Methane Hydrate Bed Formation in a Visual Autoclave: Cold Restart

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Methane Hydrate Bed Formation in a Visual Autoclave: Cold Restart and Reynolds Number Dependence Zachary M. Aman, Masoumeh Akhfash, Michael L. Johns, and Eric F. May* Centre for Energy, School of Mechanical and Chemical Engineering, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia ABSTRACT: The formation of methane hydrate beds at the gas− water interface in a high-pressure visual autoclave apparatus, under both continuous cooling/flow and shut-in/restart operating procedures, was studied. Bed formation was identified by an increase in the measured resistance-to-flow of the hydrate slurry, and supported by visual observations. During continuous cooling/flow experiments, the hydrate volume fraction required to form a moving bed increased from 15 vol % to 40 vol % over a range of initial Reynolds numbers for the stirred cell of 280 to 4500. For shut-in/restart trials, the bed formation point increased from 6.6 vol % to 33 vol % hydrate over an equivalent, stirred cell Reynolds number range of 240 to 3900. No significant differences in the dependence of the bed formation point on shear rate were observed between the constant cooling/flow and shut-in/restart experiments, suggesting both systems evolved along the same pathway to hydrate plug formation. Some differences between the two types of experiments were observed in the dependence of the initial hydrate formation rates on Reynolds number, with shut-in/restart experiments having formation rates up to an order of magnitude larger. More significantly, in both types of experiments the formation rate increased logarithmically with Reynolds number. The dependences of bed formation and growth rate on shear are crucial results for assessing the risk of forming a hydrate plug in mature production systems in which water is the dominant phase. High shear increases hydrate growth rate, but delays the onset of hydrate bed formation, which is the precursor to plugging. By trading-off these competing effects, it may be possible to develop an optimum restart strategy to minimize the risk of hydrate plug formation. water-continuous systems, based on several flowloop experiments performed over a wide range of liquid loading and velocity conditions. In these tests, the pressure drop measured across the flowloop was observed to increase only after a finite amount of hydrate had formed in the system. This transition in the pressure drop behavior as a function of hydrate volume fraction was interpreted to correspond to the transition from a homogeneous distribution of hydrate particles to a heterogeneous distribution and the formation of a moving hydrate bed,15 where hydrate particles would collect at the gas−water interface due primarily to the density difference between the hydrate and external (water) phase. Joshi et al.14 labeled the hydrate fraction at which the hydrate-in-water particle distribution transitioned from homogeneous to heterogeneous as Φtransition, which is shown conceptually in Figure 1. This transition is crucial to the system’s evolution because while initial hydrate formation will lead to a sparse and noninteracting distribution of hydrate particles in the water phase (labeled as a homogeneous dispersion), continued hydrate growth will lead

1. INTRODUCTION Gas hydrates are ice-like solids, where molecular water cages surround light hydrocarbon species (e.g., methane) at high pressure and low temperature.1 Hydrates readily form in subsea oil and gas flowlines due to heat exchange with the cool ocean water surrounding the pipe. Over the past three decades, the laboratory of Sloan2 has systematically devised and validated the conceptual mechanisms describing hydrate plug formation, starting with systems containing dominant crude oil fractions. Turner et al.3 in collaboration with J. Abrahamson described four critical stages in the formation of a hydrate plug in an oildominant system: (i) emulsification,4 where water droplets are entrained in the liquid hydrocarbon phase through a mechanical equilibrium between fluid shear stress and interfacial tension;5 (ii) hydrate nucleation6 and film growth7 at the oil−water interface; (iii) aggregation8 between hydrate particles or shells3 via capillary cohesion or sintering,9 which increases slurry viscosity;10 and (iv) jamming11 of large aggregates to restrict flow and create catastrophic pressure drop fluctuations.12 As the field life of a produced reservoir increases, the volume fraction of produced water increases.2 When the water content is in the range of 70 vol % to 90 vol %, any emulsion present will undergo a phase inversion,13 where oil droplets flow in the continuous water medium. Recently, Joshi et al.14 proposed a conceptual mechanism (Figure 1) for hydrate plug formation in © XXXX American Chemical Society

Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: July 17, 2014 Accepted: October 7, 2014

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dx.doi.org/10.1021/je500670h | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Conceptual mechanism proposed by Joshi et al.14 for hydrate plug formation in water-continuous systems, where resistance-to-flow is observed only after a moving hydrate bed has been established via the transition from a homogeneous to heterogeneous particle distribution.

Figure 2. Schematic (left) and photograph (right) of the high-pressure visual autoclave, where methane hydrate is formed in the sapphire cell and monitored visually with time.

system’s resistance to flow evolved along the pathway observed for flow velocities of 1 m/s. These experiments indicated that the transition to a heterogeneous particle distribution was both irreversible and path-dependent. Joshi et al.14 suggested that hydrate agglomeration or particle deposition on the wall may explain this behavior. The present work quantifies the hydrate bed formation point over 20 different turbulence conditions and reports an observation of deposition and film growth that with further research might help identify the cause of the irreversibility and path dependence of Φtransition.

to particle interactions, which when sufficiently large will result in a buoyant collection of hydrate particles at the water−gas interface. Akhfash et al.16 provided the first direct visual confirmation of the existence of this transition and the formation of a hydrate bed, confirming the hypothesis of Joshi et al.14 through a series of experiments conducted in a high-pressure visual autoclave, which is an experimental geometry very different to that of a flowloop. In these autoclave experiments, Φtransition could be measured by up to three independent methods: (i) direct visual observation of the spatial distribution of hydrate particles in the sapphire autoclave; (ii) an increase in the rate of pressure consumption to form hydrate; and (iii) an increase in the motor current required to maintain constant mixing velocity. The quantity utilized in this third method, which was the most reliable for determining Φtransition, is an equivalent measure of the hydrate slurry’s resistance to flow and is an analogue of the pressure drop measured in flowloop experiments. The present study extends the work initiated by Akhfash et al.16 to quantify the effect of turbulence on Φtransition in a methane−water system using the visual autoclave. In addition, this work studies the effect of transient operations (simulated shut-in and restart) on Φtransition, during which subsea oil and gas operations are at the highest risk of hydrate plug formation.2 Joshi et al.14 proposed a linear relationship between Φtransition and mixture velocity; over six flowloop experiments, Φtransition was estimated at 10, 18, and 28 vol % for mixture velocities of 1.0, 1.75, and 2.5 m/s, respectively. Furthermore, they conducted experiments where the mixture velocity was changed from 1 to 2.5 m/s before and after 10 vol % hydrate had formed (the value of Φtransition for 1 m/s). In both experiments, the

2. EXPERIMENTAL METHODS A high-pressure visual autoclave (HPVA) apparatus was deployed in the present study and is discussed in detail by Akhfash et al.16 The apparatus consisted of a DB Robinson-type sapphire cylinder (2.54 cm inner diameter, 15.0 cm height, 0.64 cm thick), which was fitted with a magnetically coupled (Dyna/ Mag #MM-T06), four-blade vane-and-baffle geometry impeller driven by a Groschopp direct-drive DC motor (180 V, 1000), the maximum resistance-to-flow decreased with increasing Reynolds number, with an abrupt transition being apparent from the laminar flow value. This latter observation may be the result of increased shear forces and turbulent eddies acting to reduce the degree of hydrate deposition and annealing to the walls.24 An alternative mechanism is required to explain the reverse observation in the laminar and transition flow regimes. One possibility is that, at these low shear rates, the adhesive strength of the hydrate deposit exceeds the shear stress from the bulk fluid. The growing hydrate deposit provides a surface for continued hydrate growth, occluding the flow channel and interacting directly with the mixing blades. As is demonstrated in the following section, hydrate growth rate increases directly with the turbulence of mixing. Consequently, the observed peak in relative motor current at Reinit = 1145 (Figure 5, bottom panel) represents a worst-case mixing velocity, resulting in the highest possible growth rate that does not enable sufficient sloughing of hydrate deposited on the wall. The trend observed for the shut-

Table 1. Summary of Experiments Conducted exp 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

Pinitial

Tinitial

bar

°C

Φtransition RPM

Reinitial

Reequivalent

Constant Cooling and Flow Operating Mode 63.8 18.5 50 285 63.9 18.5 50 286 64.3 21.0 100 574 63.8 20.0 150 851 65.7 25.3 200 1145 63.2 18.4 250 1449 62.7 16.1 300 1739 64.7 23.7 350 2033 64.2 21.0 400 2323 64.6 22.0 400 2319 64.5 22.1 550 3213 64.0 20.4 550 3182 63.1 19.6 600 3387 64.5 25.3 600 3404 63.2 18.4 700 4052 63.5 18.2 700 4054 64.0 18.6 800 4515 64.2 18.4 800 4275 Shut-in and Restart Operating Mode 64.0 17.9 50 242 64.4 19.8 50 242 64.0 17.0 100 485 63.0 18.0 200 989 63.2 18.8 200 976 62.6 16.3 300 1453 62.7 17.0 400 1921 62.6 16.4 400 1924 63.8 20.5 600 2910 62.9 17.4 600 2897 63.9 18.0 700 3395 63.8 18.0 800 3879

vol % 16.6 14.6 18.9 16.1 17.8 19.5 20.7 20.1 18.1 22.5 31.5 32.6 34.8 32.1 40.2 37.5 24.3 26.5 6.6 12.7 18.5 15.5 14.5 15.8 16.5 14.6 24.4 22.1 26.8 32.7

Figure 4. Onset of hydrate bed formation (Φtransition) as a function of initial (or equivalent) Reynolds number in the HPVA apparatus.

Second, Φtransition increases directly with Reynolds number. That is, the number of hydrate particles required to stabilize a bed increases with the system’s degree of turbulence. This observation qualitatively agrees with flowloop observations by Joshi et al.,14 but it is not quantitatively consistent with the values for the Reynolds number dependence of Φtransition indicated by Sum et al.22 from those flow loop data. In addition, Joshi23 reported the results of three experiments made using a 4 in. internal diameter autoclave, where Φtransition increased from about 14 vol % to 19 vol % over a corresponding stirred cell Reynolds numbers of 35 000 to 58 000 (300 rpm to 500 rpm). Comparing these ranges from Joshi23 and Sum et al.22 to the data reported in Figure 4 E



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contribution may come to dominate the flow resistance of the entire system. This hypothesis suggests that avoiding the formation of a hydrate bed will be crucial to the management of plugging risk in a water-dominant flow. Antiagglomerant-type strategies, which aim to reduce the effective viscosity of the slurry by lowering interparticle forces between hydrate, are unlikely to be effective in water-dominant systems because (i) the interparticle forces are already very low and (ii), regardless of how small they are, the slurry’s effective viscosity will increase when localized regions within the bed reach very high particle densities. Evolution and Shear Rate Dependence of Hydrate Growth. The hydrate formation rates measured during the constant cooling/flow experiments were compared to the mass transport-limited hydrate growth model introduced above. Experimental values of temperature, pressure, and hydrate volume fraction for all experiments are listed in Table 2 and compared with values calculated using either the MultiFlash equilibrium model19 or the mass transport-limited hydrate growth model discussed by Akhfash et al.16 The measured and calculated temperatures indicate that, in all cases, a positive subcooling (Teq − T) was maintained for the duration of the experiment, indicating the absence of heat transport limitations between the cell and bath. In all experiments, the cell temperature was always maintained below the hydrate equilibrium temperature corresponding to the cell pressure, confirming the appropriate application of a mass transportlimited growth model. At low mixing speeds (e.g., 250 rpm in Figure 6), the hydrate volume fraction was observed to plateau at an intermediate value, while the system remained in the hydrate formation region (Table 2). Under highly turbulent conditions (e.g., 800 rpm in Figure 6), the initial hydrate growth rate was 2 orders of magnitude larger and growth was sustained for a longer period, which resulted in a volume fraction well above 50 % at steadystate. Figure 7 shows that the maximum amount of hydrate formed in each continuous cooling/flow experiment increased with initial Reynolds number over the range 285 to 851, but remained constant at (71 ± 7) vol % for turbulent shear rates (1000 < Reinitial ≤ 4515). The largest hydrate volume fraction achieved was 84 % at a shear rate of 550 rpm (experitment 11), where the final subcooling was 0.1 °C. The experiments presented herein behaved similarly to those reported by Akhfash et al.,16 with the water phase not being completely converted even after 24 h of mixing. This was likely the consequence of forming a stationary hydrate bed at and above the water−gas interface, which prevented the convective transport of methane to unreacted water at the bottom of the cell. The results for the shut-in/restart experiments shown in Figure 7 are somewhat different to those for the continuous cooling/flow experiments. For Reequiv < 3000, the final hydrate volume fraction achieved was quite consistent and remained below 50 vol %, which is nearly a factor of 2 lower than that produced in the continuous cooling flow experiments with Reinit > 1000. This indicates that unless the shear was very high, the breakup of initial hydrate film upon restart ultimately led to a stationary bed that was more effective at preventing the conversion of water to hydrate than was the case in the continuous cooling/flow experiments. However, the opposite was true when one considers the hydrate formation rates achieved at the beginning of each experiment.

Figure 5. Top: Relative motor current (resistance-to-flow) as a function of time after nucleation for constant cooling/flow trials with initial Reynolds numbers of 1145 and 1450, with the maximum value observed in each trial circled. Bottom: Maximum relative motor current observed during constant cooling and shut-in/restart experiments, as a function of initial turbulence of mixing. The relative motor current is defined as the motor current measured during hydrate formation, normalized by the baseline motor current prior to hydrate formation.

in/restart experiments is consistent with this picture; as discussed below, the initial formation rates in these experiments were less sensitive to shear rate, and were significantly higher than those observed at low mixing velocities in the continuous experiments. In water-dominant systems, it is clear that the increase in the effective viscosity of the hydrate slurry occurs by quite a different mechanism to that reported for oil-dominant systems. Aman et al.25 demonstrated that the cohesive force between cyclopentane hydrate particles suspended in an aqueous bulk phase was minimal, and it is clear from the results shown here as well as by Joshi et al.14 and Akhfash et al.16 that there is no effective viscosification of the water-continuous hydrate slurry as long as the particle distribution remains homogeneous. However, the mechanism by which the resistance to flow increases upon the transition to a heterogeneous particle distribution remains unclear. We hypothesize that one mechanism may be the effective viscosification of localized volume elements within the heterogeneous distribution, in which the particle number density approaches the limit of particle jamming. Even though the cohesive forces between hydrate particles may be minimal, the relative viscosity of the localized volume element may become very large, particularly if, as proposed by Mills,26 it diverges to infinity as the local particle fraction approaches about 57 vol %. Since the number of localized volume elements with a large effective viscosity will increase with the thickness of the hydrate bed, their overall F



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Table 2. Summary of Experimental Results from Constant Cooling/Flow and Shut-in/Restart Experiments, Together with Values Calculated with an Equilibrium Model19 and a Mass-Transport Limited Model16 Tnuc

Pnuc

Teq,nuc

Tfinal

Teq,final

Pfinal

P(model) final

ϕ(exp) max

ϕ(model) max

exp

°C

bar

°C

°C

°C

bar

bar

vol %

vol %

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

7.1 7.4 7.5 7.0 7.4 7.9 7.8 7.9 7.9 7.8 8.1 7.8 6.8 7.0 7.8 7.8 6.8 4.8 1.2 1.2 4.1 0.4 0.1 0.8 1.2 1.2 1.4 0.8 4.8 1.8

60.9 60.9 60.3 60.0 60.2 60.5 60.5 60.5 60.5 60.4 60.5 60.4 59.5 59.5 60.5 60.5 61.4 60.7 59.5 58.9 60.1 58.0 58.0 58.1 58.3 58.2 58.1 58.1 60.3 58.9

8.4 8.4 8.3 8.2 8.3 8.3 8.3 8.3 8.3 8.3 8.3 8.3 8.1 8.1 8.3 8.3 8.5 8.4 8.1 8.0 8.2 7.9 7.9 7.9 7.9 7.9 7.9 7.9 8.3 8.0

1.2 1.2 1.9 1.3 1.8 2.2 1.2 1.2 1.3 2.0 1.2 1.2 2.5 2.6 1.2 1.2 1.5 1.2 1.2 1.2 1.2 0.2 1.1 0.6 1.0 0.9 1.0 1.5 1.2 1.2

6.5 6.4 5.5 5.4 3.2 3.0 2.6 2.6 3.0 2.8 1.3 3.0 3.8 3.3 3.0 1.9 3.7 3.5 6.2 5.7 5.4 6.0 6.3 5.3 5.1 5.3 5.1 5.2 2.2 1.4

50.2 49.9 45.5 45.3 36.4 35.5 34.2 34.4 35.3 35.1 30.1 35.7 38.6 36.5 35.5 31.9 38.2 37.4 48.7 46.6 45.2 48.0 49.4 44.5 43.9 44.8 43.8 44.3 32.8 30.4

53.7 54.0 35.8 32.0 32.7 33.1 30.8 31.2 31.6 32.5 30.0 30.0 31.5 31.5 30.0 30.0 32.3 30.0 50.1 48.9 31.9 36.2 52.4 32.7 35.7 34.3 28.9 30.6 30.0 30.0

29.3 30.2 45.0 44.7 69.3 71.8 74.7 74.2 71.7 72.8 84.0 71.1 57.0 − 71.8 80.2 62.8 64.5 33.5 37.6 42.1 31 38.0 41.2 43.2 40.4 42.8 42.2 70.7 74.5

49.2 51.4 59.6 66.3 66.1 65.8 70.4 69.6 68.8 66.8 72.0 72.0 60.7 60.7 72.2 72.2 63.8 67.8 23.4 24.8 60.5 50.7 16.1 57.4 51.6 54.2 64.6 61.5 64.8 63.3

Figure 7. Maximum hydrate volume fraction (of original water volume) for constant cooling/flow (solid circles) and shut-in/restart (open circles) data, as a function of the initial turbulence of mixing; the cell is expected to be fully turbulent at Reynolds numbers above approximately 1000.20

Figure 6. Hydrate volume fraction, with respect to the liquid phase, as a function of time after nucleation for experiments 6 and 17 (250 rpm and 800 rpm, respectively); the dashed curve represents predictions from the mass-transfer limited model (eq 2) for each experiment.

the growth of an initial hydrate film at the gas−water interface. Second, the minimum growth rate attained (at 50 rpm) was approximately 1 order of magnitude larger than that for the constant cooling experiments at the same shear rate. This result is expected as the initial hydrate formation period for shut-in and restart experiments begins with a substantially larger hydrate−water surface area. The kinetic-type relationship discussed by Bishnoi27 indicates that the rate of hydrate growth will depend linearly on the existing hydrate crystal surface area. In contrast, when considering mass-transport limited growth, Skovborg et al.21

Figure 8 shows the initial hydrate growth rate achieved for both the constant cooling/flow and shut-in/restart experiments, determined from the rate of pressure consumption for the first volume percent of hydrate formed after nucleation or after restart, respectively. For the continuous cooling/flow experiments, the initial hydrate growth rate increased exponentially with initial Reynolds number. A similar pattern was observed for the shut-in/restart experiments with two notable changes. First, the scatter between replicate data points was consistently larger, which may be the result of surface area variance during G



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The third conclusion, coupled with the observations of the system evolution possible in the visual autoclave, suggests that hydrate film growth and particle deposition at the wall represent critical steps in the formation of a hydrate plug, and possibly explain the path dependence and irreversibility of Φtransition with Reynolds number. Quantitative comparisons of the shear dependence of Φtransition reported here and for different flow geometries by Joshi23 and Sum et al.22 indicate that a universal relation is yet to be found. Additional comparative tests between autoclave and flowloop geometries are required to determine whether Reynolds number, as currently used, is the most appropriate relation through which to scale autoclave results to pipeline-type geometries. Finally, the results reported here indicate that for water-dominant systems, the more turbulence that is applied during a restart process, the greater is the transportability of the resulting hydrate slurry, and the lower is the risk of plug formation.

Figure 8. Initial hydrate growth rate for continuous cooling/flow (filled data points) and shut-in/restart (open data points) experiments listed in Table 1 as a function of initial turbulence of mixing in the cell. Growth rate was defined for the first volume percent of hydrate following nucleation or the restart of shear. The solid curve corresponds to the model from Skovborg et al.,21 (eq 2) based on diffusion-limited hydrate growth in an the aqueous phase.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +61 8 6488 2954. Fax: +61 8 6488 1024. Funding

identified the gas−water interfacial area and transport through the aqueous phase to be the quantities that limit the formation of the hydrate phase. The initial growth rate predictions of the model by Skovborg et al.21 are shown in Figure 8, which were calculated using eq 2. These model calculations use the geometric formalism described by Akhfash et al.16 for a stirred autoclave cell and are in reasonable agreement with the initial rates measured for constant cooling/flow experiments, particularly if the shear rate was sufficiently high. The substantially higher growth rates observed in the first stages of the shut-in/restart experiments indicate that, at least initially, the formation was kinetically limited rather than mass-transport limited. It is very difficult to estimate the hydrate−water surface area involved with this kinetic reaction and between repeat experiments this surface area would very likely have varied significantly. However, the data indicate that this kinetically limited initial growth rate increased with Reequivalent, which could reflect that higher rates of initial shear caused the film to be broken into smaller pieces with a correspondingly larger hydrate−water surface area.

Z.M.A. acknowledges the University of Western Australia for a Research Development Award. E.F.M. acknowledges Chevron for their support of the research through the Gas Process Engineering endowment. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Louis Pieterse for his efforts in collecting experimental data, Prof. Ken Marsh for the donation of the sapphire cell used in this work, and Mr. David Amm for constructing and maintaining the autoclave system.

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REFERENCES

(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press, Taylor & Francis Group: Boca Raton, FL, 2007. (2) Sloan, E. D.; Koh, C. A.; Sum, A. K. Natural Gas Hydrates in Flow Assurance; Gulf Professional Publishing, Elsevier Inc.: Houston, Texas, 2011. (3) Turner, D.; Miller, K.; Sloan, E. Methane Hydrate Formation and an Inward Growing Shell Model in Water-in-Oil Dispersions. Chem. Eng. Sci. 2009, 64, 3996−4004. (4) Boxall, J. A.; Koh, C. A.; Sloan, E. D.; Sum, A. K.; Wu, D. T. Droplet Size Scaling of Water-in-Oil Emulsions under Turbulent Flow. Langmuir 2012, 28 (1), 104−110. (5) Janssen, J. J. M.; Boon, A.; Agterof, W. G. M. Droplet break-up in simple shear flow in the presence of emulsifiers. Colloids Surf., A 1994, 91, 141−148. (6) Walsh, M. R.; Koh, C. A.; Sloan, E. D.; Sum, A. K.; Wu, D. T. Microsecond Simulations of Spontaneous Methane Hydrate Nucleation and Growth. Science 2009, 326 (5956), 1095−1098. (7) Taylor, C. J.; Miller, K. T.; Koh, C. A.; Sloan, E. D., Jr. Macroscopic Investigation of Hydrate Film Growth at the Hydrocarbon/Water Interface. Chem. Eng. Sci. 2007, 62 (23), 6524−6533. (8) Yang, S.-o.; Kleehammer, D.; Huo, Z.; Sloan, J.; Dendy, E.; Miller, K. Temperature Dependence of Particle−Particle Adherence Forces in Ice and Clathrate Hydrates. J. Colloid Interface Sci. 2004, 277, 335−341. (9) Aman, Z. M.; Brown, E. P.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Interfacial Mechanisms Governing Cyclopentane Clathrate Hydrate Adhesion/Cohesion. Phys. Chem. Chem. Phys. 2011, 13, 19796−19806.

4. CONCLUSIONS Methane hydrate growth, bed formation, and resistance-to-flow in a high-pressure visual autoclave were studied. Hydrate plugs were formed through both constant cooling/flow and shut-in/ restart procedures, leading to four primary conclusions: (1) The hydrate volume fraction required to produce a moving hydrate bed at the water−gas interface (at a volume fraction Φtransition) increases directly with the degree of turbulence in the system. (2) Forming hydrate plugs from a cold restart did not affect the relationship between Φtransition and turbulence. (3) For constant cooling/flow experiments with systems under laminar or transition turbulent regimes, the maximum resistance-to-flow observed (following Φtransition) increased with turbulence. Otherwise the maximum resistance-to-flow decreased with shear. (4) Initial hydrate growth rates for cold restart experiments were approximately 1 order of magnitude larger than for comparable constant cooling experiments. H



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Methane Hydrate Bed Formation in a Visual Autoclave: Cold Restart

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