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Kinetic Hydrate Inhibitor Studies for Gas Hydrate Systems – A Review of Experimental Equipment and Test Methods Malcolm A. Kelland, and Wei Ke Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02739 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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Kinetic Hydrate Inhibitor Studies for Gas Hydrate Systems – A Review

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of Experimental Equipment and Test Methods

3 Wei Ke1, Malcolm A. Kelland2*

4 5 6

1

7

Stavanger, NO-4036 Stavanger, Norway

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University of Stavanger, NO-4036 Stavanger, Norway

Department of Petroleum Engineering, Faculty of Science and Technology, University of

Department of Mathematics and Natural Sciences, Faculty of Science and Technology,

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Abstract

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Kinetic hydrate inhibitors (KHIs) have been studied, developed and used in the oil and gas

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industry for more than two decades. The main active ingredients in commercial KHI

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formulations are water-soluble polymers. When dosed at low concentrations (0.1–2.0 wt. %

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active chemical) they are able to retard the gas hydrate formation process and facilitate reliable

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oil and gas transportation. A considerable amount of research effort on KHI technologies has

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contributed to an abundance of KHI knowledge, applications, and tailor-made solutions.

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Whereas previous reviews have concentrated on the chemistry of KHIs, this review article has a

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particular emphasis on the experimental equipment, hydrate detection tools and test methods

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commonly applied in KHI investigations. The underlying mechanisms of KHIs still are not fully

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understood. The major hypotheses proposed in the literature and supporting experimental and

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computational evidence are also reviewed.

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Keywords: petroleum, gas hydrates, kinetic hydrate inhibitors, kinetics, mechanisms

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*

Corresponding author. E-mail address: [email protected] (M.A. Kelland).

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Introduction

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Gas hydrates are crystalline, non-stoichiometric clathrate inclusion compounds. At the

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microscopic level, they are composed of hydrogen-bonded water molecules as hosts and gas

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molecules entrapped in the water cavities as guests. They are not chemical compounds since no

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strong chemical bonds exist between the water and gas molecules. Generally, high pressures and

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low temperatures are required for their stable existence. The gas molecules able to be

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enclathrated into the water lattices are usually small (< 10 Å),1 such as methane, ethane, propane,

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iso-butane, and inorganic gases including nitrogen, hydrogen, and carbon dioxide.2 Different

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hydrate structures have been discovered. Cubic structure I (sI) and structure II (sII), and

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hexagonal structure H (sH) are the three most common gas hydrate structures. This is determined

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by how the hydrate unit cells are assembled at the molecular level.2

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Since it was first reported3 as an industrial nuisance, gas hydrate formation continuously leads to

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pipeline plugging issues during oil and gas transportation. There are three major stages of phase

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transitions associated with hydrate plug formation, that is, nucleation,2,

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agglomeration.6, 7 Numerous experimental investigations2, 8-16 and modeling and simulations12, 17-

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21

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innovations and technical improvements, hydrate formation remains the number one problem in

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flow assurance.22 In fact, we are facing more severe technical challenges due to the formation of

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gas hydrates than before, especially when offshore drilling activities move towards geological

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sites of deeper waters and colder temperatures. The operational regions are often well within the

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hydrate stability zone. An enormous expense for hydrate prevention and mitigation is associated

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with the use of traditional thermodynamic hydrate inhibitors (THIs) such as methanol and mono 3

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growth,2,

5

and

have contributed to the current understanding of such phase transitions. Despite scientific

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ethylene glycol (MEG).23 In such a context, robust and economical hydrate management

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strategies24 have become an urgent need for safe and reliable production of oil and gas.

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To mitigate the hydrate formation and plugging issues, an important area of efforts is the

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research and development of low dosage hydrate inhibitors (LDHIs).25 The concept was initiated

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since the mid-1980s and early 1990s.26 Research on LDHIs has been active all the time.7, 20, 22, 24,

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27-33

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called so to differ from the traditional THIs based on their different hydrate inhibition

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mechanisms. THIs shift the hydrate equilibrium to lower temperatures and higher pressures,

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forcing the system to stay within the hydrate-free region. On the contrary, KHIs, mostly

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polymeric compounds, delay the hydrate nucleation and/or growth and extend the hydrate

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induction time to exceed the residence time of the reservoir fluid; AAs, mostly surfactants, allow

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the hydrate particles to form and keep them dispersed in the reservoir fluid to generate

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transportable slurries. For the time being, AAs are used more often than KHIs in the industrial

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practices, especially at harsh conditions with higher subcooling, longer transportation distances,

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or higher water-cuts. However, the research progress in recent years on KHIs has made KHI

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technologies equally promising. Laboratory investigations into the biological KHIs34 also have

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been going on for years. High-performance KHIs have been applied to varied fluid systems

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including gas, condensate, and black oil.35 To mimic the real multi-phase reservoir fluid flow in

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oil and gas pipelines, salt36-39 could be added to make saline solutions, and heptane39-42 or

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decane43-45 could be added as a liquid hydrocarbon phase. In principle, KHIs are designed for

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multi-phase pipeline applications, but may also be applied in other circumstances. For example,

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KHIs may be used during drilling while exposed to the drilling fluid. In such a case, drilling

LDHIs consist of kinetic hydrate inhibitors (KHIs) and anti-agglomerants (AAs). They are

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mud36,

46-50

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environment. Porous media such as silica sand51, 52 have also been tested. Hydrate formation and

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dissociation behavior in the interstitial pores in the porous media can be very different from that

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in the usual liquid-gas-hydrate systems. Establishment and testing of hydrate systems with one or

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more of these components or additives is able to offer additional insights on how the selected

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KHIs can behave in close-to-reality circumstances. Kelland53 has made a comprehensive review

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on gas hydrate control, with special focus on the lately developed KHIs, their chemistry,

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synthesis, and laboratory performances. A review on LDHI–KHI applications from an industrial

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perspective was given by Klomp.35

could be added to evaluate the performance of KHIs in the deep-water drilling

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Whereas previous reviews on KHIs have concentrated on their chemistry, this work has focused

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on the tools and methods commonly used for experimental KHI investigations in natural gas

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hydrate systems in the past decade. The proposed KHI inhibition mechanisms with experimental

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and computational evidence in the literature are also reviewed. The following areas of KHI

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research will not be considered in details in the current work: a) anti-freeze proteins (AFPs) from

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living organisms, also called ice-structuring proteins (ISPs), and their potential as KHIs.54-60 An

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up-to-date review of AFP studies is available by Walker et al;61 b) ionic liquids (ILs)62-64 as

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potential KHIs. Tariq et al.65 have recently reviewed ILs-related research activities; c)

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tetrahydrofuran (THF)66-69 or cyclopentane66,

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cyclopentane hydrates are often used to facilitate the screening of KHIs for gas hydrate

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inhibition, there are two major differences that are worth mentioning. First, THF and

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cyclopentane hydrates form without the need of elevated pressures, thus not able to fully

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represent the real conditions for gas hydrate formation. For the latter, the dissolution and

70

hydrate systems. Although THF and

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diffusion of the pressurized gas into the aqueous phase is critical. Second, the actual performance

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of KHIs on THF and cyclopentane hydrates can be different or even opposite to that on gas

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hydrate systems due to probably different KHI working mechanisms;71 d) recycling72,

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removal74, 75 of KHIs for future field applications; and e) compatibility studies of KHIs with

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other types of production chemicals,76 in particular, corrosion inhibitors.77, 78 The current survey

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remains its focus on the tools and methods of experimental KHI investigations, and experimental

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and computational evidence for the proposed inhibition mechanisms in the literature.

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Major KHI Categories

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There are three main categories of KHIs developed for field applications, all of which are

73

or

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polymeric compounds. These are:

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• Poly-N-vinyllactam polymers, including a variety of copolymers and grafted polymers. Typical

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examples are five-ring polyvinylpyrrolidone79 (PVP), six-ring polyvinylpiperidone80 (PVPip),

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seven-ring polyvinylcaprolactam81 (PVCap), and eight-ring polyvinylazacyclooctanone82

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(PVACO). Their chemical structures are shown in Figure 1. Their performances as KHIs were

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found to improve with increasing lactam ring sizes.80, 82 Only polymers with the five- and seven-

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rings are used in commercial KHI formulations.

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Figure 1. Typical poly-N-vinyllactam polymers with increasing lactam ring sizes. From left to

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right: PVP (five-ring), PVPip (six-ring), PVCap (seven-ring), PVACO (eight-ring).

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• Hyper-branched poly(esteramide)s.83 An illustrative structure of a hyper-branched

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poly(esteramide) is shown in Figure 2. It is feasible to modify the tips on its molecular structure

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to make the polymer more or less hydrophilic.84 This class of KHIs is claimed to have better

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performance on structure-I hydrates than the VCap-based polymers.25

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

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• N-Isopropylmethacrylamide85 (IPMA) polymers and copolymers. This is another widely

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studied KHI category. The structure of poly(N-isopropylmethacrylamide) is shown in Figure 3.

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IPMA copolymers86 may have higher cloud points and better tolerance to saline environments.

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Figure 3. Poly(N-isopropylmethacrylamide).

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Other polymeric KHIs that have been developed and tested include pyroglutamate polymers (a

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few field applications have been carried out),87,

88

maleic copolymers and alkylamide

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derivatives,89 polyalkyloxazolines,90, 91 polymaleimides,92 polyallylamides,93 polyaspartamides,45,

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94, 95

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inorganic and traditional THI, was reported to possess kinetic inhibition properties towards

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hydrate formation in porous media.52 However, it appears to only slow down the growth rate of

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hydrate formation and has no effect on the induction time. An introductory overview of KHIs

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was given by Erfani et al.98 With abundant details and a special focus on the chemistry, the

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structure, functional groups, and synthesis processes, Kelland25, 53, 84 has made comprehensive

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reviews on all categories of KHIs along with their histories of development. A general guideline

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for developing a polymeric KHI is that a certain level of hydrophobicity should be reached for

modified starch and starch derivatives96, 97, and proteins61. Recently, sodium chloride, an

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the KHI to effectively disrupt the water structures, while it has to remain water soluble to get into

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the liquid system.

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Experimental Apparatus for Testing of KHIs

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Experimental device for testing and screening of KHIs for gas hydrate systems, from laboratory

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scale to pilot scale, have evolved over years. The most commonly used apparatus include

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autoclave, rocking cell, stirred reactor, batch or semi-batch crystallizers, automated lag time

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apparatus (HP-ALTA), and (micro) differential scanning calorimetry (DSC or µ-DSC), pipe-

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wheel, and flow loop. A brief description of each is given below, accompanied with a list of

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apparatus and selected research groups who often apply them is given in Table 1.

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Table 1. Apparatus/reaction vessels used in gas hydrate studies in the presence of KHIs.

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The selection of a proper experimental apparatus largely depends on the purpose and perspective

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of the study. The quality and convenience of data collection are also priorities for consideration.

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Autoclaves, crystallizers, and other types of stirred reactors are frequently used in gas hydrate

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studies with adjustable agitation strength and pressure and temperature monitoring. Semi-batch

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autoclaves or semi-batch crystallizers could be considered, when adding components or taking

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samples for analysis along the process is required. Stainless steel and titanium are common

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materials for construction of the cell body, allowing varied designs of the inner volume, the cell 9

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geometry, and the pressure grading. Sapphire cells often provide high pressure grading and the

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possibility to mount a window on the cell body for visual inspection of the hydrate formation

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process. Rocking cells were first introduced by Shell oil company. Commercial equipment with

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several steel cells mounted in parallel, or up to 20 sapphire cells (which are particularly useful

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for studying LDHI anti-agglomerants where visual data is critical) are now available. This gives

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the researcher access to multiple results in the same cooling unit in the same time that it takes to

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run an autoclave, with at least as good reproducibility and reliability. Autoclaves and rocking

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cells that can be used for sour natural gas mixtures (containing H2S) are also available. Rocking

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cells are especially suitable for fast screening of KHI candidates under continuous cooling by

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recording hydrate formation temperatures. Video cameras, high-resolution digital cameras, or

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microscopes with CCD could be mounted or connected to the high-pressure cells to allow real

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time photo and video recordings. Specially engineered PVT cells could also be used to study the

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three-phase properties of water, gas, and hydrate with accurate monitoring of the system

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pressure, temperature, and reaction volume. Compared to other types of reaction vessels, HP-

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ALTA and HP-micro DSC have the advantage of being especially suitable for conducting a large

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number of experimental series. They both take small sample volumes (about 1 ml or less) and the

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same sample could form hydrates, then melted, for repeated times. The sample transparency

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reflected by the passing light beam (ALTA), or the heat transfer measured by the differential

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scanning calorimetry (DSC), leads to reliable and accurate detection of the hydrate onset point.

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Hydrate equilibrium cell is another type of high-pressure cell that normally allows visual

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inspection through sapphire windows. Sealed glass ampoules are less common reaction vessels

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in hydrate studies. They could be used to evaluate the performance of selected KHIs and can

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operate collectively with advanced monitoring techniques such as the NMR spectroscopy. 10

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The vertical placed pipe-wheel or loop-wheel and the horizontal flow loop of varied sizes are the

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two types of apparatus often used for industrial pilot-scale KHI evaluations under simulated field

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conditions. They could be used when the KHI candidates have passed the preliminary lab-scale

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tests and need to be further examined in the industrial multi-phase flow situations. The

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pressurized and rotating pipe-wheel in a cooling chamber usually has an inner diameter (i.d.) of

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1-3 inches, with an optional window for visual observation. For instance, Urdahl and co-

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workers99, 100 from Statoil and SINTEF adopted a high-pressure stainless steel wheel to study gas

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hydrate formation and inhibition. The wheel with a video camera on a Perspex window could

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rotate at a constant angular velocity, and was equipped with sensors for monitoring pressure,

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temperature, and torque exerted on the wheel. One advantage of the wheel is that there is no need

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for a pump to move liquids around, as is usually necessary in a horizontal flow loop. Some

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pumps can affect the consistency of hydrate particles, which is more of a problem for testing

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LDHI anti-agglomerants.

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Bench-top horizontal “wheels” have also been developed for studying LDHIs. The first such

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high pressure wheel used a magnet to move fluids around the pipe, avoiding rotation of the

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wheel.101 Rotation in another bench-top wheel design was avoided by using the Euler movement

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to provide fluid movement in the pipe.102

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A simple horizontal flow loop can be built to study THF hydrates at atmospheric pressure with

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limited scope of use.103 Most high-pressure flow loops used today are built to study hydrates in

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systems consisting of natural gas, condensate or oil, and water, in the presence or absence of

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hydrate inhibitors. The inner diameter of a flow loop could vary from ¼ inch (mini loop) to 4

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inches or larger. For example, Reed et al.104 and Talley et al.105 from Exxon Upstream Research 11

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Company constructed stainless steel hydrate flow loops with ½ and 4 inches in i.d. to evaluate

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kinetic hydrate inhibitors. Talley, and other co-workers utilized the ½-inch mini-loop (about 3 m

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long, with a transparent section for visual observation) and performed a series of KHI studies in

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the mid-1990s.106

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This led to the filing of multiple patents on several classes of polymers containing amide groups,

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including poly(N-alkylacrylamide)s and polyvinylamides.90, 92, 93, 107-110 Klomp and co-workers at

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Shell first utilized a flow loop with ¼ inch in i.d. and 16 m in length to study PVP and butylated

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PVP,111 and later used a larger loop with ¾ inch in. i.d. and 108 m in length in their studies of a

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hyperbranched poly(esteramide).83 Shell also carried out a field trial in Michigan with PVP using

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a flow loop with 3 inch in i.d. and a total length of around 2.5 km, where PVP failed to prevent

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hydrate formation at 10 °C or higher subcoolings.112 Sinquin and co-workers at Institut Francais

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du Petrole (IFP) in their initial search of effective KHIs used a pilot loop with 0.3 inch in i.d. and

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about 6 m in length as the experimental apparatus. They filed patent applications on two novel

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polymeric KHIs, whose performances were moderate and no better than PVP.113, 114 Palermo et

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al.115 at BP commissioned their KHI studies with the IFP loop at Solaize (2 inch in i.d. and 140

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m in length) in the late 1990s. Peytavy et al at TOTAL constructed a flow loop with 1 inch in i.d.

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and 35.6 m in length and used it for KHI studies with increased repeatability when applying a

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special in-house operation protocol using water formed from melted hydrates.116

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To conclude this section of the review, with the large variance in the available experimental

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apparatus for gas hydrate and KHI studies, there are several considerations to make in the

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selection and operation of the hydrate formation vessels. These include the cost of construction,

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the design of geometry and stirring vortex, other flow regimes obtainable, the inner volume and 12

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pressure grading, the ease of operation, maintenance and repair, and not the least, the

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convenience of data collection and the possibility of connecting to external monitoring and

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characterizing devices.

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Other Monitoring and Characterizing Techniques

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With an appropriate experimental apparatus in place where hydrate formation and dissociation

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could occur, we need auxiliary monitoring and characterizing techniques to have the whole

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process under control and have the experimental data properly recorded for analysis. Depending

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on the aim of study, pressure and temperature transducers for recording of the P/T profiles, and

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occasionally, cell windows for visual inspection, may or may not be sufficient. More advanced

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techniques have been frequently involved for specifically monitoring those system variables of

12

interest and facilitating the data collection. Examples include the applications of viscometer,

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microscope with video camera or CCD, gas chromatography (GC), Raman spectroscopy,

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infrared (IR) spectroscopy, powder X-ray diffraction (PXRD), nuclear magnetic resonance

15

(NMR), magnetic resonance imaging (MRI), and neutron diffraction. These specialized

16

techniques are powerful in gaining further insights, either at the macroscopic level such as

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hydrate particle morphology and fluid viscosity, or at the microscopic level such as hydrate-

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water interactions, polymer structures, and the identification of molecules. A short description of

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these techniques with references demonstrating how they were applied in hydrate and KHI

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studies is given in Table 2. The differential scanning calorimetry (DSC) has also been listed here

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as a monitoring technique, since it offers a place for sampling tubes and measures the heat

22

exchange during hydrate formation. 13

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Table 2. Monitoring & characterizing techniques frequently used in gas hydrate and KHI studies.

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Experimental Methods for Evaluation of KHI Performance

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Even for the same experimental apparatus and the same monitoring techniques, there is a variety

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of experimental procedures to choose from when performing laboratory KHI performance

7

investigations. There is currently no standard test procedure for the screening and evaluation of

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KHI performances. Results from different laboratories with different equipment and testing

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procedures are not always exchangeable or transferable. Depending on the type of equipment and

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the purpose of study, a careful selection of experimental method and procedure is critical for

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convincing evaluations. Table 3 summarizes the most commonly used laboratory procedures for

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conducting experimental investigations on gas hydrate systems, in the presence or absence of

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KHIs.

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All KHI experiments under pressure require loading the test equipment with at least hydrate-

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forming gas and aqueous fluid, and maybe also liquid hydrocarbons. However, sometimes

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reports on KHI studies lack an important detail in the experimental information that can affect

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the performance. An example is the time between when the KHI test solution is made and the

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start of the experiment. Particularly if the solution contains long polymer chains and the polymer

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cloud point is low, solution equilibrium may not be reached as the polymer has not had time to

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fully unravel and fully interact with the aqueous phase. Another detail that can affect the KHI

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result is whether pressurization of the system was carried out before or after cooling in an 14

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isothermal test. Another detail that affects KHI performance is the percentage liquid volume in

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the test equipment.117

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Table 3. Experimental procedures commonly applied in gas hydrate studies and testing of KHIs.

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Among the experimental procedures listed in Table 3, induction time measurements under the

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isothermal or isothermal-isobaric conditions stabilize the hydrate system at the set temperature

6

and pressure before the start of an experiment. The results of a series of experiments could be

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compared with a same degree of subcooling (as an approximation for the nucleation driving

8

force118, 119). This facilitates the study of nucleation mechanisms and the effects of KHIs on the

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kinetics of hydrate nucleation and growth. After the start of hydrate onset, the system pressure

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will either decrease (isothermal) due to the consumption of gas components during hydrate

11

formation, or maintain constant (isothermal-isobaric) with continuous gas supply. Formation

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temperature measurements with the constant cooling procedure, isobaric or not, fits well the

13

purpose of fast KHI screening and has been used intensively for preliminary evaluations of a

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large number of KHI candidates.53 In both cooling experiments, the first sign of pressure drop

15

that is not due to the drop in temperature is taken as the earliest evidence of hydrate formation.

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However, true nucleation may have occurred sometime before but was undetected on the

17

macroscopic scale. The same is also true of the first sign of pressure drop in an isothermal

18

experiment, assuming pressure drop due to gas dissolution in the liquid phases has reached

19

equilibrium. The isobaric constant cooling sequence mimics well the cooling process during the

20

oil and gas pipeline transportation. The temperature ramping procedure with the monitoring of

21

formation temperatures has the advantage of intermediate stabilization of the tested sample with

22

short isothermal intervals in between. The above procedures of induction time measurements and 15

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formation temperature measurements have been compared10, 120 to examine their equivalence in

2

data collection and analysis for the highly stochastic hydrate nucleation process. The sudden

3

cooling procedure121 is a variant of the constant cooling procedure. It allows the reactor to cool

4

down gradually into the hydrate region after the water jacket surrounding the reactor is cooled

5

down in a very short time. Cooling-heating cycles take advantage of the memory effect42, 122 that

6

hydrate reformation is facilitated due to the existence of residual water structures or semi-cages

7

in the solution. By doing this, the reproducibility of experimental data is expected to improve as

8

compared to fresh water nucleation. This procedure has been developed into the hydrate

9

precursor method by Duchateau and co-workers.123, 124 They melted the formed hydrates just a

10

few degrees of centigrade above the equilibrium temperature for a limited period of time. In this

11

manner, the water history would be preserved. The cooling-heating cycle procedure bears other

12

names such as the precursor constant cooling procedure,125 or the superheated hydrate melting

13

method.44 Other effective ways to reduce the data scattering and improve the reproducibility

14

include utilizing the ice-memory effect,126 adding external impurities39 as heterogeneous

15

nucleation sites, and using water-in-oil emulsion127 for hydrate formation.

16

Two or more of the above-mentioned procedures could be applied collectively and the results

17

from different procedures could be compared.10,

18

examined the equivalence of the isothermal and constant cooling procedures in data collection

19

during the hydrate formation process. They found several differences although the nucleation

20

data collected by either procedure could well represent the stochastic nature of hydrate

21

nucleation. A general trend is that with the constant cooling procedure hydrate formation

22

becomes less stochastic with less scattered data points than that under constant temperature.2

120, 128

Wu et al.120 and Kulkarni et al.10

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This is probably because both the critical nuclei size and the free energy barrier of nucleation

2

would decrease along the cooling sequence (i.e. increasing degree of subcooling).4 As a result,

3

studies performed at isothermal conditions usually require many experimental parallels in order

4

to collect representative data. On the other hand, at isothermal conditions, the gas solubility

5

would remain the same while during cooling or cooling ramps the gas solubility increases with

6

decreasing temperatures. When cooling proceeds, the system requires an increasing amount of

7

dissolved gas to attain a sufficient supersaturation level for nucleation.120 At moderate to fast

8

cooling rates, it is no longer reasonable to assume quasi-steady states and the analysis of the

9

formation temperature data could become difficult due to the varying levels of the

10

supersaturation. A similar situation could occur if the system is pressurized at the room

11

temperature instead of the experimental temperature. For high-solubility gases such as carbon

12

dioxide and hydrogen sulfide, the chosen pressurizing/cooling procedure may lead to a giant

13

difference in the resulted experimental data. Although the cooling and temperature ramping

14

procedures seem to be less labor intensive for data collection, the induction time data measured

15

at isothermal conditions are usually more accurate and easier to analyze.10

16

Specifically designed for the fast screening of KHIs, a more recent crystal growth inhibition

17

(CGI) test method has drawn increasing interest from the oil and gas industry. Tohidi’s group129

18

took the concept of the hydrate precursor method one step further. Their standardized CGI

19

method offers a convenient tool for evaluation of KHI performances. The idea is to bypass the

20

stochastic nucleation process by purposely retaining a small yet measurable fraction of the

21

hydrate particles in the system. This is followed by cooling the system into the hydrate region to

22

observe the further growth of the existing hydrate crystals. As a result, any observed growth 17

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1

behavior would be solely associated with the performance of the tested KHIs on hydrate growth.

2

Intriguingly, there exists clear boundaries of varied growth inhibition regions when such systems

3

are tested upon increasing degrees of subcooling. Figure 4 illustrates the concept of the CGI

4

method with distinctive growth inhibition regions.

5 6 7

Figure 4. Determination of the crystal growth inhibition (CGI) test method129 for evaluation of

8

KHI performance.

9 10

As shown in Figure 4, the inhibition performance of individually tested KHIs by the CGI method

11

can be mapped out as three identifiable growth inhibition regions that correspond to the 18

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increasing degrees of subcooling (from right to left). They are termed the complete inhibition

2

region (CIR) where the applied subcooling level is fully suppressed and no hydrate forms, the

3

slow growth region (SGR) with slow to moderate growth rates of hydrate particles, and the rapid

4

growth region (RGR) with fast and catastrophic hydrate growth, respectively. The rightmost

5

SDR region marked in Figure 4 refers to the slow dissociation region. As Svartaas et al.130

6

observed earlier for methane-propane hydrate, the hydrate dissociation rate reduced significantly

7

in the presence of tested KHIs. This is because the polymer bonding would contribute to the

8

stabilization of the formed hydrates, and consequently, the increase of hydrate dissociation

9

temperature. The CGI method has been successfully applied to quantitatively evaluate the

10

performance of KHI candidates.131-134 An apparent advantage is that it can test KHI

11

performances with the worst-case scenarios under flowing or shut-in conditions, when a small

12

fraction of hydrates may already be present.131

13

Hase et al.135 compared the varied experimental procedures including the isothermal, constant

14

cooling, and the CGI methods by examining the behaviors of hydrate formation in the presence

15

of five VP/VCap-based polymers using autoclave and rocking cell as the reaction vessels. Their

16

results suggested a following testing order for the evaluation of KHIs. First, the constant cooling

17

procedure could be applied for pre-screening and elimination of poor KHIs. Then, the CGI

18

method could be adopted to further distinguish among the best performers. Finally, the

19

isothermal induction time measurements can be utilized for verification of the most promising

20

KHIs.

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KHI Mechanisms–Experimental and Computational Evidence

2

A large amount of experimental studies on KHIs has enriched our understanding of the gas

3

hydrate systems and the effects of various KHIs on gas hydrate formation. During the last two

4

decades computational modeling and simulations, in particular, the molecular dynamics (MD)

5

and Monte-Carlo (MC) simulations have become more developed and have been used to study

6

KHI mechanisms. Used correctly, they can offer insights into the mechanisms operating by

7

simulating and revealing the KHI-water-hydrate interactions at the molecular level. These

8

computational simulations have shown agreements with experimental studies,136 although they

9

may also give different or even opposite indications as those suggested by experiments.137 Thus,

10

although modeling and simulation results can be informative, conclusions from such studies need

11

to be verified by laboratory experimental investigations.

12

Kinetic inhibition is well known for other crystals than clathrate hydrates. For example, scale

13

inhibitors for the inhibition of calcium carbonate and barium sulfate, are presumed to inhibit

14

either nucleation or crystal growth, or both.25 KHIs are also presumed to operate by one or both

15

of these mechanisms.

16

The crystal growth inhibition mechanism involves adsorption of the inhibitor onto some part of

17

the growing hydrate crystal surface, altering the morphology and/or lowering the rate of crystal

18

growth. However, there is no reason why this mechanism is confined to crystals, i.e. particles

19

that have reached the critical nucleus size at which point the change in Gibbs free energy (∆G) is

20

negative for spontaneous growth. It is just as likely that adsorption of inhibitors can occur on

21

sub-critical hydrate nuclei also. For gas hydrates this therefore becomes a nucleation inhibition

22

mechanism with short direct interactions between the KHI and hydrate surfaces. A second 20

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mechanism involves more long-range water perturbation effects whereby the KHI destabilizes

2

nuclei formation, preventing sufficient particle growth and clustering to occur to reach the

3

critical nucleus size. Other mechanisms proposed include deactivation of gas hydrate hetero-

4

nucleation sites and enclathration of dissolved hydrate-forming molecules in the water phase

5

rendering them unavailable for hydrate formation. Kuznetsova et al.138 suggested that KHI

6

polymers modified the interfacial tension of the water-methane surface, converting the initially

7

dispersed methane phase into separated bubbles, and then built a system-wide network that

8

partially covered the surface of the methane bubbles.

9

The first KHI mechanism to find experimental support was the crystal growth inhibition. Studies

10

on tetrahydrofuran (THF) hydrate crystal growth proved that polymers such as poly(N-vinyl

11

caprolactam) (PVCap) adsorbed to the growing hydrate crystal surface, perturbing the normal

12

crystal morphology and/or inhibiting further growth.139, 140 Examples of gas hydrate experimental

13

studies in favor of the adsorption–inhibition mechanism include work by Posteraro et al.141 and

14

Ivall et al.142 who both studied the effect of PVP on sI methane hydrate formation. Evidence for

15

surface adsorption of KHI polymers on gas hydrates has also been obtained from neutron

16

scattering experiments.143

17

Rojas González71 examined the effects of various N-vinyl lactam polymers on sII hydrate of a

18

natural gas mixture. They found that the adsorption of KHIs was directly related to their

19

effectiveness of hydrate inhibition. Interestingly, two polymers that showed the worst and the

20

best inhibition performances for THF hydrates respectively, exhibited the opposite inhibition

21

performances for gas hydrates. These observations have been seen in many other studies. For

22

example, Shell’s work in the early 1990’s and later work by Kelland showed that quaternary 21

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ammonium and phosphonium salts are excellent THF hydrate crystal growth inhibitors but have

2

very little effect as gas hydrate KHIs when used alone.144, 145 In contrast, some polymers, such as

3

poly(esteramide)s and poly(N-alkylacrylamide)s are fairly poor THF hydrate crystal growth

4

inhibitors yet show good performance as sII-forming gas hydrate KHIs.146 This indicates that

5

KHI evaluations using THF hydrate crystal growth systems alone need to be verified on high

6

pressure using gas hydrate systems to include possible nucleation inhibition.

7

Ohno et al.147 studied the effect of PVCap on methane-ethane hydrate and observed a structural

8

interconversion between sI and sII. They claimed that the polymer adsorption had contributed to

9

the gas and water rearrangement at the crystal interfaces, which eventually led to the structural

10

conversion. Various computer molecular simulation studies have also provided evidence for

11

hydrate surface adsorption by KHI polymers.139, 143, 148-150

12

The most studied class of KHI polymers are those based on N-vinyl lactams. Several studies

13

indicate that both nucleation and crystal growth inhibition mechanisms are operating with this

14

polymer class.47, 126, 132 In many gas hydrate KHI studies it is impossible to differentiate between

15

nucleation and crystal growth inhibition. However, there is experimental support for the

16

nucleation inhibition mechanism by water perturbation, where it can be differentiated from

17

mechanisms involving hydrate particle surface adsorption. For example, Chua and Kelland151

18

showed that the KHI performance on a sII gas hydrate system of a 1:1 weight mixture of tetra(n-

19

hexyl)ammonium bromide (THexAB) and PVCap gave very similar performance to PVCap

20

blended with tetra(n-pentyl)ammonium bromide (TPAB) and much better performance than

21

PVCap blended with tetra(n-butyl)ammonium bromide (TBAB). However, as THF hydrate

22

crystal growth inhibitors, TPAB is by far the superior inhibitor of the three, followed by TBAB, 22

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with THexAB being a very poor inhibitor. This suggests that the synergistic enhancement by

2

THexAB is not related to crystal growth inhibition but by some other mechanism. The authors

3

surmise that the long hydrophobic n-hexyl groups in THexAB cause enhanced water perturbation

4

relative to the other quaternary ammonium salts, improving the hydrate nucleation inhibition.

5

Perturbation of the bulk water phase causing nucleation inhibition has been suggested to occur

6

for KHI polymers where they have been shown to give relatively poor THF hydrate crystal

7

growth inhibition relative to good growth inhibitors such as PVCap. This includes

8

polyesteramides, polyaspartamides and polyalkyl(meth)acrylamides. Exxon Mobil was the first

9

to imply nucleation inhibition by water-perturbation in their work on the latter of these polymer

10

classes.25, 84, 108

11

There is also computer molecular simulation evidence for hydrate nucleation inhibition by KHIs.

12

Moon et al.18 showed that PVP molecules were held a distance of 5–10 Å from the crystal

13

surface, instead of adsorbing directly onto the growing crystal planes, yet hydrate formation was

14

destabilised. Kvamme et al.152 evaluated the effects of N-vinyl lactam polymers on sI and sII

15

hydrates with molecular dynamics simulations. The results showed that the KHIs could interact

16

with the hydrate-water clusters and trigger nuclei dissolution without having direct contact.

17

Hawtin

18

poly(dimethylaminoethylmethacrylate) (PDMAEMA) on methane hydrate formation. They

19

showed that the polymer had long-range effects in solution that perturbed the water structures.

20

Monte-Carlo simulations by Wathen et al.154 indicated that PVP could either inhibit or promote

21

sII hydrate formation when the polymers were within the working distance to the embryo

22

surfaces. Promotion of hydrate formation is also observed for N-vinyl lactam polymers on THF

and

Rodger153

with

MD

simulations

studied

the

effect

of

23

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hydrate, whereby accelerated plate growth is observed below the total inhibition concentration.80,

2

139

3

Unlike experimental studies, the limited computational capacity poses a challenge to the current

4

modeling and simulation studies. Only a confined simulation module with a limited number of

5

molecules (typically, a few hundred to several tens of thousand) can be established and simulated

6

for a limited period of time (typically, a few hundred nanoseconds to a few microseconds).155, 156

7

It partly explains why the formation of the structurally simpler sI methane hydrate is most often

8

simulated for the evaluation of KHI performance. However, with increased computational

9

capacity and complexity, future molecular modeling and simulations will become more powerful

10

and offer more insights into the gas hydrate phenomena and the effects of KHIs.

11 12

Conclusion

13

Kinetic hydrate inhibitors (KHIs) as a main category of the low dosage hydrate inhibitors

14

(LDHIs) are actively in use for hydrate prevention and mitigation in the petroleum industry in

15

the past two decades. Various chemicals that act as KHIs, mostly polymeric compounds, are

16

capable of retarding gas hydrate formation at low concentrations and facilitating smooth and

17

reliable oil and gas transportations. This article with literature survey has reviewed the most

18

commonly used experimental apparatus, monitoring and characterizing techniques, and

19

experimental methods for conducting KHI-related gas hydrate studies. The major kinetic

20

inhibition mechanisms proposed in the literature and supporting experimental and computational

21

evidence are also reviewed. More research is crucial for developing better or greener KHIs, 24

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understanding their working mechanisms, and optimizing test methods for increasing the number

2

of field applications.

3 4

Acknowledgements

5

The authors thank the Norwegian Ministry of Education and Research and University of

6

Stavanger for their financial support of this work.

7 8

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Tables

2

Table 1. Apparatus/reaction vessels used in gas hydrate studies in the presence of KHIs. Apparatus/Vessel Autoclave Autoclave Autoclave Autoclave Autoclave Autoclave Autoclave Autoclave Autoclave Autoclave Autoclave Autoclave Semi-batch autoclave Crystallizer Crystallizer Crystallizer Crystallizer Crystallizer Semi-batch crystallizer Semi-batch crystallizer Stainless-steel cell Stainless-steel cell Stainless-steel cell Sapphire crystallizer Sapphire crystallizer Sapphire crystallizer Stirred reactor Stirred reactor Stirred reactor Stirred reactor Stirred reactor Stirred reactor Rocking cell (RC5)

Sapphire Rocker rig (RCS20) T-Piece rocking cell HP cell HP cell

Inner Volume (cm3) 145 200 280

Pressure Grade (MPa) 40 15 41

290 300 23 280 300 500 750 3000 25000 58 500 610 211 400 2182 600 N/A 200 235 288 77 100 50.7 43.3 90 300 1072 372,5 750 40

14.7 41 N/A N/A N/A N/A N/A N/A N/A N/A 20 20 N/A N/A N/A 12 N/A N/A N/A N/A 20 50 N/A 15 15 20 20 N/A N/A 20

20 N/A 100 280

N/A N/A 20 10

Research Group Svartaas and co-workers157, 158 Cha et al.159 Llamedo C and Yánez;67 Anderson et al.129 Ferreira et al.160 Luna-Ortiz et al.131 Chua et al.85 Glénat et al.132 Shin et al.43 Kelland and Iversen36 Zhao and co-workers46, 47 Hould et al.136 Salamat et al.121 Daraboina and co-workers161, 162 Xu et al.163 Al-Adel et al.164 Sharifi et al.38 Kumar et al.165 Chong et al.52 Posteraro and co-workers141, 166 Lee and co-workers41, 97 Villano and Kelland44 Svartaas et al.167 Lee et al.168 Bruusgaard et al.169 Li et al.170 Chen et al.37 Sharifi et al.171 Rasoolzadeh et al.172 Xu et al.173 Tang et al.174 Seo et al.175 Zare et al.176 Kelland and co-workers;117, 177, 178 Daraboina et al.;179 Li et al.170 Li et al.170 Cook et al.180 Duchateau et al.124 Jensen et al.39

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

HP cell HP cell HP cell HP cell HP cell HP cell HP cell HP-Video cell Micro-Cell PVT cell HP-ALTA HP-micro DSC

1

300 300 1 30 90 556 600 N/A 60 N/A 0.15 0.5-1

12 15 N/A N/A N/A N/A N/A 15 N/A N/A N/A 40

Hydrate equilibrium cell Sealed glass ampoules Pipe-wheel Flow loop

66.5 0.2 13400 Up to 106

15 N/A 15 10-15

Mini/micro flow loop

100-1000

~10

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Seo and Kang181 Sina et al.182 Ohno et al.147 Ohno et al.183 Kang et al.184 Yang and Tohidi185 Park et al.186 Wu et al.187 Rojas González71 Semenov et al.188 Lou et al.189 McNamee;126 Sharifi and Englezos;190 Daraboina and Linga;51, 191 Jensen et al.192 Ohno et al.193 Urdahl and co-workers99, 100 Reed et al.;104 Talley et al.;105 Klomp and co-workers;83, 111 Peytavy et al;116 Turner et al.194 Talaghat and co-workers;195, 196 Cook et al.180

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

2

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1 2 3 4 5Nr. 61 7 82 9 10 3 11 12 4 13 14 5 15 16 6 17 18 19 7 20 21 8 22 23 9 24 25 10 26 27 11 28 29 12 30 31 32 13 33 34 14 35 36 15 37 38 16 39 40 17 41 42 43 18 44 45 19 46 47 20 48 21 49 22 50 23 51 52 53 24 54 25 55 56 57 58 59 60

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Table 2. Monitoring & characterizing techniques frequently used in gas hydrate and KHI studies.

Technique Neutron diffraction Proton (1H) NMR Solid-state 13C NMR Solid-state 13C magic angle spinning (MAS) NMR Raman spectroscopy Polarized Raman spectroscopy In-situ laser Raman spectroscopy Infrared (IR) spectroscopy Magnetic resonance imaging (MRI) Powder X-ray diffraction Gas chromatography (GC) Size-exclusion chromatography (SEC) Gel permeation chromatography (GPC) Microscope with CCD/digital camera Focused beam reflectance measurement (FBRM) Ultrasonic signal processing

Function & short description Determines the atomic and/or magnetic structure of a material via neutron scattering Applies NMR with respect to hydrogen nuclei to determine the molecular structures Complements to X-ray diffraction and provides structural information about solids and polymers Spins the sample with respect to the direction of the magnetic field for increased resolution Observes vibrational, rotational, and other lowfrequency modes for identification of molecules Reveals the molecular orientation and symmetry of the bond vibrations to identify chemicals Enhances the Raman effect for identification of substances by using lasers Identifies molecular information (e.g., can measure the degree of polymerization) Quantifies molecular information during crystallization and phase transition Identifies phases of a crystalline material and provides information on the unit cell dimensions Separates and analyzes compounds to measure the content of various components in a sample Separates molecules by sizes or molecular weights. Often applied to proteins or polymers Separates analytes on the basis of size; a type of size-exclusion chromatography Captures images with modern optical microscopy Measures in-situ particle size

Quartz crystal microbalancedissipation (QCM-D) Differential scanning calorimeter (DSC) Micro-differential scanning calorimeter (micro DSC) Pendant drop tensiometer Laser interferometer Photodetector Optical/video camera

Identifies integrity or geometry of the substance; can characterizes phase change or defect Detects interfacial acoustic signals to examine surface adsorption, film thickness or softness Measures heat exchange between the sample and the reference as a function of temperature Operates in wider temperature and pressure ranges, as compared to the normal DSC Measures interfacial tension Measures hydrate film thickness Detects change in the light transmission Visually observes hydrate formation

Boroscope Viscometer

Inspects the inner structure through a small hole Measures torque and viscosity in a fluid system

Selected references Koh and co-workers;137, 197, 198 Lokshin et al.199 Kelland et al.;177 Ferreira et al.;160 Semenov et al.188 Park et al.;186 Cha et al.;159 Ohno et al.;183 Ferreira et al.;160 Semenov et al.;188 Daraboina et al.;200 Ohno et al.193 Daraboina et al.;201 Ohno et al.147 Sa et al.202 Hong et al.203 Ferreira et al.;160 Xu et al.163 Kvamme and co-workers204, 205 Sa et al.;206 Park et al.;186 Ohno et al.;183 Daraboina et al.201 Daraboina et al.;201 Kumar et al.42 Kelland et al.;177 Seo et al.175 Nakarit et al.69 Lee et al.;168 Daraboina et al.;161 Kumar et al.42 Greaves et al.;207 Clarke and Bishnoi.208 Yang and Tohidi185 Walker et al.209 Varma-Nair et al.210 McNamee;126 Sharifi and Englezos;190 Daraboina and Linga;51 Lachance et al.;127 Duchateau et al.211 Mori and co-workers212-214 May et al.;215 Lou et al.189 Shin et al.;43 Li et al.;170 Wu et al.;187 Seo and Kang;181 Zheng et al.;216 Zheng et al.216 Zhao et al.46

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1 2 3 1 4 2 Table 3. Experimental procedures commonly applied in gas hydrate studies and testing of KHIs. 5 Procedure/condition Key variable Comments 6 to monitor 7 Induction time Maintains a constant temperature and degree of subcooling from the 8 Isothermal 9 start of an experiment 10 Isothermal-isobaric Induction time Keeps both the system temperature and pressure constant for hydrate 11 formation 12 Constant cooling Hydrate formation Applies fixed cooling rates and allows the pressure to decrease with 13 temperature decreasing temperatures 14 Constant cooling-isobaric Hydrate formation Ensures a constant pressure along the cooling sequence and the 15 temperature hydrate formation process 16 Temperature ramping Hydrate formation Performs multiple cooling ramps with isothermal intervals in 17 temperature between 18 Induction time or hydrate Forms the hydrates, then melted, for repeated times, with the 19 Cooling-heating cycles formation temperature memory effect involved 20 21 Sudden cooling Hydrate formation Cools down the surrounding water jacket of the reactor rapidly and 22 temperature allows the reactor to cool down gradually into the hydrate region 23 Constant heating Decomposition temperature Decomposes slowly the formed hydrates and measures the hydrate 24 and pressure equilibrium data 25 3 26 27 4 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 54 58 59 60 ACS Paragon Plus Environment