International Conference on Gas Hydrates (ICGH) (6th : 2008)

STUDY OF AGGLOMERATION OF ICE PARTICLES AND OF TRICHLOROFLUOROMETHANE HYDRATE PARTICLES SUSPENDED IN.. Colombel, Emilie; Palermo, Thierry; Barré, Loic; Gateau, Patrick; Gruy, Frédéric 2008-07-31

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STUDY OF AGGLOMERATION OF ICE PARTICLES AND OF TRICHLOROFLUOROMETHANE HYDRATE PARTICLES SUSPENDED IN A HYDROCARBON PHASE  Emilie Colombel****, Thierry Palermo, Loic Barr?, Patrick Gateau Department of applied chemistry and physico-chemistry IFP - 1 avenue de Bois Pr?au 92 852 Rueil Malmaison Cedex - FRANCE  Fr?d?ric Gruy Department SPIN Ecole Natinale Sup?rieure des mines de Saint Etienne 158 Cours Fauriel 42023 Saint Etienne Cedex 2 - FRANCE   ABSTRACT  This work deals with the problem of pipeline plugging by gas hydrates during oil production. Gas hydrates are crystals resulting from water and gas molecules association under high pressure and low temperature conditions.  Such  thermodynamical  conditions  are  generally  encountered  during  oil  production  and transport, particularly in deep offshore fields or in cold areas. Due to an agglomeration process which is still debated, hydrate occurrence can lead to plug formation. This  study  aims  at  improving  the  understanding  in  this  mechanism  process,  in  the  case  of  water-in-oil emulsions.  Therefore,  ice  or  hydrate  particle  agglomeration  is  compared.  Ice  or  trichlorofluoromethane (CCl3F) hydrate particles dispersed in xylene with asphaltenes as surfactant is chosen as a model system. As CCl3F hydrates are stable under atmospheric pressure, it allows us to apply different techniques without being limited by high pressure conditions. The Nuclear Magnetic Resonance (NMR) technique is used. The very different relaxation rate for solids or liquids is used to monitor in situ the ratio between solid and total hydrogen  or  fluorine  as  a function  of  time  with  controlled shearing  conditions.  Thus, a  kinetic  study  is realized,  that  enabled  to  know  the  amount  of  ice  formed.  The  apparent  viscosity  of  the  system,  during crystallization and plugging, is also followed with rheometry in order to characterize agglomeration.  This  experimental  approach  allows  us  to  highlight  that  physico-chemistry  of  interface  water/oil  has  an important role in agglomeration. It enables us to discuss different mechanisms of agglomeration of ice and hydrate particles in a hydrocarbon phase.  Key Words: Hydrates, Freon, Ice, Emulsion, Crystallization, Agglomeration, NMR, Rheometry                                                       * Corresponding author: Phone: +33 147 52 73   Fax +33 147 52 70 58  E-mail: emilie.colombel@ifp.fr NOMENCLATURE a Primary Particle Radius [?m] B0 Magnetic Field at Thermodynamic Equilibrium                  [T] D Fractal Dimension  t Crystallization Time [min] F Force of Adhesion [N]    Shear Rate [s-1] kB   Boltzmann Constant [J.K-1] m    coefficient  which  represents  breakage mechanism M0 Initial Magnetization [a.u] Mliq Liquid Magnetization [a.u] Mtot Total Magnetization [a.u] My  Magnetization in the (x,y) plan on y axis [a.u] ? Suspension Viscosity [Pa.s] ?0 Continuous Phase Viscosity [Pa.s] N  Molecule Number per Volume Unit [m-3]   Volume Fraction  Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  ?g eff  Effective Volume Fraction  max  Maximum Volume Fraction R Aggregate Radius [?m]   Energy of Adhesion per Area Unit [J.m-2] t Time [min] tL Induction Time [min] T Temperature [K] T2 Transversal Relaxation Time [ms] T2*Characteristic  relaxation  Time  of  Transversal Magnetization [ms] T2+ Relaxation Time due to Field Inhomogeneities [ms]    Shear Stress [Pa]  0 Critical Shear Stress [Pa]  INTRODUCTION Gas  hydrates  are  crystalline  compounds  which may lead to pipeline blockage during offshore oil production.  These  compounds  form  at  high pressure and low temperature when water and gas molecules are in contact. These conditions may be  encountered during oil production in deep offshore where gas molecules (methane for example) are in contact  with  water  droplets  of  water  in  oil emulsions.  These  hydrate  particles,  by  an agglomeration  mechanism,  progressively  tend  to form  a  blockage  in  pipelines  and  impair production.            This  work  aims  at  trying  to  understand agglomeration mechanism of gas hydrate particles in  water  in  oil  emulsions.  To  study  this mechanism,  a  model  system  is  chosen:  a  model emulsion water in xylene/asphaltene and a hydrate model: Freon hydrate. This system illustrates what it happens in pipelines as produced water with oil is often dispersed by surfactants naturally present in  oil  phase.  Moreover,  Freon  well  models methane  behavior  because  both  methane  and Freon are soluble in oil and insoluble in water. It enables to mimic a transfer phenomenon between oil  and  water  phase  during  hydrate  formation.      As  Freon  hydrate  forms  at  atmospheric  pressure and  low  temperature,  there  is  no  need  of pressurized systems in the experimental techniques used.  Freon  boiling  point  is  297K,  so  it  is  very volatile  at  ambient  temperature.  So  it  was necessary  to  find  solutions  to  keep  installations airtight.  To  better  understand  particle agglomeration  process  in  pipeline,  two  kinds  of particles  are  studied:  ice  and  Freon  hydrate particles  in  the  same  oil  phase.  Rheology  is  the main  characterization  method  used  for  this.  It  is completed  by  NMR  to  study  agglomeration kinetics and quantify amount of formed crystals.  EXPERIMENTAL METHODS Model system: a model oil and a model hydrate Hydrate agglomeration properties in emulsion are studied thanks to a model emulsion. It is composed of  oil  phase  (continuous  phase),  deionised  water (dispersed  phase)  and  asphaltenes  (surfactant) coming  from  SINCOR  crude  oil,  insoluble  in heptan.  Different  oil  phases  are  studied.  For  ice system,  two  oil  phases  are  used:  orthoxylene (named XA below)  and dodecane + orthoxylene (named XDA below). For hydrate the oil phase is constituted of orthoxylene + freon.                     Orthoxylen  is  a  dispersing  agent  of  asphaltenes whereas dodecane or freon precipitate asphaltenes. Dodecane is added to xylene for ice in order to be close  to  the  flocculation  onset  of  asphaltenes.  In these conditions, asphaltenes is a good surfactant; emulsion  is  better  stabilized  [1].  Flocculation conditions  are  determined  by  microscopy  and refractive index measurements.   Asphaltenes as surfactant  Asphaltenes  are  the  most  polar  and  highest molecular weight compounds in crude oils.           It  is  not  possible  to  associate  a  single  chemical formula  to  all  asphaltenes  present  in  a  crude  oil and so impossible to generalize a formula for all origins of crude oil. There is a wide polydispersity in  the  same  crude  oil  too.  However,  asphaltene molar  mass  is  estimated  between  500  and  1,000 g.mol-1 while micelles molar mass is about 10,000 g.mol-1.  Carbon  and  hydrogen  are  the  two  main constituents of asphaltenes. Their atomic ratio H/C is  estimated  between  0.9  and  1.2.  They  are represented by an opened structure with aromatic nucleus linked by alkyl (figure 1)      Figure 1: Example of schematic representation of asphaltene molecule The  asphaltene  structure  give  them  amphiphilic properties.  Indeed,  carbon  and  hydrogen  are  non polar  and  heteroatoms  are  polar  so  asphaltenes have  surfactant  properties.  Asphaltenes  adsorb  10 ?        = maRgt themselves naturally to the water/oil interface such away  the  heteroatoms  are  in  contact  with  water phase  and  hydrocarbon  components  with  the  oil phase.  Moreover,  emulsion  stability  is  linked  to asphaltenes  capacity  to  form  aggregates  and  so thick  layer  in  interface  water/oil.  Crude  oil  are constituted  by  asphaltenes  (insoluble  in  heptan (371K)  but  soluble  in  toluene),  maltenes (components  Saturated,  Aromatic  and  Resins). Asphaltenes are by definition insoluble in heptan. This  properties  is  used  to  precipitate  them  and separate of liquid fraction (maltenes + heptan) by filtration. This method is named SARA separation. This  separation  processus  is  defined  by  AFNOR NF T60-115 standard.  Elaboration of emulsion A model emulsion is used for this study in order to simplify the system. Indeed, a petroleum emulsion, containing crude oil and water, has a very complex composition with numerous different components in  oil  phase.  Here,  the  oil  is  Xylene,  Xylene  + Dodecane  or  Xylene  +  Freon  and  surfactant  is asphaltenes.  They  are  extracted  from  the Venezuelan  crude  oil  SINCOR.  Asphaltene extraction  is  realized  with  heptan  so  asphaltenes are  named  SINCOR  I7  (insoluble  in  heptan).      To prepare emulsion, asphaltenes are dissolved in xylene  C8H10  which  is  a  good  solvent  of asphaltenes.  Emulsions  are  prepared  with  a  Ultra Turrax of 8 mm diameter at 13500 rpm during a minute. Deionised water is added drop by drop in oil + asphaltene phase;  Freon hydrate: Model hydrate Studied hydrates are Freon 11 hydrates. Freon 11, named  trichlorofluoromethane  too,  has  for chemical  formula  CCl3F.  Freon  hydrate  forms  at atmospheric  pressure  which  enables  us  to  use numerous  characterization  techniques  without needing to use pressure systems. Moreover, Freon well models methane action because, as itself, it is soluble in oil and insoluble in water. It enables to mimic  a  transfer  phenomenon  between  oil  and water phase during hydrate formation. By contrast, Freon inconvenient is its volatility. Indeed, Freon boiling  point  is  297K,  so  it  is  very  volatile  at ambient temperature.  So it  was  necessary  to find solutions to keep installations airtight.       Freon  molecule  has  a  radius  of  gyration  of  3.3  which  predicts  a  hydrate structure type  II  (figure 2). Crystalline network of structure II is diamond type,  cubic,  parameter  mesh  17.2     and  given composition 16M1.8M2.136H20 [2].       Figure 2: Type II hydrate structure With the size of Freon molecule, only wide cage are full which predict a composition 8M2.136H20. We can write:              CCl3F + 17 H20   CCl3F.17H20           (1) Rheology Agglomeration  of  hydrate  particles  is  studied  by rheology.  The  comprehension  of  rheological results  is  based  on  Mill?s  theory  [3]  and  R. Camargo?s  results  [4].  Hydrate  crystals  are expected to form at the water-oil interface. Thus, a solid  shell  forms  around  water  droplet  making them to behave as solid particles. In the following, hydrate particles will be assumed to be spherical, with an identical diameter as water droplet. Considering that the final size of hydrate particle aggregates is controlled by shear rate [5], we have:                        (2) where  R  and  a  are  respectively the  aggregate  radius  and  the  primary  particle radius  which  is  the  average  radius  of  a  water droplet, ? the apparent viscosity of the suspension,  ?g  the shear rate,  0 the critical shear stress below which  aggregates  can  form  and  m  an  exponent which  represents  breakage  mechanism  of aggregate and generally reported around 0.5 in the literature [6].  The critical shear stress is related to the force of adhesion F between particles as:             (3)  where  aF  is the energy of adhesion per area unit  and  depends  only  on  the  physico-chemical properties of the system.  aF sat =20 ( )fgmtmeff aR --        =      = 303 The effective volume fraction  eff  scales with the actual volume fraction  , which represents water cut, as:                       (4)  D represents fractal dimension.  Moreover,  effective  volume  fraction   eff  and viscosity of hydrate suspension   are linked by (5)               (5)  with  ?0  the  oil  viscosity  and   max  the  maximum packing.  This equation is verified for  eff <  max = 4/7 and m1max0-        >fftt.If  this  condition  is  not fulfilled, blockage appears.  NMR Nuclear Magnetic Resonance NMR  has  been  often  used  in  hydrate  studies  to determine  their  structure  by  spectroscopy  but relaxation  properties  hasn? much  been  used  [8]. Study  of  relaxation  time  in  NMR  allows  us  to determine ice or hydrate solid fraction as function as time. All these informations come from 2 measurement sequences: -  Carr-Purcell  and  Meiboom-Gill  spin  echo   sequence (CPMG)  -  Free Induction Decay measurement (FID) One of the advantages of this technique is its non destructive  character  and  its  ability  to  measure high solid concentrations. NMR notions [9] NMR relaxometry is based on radiofrequency field application  on  atoms  nuclear  spins  in  order  to manipulate them. A spin population is considered at thermodynamic equilibrium in a magnetic field B0.  So  spins  are  manipulated  in  applying  a radiofrequency field named pulse. The process of relaxation  involve  with  the  particle  which  comes back to its original energy state by loss of achieved energy.  Relaxation  occurs  through  exchange  of energy  between  the  particle  and  its  environment according to two processes: longitudinal relaxation (or spin-network) and transverse relaxation (spin-spin).  In  our  studies,  transverse  relaxation  is considered.                  The transverse relaxation insures the return of the particle  in  its  fundamental  state,  causing  the decrease of the magnetization My in the (x, y) plan. The effects that tend to reduce the magnetization in  the  plan  (x,  y)  is  the  spin-spin  relaxation  and non  uniform  field.  The  spin-spin  relaxation  is described according to Bloch by:  (6)  The time constant T2 is called transverse relaxation time.  The  spin-spin  relaxation  is  due  to  the presence  within  the  molecule  many  other  nuclei, all  at  their  own  precession  Larmor  frequency. They  can  provide  additional  varying  magnetic fields  quickly  and  regularly,  which  may  interact with  the  molecule.  The  result  is  a  loss  of  phase coherence  between  the  particles  at  resonance which induces a decrease in My. The  magnitude  of  the  magnetization  M0  of  an NMR  signal  is  proportional  to  the  number  of elements  detected  by  the  probe  according  to  the law of Curie (for a spin I = ?):       (7)  Relaxation time  measurement T2: Measurement of liquid species in sample A spin population in an inhomogeneous field after a  /2 pulse has his Larmor frequency distribution which  widens  out.  So,  inhomogeneity  field induces  a  delay  for  some  spins  and  an  lead  for others. If spins can move  during a time   after a  /2  pulse  then  a     pulse  is  them  applied  in comparison  with  the  same  phase,  so  there  is  a symmetrical  spin  turning  which  will  evolve  in opposite  direction  during  time   .  So  spins  will come back in their initial configuration which will bring about a spin echo (figure 3):      Figure 3: Spin echo     )exp( 20 TtMM y -=  7411max2max0 =         --= ffffmmeffeff  TBNMB4020 =Carr-Purcell and Meiboom-Gill CPMG method This  method  consists  in  an  echo  succession. Diffusion effects are corrected by inter echo time.     Figure 4: CPMG sequence This sequence (figure 4) enables to know number of entities presents in liquid phase and relaxation times  T2  of  liquid  entities  presents  in  sample. Signal is a decreasing exponential which, with its extrapolation,  enables  to  know  liquid  amplitude and so know number of entities presents in liquid phase (figure 5).        Figure 5 : Example of T2 cpmg measurement  FID measurement: Measurement of all species of sample This is the simplest measurement. Pulse sequence of  FID  measurement  is  an  impulsion  application ( )x and an acquisition on Oy axis (figure 6).     Figure 6: Free Induction Decay sequence This  sequence  enables  characteristic  time  of transversal  magnetization  disappearance  T2* established by the relation (8) to be measured:  (8)  T2+  is  the  characteristic  time  of  the  transversal magnetization  disappearence  due  to  other  effects that  the  spin-spin  coupling  effects  including mainly  inhomogeneity  field.  The  order  of magnitude  T2+,  which  can  be  seen  mainly  as  an apparatus  characteristic  is  few  milliseconds.  By contrast, T2 is governed by dipolar relaxation and depends on entities environments.  The  number  of  protons  is  measured  by  the hydrogen probe (and the number of fluorine by the fluorine  probe  in  the  case  of  measurements relating to hydrate Freon). This sequence has the advantage of not neglecting the to short relaxation time entities. So if the NMR signal is calibrated on a  sample  quantity  of  protons  (for  the  hydrogen probe)  is  known,  it  is  possible  to  determine  the amount of protons present in any sample. A solid relaxes quickly: T2   10  s. A liquid has a longer T2  (10  to  3000ms).  An  example  of  hydrate suspension in emulsion is shown (figure 7). In this system,  the  T2  relaxation  time  of  Freon  hydrate particles is very short (a few  s) compared to that of Freon (about 1000ms). That is why we find the two behaviors and the two environments fluorine in Figure 7. In addition, the relative values of the amplitudes  are  proportional  to  the  relative quantities  of  fluorine  of  each  species  which  will enable  measure  quantitatively  the  number  of fluorine present in the sample.                    This  technique  is  used  to  detect  the  quantity  of solid and liquid within the same sample.          Figure 7: FID echo solid signal of Freon hydrate So when solid echo FID and CPMG sequences are coupled, one can access to total and liquid entities number and know solid concentrations in sample:  (9)  This  methodology  enables  us  to  measure ice and hydrate concentration  in  our  sample. To  simplify measurement  and  after  verifying  that  Mliq  found   +?+=2*21TT 0,0 0,1 0,2 0,3 0,4 0,5110                                                        0 1000 2000 3000 4000 5000024681012                           totliqliquidsolidMMNNionsolidfract -=+=from  an  linear  extrapolation  with  FID  (figure  7) and  from  an  exponential  extrapolation  with  T2 CPMG measurement (figure 5) are the same: Mliq = 10, we just show and use results obtained from FID. NMR experimental system  Used  equipment  is the  Br?ker  minispec  mq-serie 2001. Field is at 19.65 MHz frequency (permanent magnet).  Probes  which  are  used  for  our experiments are a temperature hydrogen probe and a temperature Fluor probe. They are controlled by a  BVT  3000  which  can  be  used  from  173K  to 473K.  Temperature  is  regulated  with  liquid nitrogen, gaz nitrogen and a PID.   Experiments under shear rate  Study  of  ice  and  hydrate  agglomeration  is  done under  controlled  shear  rate.  To  avoid  particles sedimentation,  an  helix  mobile  is  used.  The Couette analogy described by Ait-Kadi et. al. [7] is used to access the true shear rate. A schematic description of the helix mobile is shown on figure 8.          Figure 8: Helix and cylinder geometry dimensions h  =  34  mm;  Re  =  18.35  mm;  R  =  16.5  mm;            Ri = 13.95 mm; l = 8 mm To  complete  rheological  measurements, particularly for kinetics study, and to evaluate the amount  of  crystals  formed,  NMR  measurements are done under the same shear rate conditions than in rheology measurements. Indeed, an helix (a in figure 9) located at the bottom of an NMR tube (b) and  centered  by  stoppers  (c),  is  connected  to  a motor  (d)  which  enables  rotation  at  controlled speed.  This  helix  has  been  calibrated  using  the same procedure used for the previous device.          Figure  9:  NMR  Helix  and  assembly  on  NMR equipment. Thus,  comparison  of  induction  times  and crystallization  times  can  be  done  with  these  two experimental techniques. For ice particles, an helix made of Teflon is used and for hydrate particles an helix in PEEK (PolyEthylEtherKetone) is used.  EXPERIMENTAL  RESULTS  AND DISCUSSION           Ice  and  hydrate  suspensions  are  studied  by rheometry  and  NMR.  In  these  2  cases,  samples undergo the same program: -     Cooling to 266K    Two rheograms (only in rheometry)    An  isotherm  at  266K  under  a  constant shear rate during crystallization    Two rheograms after crystallization (only in rheometry)    Dissociation of crystals Crystallization  and  agglomeration  are  observed and  so  studied  during  isotherm  stage.  Viscosity evolution  is  a  consequence  of  agglomeration  of crystals  in  the  suspension.  Moreover,  NMR measurements  give  information  on  kinetics  of crystals  formation  and  conversion  from  water  to crystals over time.  Ice suspensions Two  different  oils  are  studied  for  ice  case.  The first is Xylene (named XA below) and the second is Xylene + Dodecane (named XDA below), with Asphaltenes  as  surfactant  in  these  2  cases.  For these two different emulsions four water cuts (2%, 5%,  10%  and  15%)  and  three  shear  rates  (25s-1, 50s-1  and  100s-1)  are  studied  by  NMR  and rheometry.  The  case  of  xylene/dodecane  in  15% water emulsion is presented in the figure 10 as an example.   abc abcdMethodology  to  determine  induction  and crystallization times In rheology, for every water cuts and shear rates,  the  apparent  viscosity  shows  the  same  trend:  an increase, an overshoot and then a stabilization of the viscosity.            Figure 10: Example of viscosity curve for ice suspensions in emulsion 15% water cut and 25s-1 Agglomeration of particles leads to an increase in the  effective  volume  fraction   eff  of  suspension which is translated by an increase viscosity (5). In rheology,  tL  is  determined  at  the  onset  of increasing viscosity and   t is associated with the time of increasing viscosity (figure 10). In  NMR,  tL  is  determined  when  the  first  solid amplitude appears (3 min in figure 11)  and  t is determined  when  the  whole  water  is transformed in ice (141 min in figure 11).               Figure 11: FID Echo solid measurements for water in ice conversion for XDA emulsion 15% water cut and 25s-1            Figure 12: Water in ice conversion for XDA emulsion 15% water cut and 25s-1 In the figure 12, for this same sample, conversion water  in  ice  is  presented  and  obtained  by  NMR. The same shape of conversion curve is found for all  water  cut  and  shear  rates,  and  we  can  later apply  them  a  kinetic  model  and  characteristic kinetics constants could be determined. The  same  shear  rate  is  imposed  for  the  2 techniques  with  a  same  geometry  helix  (Cf description  in  experimental  methods).  We compare,  for  the  XA  and  XDA  emulsions, induction  time  tL  (figures  13  and  14)  and crystallization times  t  (figures 15 and 16) Induction times                     Figure 13: Induction time tL for XA emulsion Comparison between rheology   and NMR     rheology XA 2% 25s-1NMR XA 2% 25s-1rheology XA 2% 50s-1NMR XA 2% 50s-1rheology XA 2% 100s-1NMR XA 2% 100s-1rheology XA 5% 25s-1NMR XA 5% 25s-1 rheology XA 5% 50s-1NMR XA 5% 50s-1rheology XA 5% 100s-1NMR XA 5% 100s-1rheology XA 10% 25s-1NMR XA 10% 25s-1rheology XA 10% 50s-1NMRXA 10% 50s-1rheology XA 10% 100s-1NMR XA 10% 100s-1rheology XA 15% 25s-1NMR XA 15% 25s-1rheology XA 15% 50s-1NMR XA 15% 50s-1rheology XA 15% 100s-1NMR XA 15% 100s-10102030405060708090100Time (min)0,00 0,02 0,04 0,06 0,08 0,10567   0   3    6 min 12  25   39min 47  53   141minMagnetisation (%)Time (ms)0 20 40 60 80 100 120 140 160 180 2000,00,20,40,60,81,0 Water in ice conversion (%)Time (min) t(NMR)tL(NMR)0 20 40 60 80 100 120 140 160 180 2000,00,20,40,60,81,0 Water in ice conversion (%)Time (min) t(NMR)tL(NMR)  XDA 15% 25 s-1XDA 15% 50 s-1XDA 15% 100 s-1XDA 15% 25 s-1XDA 15% 50 s-1XDA 15% 100 s-100.511.522.533.544.550 2 4 6 8 10Time (hours)Viscosity (Pa.s) t(rheology)tL(rheology)00.511.522.533.544.550 2 4 6 8 10Time (hours)Viscosity (Pa.s) t(rheology)tL(rheology)               Figure 14: Induction time tL for XDA emulsion Comparison between rheology   and NMR     For XA emulsions (figure 13), tL  20 min in each case in NMR and rheology. So tL is independent of water  cut  and  shear  rate.  For  XDA  emulsions (figure 14), tL is dependent of shear rate and water cut. The more important they are, the smaller tL is. It  is  really  visible  with  rheology  results  and  less with NMR results.  In general, tL is smaller in NMR than in rheology which  is  explained  by  the  volume  of  sample. Indeed, in NMR sample is about 1mL and in NMR it is 50 mL. So in NMR entire sample is quicker at the  good  temperature  than  in  rheology.  So crystallization can occur faster in NMR.   Crystallization times                 Figure 15: Crystallization time  t for XA emulsion Comparison between rheology   and NMR                    Figure 16: Crystallization time  t for XDA emulsion Comparison between rheology   and NMR     For XA emulsions (figure 15), crystallization time  t is about 50 min, and seems to be independent of water cut and shear rate. Contrary to this, for XDA emulsions (figure 16), the higher the water cut and the shear rate are, the smaller  t is.  The difference in  t (and in tL) for the two systems is explained by the different physico chemistry of the two  emulsions.  In  the two  systems,  the  same conditions  on  water  cut  and  shear  rate  are imposed.  There  is  only  the  physico  chemistry which  changes.  Indeed,  for  XA  emulsion, asphaltenes  are  soluble  in  xylene  and  so  water droplet are not very stabilized and "protected" by asphaltenes. In opposition, for XDA emulsions, we are  in  the  best  conditions  of  stabilization  [1]  so water  droplets  are  very  isolated.  Thus,  steric barrier prevents contact between particles in XDA system  in  opposite  to  system  XA.  So crystallization  spreads  by  contact  between  water droplet.  Moreover,  NMR  measurements  enable  us  to determine water conversion in ice (figure 17).           Rheology XA 2% 25s-1NMR XA 2% 25s-1Rheology XA 2% 50s-1NMR XA 2% 50s-1RheologyXA 2% 100s-1NMR XA 2% 100s-1Rheology XA 5% 25s-1NMR XA 5% 25s-1Rheology XA 5% 50s-1NMR XA 5% 50s-1Rheology XA 5% 100s-1NMR XA 5% 100s-1RheologyXA 10% 25s-1NMR XA 10% 25s-1Rheology XA 10% 50s-1NMR XA 10% 50s-1Rheology XA 10% 100s-1NMR XA 10% 100s-1Rheology XA 15% 25s-1NMR XA 15% 25s-1Rheology XA 15% 50s-1NMR XA 15% 50s-1Rheology XA 15% 100s-1NMR XA 15% 100s-1020406080100120140160Time (min)rheology XDA 2% 25s-1NMR XDA 2% 25s-1rheology XDA 2% 50s-1NMR XDA 2% 50s-1rheology XDA 2% 100s-1NMR XDA 2% 100s-1rheology XDA 5% 25s-1NMR XDA 5% 25s-1rheology XDA 5% 50s-1NMR XDA 5% 50s-1rheology XDA 5% 100s-1NMR XDA 5% 100s-1rheology XDA 10% 25s-1NMR XDA 10% 25s-1rheology XDA 10% 50s-1NMR XDA 10% 50s-1rheology XDA 10% 100s-1NMR XDA 10% 100s-1rheology XDA 15% 25s-1NMR XDA 15% 25s-1rheology XDA 15% 50s-1NMR XDA 15% 50s-1rheology XDA 15% 100s-1NMR XDA 15% 100s-1                             rheology XDA 2% 25s-1NMR XDA 2% 25s-1rheology XDA 2% 50s-1NMR XDA 2% 50s-1rheology XDA 2% 100s-1NMR XDA 2% 100s-1rheologyXDA 5% 25s-1 NMR XDA 5% 25s-1rheology XDA 5% 50s-1NMR XDA 5% 50s-1rheology XDA 5% 100s-1NMR XDA 5% 100s-1rheology XDA 10% 25s-1NMR XDA 10% 25s-1rheology XDA 10% 50s-1NMR XDA 10% 50s-1rheology XDA 10% 100s-1NMR XDA 10% 100s-1rheology XDA 15% 25s-1NMR XDA 15% 25s-1rheology XDA 15% 50s-1NMR XDA 15% 50s-1rheology XDA 15% 100s-1NMR XDA 15% 100s-1050100150200Time (min)              Figure 17: conversion water in ice in emulsion XA et XDA  In each cases, the whole water is converted in ice. According to previously, effective volume fraction of suspension evolves with a power law:  eff     x . In  ice  case,  for  XA  and  XDA  emulsions,   eff  is very  high,  nearly  0.57  which  is  the  maximum volume  fraction.  So  ice  particles  agglomerate quickly and strongly and conditions blockage are encountered.   Freon Hydrate suspensions As dodecane, Freon is a floculant of asphaltenes. So  we  are  placed  in  the  same  physico  chemical conditions  than  the  case  xylene/dodecane  to compare the 2 systems. Same parameters are studied: shear rate and water cut as for ice. Moreover, subcooling temperature is studied in the case of hydrates.  Water cut influence water-cut  Induction time/min   t/min  0.05  960  150 0.07  150  90 0.1  30  60 0.3  30  30 Table 1: Induction times and  t obtained with a 268 K temperature and a 250 s-1 shear rate water-cut  Induction time/min   t/min  0.1  10  30 0.15  5  60 0.15  0  90 0.2  5  10 0.2  10  30 0.3  15  5 Table 2: Induction times and  t obtained with a 268 K temperature and a 100 s-1 shear rate The greater the water cut, the lower the induction time. So, it seems to correspond to nucleation by contact.  The  same  influence  is  observed  for   t which show that agglomeration is by contact.  Shear rate influence shear rate/s-1  Induction time/min   t/min  100  30  10 250  30  30 500  0  5 Table 3: Induction times and  t obtained with a 268 K temperature and a 30% water cut shear rate/s-1  Induction time/min   t/min  100  10  600 250  30  60 500  30  40 Table 4: Induction times and  t obtained with a 268 K temperature and a 10% water cut A little effect on induction time is observed with shear  rate.  Contrary  to  this,  there  is  a  strong relation between shear rate and  t: the greater the shear rate, the lower is  t. Moreover,  viscosity  depends  on  imposed  shear rate so aggregates size depends on shear rate.  Subcooling temperature influence Temperature/K  Induction time/min   t/min  274  5520  10 271  3120  10 268  30  30 Table 5: Induction times and  t obtained with 250s-1 shear rate and a 10% water cut For hydrate particle suspensions, aggregate size is controlled by shear rate (figure 18) y = 5.8734x-0.23080246810120 100 200 300   eff/  Figure 18: power law for hydrate particles suspensions 0 2 4 6 8 10 12 14 16 18 2002468101214161820 XA XDA parity plotconverted water (%)initial water cut (%)Thanks to equation (4), we can determine (3-D)m = 0.23 and  0(3-D)m = 5.87 Pa so  0   2000 Pa. Thanks first results of modeling, fractal dimension D  would  be  around  2.1  so,  which  m  represents breakage mechanism of aggregate, will be around 0.25. So,  ice  agglomeration  is  stronger  than  hydrate agglomeration.  moreover,  hydrate  agglomeration seems to be reversible.  CONCLUSIONS Gas  hydrates  are  crystalline  compounds  which may  lead  to  pipelines  blockage  during  oil production in offshore conditions. These particles, by  an  agglomeration  mechanism,  tend  to progressively  form  a  plug  and  so  prevent production in pipelines. They can be modelled and compared with ice crystals.  So,  this  work  deals  with  study  of  kinetics  of formation  and  agglomeration  and  mechanisms agglomeration  of  hydrate  and  ice  particles  in emulsion in a hydrocarbon phase.  For  this,  two  main  characterisation  techniques have  in  particular  been  implemented  and  been optimized for our system: NMR relaxometry and rheology.  A  calibrated  shear  rate  from  the  shear rate  imposed  in  rheology  have  been  adapted  in NMR  in  order  to  reproduce  same  experimental conditions  and  be  able  to  compare  the  two measurement techniques. This is the originality of our works. Moreover, NMR gives information on the  formation  kinetics  and  the  conversion  rate water in ice crystals. The main perspective for the rest  of  our  work  is  to  study  formation  of  Freon hydrate crystals formation too.   REFERENCES [1]  Mc  Lean  J.D.  ,  P.K.  Kilpatrick,  Effects  of Asphaltene  Aggregation  in  Model  Heptane-Toluene  Mistures  on  Stability  of  Water-in-Oil Emulsions.  Colloid  Interface  Sci.,  1997,  196,  23-34 (1) [2]    Fouconnier  B.,  Komunger  L.,  Clausse  D., Ollivon M., Study of CCl3F hydrate in emulsions by  the  coupled  DSC/XRDT  technique.  3rd  World Congress on Emulsion (CME), 2002, Lyon, France [3] Snabre P., Mills P., Rheology of concentrated suspensions  of  viscoelastic  particles,  1998,  Journal of Colloids and Surfaces 152 79-88 [4]  Camargo  R.,  Palermo  T.,  Rheological Properties  of  Hydrate  suspensions  in  an asphaltenic  Crude  Oil,  4th  International Conference on Gas Hydrate, 2002. [5]  Pauchard  V.,  Darbouret  M.,  Palermo  T.,  Gas Hydrate Slurry Flow in a Black Oil. Prediction of Gas Hydrate Particles Agglomeration and Linear Pressure  Drop,  Proceedings  of  the  13th International Conference of Multiphase Production Technology, 2007, Edinburgh, UK;343-355 [6]  Potanin  Andrew  A.,    On  the  mechanism  of aggregation  in  the  shear  flow  of  suspensions, Journal  of  colloid  and  Interface  Science, 1991,145(1) 140-157 [7]  Ait-Kadi  A.,  Marchal  P.,  Choplin  L., Quantitative  analysis  of  mixer-Type  Rheometers using the Couette Analogy, The Canadian Journal of  Chemical  Engineering,  2002,  Vol.  80,  1166-1174 [8]  Gao  S.,  Chapman  W.,  Measuring  Hydrate Behaviour  in  Black  Oil  Emulsions  using  NMR, American  Chemical  Society-Division  of Petroleum Chemistry, 2005, 49 (3 et 4) [9]  Canet  D.,  Boubel  J.C.,  Canet-Soulas  E.  La RMN:  Concepts,  M?thodes  et  Applications:  2?me ?dition, Dunod, 2002.    

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