. 8.

Characterization of Hydrogen Bonding Interactions

Hydrogen bonds are an attractive interaction between mole-cules with electron deficient hydrogen, i.e. hydrogen bonddonors, and molecules with high electron density, i.e., hydro-gen bond acceptors. The atoms that are usually involved in hy-drogen bond interactions are N, F and O. In principle, whenthe hydrogen bondings (X-H … Y) form, significant chargesare transferred to the proton-acceptor, Y, from the donor, X-H,resulting in weakening and elongation of the X-H bond. Inconsequence, it decreases the X-H vibration frequency com-pared to the normal bond measured by infrared spectroscopy.The shift of bond frequency to a lower frequency, or energy, iscalled a red shift, and in the opposite case, the shift of bondfrequency to a higher frequency is called a blue shift10. Theshift in frequency is correlated with the strength of the hydro-gen bonding, and so is the hydrate inhibition. The hydrogenbonding interactions play an important role in hydrate inhibi-tion using KHIs. To measure the polymer-hydrate surrogatemolecule, i.e., D2O interactions, ATR-FTIR was employed.

First, the analysis was done for both neat and hydratedsamples of PAC-AA and PEOx. The spectra, Fig. 9a, show fivemajor ATR-FTIR bands assigned to the polymer of PAC-AA.The bands are attributed to both the amide group and theacrylic acid group, so specific interpretation is challenging.Features consistent with the carbonyl stretching are at 1,662cm-1 and 1,548 cm-1. The carbonyl band at 1,662 cm-1 redshifted when the polymer was hydrated, but the band at 1,548cm-1 also broadened, so a specific shift could not be measuredaccurately8. Because of the limitation of the ATR-FTIR spec-troscopy, the computational modeling had to be used to investigate the interactions of the PAC-AA with water. TheATR-FTIR spectra of PEOx could be more accurately analyzedbecause PEOx is not a copolymer like PAC-AA. The majorATR-FTIR band of PEOx is attributed to the amide group at1,620 cm-1. The red shift of the amide group was noticed alsofor PEOx from 1,620 cm-1 to 1,603 cm-1. The intensity of theamide peak increased, but the broadness did not change. Thisshift indicates the presence of hydrogen bonding betweenPEOx and D2O.

Modeling the Interactions between the KHI Monomers of

PEOx, PAC-AA and Water

To investigate potential polymers that could be used as KHIs, asimple model of a monomer — containing the correspondingfunctional group — was created, and the hydrogen bonding in-teraction energies were calculated, as well as other importantmolecular characteristics, i.e., bond length, frequency andatomic charges.

Figure 10a shows the geometrically optimized ethyl oxazo-line monomer with the amide functional group, where nitro-gen is in the backbone of the PEOx polymer. The minimumenergy structure due to the hydrogen bonding interaction be-tween the ethyl oxazoline monomer and water is shown in Fig.10b. Similarly, the geometrically optimized acrylamide-co-acrylic acid monomer is shown in Fig. 10c with the character-istic amide and carboxylic acid functional groups. Figure 10dillustrates the hydrogen bonding interaction between water andthe acrylamide-co-acrylic acid monomer through the carbonyl,

Fig. 8. (a) PEOx, and (b) PAC-AA where R is either OH or NH2.

Fig. 9. ATR-FTIR spectra of (a) PAC-AA, and (b) PEOx in dry (red) and hydratedwith D2O states (black) samples.

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C=O, in the amide functional group. Table 2 is a summarizationof the calculation results, including geometrical and atomic parameters.

To calculate hydrogen bonding interaction energy, a watermolecule, H2O, was optimized in the same way as the KHImonomers. Geometrical parameters, namely H-O and ∠H-O-H, agreed well with the experimental data. The total energy,EH2O, was calculated in vacuum and aqueous phases. Resultsare indicated in Table 2, where 1 Ha = 2625.5 kJ/mol.

Table 2 also indicates that the calculated hydrogen bondinginteraction energy for the PEOx monomer, -11.82 kJ/mol, isgreater than that for the PAC-AA monomer, -10.14 kJ/mol.Therefore, a better hydrate inhibition is expected by PEOx.This better PEOx performance is also reflected in other geo-metrical and atomic parameters. In more detail, the hydrogenbond distance, H-O-H…O=C, for the optimized PEOx…H2O

structures of 1.938 Å, Fig. 10b and Table 2, is slightly shorterand stronger than that for the PAC-AA...H2O structures of1.989 Å, Fig. 10d and Table 2, which agrees with the calcu-lated interaction energy order.

Structural changes for the PEOx and PAC-AA monomers,when optimized in vacuum and aqueous phases, were obtainedto correlate the physiochemical properties and KHI perform-ance. For example, the C=O bond length of the PEOxmonomer increases slightly in the aqueous phase to 1.253 Åwhen compared to the vacuum phase increase to 1.238 Å, dueto hydrogen bonding. A similar trend was observed in the caseof the PAC-AA monomer; the C=O bond length in the aqueousphase is 1.248 Å as compared with 1.237 Å in the vacuumphase. In the next stage, the oxygen atomic charge in the car-bonyl C=O group of benchmark KHI polymers, PEOx andPAC-AA, was calculated as previously presented in Table 2.The negative atomic charge of the aqueous PEOx monomer, -0.580e, is slightly larger than that in vacuum, -0.484e, due tohydrogen bonding. Similarly, the negative atomic charge of theaqueous PAC-AA monomer, -0.559e, is slightly larger thanthat in vacuum, -0.480e. As a result of the calculated oxygenatomic charge on the PEOx monomer, i.e., -0.580e, and on thePAC-AA monomer, i.e., -0.559e, in the aqueous phases, it isconcluded that PEOx has stronger hydrogen bond interactions,which agrees with the calculated interaction energies.

Linking computational modeling calculations with experi-mental results is crucial to present a reliable fundamental un-derstanding of KHI performance, and ultimately, lead to therational design of novel KHIs. For achieving this, the asym-metric stretching frequency of C=O bonds in the amide func-tional group of PEOx and PAC-AA monomers was calculatedin vacuum and aqueous phases. The calculated red shift for thePEOx monomer, Δ72 cm-1, is larger than that for PAC-AAmonomer, Δ26 cm-1. This finding agrees well with the strongerinteractions of water with the PEOx than with the PAC-AA,which leads to the conclusion that PEOx has better inhibitionefficiency than PAC-AA, as was measured by the RC-5. Thecomputational modeling also enables accurate analysis of theFTIR data when the experimental FTIR is complicated by the

Fig. 10. (a) Geometrically optimized structures for ethyl oxazoline monomer, (b)ethyl oxazoline with H2O, (c) acrylamide-co-acrylic acid monomer, and (d)acrylamide-co-acrylic acid with H2O. Hydrogen, carbon, nitrogen and oxygenappear in white, gray, blue and red color, respectively.

ParameterPEOx PAC-AA

vac aq vac aq

qO (e) -0.484 -0.58 -0.48 -0.559

C=O (Å) 1.238 1.253 1.237 1.248

oC=O (amide) cm-1 1664 1592 1680 1654

H-O-H…O=C (Å) – 1.938 – 1.989

Total energy (Ha) -366.592998 -366.604541 -554.024719 -554.046043

EInter (kJ/mol) -11.82 -10.14

Table 2. Calculated structural and atomic parameters of PEOx and PAC-AA monomers in the vacuum and aqueous phases as well as their hydrogen bonding interactionenergies

SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2015

presence of overlapping carbonyl groups.A further step toward the rational design of potential KHI

polymers is to investigate the effect of additional functionalgroups on the parent amide group in an oxazoline monomer,namely, the ether, fluoride, ethyl and imide functional groups.The calculated hydrogen bonding interaction energies followedthis order: imide, -21.0 kJ/mol, > ether, -10.9 kJ/mol, > ethyl, -10.1 kJ/mol, > fluoride, -8.1 kJ/mol. It can be deduced thatthe hydrogen bond strength is influenced by the added func-tional groups, increasing with electron donating groups likeimide and ether, and decreasing with electron withdrawinggroups like fluoride. The hydrogen bond strength is also re-flected by the shorter hydrogen bond distance; 1.394 Å in thecase of imide as compared to 2.025 Å for fluoride. Based onthis approach, molecular computational modeling can be usedto either optimize current synthesized KHI or propose new formulated KHIs.

Performance of Novel Synthesized KHIs

By considering the previous results, 12 new synthesized poly-mers were evaluated as KHIs in the RC-5, Fig. 11. P-9 and P-12 yielded less hydrate — compared to the control — by inhibiting hydrate growth at the sub-cooling level of 4 °C and5.6 °C, respectively. At the same time, no nucleation inhibitionwas measured. At the sub-cooling temperature of 10.5 °C, P-9promoted the growth of hydrates. Also very important to anunderstanding of the fundamentals is that the P-9 polymer stabilized the hydrate, as can be seen from Fig. 11. When thesystem was heated to temperatures where hydrate is thermo-dynamically unstable, a kinetic aspect of the hydrate-polymerinteraction led to stable hydrate. P-7 performed exceptionallyin comparison to other polymers, as can also be seen in Fig.11. Even in the presence of this polymer, however, hydrate

growth was promoted at the sub-cooling temperature of 10.5 °C. The promotion of hydrate growth is also observed at lowersystem pressures compared to the control, e.g., see P-7 and P-9in Fig. 11. Among the other 12 polymers, P-12 was also inter-esting because it extended the induction time to 7½ hours andpromoted hydrate growth at a sub-cooling temperature of 10.5°C. P-8 performed similarly to P-7, suggesting another polymerhas been discovered to be a KHI. The other polymers behavedsimilarly to P-11: not inhibiting the hydrate nucleation butchanging the hydrate growth properties. The mechanisms ofthese hydrate-polymer interactions are now being investigatedwith computational modeling and structural characterization,as was presented here for the PAC-AA and PEOx systems.

CONCLUSIONS

The issue of gas hydrates occurring in natural gas fields at highsub-cooling temperatures has triggered the need for a reliableway to mitigate such a problem. Many challenges are associ-ated with this type of field that prevents hydrate inhibition,such as sI hydrate formation and a high concentration of H2S.As a result, we have developed a comprehensive approachbased on experimental and computational methods to mitigatethis issue by developing a novel class of KHIs.

Our approach consists of KHI performance testing, compu-tational molecular modeling and acquiring an understandingof fundamentals through physiochemical characterization ofpolymers and model components. The performance testingwas conducted on ready-made polymers, commercial KHIsand new synthesized polymers. The amide functional groupwas found to be an essential chemical group in any KHI,where the carbonyl group forms hydrogen bonding with thehydrate surface to delay the enclathration process. The studyof polymer-water hydrogen bonding interactions revealed thatthe interaction of PAC-AA and PEOx with water is consistentwith their inhibition performance. Computational molecularmodeling calculations of hydrogen bonding interaction energiesand carbonyl bond frequencies agree with the experimental results. The calculated molecular characteristics, i.e., atomiccharges and bond distances, were consistent with the perform-ance of PEOx and PAC-AA. Therefore, molecular modeling isa very useful approach for describing KHIs at the atomic scale.Linking of the atomic scale properties to the macro scale per-formance has led to a crucial understanding of KHI mechanisms.Our findings have helped us to develop two novel polymericKHIs. These new KHIs inhibited the gas hydrates of syntheticnatural gas with high concentrations of H2S and CO2 at andbeyond sub-cooling temperatures of 4 °C and 5.6 °C.

ACKNOWLEDGMENTS

The authors would like to thank Saudi Aramco for permissionto publish this article. Special thanks go to R&DC manage-ment for their support and encouragement. The authors wouldFig. 11. Representative curves from evaluating the 12 new polymers in the RC-5.

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like to thank Dr. Ashrof Ali from the Chemistry Department atKing Fahd University of Petroleum and Minerals for synthesiz-ing the new KHI polymers, and Dr. Mohammed Al-Daous forthe fruitful discussions.

REFERENCES

1. Kelland, M.A.: “History of the Development of LowDosage Hydrate Inhibitors,” Energy & Fuels, Vol. 20, No.3, April 2006, pp. 825-847.

2. Sloan Jr., E.D.: “Fundamental Principles and Applicationsof Natural Gas Hydrates,” Nature, Vol. 426, November20, 2003, pp. 353-359.

3. Lederhos, J.P., Long, J.P., Sum, A., Christiansen, R. andSloan Jr., E.D.: “Effective Kinetic Hydrate Inhibitors forNatural Gas Hydrates,” Chemical Engineering Science,Vol. 51, No. 8, April 1996, pp. 1221-1229.

4. Pickering, P.F., Edmonds, B., Moorwood, R.A.S.,Szczepanski, R. and Watson, M.J.: “Evaluating NewChemicals and Alternatives for Mitigating Hydrates in Oiland Gas Production,” paper presented at the IIRConference, Aberdeen, Scotland, 2001.

5. Notz, P.K., Bumgartner, S.B., Schaneman, B.D. and Todd,J.L.: “The Application of Kinetic Inhibitors to Gas HydrateProblems,” OTC paper 7777, presented at the OffshoreTechnology Conference, Houston, Texas, May 1, 1995.

6. Anderson, B., Tester, J.W., Borghi, G-P. and Trout, B.L.:“Properties of Inhibitors of Methane Hydrate Formationvia Dynamics Simulations,” Journal of the AmericanChemical Society, Vol. 127, No. 50, December 2005, pp.17852-17862.

7. Boese, A.D. and Handy, N.C.: “A New Parametrization of Exchange-Correlation Generalized GradientApproximation Functionals,” Journal of Chemical Physics,Vol. 114, No. 13, March 2001, pp. 5497-5503.

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BIOGRAPHIES

Rashed M. Al-Eisa is a Scientist in theChemicals Research Division of SaudiAramco’s Research & DevelopmentCenter (R&DC). He joined SaudiAramco in 2010 after he finished hisdegree as a member of the CollegeDegree Program for Non-Employees

(CDPNE). Rashid started working with the AdvancedAnalysis Unit in the online group specializing in analyzerand sensor technologies. Rashid then joined the innovativechemicals sub-team under the Chemicals Research Divisionwhere he focused on the kinetic hydrate inhibitors project.His areas of interest include polymer science, water-solublepolymers, and catalyst synthesis and characterization.

Rashed received his B.S. degree in Chemistry fromAston University, Birmingham, U.K., and his M.S. degree inPolymer Science from the University of North Carolina atWilmington, Wilmington, NC.

He is a member of the American Chemical Society(ACS) and the International Society of Automation (ISA).

Dr. Mohamed S. Elanany is a ScienceSpecialist with the Chemicals ResearchDivision of Saudi Aramco’s Research& Development Center (R&DC). Heis part of a team focused on thedevelopment of in-house gas hydrateinhibitors. Mohamed is an expert in

computational molecular modeling, advanced materialsdevelopment and characterization.

He has published more than 30 international journalarticles and two patents, and he has presented his researchresults at many international conferences.

Mohamed received his B.S. and M.S. degrees in PhysicalChemistry from Egypt. In 2004, he received his Ph.D.degree in Materials Chemistry from the Graduate School ofEngineering, Tohoku University, Japan. Mohamed gainedfurther postdoctoral experience in advanced materials andmolecular modeling at the University of Namur, Belgium,before joining Saudi Aramco in 2007.

SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2015

Dr. Ibrahim A. Abba is ChiefTechnologist of the ChemicalsResearch Division at Saudi Aramco’sResearch & Development Center(R&DC). Prior to joining SaudiAramco, he taught in academia andworked for chemical companies, both

in the Middle East and in North America. Ibrahim iscurrently responsible for directing technology developmentefforts spanning the catalytic and thermal cracking ofunconventional feeds to valorization of low value streamsto high value chemicals.

He received his M.S. degree in Chemical Engineeringfrom King Fahd University of Petroleum and Minerals(KFUPM), Dhahran, Saudi Arabia, and his Ph.D. degree inChemical Engineering from the University of BritishColumbia, Vancouver, British Columbia, Canada.

Ibrahim has authored/coauthored over 40 articles andpatents.

Dr. Abdullah R. Al-Malki is aResearch Science Consultant with theChemicals Research Division of SaudiAramco’s Research & DevelopmentCenter (R&DC). He has 25 years ofexperience in crude oil and fuelscharacterization, processing, quality

and specifications. Abdullah joined Saudi Aramco in 1988after graduation and has spent most of his career withR&DC, although he has had several assignments workingat the Ras Tanura refinery and in the Oil Supply Planningand Scheduling (OSPAS) Department.

Abdullah received his B.S. degree in Chemistry fromKing Abdulaziz University, Jiddah, Saudi Arabia, and M.S.and Ph.D. degrees in Chemistry from King Fahd Universityof Petroleum and Minerals (KFUPM), Dhahran, SaudiArabia.

Dr. Khalid A. Al-Majnouni is theleader of the innovative chemicals sub-team under the Chemicals ResearchDivision of Saudi Aramco’s Research& Development Center (R&DC). Hisexperience includes working at theJiddah Refinery for more than two

years as a Process Engineer in crude distillation andnaphtha reforming, and with fluid catalytic cracking (FCC)units.

In 1994, Khalid joined Saudi Aramco’s College DegreeProgram for Non-Employees (CDPNE) and received hisB.S. degree in Chemical Engineering from King FahdUniversity of Petroleum and Minerals (KFUPM), Dhahran,Saudi Arabia. Then, in 2005, he received his M.S. degree inChemical Engineering from the Colorado School of Mines,Golden, CO. In 2011, Khalid received his Ph.D. degree inChemical Engineering from the University of Delaware,Newark, DE.

Ali A. Al-Jabran is a Senior DigitalSystem Technician currently workingin the Materials Performance Unit ofthe Technical Services Division ofSaudi Aramco’s Research &Development Center (R&DC). Hejoined Saudi Aramco in July 1983 as a

trainee in the Industrial Training Center (ITC) andcompleted the program in 1985.

Ali is specialized in high-pressure high temperature(HPHT) operations and specifically in flow acceleratedcorrosion, electrochemistry and the process control ofHPHT equipment. He has also been involved in developingnew competencies in corrosion testing.

Ali completed six months of specialized training withC&C Technologies, Houston, TX.

Hassan A. Al-Ajwad is a ScienceSpecialist working with the MaterialsPerformance Unit of the TechnicalServices Division at Saudi Aramco’sResearch & Development Center(R&DC). His focus areas in researchand technical service include high-

pressure high temperature (HPHT) corrosion simulation,flow accelerated corrosion, oil field chemicals performanceand design of sophisticated laboratory systems. Over thepast few years, Hassan has led the R&DC efforts for in-house development of world-class capabilities in the areasof HPHT corrosion testing and flow corrosion simulationand measurement systems.

He received his B.S. degree in Industrial Chemistry fromKing Fahd University of Petroleum and Minerals (KFUPM),Dhahran, Saudi Arabia.

FALL 2015 SAUDI ARAMCO JOURNAL OF TECHNOLOGY