Kinetic Hydrate Inhibitor Studies for Gas Hydrate Systems: A Reviewof Experimental Equipment and Test MethodsWei Ke† and Malcolm A. Kelland*,‡

†Department of Petroleum Engineering and ‡Department of Mathematics and Natural Sciences, Faculty of Science and Technology,University of Stavanger, NO-4036 Stavanger, Norway

ABSTRACT: Kinetic hydrate inhibitors (KHIs) have been studied, developed, and used in the oil and gas industries for morethan two decades. The main active ingredients in commercial KHI formulations are water-soluble polymers. When dosed at lowconcentrations (0.1−2.0 wt % active chemical), they are able to retard the gas hydrate formation process and facilitate reliable oiland gas transportation. A considerable amount of research effort on KHI technologies has contributed to an abundance of KHIknowledge, applications, and tailor-made solutions. Whereas previous reviews have concentrated on the chemistry of KHIs, thisreview article has a particular emphasis on the experimental equipment, hydrate detection tools, and test methods commonlyapplied in KHI investigations. The underlying mechanisms of KHIs are still not fully understood. The major hypothesesproposed in the literature and supporting experimental and computational evidence are also reviewed.

■ INTRODUCTION

Gas hydrates are crystalline, nonstoichiometric clathrateinclusion compounds. At the microscopic level, they arecomposed of hydrogen-bonded water molecules as hosts andgas molecules entrapped in the water cavities as guests. Theyare not chemical compounds because no strong chemical bondsexist between the water and gas molecules. Generally, highpressures and low temperatures are required for their stableexistence. The gas molecules able to be enclathrated into thewater lattices are usually small (<10 Å),1 such as methane,ethane, propane, and 1-butane, and inorganic gases, includingnitrogen, hydrogen, and carbon dioxide.2 Different hydratestructures have been discovered. Cubic structure I (sI) andstructure II (sII) and hexagonal structure H (sH) are the threemost common gas hydrate structures. This is determined byhow the hydrate unit cells are assembled at the molecular level.2

Since it was first reported3 as an industrial nuisance, gashydrate formation continuously leads to pipeline pluggingissues during oil and gas transportation. There are three majorstages of phase transitions associated with hydrate plugformation: nucleation,2,4 growth,2,5 and agglomeration.6,7

Numerous experimental investigations2,8−16 and modelingand simulations12,17−21 have contributed to the currentunderstanding of such phase transitions. Despite scientificinnovations and technical improvements, hydrate formationremains the number one problem in flow assurance.22 In fact,we are facing even more severe technical challenges due to theformation of gas hydrates than before, especially when offshoredrilling activities move toward geological sites of deeper watersand colder temperatures. The operational regions are often wellwithin the hydrate stability zone. An enormous expense forhydrate prevention and mitigation is associated with the use oftraditional thermodynamic hydrate inhibitors (THIs), such asmethanol and monoethylene glycol (MEG).23 In such acontext, robust and economical hydrate management strat-egies24 have become an urgent need for safe and reliableproduction of oil and gas.

For hydrate formation and plugging issues to be mitigated, animportant area of effort is the research and development of lowdosage hydrate inhibitors (LDHIs).25 The concept was initiatedin the mid-1980s and early 1990s.26 Research on LDHIs hasbeen active since that time.7,20,22,24,27−33 LDHIs consist ofkinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs).They are called so to differ from the traditional THIs based ontheir different hydrate inhibition mechanisms. THIs shift thehydrate equilibrium to lower temperatures and higherpressures, forcing the system to stay within the hydrate-freeregion. On the contrary, KHIs, mostly polymeric compounds,delay the hydrate nucleation and/or growth and extend thehydrate induction time to exceed the residence time of thereservoir fluid; AAs, mostly surfactants, allow the hydrateparticles to form and keep them dispersed in the reservoir fluidto generate transportable slurries. Both KHIs and AAs are usedfor continuous injection applications, but AAs are especiallyused in shut-in/start-up scenarios and at high subcoolings.Research progress in recent years on test methodologies hasincreased the confidence in KHI technology and therefore thenumber of field applications. Laboratory investigations into thebiological KHIs34 have also been ongoing for years. High-performance KHIs have been applied to varied fluid systemsincluding gas, condensate, and black oil.35 For mimicking thereal multiphase reservoir fluid flow in oil and gas pipelines,salt36−39 could be added to make saline solutions, andheptane39−42 or decane43−45 could be added as a liquidhydrocarbon phase. In principle, KHIs are designed formultiphase pipeline applications but may also be applied inother circumstances. For example, KHIs may be used duringdrilling while exposed to the drilling fluid. In such a case,drilling mud36,46−50 could be added to evaluate the perform-ance of KHIs in the deep-water drilling environment. Porousmedia such as silica sand51,52 have also been tested. Hydrate

Received: October 20, 2016Published: November 22, 2016

Review

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formation and dissociation behavior in the interstitial pores inthe porous media can be very different from that in the usualliquid-gas-hydrate systems. Establishment and testing ofhydrate systems with one or more of these components oradditives is able to offer additional insights on how the selectedKHIs can behave in close-to-reality circumstances. Kelland53

has presented a comprehensive review on gas hydrate controlwith special focus on the recently developed KHIs, theirchemistry, synthesis, and laboratory performances. A review onLDHI−KHI applications from an industrial perspective hasbeen presented by Klomp.35

Whereas previous reviews on KHIs have concentrated ontheir chemistry, this work focuses on the tools and methodscommonly used for experimental KHI investigations in naturalgas hydrate systems over the past decade. The proposed KHIinhibition mechanisms with experimental and computationalevidence in the literature are also reviewed. The following areasof KHI research will not be considered in detail in the currentwork: (a) antifreeze proteins (AFPs) from living organisms,also called ice-structuring proteins (ISPs), and their potential asKHIs54−60 (an up-to-date review of AFP studies is availablefrom Walker et al.61), (b) ionic liquids (ILs)62−64 as potentialKHIs (Tariq et al.65 have recently reviewed IL-related researchactivities), (c) tetrahydrofuran (THF)66−69 or cyclopen-tane66,70 hydrate systems (Although THF and cyclopentanehydrates are often used to facilitate the screening of KHIs forgas hydrate inhibition, there are two major differences that areworth mentioning: First, THF and cyclopentane hydrates formwithout the need for elevated pressure; thus, they are not ableto fully represent the real conditions for gas hydrate formation.For the latter, the dissolution and diffusion of the pressurizedgas into the aqueous phase is critical. Second, the actualperformance of KHIs on THF and cyclopentane hydrates canbe different or even opposite to that on gas hydrate systemslikely due to different KHI working mechanisms.71), (d)recycling72,73 or removal74,75 of KHIs for future fieldapplications, and (e) compatibility studies of KHIs with othertypes of production chemicals,76 in particular, corrosioninhibitors.77,78 The current survey maintains its focus on thetools and methods of experimental KHI investigation andexperimental and computational evidence for the proposedinhibition mechanisms in the literature.

■ MAJOR KHI CATEGORIESThere are three main categories of KHIs developed for fieldapplications, all of which are polymeric compounds. These are• Poly(N-vinyllactam) polymers, including a variety of

copolymers and grafted polymers. Typical examples are five-ring polyvinylpyrrolidone79 (PVP), six-ring polyvinylpiperi-done80 (PVPip), seven-ring polyvinylcaprolactam81 (PVCap),and eight-ring polyvinylazacyclooctanone82 (PVACO). Theirchemical structures are shown in Figure 1. Their performancesas KHIs were found to improve with increasing lactam ringsize.80,82 Only polymers with the five- and seven-rings are usedin commercial KHI formulations.• Hyper-branched poly(esteramide)s.83 An illustrative

structure of a hyper-branched poly(esteramide) is shown inFigure 2. It is feasible to modify the tips on its molecularstructure to make the polymer more or less hydrophilic.84 Thisclass of KHIs is claimed to have better performance onstructure-I hydrates than the VCap-based polymers.25

• N-Isopropylmethacrylamide85 (IPMA) polymers andcopolymers. This is another widely studied KHI category.

The structure of poly(N-isopropylmethacrylamide) is shown inFigure 3. IPMA copolymers86 may have higher cloud pointsand better tolerance to saline environments.

Other polymeric KHIs that have been developed and testedinclude pyroglutamate polymers (a few field applications havebeen carried out),87,88 maleic copolymers and alkylamidederivatives,89 polyalkyloxazolines,90,91 polymaleimides,92 poly-allylamides,93 polyaspartamides,45,94,95 modified starch andstarch derivatives,96,97 and proteins.61 Recently, sodiumchloride, an inorganic and traditional THI, was reported topossess kinetic inhibition properties toward hydrate formationin porous media.52 However, it appears to only slow the growthrate of hydrate formation and has no effect on the inductiontime. An introductory overview of KHIs was given by Erfani etal.98 With abundant details and a special focus on the chemistry,structure, functional groups, and synthesis processes, Kel-land25,53,84 has made comprehensive reviews on all categories ofKHIs along with their histories of development. A generalguideline for developing a polymeric KHI is that a certain levelof hydrophobicity should be reached for the KHI to effectivelydisrupt the water structures, whereas it has to remain water-soluble to get into the liquid system.

Figure 1. Typical poly(N-vinyllactam) polymers with increasinglactam ring sizes. From left to right: PVP (five-ring), PVPip (six-ring), PVCap (seven-ring), and PVACO (eight-ring).

Figure 2. An example structure of a hyper-branched poly(esteramide).

Figure 3. Structure of poly(N-isopropylmethacrylamide).

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■ EXPERIMENTAL APPARATUS FOR TESTING OFKHIS

Experimental devices for testing and screening of KHIs for gashydrate systems, from laboratory scale to pilot scale, haveevolved over the years. The most commonly used apparatusinclude autoclave, rocking cell, stirred reactor, batch orsemibatch crystallizers, automated lag time apparatus (HP-ALTA), (micro)differential scanning calorimetry (DSC or μ-DSC), pipe wheel, and flow loop. A brief description of each,accompanied by a list of apparatus and selected research groupswho often apply them is provided in Table 1.The selection of a proper experimental apparatus largely

depends on the purpose and perspective of the study. Thequality and convenience of data collection are also priorities forconsideration. Autoclaves, crystallizers, and other types ofstirred reactors are frequently used in gas hydrate studies withadjustable agitation strength, pressure, and temperaturemonitoring. Semibatch autoclaves or semibatch crystallizerscould be considered when adding components or takingsamples for analysis along the process is required. Stainless steeland titanium are common materials for construction of the cell

body, allowing varied designs of the inner volume, cellgeometry, and pressure grading. Sapphire cells often providehigh pressure grading and the possibility to mount a window onthe cell body for visual inspection of the hydrate formationprocess. Rocking cells were first introduced by Shell oilcompany. Commercial equipment with several steel cellsmounted in parallel, or up to 20 sapphire cells (which areparticularly useful for studying LDHI antiagglomerants wherevisual data is critical) are now available. This gives theresearcher access to multiple results in the same cooling unit inthe same time that it takes to run an autoclave with at least asgood reproducibility and reliability. Autoclaves and rockingcells that can be used for sour natural gas mixtures (containingH2S) are also available. Rocking cells are especially suitable forfast screening of KHI candidates under continuous cooling byrecording hydrate formation temperatures. Video cameras,high-resolution digital cameras, or microscopes with CCDcould be mounted or connected to the high-pressure cells toallow real time photo and video recordings. Speciallyengineered PVT cells could also be used to study the three-phase properties of water, gas, and hydrate with accurate

Table 1. Apparatus/Reaction Vessels Used in Gas Hydrate Studies in the Presence of KHIs

apparatus/vessel

innervolume(cm3)a

pressuregrade(mpa)a research group

autoclave 145 40 Svartaas and co-workers157,158

autoclave 200 15 Cha et al.159

autoclave 280 41 Llamedo and Yanez;67 Anderson etal.129

autoclave 290 14.7 Ferreira et al.160

autoclave 300 41 Luna-Ortiz et al.131

autoclave 23 N/A Chua et al.85

autoclave 280 N/A Glenat et al.132

autoclave 300 N/A Shin et al.43

autoclave 500 N/A Kelland and Iversen36

autoclave 750 N/A Zhao and co-workers46,47

autoclave 3000 N/A Hould et al.136

autoclave 25000 N/A Salamat et al.121

semibatchautoclave

58 N/A Daraboina and co-workers161,162

crystallizer 500 20 Xu et al.163

crystallizer 610 20 Al-Adel et al.164

crystallizer 211 N/A Sharifi et al.38

crystallizer 400 N/A Kumar et al.165

crystallizer 2182 N/A Chong et al.52

semibatchcrystallizer

600 12 Posteraro and co-workers141,166

semibatchcrystallizer

N/A N/A Lee and co-workers41,97

stainless-steelcell

200 N/A Villano and Kelland44

stainless-steelcell

235 N/A Svartaas et al.167

stainless-steelcell

288 N/A Lee et al.168

sapphirecrystallizer

77 20 Bruusgaard et al.169

sapphirecrystallizer

100 50 Li et al.170

sapphirecrystallizer

50.7 N/A Chen et al.37

stirred reactor 43.3 15 Sharifi et al.171

stirred reactor 90 15 Rasoolzadeh et al.172

stirred reactor 300 20 Xu et al.173

apparatus/vessel

innervolume(cm3)a

pressuregrade(mpa)a research group

stirred reactor 1072 20 Tang et al.174

stirred reactor 372,5 N/A Seo et al.175

stirred reactor 750 N/A Zare et al.176

rocking cell(RC5)

40 20 Kelland and co-workers117,177,178

Daraboina et al.;179 Li et al.170

sapphirerocker rig(RCS20)

20 N/A Li et al.170

T-piecerocking cell

N/A N/A Cook et al.180

HP cell 100 20 Duchateau et al.124

HP cell 280 10 Jensen et al.39

HP cell 300 12 Seo and Kang181

HP cell 300 15 Sina et al.182

HP cell 1 N/A Ohno et al.147

HP cell 30 N/A Ohno et al.183

HP cell 90 N/A Kang et al.184

HP cell 556 N/A Yang and Tohidi185

HP cell 600 N/A Park et al.186

HP-video cell N/A 15 Wu et al.187

microcell 60 N/A Rojas Gonzalez71

PVT cell N/A N/A Semenov et al.188

HP-ALTA 0.15 N/A Lou et al.189

HP-microDSC

0.5−1 40 McNamee;126 Sharifi andEnglezos;190 Daraboina andLinga51,191

hydrateequilibriumcell

66.5 15 Jensen et al.192

sealed glassampules

0.2 N/A Ohno et al.193

pipe wheel 13400 15 Urdahl and co-workers99,100

flow loop up to 106 10−15 Reed et al.;104 Talley et al.;105

Klomp and co-workers;83,111

Peytavy et al;116 Turner et al.194

mini/microflow loop

100−1000 ∼10 Talaghat and co-workers;195,196

Cook et al.180

aN/A: not applicable or not given in the literature.

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monitoring of the system pressure, temperature, and reactionvolume. Compared to other types of reaction vessels, HP-ALTA and HP-micro DSC have the advantage of beingespecially suitable for conducting a large number ofexperimental series. They both take small sample volumes(∼1 mL or less), and the same sample could form hydrates andthen be melted repeatably. The sample transparency reflectedby the passing light beam (ALTA), or the heat transfermeasured by the differential scanning calorimetry (DSC), leadsto reliable and accurate detection of the hydrate onset point. Ahydrate equilibrium cell is another type of high-pressure cellthat normally allows visual inspection through sapphirewindows. Sealed glass ampules are less common reactionvessels in hydrate studies. They could be used to evaluate theperformance of selected KHIs and can operate collectively withadvanced monitoring techniques such as the NMR spectros-copy.Vertically placed pipe or loop wheels and the horizontal flow

loop of various sizes are the two types of apparatus often usedfor industrial pilot-scale KHI evaluations under simulated field

conditions. They could be used when the KHI candidates havepassed the preliminary lab-scale tests and need to be furtherexamined in industrial multiphase flow situations. Thepressurized and rotating pipe wheel in a cooling chamberusually has an inner diameter (i.d.) of 1−3 in. with an optionalwindow for visual observation. For instance, Urdahl and co-workers99,100 from Statoil and SINTEF adopted a high-pressurestainless steel wheel to study gas hydrate formation andinhibition. The wheel with a video camera on a Perspex windowcould rotate at a constant angular velocity and was equippedwith sensors for monitoring pressure, temperature, and torqueexerted on the wheel. One advantage of the wheel is that thereis no need for a pump to move liquids around, as is usuallynecessary in a horizontal flow loop. Some pumps can affect theconsistency of hydrate particles, which is more of a problem fortesting LDHI antiagglomerants.Bench-top horizontal “wheels” have also been developed for

studying LDHIs. The first such high-pressure wheel used amagnet to move fluids around the pipe, thus avoiding rotationof the wheel.101 Rotation in another benchtop wheel design was

Table 2. Monitoring and Characterizing Techniques Frequently Used in Gas Hydrate and KHI Studies

no. technique function and short description selected refs

1 neutron diffraction determines the atomic and/or magnetic structure of a materialvia neutron scattering

Koh and co-workers;137,197,198 Lokshin et al.199

2 1H NMR applies NMR with respect to hydrogen nuclei to determine themolecular structures

Kelland et al.;177 Ferreira et al.;160 Semenov et al.188

3 solid-state 13C NMR complements X-ray diffraction and provides structuralinformation about solids and polymers

Park et al.;186 Cha et al.;159 Ohno et al.;183 Ferreira etal.;160 Semenov et al.;188

4 solid-state 13C magic anglespinning (MAS) NMR

spins the sample with respect to the direction of the magneticfield for increased resolution

Daraboina et al.;200 Ohno et al.193

5 Raman spectroscopy observes vibrational, rotational, and other low-frequency modesfor identification of molecules

Daraboina et al.;201 Ohno et al.147

6 polarized Raman spectroscopy reveals the molecular orientation and symmetry of the bondvibrations to identify chemicals

Sa et al.202

7 in situ laser Raman spectroscopy enhances the Raman effect for identification of substances usinglasers

Hong et al.203

8 infrared (IR) spectroscopy identifies molecular information (e.g., can measure the degree ofpolymerization)

Ferreira et al.;160 Xu et al.163

9 magnetic resonance imaging(MRI)

quantifies molecular information during crystallization andphase transition

Kvamme and co-workers204,205

10 powder X-ray diffraction identifies phases of a crystalline material and providesinformation on the unit cell dimensions

Sa et al.;206 Park et al.;186 Ohno et al.;183 Daraboina etal.201

11 gas chromatography (GC) separates and analyzes compounds to measure the content ofvarious components in a sample

Daraboina et al.;201 Kumar et al.42

12 size-exclusion chromatography(SEC)

separates molecules by size or molecular weights; often appliedto proteins or polymers

Kelland et al.;177 Seo et al.175

13 gel permeation chromatography(GPC)

separates analytes on the basis of size; a type of size-exclusionchromatography

Nakarit et al.69

14 microscope with CCD/digitalcamera

captures images with modern optical microscopy Lee et al.;168 Daraboina et al.;161 Kumar et al.42

15 focused beam reflectancemeasurement (FBRM)

measures in situ particle size Greaves et al.;207 Clarke and Bishnoi208

16 ultrasonic signal processing identifies integrity or geometry of the substance; cancharacterizes phase change or defect

Yang and Tohidi185

17 quartz crystal microbalance-dissipation (QCM-D)

detects interfacial acoustic signals to examine surfaceadsorption, film thickness, or softness

Walker et al.209

18 differential scanning calorimeter(DSC)

measures heat exchange between the sample and the referenceas a function of temperature

Varma-Nair et al.210

19 microdifferential scanningcalorimeter (microDSC)

operates in wider temperature and pressure ranges as comparedto normal DSC

McNamee;126 Sharifi and Englezos;190 Daraboina andLinga;51 Lachance et al.127

20 pendant drop tensiometer measures interfacial tension Duchateau et al.211

21 laser interferometer measures hydrate film thickness Mori and co-workers212−214

22 photodetector detects change in the light transmission May et al.;215 Lou et al.189

23 optical/video camera visually observes hydrate formation Shin et al.;43 Li et al.;170 Wu et al.;187 Seo and Kang;181

Zheng et al.216

24 boroscope inspects the inner structure through a small hole Zheng et al.216

25 viscometer measures torque and viscosity in a fluid system Zhao et al.46

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avoided by using the Euler movement to provide fluidmovement in the pipe.102

A simple horizontal flow loop can be built to study THFhydrates at atmospheric pressure with limited scope of use.103

Most high-pressure flow loops used today are built to studyhydrates in systems consisting of natural gas, condensate or oil,and water in the presence or absence of hydrate inhibitors. Theinner diameter of a flow loop could vary from 1/4 in. (miniloop) to 4 in. or larger. For example, Reed et al.104 and Talleyet al.105 from Exxon Upstream Research Company constructedstainless steel hydrate flow loops with 1/2 and 4 in. i.d. toevaluate kinetic hydrate inhibitors. Talley and other co-workersutilized the 1/2 in. mini-loop (∼3 m long with a transparentsection for visual observation) and performed a series of KHIstudies in the mid-1990s.106

This led to the filing of multiple patents on several classes ofpolymers containing amide groups, including poly(N-alkylacrylamide)s and polyvinylamides.90,92,93,107−110 Klompand co-workers at Shell first utilized a flow loop with 1/4 in.i.d. and 16 m length to study PVP and butylated PVP111 andlater used a larger loop with 3/4 in. i.d. and 108 m length intheir studies of a hyperbranched poly(esteramide).83 Shell alsocarried out a field trial in Michigan with PVP using a flow loopwith 3 in. i.d. and a total length of ∼2.5 km, where PVP failedto prevent hydrate formation at 10 °C or higher subcoolings.112

Sinquin and co-workers at the Institut Francais du Petrole(IFP) in their initial search of effective KHIs used a pilot loopwith 0.3 in. i.d. and ∼6 m length as the experimental apparatus.They filed patent applications on two novel polymeric KHIs,whose performances were moderate and no better thanPVP.113,114 Palermo et al.115 at BP commissioned their KHIstudies with the IFP loop at Solaize (2 in. i.d. and 140 mlength) in the late 1990s. Peytavy et al. at TOTAL constructeda flow loop with 1 in. i.d. and 35.6 m length and used it for KHIstudies with increasing repeatability when applying a special in-house operation protocol using water formed from meltedhydrates.116

For concluding this section of the review, with the largevariance in the available experimental apparatus for gas hydrateand KHI studies, there are several considerations to make in theselection and operation of the hydrate formation vessels. Theseinclude the cost of construction, the design of geometry andstirring vortex, other flow regimes obtainable, the inner volumeand pressure grading, the ease of operation, maintenance andrepair, and not least, the convenience of data collection and thepossibility of connecting to external monitoring and character-ization devices.

■ OTHER MONITORING AND CHARACTERIZATIONTECHNIQUES

With an appropriate experimental apparatus in place wherehydrate formation and dissociation could occur, auxiliarymonitoring and characterizing techniques are needed to havethe whole process under control and have the experimentaldata properly recorded for analysis. Depending on the aim ofthe study, pressure and temperature transducers for recordingof the P/T profiles and, occasionally, cell windows for visualinspection may or may not be sufficient. More advancedtechniques have been frequently involved for specificallymonitoring those system variables of interest and facilitatingthe data collection. Examples include the applications of aviscometer, microscope with video camera or CCD, gaschromatography (GC), Raman spectroscopy, infrared (IR)spectroscopy, powder X-ray diffraction (PXRD), nuclearmagnetic resonance (NMR), magnetic resonance imaging(MRI), and neutron diffraction. These specialized techniquesare powerful for gaining further insights at either themacroscopic level, such as hydrate particle morphology andfluid viscosity, or the microscopic level, such as hydrate−waterinteractions, polymer structures, and the identification ofmolecules. A short description of these techniques withreferences demonstrating how they were applied in hydrateand KHI studies is given in Table 2. Differential scanningcalorimetry (DSC) has also been listed here as a monitoringtechnique because it offers a place for sampling tubes andmeasures the heat exchange during hydrate formation.

■ EXPERIMENTAL METHODS FOR EVALUATION OFKHI PERFORMANCE

Even for the same experimental apparatus and the samemonitoring techniques, there are a variety of experimentalprocedures to choose from when performing laboratory KHIperformance investigations. There is currently no standard testprocedure for the screening and evaluation of KHI perform-ance. Results from different laboratories with different equip-ment and testing procedures are not always exchangeable ortransferable. Depending on the type of equipment and thepurpose of the study, a careful selection of the experimentalmethod and procedure is critical for convincing evaluations.Table 3 summarizes the most commonly used laboratoryprocedures for conducting experimental investigations on gashydrate systems in the presence or absence of KHIs.All KHI experiments under pressure require loading the test

equipment with at least hydrate-forming gas and aqueous fluidand possibly also liquid hydrocarbons. However, reports onKHI studies sometimes lack an important detail in the

Table 3. Experimental Procedures Commonly Applied in Gas Hydrate Studies and Testing of KHIs

procedure/condition key variable to monitor comments

isothermal induction time maintains a constant temperature and degree of subcooling from the start of an experimentisothermal−isobaric induction time keeps both the system temperature and pressure constant for hydrate formationconstant cooling hydrate formation temperature applies fixed cooling rates and allows the pressure to decrease with decreasing temperatureconstant cooling−isobaric

hydrate formation temperature ensures a constant pressure along the cooling sequence and the hydrate formation process

temperature ramping hydrate formation temperature performs multiple cooling ramps with isothermal intervals in betweencooling−heatingcycles

induction time or hydrate formationtemperature

forms the hydrates, then melts, for repeated times with the memory effect involved

sudden cooling hydrate formation temperature cools the surrounding water jacket of the reactor rapidly and allows the reactor to cool downgradually into the hydrate region

constant heating decomposition temperature andpressure

slowly decomposes the formed hydrates and measures the hydrate equilibrium data

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experimental information that can affect the performance. Anexample is the time between when the KHI test solution ismade and the start of the experiment. In particular, if thesolution contains long polymer chains and the polymer cloudpoint is low, solution equilibrium may not be reached as thepolymer has not had time to fully unravel and fully interact withthe aqueous phase. Another detail that can affect the KHI resultis whether pressurization of the system is carried out before orafter cooling in an isothermal test. Another detail that affectsKHI performance is the percent liquid volume in the testequipment.117

Among the experimental procedures listed in Table 3,induction time measurements under the isothermal orisothermal−isobaric conditions stabilize the hydrate system atthe set temperature and pressure before the start of anexperiment. The results of a series of experiments could becompared with the same degree of subcooling (as anapproximation for the nucleation driving force118,119). Thisfacilitates the study of nucleation mechanisms and the effects ofKHIs on the kinetics of hydrate nucleation and growth. Afterthe start of hydrate onset, the system pressure will eitherdecrease (isothermal) due to the consumption of gascomponents during hydrate formation, or be maintained asconstant (isothermal−isobaric) with a continuous gas supply.Formation temperature measurements with the constantcooling procedure, isobaric or not, fits well the purpose offast KHI screening and has been used intensively forpreliminary evaluations of a large number of KHI candidates.53

In both cooling experiments, the first sign of pressure drop thatis not due to the drop in temperature is taken as the earliestevidence of hydrate formation. However, true nucleation mayhave occurred sometime before but was undetected on amacroscopic scale. The same is also true of the first sign ofpressure drop in an isothermal experiment, assuming thepressure drop due to gas dissolution in the liquid phases hasreached equilibrium. The isobaric constant cooling sequencemimics well the cooling process during the oil and gas pipelinetransportation. The temperature ramping procedure with themonitoring of formation temperatures has the advantage ofintermediate stabilization of the tested sample with shortisothermal intervals in between. The above procedures ofinduction time measurements and formation temperaturemeasurements have been compared10,120 to examine theirequivalence in data collection and analysis for the highlystochastic hydrate nucleation process. The sudden coolingprocedure121 is a variant of the constant cooling procedure. Itallows the reactor to cool gradually into the hydrate region afterthe water jacket surrounding the reactor is cooled in a veryshort time. Cooling−heating cycles take advantage of thememory effect42,122 that hydrate reformation is facilitated dueto the existence of residual water structures or semicages in thesolution. By doing this, the reproducibility of experimental datais expected to improve as compared to that of fresh waternucleation. This procedure has been developed into the hydrateprecursor method by Duchateau and co-workers.123,124 Theymelted the formed hydrates at just a few degrees centigradeabove the equilibrium temperature for a limited period of time.In this manner, the water history would be preserved. Thecooling−heating cycle procedure bears other names, such as theprecursor constant cooling procedure125 or the superheatedhydrate melting method.44 Other effective ways to reduce thedata scattering and improve the reproducibility include utilizingthe ice-memory effect,126 adding external impurities39 as

heterogeneous nucleation sites, and using water-in-oil emul-sion127 for hydrate formation.Two or more of the procedures mentioned above could be

applied collectively, and the results from different procedurescould be compared.10,120,128 Wu et al.120 and Kulkarni et al.10

examined the equivalence of the isothermal and constantcooling procedures in data collection during the hydrateformation process. They found several differences, although thenucleation data collected by either procedure could wellrepresent the stochastic nature of hydrate nucleation. A generaltrend is that with the constant cooling procedure hydrateformation becomes less stochastic with less scattered datapoints than that under constant temperature.2 This is probablybecause both the critical nuclei size and the free energy barrierof nucleation would decrease along the cooling sequence (i.e.,increasing degree of subcooling).4 As a result, studiesperformed at isothermal conditions usually require manyexperimental parallels to collect representative data. On theother hand, at isothermal conditions, the gas solubility wouldremain the same, whereas during cooling or cooling ramps, thegas solubility increases with decreasing temperature. Whencooling proceeds, the system requires an increasing amount ofdissolved gas to attain a sufficient supersaturation level fornucleation.120 At moderate to fast cooling rates, it is no longerreasonable to assume quasi-steady states, and analysis of theformation temperature data could become difficult due tovarying levels of supersaturation. A similar situation could occurif the system is pressurized at room temperature instead of theexperimental temperature. For high-solubility gases such ascarbon dioxide and hydrogen sulfide, the chosen pressurizing/cooling procedure may lead to a giant difference in the resultingexperimental data. Although the cooling and temperatureramping procedures seem to be less labor intensive for datacollection, the induction time data measured at isothermalconditions are usually more accurate and easier to analyze.10

Specifically designed for the fast screening of KHIs, a morerecent crystal growth inhibition (CGI) test method has drawnincreasing interest from the oil and gas industries. Tohidi’sgroup129 took the concept of the hydrate precursor method onestep further. Their standardized CGI method offers aconvenient tool for evaluation of KHI performance. The ideais to bypass the stochastic nucleation process by purposelyretaining a small yet measurable fraction of the hydrate particlesin the system. This is followed by cooling the system into thehydrate region to observe further growth of the existing hydratecrystals. As a result, any observed growth behavior would besolely associated with the performance of the tested KHIs onhydrate growth. Intriguingly, there exists clear boundaries ofvaried growth inhibition regions when such systems are testedupon increasing degrees of subcooling. Figure 4 illustrates theconcept of the CGI method with distinctive growth inhibitionregions.As shown in Figure 4, the inhibition performance of

individually tested KHIs by the CGI method can be mappedout as three identifiable growth inhibition regions thatcorrespond to the increasing degrees of subcooling (fromright to left). They are termed the complete inhibition region(CIR), where the applied subcooling level is fully suppressedand no hydrate forms, the slow growth region (SGR) with slowto moderate growth rates of hydrate particles, and the rapidgrowth region (RGR) with fast and catastrophic hydrategrowth. The rightmost SDR region marked in Figure 4 refers tothe slow dissociation region. As Svartaas et al.130 observed

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earlier for methane-propane hydrate, the hydrate dissociationrate was reduced significantly in the presence of tested KHIs.This is because the polymer bonding would contribute to thestabilization of the formed hydrates, and consequently, theincrease of hydrate dissociation temperature. The CGI methodhas been successfully applied to quantitatively evaluate theperformance of KHI candidates.131−134 An apparent advantageis that it can test KHI performances with the worst-casescenarios under flowing or shut-in conditions when a smallfraction of hydrates may already be present.131

Hase et al.135 compared the various experimental proceduresincluding the isothermal, constant cooling, and the CGImethods by examining the behaviors of hydrate formation inthe presence of five VP/VCap-based polymers using autoclaveand rocking cell as the reaction vessels. Their results suggestedthe following testing order for the evaluation of KHIs. First, theconstant cooling procedure could be applied for prescreeningand elimination of poor KHIs. Then, the CGI method could beadopted to further distinguish among the best performers.Finally, the isothermal induction time measurements can beutilized for verification of the most promising KHIs.

■ KHI MECHANISMS: EXPERIMENTAL ANDCOMPUTATIONAL EVIDENCE

A large number of experimental studies on KHIs have enrichedour understanding of the gas hydrate systems and the effects ofvarious KHIs on gas hydrate formation. During the last twodecades, computational modeling and simulations, in particular,the molecular dynamics (MD) and Monte Carlo (MC)simulations, have become more developed and have beenused to study KHI mechanisms. Used correctly, they can offerinsights into the operational mechanisms by simulating andrevealing the KHI-water-hydrate interactions at the molecularlevel. These computational simulations have shown agreementswith experimental studies,136 although they may also givedifferent or even opposite indications as those suggested byexperiments.137 Thus, although modeling and simulation resultscan be informative, conclusions from such studies need to beverified by laboratory experimental investigations.Kinetic inhibition is well known for other crystals than

clathrate hydrates. For example, scale inhibitors for theinhibition of calcium carbonate and barium sulfate arepresumed to inhibit nucleation, crystal growth, or both.25

KHIs are also presumed to operate by one or both of thesemechanisms.

The crystal growth inhibition mechanism involves adsorptionof the inhibitor onto some part of the growing hydrate crystalsurface, altering the morphology and/or lowering the rate ofcrystal growth. However, there is no reason why thismechanism is confined to crystals, i.e., particles that havereached the critical nucleus size, at which point the change inGibbs free energy (ΔG) is negative for spontaneous growth. Itis just as likely that adsorption of inhibitors can occur onsubcritical hydrate nuclei also. For gas hydrates, this thereforebecomes a nucleation inhibition mechanism with short directinteractions between the KHI and hydrate surfaces. A secondmechanism involves more long-range water perturbation effectswhereby the KHI destabilizes nuclei formation, preventingsufficient particle growth and clustering to occur to reach thecritical nucleus size. Other mechanisms proposed includedeactivation of gas hydrate heteronucleation sites andenclathration of dissolved hydrate-forming molecules in thewater phase, rendering them unavailable for hydrate formation.Kuznetsova et al.138 suggested that KHI polymers modified theinterfacial tension of the water−methane surface, convertingthe initially dispersed methane phase into separated bubblesand then building a system-wide network that partially coveredthe surface of the methane bubbles.The first KHI mechanism to find experimental support was

crystal growth inhibition. Studies on tetrahydrofuran (THF)hydrate crystal growth proved that polymers such as poly(N-vinyl caprolactam) (PVCap) adsorbed to the growing hydratecrystal surface, perturbing the normal crystal morphology and/or inhibiting further growth.139,140 Examples of gas hydrateexperimental studies in favor of the adsorption−inhibitionmechanism include work by Posteraro et al.141 and Ivall etal.,142 who both studied the effect of PVP on sI methanehydrate formation. Evidence for surface adsorption of KHIpolymers on gas hydrates has also been obtained from neutronscattering experiments.143

Rojas Gonzalez71 examined the effects of various N-vinyllactam polymers on sII hydrate of a natural gas mixture. Theyfound that the adsorption of KHIs was directly related to theireffectiveness of hydrate inhibition. Interestingly, two polymersthat showed the worst and the best inhibition performances forTHF hydrates exhibited the opposite inhibition performancesfor gas hydrates. These observations have been seen in manyother studies. For example, Shell’s work in the early 1990s andlater work by Kelland showed that quaternary ammonium andphosphonium salts are excellent THF hydrate crystal growthinhibitors but have very little effect as gas hydrate KHIs whenused alone.144,145 In contrast, some polymers, such aspoly(esteramide)s and poly(N-alkylacrylamide)s are fairlypoor THF hydrate crystal growth inhibitors yet show goodperformance as sII-forming gas hydrate KHIs.146 This indicatesthat KHI evaluations using THF hydrate crystal growth systemsalone need to be verified at high pressure using gas hydratesystems to include possible nucleation inhibition.Ohno et al.147 studied the effect of PVCap on methane−

ethane hydrate and observed a structural interconversionbetween sI and sII. They claimed that the polymer adsorptionhad contributed to the gas and water rearrangement at thecrystal interfaces, which eventually led to the structuralconversion. Various computer molecular simulation studieshave also provided evidence for hydrate surface adsorption byKHI polymers.139,143,148−150

The most studied class of KHI polymers are those based onN-vinyl lactams. Several studies indicate that both nucleation

Figure 4. Determination of the crystal growth inhibition (CGI) testmethod129 for evaluation of KHI performance.

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and crystal growth inhibition mechanisms are operating withthis polymer class.47,126,132 In many gas hydrate KHI studies, itis impossible to differentiate between nucleation and crystalgrowth inhibition. However, there is experimental support forthe nucleation inhibition mechanism by water perturbation,where it can be differentiated from mechanisms involvinghydrate particle surface adsorption. For example, Chua andKelland151 showed that the KHI performance on an sII gashydrate system of a 1:1 weight mixture of tetra(n-hexyl)-ammonium bromide (THexAB) and PVCap gave very similarperformances to PVCap blended with tetra(n-pentyl)-ammonium bromide (TPAB) and much better performancethan PVCap blended with tetra(n-butyl)ammonium bromide(TBAB). However, as THF hydrate crystal growth inhibitors,TPAB is by far the superior inhibitor of the three, followed byTBAB, with THexAB being a very poor inhibitor. This suggeststhat the synergistic enhancement by THexAB is not related tocrystal growth inhibition but by some other mechanism. Theauthors surmise that the long hydrophobic n-hexyl groups inTHexAB cause enhanced water perturbation relative to theother quaternary ammonium salts, improving the hydratenucleation inhibition. Perturbation of the bulk water phasecausing nucleation inhibition has been suggested to occur forKHI polymers where they have been shown to give relativelypoor THF hydrate crystal growth inhibition relative to goodgrowth inhibitors such as PVCap. This includes polyester-amides, polyaspartamides, and polyalkyl(meth)acrylamides.Exxon Mobil was the first to imply nucleation inhibition bywater perturbation in their work on the latter of these polymerclasses.25,84,108

There is also computer molecular simulation evidence forhydrate nucleation inhibition by KHIs. Moon et al.18 showedthat PVP molecules were held at a distance of 5−10 Å from thecrystal surface, instead of adsorbing directly onto the growingcrystal planes, yet hydrate formation was destabilized. Kvammeet al.152 evaluated the effects of N-vinyl lactam polymers on sIand sII hydrates with molecular dynamics simulations. Theresults showed that the KHIs could interact with the hydrate-water clusters and trigger nuclei dissolution without havingdirect contact. Hawtin and Rodger153 with MD simulationsstudied the effect of poly(dimethylaminoethyl methacrylate)(PDMAEMA) on methane hydrate formation. They showedthat the polymer had long-range effects in solution thatperturbed the water structures. Monte Carlo simulations byWathen et al.154 indicated that PVP could either inhibit orpromote sII hydrate formation when the polymers were withinthe working distance to the embryo surfaces. Promotion ofhydrate formation is also observed for N-vinyl lactam polymerson THF hydrate, whereby accelerated plate growth is observedbelow the total inhibition concentration.80,139

Unlike experimental studies, the limited computationalcapacity poses a challenge to the current modeling andsimulation studies. Only a confined simulation module with alimited number of molecules (typically, a few hundred toseveral tens of thousands) can be established and simulated fora limited period of time (typically, a few hundred nanosecondsto a few microseconds).155,156 It partly explains why theformation of the structurally simpler sI methane hydrate is mostoften simulated for the evaluation of KHI performance.However, with increased computational capacity and complex-ity, future molecular modeling and simulations will becomemore powerful and offer more insights into the gas hydratephenomena and the effects of KHIs.

■ CONCLUSIONSKinetic hydrate inhibitors (KHIs) as a main category of lowdosage hydrate inhibitors (LDHIs) have been actively used forhydrate prevention and mitigation in the petroleum industryover the past two decades. Various chemicals that act as KHIs,mostly polymeric compounds, are capable of retarding gashydrate formation at low concentrations and facilitating smoothand reliable oil and gas transportation. This article with aliterature survey has reviewed the most commonly usedexperimental apparatus, monitoring and characterizationtechniques, and experimental methods for conducting KHI-related gas hydrate studies. The major kinetic inhibitionmechanisms proposed in the literature and supportingexperimental and computational evidence are also reviewed.More research is crucial for developing better or greener KHIs,understanding their working mechanisms, optimizing testmethods and developing commercial KHI recycling technologyin order to increase the number of field applications.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: malcolm.kelland@uis.no.ORCIDWei Ke: 0000-0003-3793-7481NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank the Norwegian Ministry of Education andResearch and University of Stavanger for their financial supportof this work.

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