Flow Characteristics and Rheological Properties of Natural Gas Hydrate Slurry in the Presence of Anti Agglomerant in a Flow Loop Apparatus

5/28/2018 Flow Characteristics and Rheological Properties of Natural Gas Hydrate Slurry in the Presence of Anti Agglomerant in a Flow Loop Apparatus

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Author's Accepted Manuscript

Flow characteristics and rheological proper-ties of natural gas hydrate slurry in the

presence of anti-agglomerant in a flow loopapparatus

Ke-Le Yan, Chang-Yu Sun, Jun Chen, Li-TaoChen, De-Ji Shen, Bei Liu, Meng-Lei Jia, MengNiu, Yi-Ning Lv, Nan Li, Zhi-Yu Song, Shu-ShanNiu, Guang-Jin Chen

PII: S0009-2509(13)00750-1DOI: http://dx.doi.org/10.1016/j.ces.2013.11.015Reference: CES11395

To appear in: Chemical Engineering Science

Received date: 28 July 2013Revised date: 22 October 2013Accepted date: 10 November 2013

Cite this article as: Ke-Le Yan, Chang-Yu Sun, Jun Chen, Li-Tao Chen, De-JiShen, Bei Liu, Meng-Lei Jia, Meng Niu, Yi-Ning Lv, Nan Li, Zhi-Yu Song, Shu-Shan Niu, Guang-Jin Chen, Flow characteristics and rheological properties ofnatural gas hydrate slurry in the presence of anti-agglomerant in a flow loopapparatus, Chemical Engineering Science, http://dx.doi.org/10.1016/j.ces.2013.11.015

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Flow characteristics and rheological properties of natural

gas hydrate slurry in the presence of anti-agglomerant in a

flow loop apparatus

Ke-Le Yana, Chang-Yu Sun

a,, Jun Chena, Li-Tao Chen

b, De-Ji Shen

a, Bei Liu

a, Meng-Lei Jia

a,

Meng Niua, Yi-Ning Lv

a, Nan Li

a, Zhi-Yu Song

a, Shu-Shan Niu

a, Guang-Jin Chen

a,*

a. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249,

China

b. Center for Hydrate Research, Chemical & Biological Engineering Department, Colorado

School of Mines, Golden, Colorado, United States

ABSTRACT: The flow characteristics and rheological properties of natural gas

hydrate slurry, with initial water cuts ranging from 5 to 30 vol%, were investigated in

a flow loop. The experimental results indicate that the hydrate slurry can be

considered a pseudoplastic fluid and presents more obvious shear-thinning behaviour

with the increase in the hydrate volume fraction. The study on the fluid morphology

demonstrated that the original structure of the water-in-oil emulsion is destroyed by

the formation of gas hydrate, and the hydrate slurry is ultimately transported as a solid

dispersion system. An empirical Herschel-Bulkley-type equation that considers the

hydrate volume fraction was developed to improve the description of the rheological

properties of the hydrate slurry. The apparent viscosities of the hydrate slurry

calculated by the new equation were in accordance with the experimental data.

Shutting-down/restarting tests using three shutting-down times (2 h, 4 h, and 8 h)

were performed.The experimental results indicate that the hydrate slurry can be easily

and safely restarted from the static state after a long shutting-down period and

exhibits obvious thixotropic behaviour with increasing shutting-down time.Keywords: hydrate; slurry; rheological properties; flow characteristics;

anti-agglomerant

To whom correspondence should be addressed. Fax: +86 10 89733156. E-mail: [email protected] (C. Y. Sun),[email protected] (G. J. Chen).


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1. Introduction

Gas hydrates are ice-like clathrate-type crystals in which cages of water

molecules are stabilised by the host molecules (Sloan and Koh, 2008). During oil/gas

exploitation, light alkanes, such as methane, ethane, and propane, can form gas

hydrates with the water produced in pipelines under high pressure and relatively low

temperature. The hydrate particles can agglomerate into hydrate plugs, which may

cause total blockage (Gao, 2009). Recently, the problem caused by natural gas hydrate

blocks has become increasingly severe with the increasing water depth of the offshore

oil and gas pipelines. According to a survey, the annual cost of preventing the hydrate

issue is over U.S. $200 million (Sloan, 2003) and accounts for 5 to 8% of the total

product plant cost (Sloan et al., 2011; Chandragupthan, 2011).

Many approaches are used to prevent hydrate plugs (Kelland, 2006). The most

commonly method is the addition of thermodynamic inhibitors, e.g., methanol or

ethylene glycol, which prevent hydrate formation by shifting the hydrate equilibrium

curve toward higher pressures and lower temperatures to keep the operation

conditions outside the hydrate stability region. However, the concentration usually

required for these inhibitors to be effective is 30 to 50 wt% of the water mass. Such a

high concentration requires a large amount of additives to be used, increasing the cost

of project operation and requiring the reprocessing of wastewater. Low-dosage

hydrate inhibitors (LDHIs), including kinetic hydrate inhibitors (KHIs) and

anti-agglomerants (AAs), have been researched and developed for many years as an

alternative method to control gas hydrates (Kelland, 2006; Arjmandi et al., 2005).


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KHIs are a type of water-soluble polymer with functional groups that can be

accommodated into clathrate hydrate cages. Unlike thermodynamic inhibitors, they

can delay hydrate nucleation (usually crystal growth as well), providing sufficient

time to transport the fluids to the process facilities before hydrate plugs build up in the

pipeline. KHIs have been widely applied for hydrate inhibition in gas-dominated

systems in Qatar and Iran, where the subcooling is low (


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(CH3CCl2F or HCFC-141b) clathrate hydrate slurry and developed a model to

determine the safe flow of hydrate slurries. Delahaye et al. (2008) studied the

rheological characteristic of CO2hydrate slurry using an experimental dynamic loop,

and an empirical model was developed to describe its rheological behaviour. Delahaye

et al. (2011) also studied the flow properties of CO2hydrate slurry in the presence of

additives (EO/PO copolymer). Clain et al. (2012) investigated the rheological

properties of tetra-n-butylphosphonium bromide (TBPB) hydrate slurry flow for

hydrate fractions between 0 and 28.2 vol% and shear rates between 100 and 700 s -1

and deduced that TBPB hydrate slurries exhibit a shear-thinning behaviour. Darbouret

et al. (2005) studied the rheological properties of tetra-n-butyl ammoniumbromide

(TBAB) hydrate suspensions and determined the apparent viscosity and yield shear

stress for different hydrate contents. Hashimoto et al. (2011) studied TBAB and

tetra-n-butylammonium fluoride (TBAF) hydrate slurries and showed that both

systems present pseudoplastic behaviour. They also studied the effect of the surfactant

on the flow properties of TBAB and TBAF hydrate slurries. Suzuki et al. (2013)

studied the flow and hear transfer characteristics of ammonium alum hydrate slurries.

Recently, Joshi et al. (2013) presented a detailed analysis of hydrate formation

experiments performed in a 95-m-long flowloop (9.7-cm internal pipe diameter) in

high-water-cut systems. They proposed a hydrate plugging formation mechanism,

which involves the transition from a homogeneous suspension (region I) of hydrate

particles to a heterogeneous suspension (region II), leading to increased particle

interaction and agglomeration and ultimately causing the formation of a hydrate bed


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and wall deposit (region III).

Gas and oil generally coexist in multiphase transportation pipelines. However,

there are only a few reports of studies on the flow characteristics and rheological

properties of hydrate slurries formed from the liquid hydrocarbon phase. Using a

high-pressure rheology apparatus, Webb et al. (2012) studied the in situ formation and

flow properties of gas hydrates from a water/crude oil emulsion. Fidel-Dufour et al.

(2006) investigated the crystallisation and rheology of a methane/water/dodecane

system and demonstrated that it behaves as a Newtonian fluid. According to the

studies on the rheological and flow properties of gas hydrate suspensions, Sinquin et

al. (2004) demonstrated that hydrate particle formation in the liquid phase modifies

the flow properties and that the pressure drop is controlled by the friction factor under

turbulent flow conditions or the apparent viscosity under a laminar flow regime. Shi et

al. (2011) and Gong et al. (2010) investigated natural gas hydrate formation and

growth at different water cuts for a water-in-condensate oil emulsion in a flow loop.

Zylyftari et al. (2013) studied the salt effects on the rheological properties of a

hydrate-forming emulsion. Recently, in our group, Peng et al. (2012) investigated the

flow characteristics, shutting-down/restarting behaviour, and morphology of hydrate

slurries formed from a (natural gas + diesel oil/condensate oil + water) system

containing an anti-agglomerant. Based on the analysis of the rheology parameters and

apparent viscosity of hydrate slurry during the formation of gas hydrate, they declared

that the hydrate slurry exhibits shear-thinning behaviour and is a pseudoplastic fluid.

However, the range of shear rate studied in their work (Peng et al., 2012) was only


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from 120 to 360 s-1, which is insufficient to accurately describe the rheology of

hydrate slurry for a wider range of shear rates. In addition, the restarting effect from

the static state is important for flow safety assurance in the pipeline. Peng et al. (2012)

studied the restarting effect of hydrate slurry, but the shutting-down time only ranged

from a few minutes to less than 1 h.

In gas/oil transport pipelines, the water cut is usually less than 30 vol%. In this

work, the flow characteristics and rheological properties of hydrate slurry in a flow

loop were examined for initial water cuts from 5 to 30 vol% and shear rates from 50

to 350 s-1. A new type of anti-agglomerant, different from that of Peng et al. (2012),

was added to these systems. The flow rate and pressure drop of the hydrate slurry

formed were systematically investigated. The morphologies of the hydrate slurry at

different stages were recorded and analysed. Combined with the experimental data, an

empirical rheological model based on Herschel-Bulkley-type equation was proposed

to describe the rheological behaviour of hydrate slurry. In addition,

shutting-down/restarting tests with three different shutting-down times (2 h, 4 h, and 8

h) were performed for all water-cut systems to investigate the rheological properties

of the hydrate slurry.

2. Experimental

2.1. Materials and apparatus

The experimental materials include water, diesel oil, natural gas, and an

anti-agglomerant. Diesel oil, with a freezing point of 253.2 K, serves as the liquid oil

phase, and its composition is shown in Table 1, as analysed by a crude oil true boiling


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point (TBP) distillation system. The natural gas used is the associated gas from an

oilfield, and its composition was analysed by a HP7890A gas chromatograph and

listed in Table 2. An anti-agglomerant patented by Chen et al. (2011) was chosen in

this work, and its performance was assessed using a high-pressure sapphire cell. It is

well known that the produced water contains salt; therefore, a 0.81 wt% NaCl

aqueous solution was prepared and used as the aqueous phase in the experiments.

The experimental flow loop illustrated in Figure 1, similar to that described in

our previous work (Peng et al., 2012; Shi et al., 2011), was used to measure the flow

characteristics and rheological properties of natural gas hydrate slurries with six

different initial water cuts from 5.0 to 30.0 vol%. It mainly consists of a U-bend

double pipe (20 m long, 25.4-mm inner diameter) made of 316L stainless steel, with a

maximum operation pressure of 10.0 MPa. The pipes were maintained at constant

temperature with two fluid circulation baths (Neslab RTE 111D). Five thermocouples

(0.1 K) and a pressure gauge (0-10 MPa, 0.1%) were adopted to measure the

temperature and pressure during the experiment, respectively. An IH-type single-stage,

single-suction, cantilever centrifugal pump (Tianjin Pumps & Machinery Group Co.,

Ltd., China) was equipped to circulate the liquid through the pipe loop. A turbine flow

meter was used to measure the volumetric flow rate, and a differential pressure

transducer was placed between the inlet and outlet of the U-bend pipe to measure the

pressure drop generated by the fluid flow. An observation window was placed in the

middle of the flow loop to observe the variation of the morphology of the fluid. A

mixing tank (approximately 20 L) was used to separate the gas and liquid phases, in


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which a funnel shape in the bottom was designed to ensure that the fluid in the mixing

tank and all of the slurry would be transported to the loop. All of the sensors were

connected to a PC-based acquisition system.

2.2. Experimental procedure

The experimental procedure for investigating the flow characteristics and

rheological behaviour of hydrate slurry at fixed temperature and pressure is described

as follows. First, the flow loop was cleaned by flushing with a detergent and pure

water in cycles and drained using hot gas. Thereafter, a given amount of (diesel oil +

water + anti-agglomerant) fluid was charged into the mixing tank and loop pipe. The

flow loop was evacuated for 90 min to remove the residual air. An initial flow rate of

approximately 1.5 m3/h was applied, and the temperatures of two circulating baths

were cooled to the fixed experimental value (274 K in this work). After the

temperature was stable, a fixed quantity of liquefied petroleum gas was slowly

injected into the mixing tank to saturate the fluid phase. After the system had

stabilised, the original natural gas was injected into the mixing tank until the pressure

reached the experimental value (2.10 MPa in this work). To sustain the system at a

constant pressure, natural gas was continuously charged to compensate for its

consumption due to hydrate formation, and this process may last for several hours.

The amount of gas charged into the loop was recorded online using a mass flow meter.

The variations of flow rate and pressure drop with time were also recorded. When no

gas was added, the hydrate slurry could be considered to be stable in the loop. The

flow rate was then changed, and the corresponding pressure drop was measured to


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determine the rheological properties of the gas hydrate slurry. In addition, the

equilibrium gas was sampled from the flow loop and analysed. During the entire

experiment, the morphology of the hydrate slurry could be observed through the

observation window and was continuously monitored using a video camera.

Because hydrate plugs easily form in deep-sea multi-phase transportation

pipelines during shutting-down and restarting periods, it is important to study the

restarting effect of hydrate slurry to further understand its flow characteristics. In

general, the shutting-down time used in laboratory studies have ranged from tens of

seconds to several hours (Peysson et al. 2007; Lachance et al. 2012; Estanga et al.

2008; Harun et al. 2008), whereas some field tests have used shutting-down times of

several days (Frostman, et al. 2001; Fu, et al. 2001). Therefore, in this work, three

different shutting-down times, 2 h, 4 h, and 8 h, were adopted for the

shutting-down/restarting tests of the hydrate slurry. To investigate the restarting effect

of the hydrate slurry, the pump was turned off when the slurry was under steady-flow

conditions. After shutting down for several hours, the pump was restarted. The flow

rate and corresponding pressure drop of the hydrate slurry at the restarting state were

recorded to study the restarting effect. The variation of the morphology of the hydrate

slurry during this process was also observed.

3. Results and discussion

3.1. Evaluation of the new anti-agglomerant

The method adopted in this work to evaluate the anti-agglomerant is the same as

that used by Peng et al. (2012). A high-pressure sapphire test system devised and


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constructed in our group was used to perform the evaluation experiments. More

details about the test system can be found in our previous papers (Chen et al., 2009;

Sun et al., 2003). The change in the morphology of the hydrate slurry can be observed

directly through the transparent cell wall. The anti-agglomerant performance can also

be evaluated by observing whether the stirrer in the fluid can move smoothly up and

down after almost all water has been converted into hydrate.

Figure 2 shows the morphologies of the hydrate slurries formed from the (water

+ diesel oil + natural gas) system, where the initial water cuts range from 5 to 30

vol%. The composition of the original gas used is listed in Table 2. The effective

dosage of anti-agglomerant added is 3.0 wt% of the water mass for each run. The

maximum subcooling tested by the cooling method at constant pressure is over 20 K.

As shown in Figure 2, for water cuts ranging from 5 to 30 vol%, we can see that the

hydrate particles are homogeneously dispersed in the diesel oil phase and do not

agglomerate after almost all of the water has been converted into hydrate. Although

the hydrate slurry becomes stickier with increasing initial water cut, the stirrer could

still move smoothly up and down. In addition, after the hydrate slurry was allowed to

rest at the maximum subcooling without stirring for 12 h, it was found that the stirrer

could be successfully restarted and that the hydrate could be redispersed into the oil

phase.

3.2. Flow characteristics and morphology of the hydrate slurry

The flow characteristics and morphology of hydrate slurry were investigated at

six different initial water cuts of 5, 10, 15, 20, 25, and 30 vol%. In each of the six


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experimental runs, 3.0 wt% anti-agglomerant dosages were used. The system pressure

and water bath temperature were kept constant at 2.1 MPa and 274.2 K, respectively.

The system with an initial water cut of 5 vol% was used as an example to present the

experimental results. Figure 3 shows the variations of the flow rate and pressure drop

of hydrate slurry with the elapsed time, where the zero time refers to the beginning of

the charge of natural gas. The variations of the flow rate and pressure drop of the

hydrate slurry within the first 5 h are shown separately in Figure 4 for clarity. It can be

observed that the flow rate decreases with the formation of hydrate and then becomes

stable, whereas the pressure drop first increases with increasing hydrate quantity

formed, then gradually decreases with some fluctuation, and finally reaches a stable

value. In particular, at the initial stage after the stabilisation of the temperature,

pressure, and flow rate, a sudden temperature rise occurs (see Figure 5) when hydrate

appears due to the exothermic effect of hydrate crystallisation. At the same time, a

sudden increase of the pressure drop and decrease of the flow rate (See Figure 4)

occur because the formation of solid hydrate changes the flow characteristics of the

fluid in the flow loop.

In the work of Peng et al. (2012),for an anti-agglomerant comprised of a mixture

of sorbitan monolaurate and polymer esters, when the initial water cut is less than 20

vol%, the flow rate is 1.0 m3/h, corresponding to a mean fluid velocity of 0.55 m/s.

However, for higher-water-cut systems (20 vol%), the mean fluid velocity is less

than 0.44 m/s. Based on the experimental results in this work, hydrate slurries with

the anti-agglomerant adopted can flow steadily, and the mean velocity can reach more


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than 0.55 m/s for all six groups of experiments.

The temperature of the hydrate slurry was monitored and recorded by the

thermocouples placed in five different positions in the flow loop. The average value

of these five temperature points was considered as the temperature of the hydrate

slurry. The variation of temperature with time for different water cuts is shown in

Figure 5. A sudden rise can be clearly observed when gas hydrate appears in the flow

loop, and then the temperature continues to increase and reaches a maximum value.

The maximum temperature is higher for higher initial water cuts: 276.5, 276.8, 277.0,

277.5, 277.8, and 278.2 K for 5, 10, 15, 20, 25, and 30 vol%, respectively. This

phenomenon is attributed to the formation of more hydrate and the release of more

energy for higher initial water cuts. Gradually, with the decrease of the hydrate

formation rate and the convective heat transfer between the cooling medium and

hydrate slurry, the temperatures tend towards that of the water bath, 274.2 K, within

the first 3 h of the experiments. Thereafter, the temperatures remained constant at

approximately 274.2 K.

The morphology of gas hydrate slurry in the flow loop was observed through an

observation window, as shown in Figure 1. Four images taken at different stages for

each water cut system are shown in Figure 6: the beginning stage before hydrate

formation, the stable flow stage of the hydrate slurry, the shutting-down stage, and

stable flow after restarting (the shutting-down/restarting tests will be discussed in

Section 3.4). The images of the first two stages and the last stage were taken during

the flow conditions. To obtain a better visualisation of the morphology of the hydrate


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slurry at the shutting-down stage, the pictures of the third stage were taken after the

circulating pump had been stopped for over 1 h. As shown in Figure 6, the fluid was

in the form of a water-in-oil emulsion before natural gas hydrate appeared in the loop.

With the charge of the original natural gas, hydrate particles were observed through

the observation window, and the amount of hydrate particles formed at the beginning

stage increases obviously with the increase of the initial water cut. Although the

hydrate particles are heavier than the oil phase, the fluid system is homogeneously

distributed in the pipeline in the form of a slurry, which could fill the entire flow loop.

The variation of the morphology during hydrate formation is similar to that reported

by Peng et al. (2012) for (natural gas + diesel oil + water) systems containing an

anti-agglomerant comprised of sorbitan monolaurate and polymer esters in a mixing

ratio of 4:1. However, in contrast to the homogeneous hydrate slurry observed in this

work, even at the 30 vol% initial water cut, the heterogeneity becomes obvious with

increasing initial water cut in Peng et al.s work, especially for the systems with water

cuts of 20, 22, and 24 vol%.

After the circulating pump was stopped, the separation of the liquid phase and

solid phase occurred due to the difference in the density between gas hydrate and

diesel oil. The result is that the oil phase is at the top of the flow loop, while the

hydrate phase is at the bottom. This phenomenon can be clearly seen for the 5 vol%

water cut in Figure 6, indicating that the original water-in-oil emulsion structure was

destroyed when almost all water was converted into hydrate. This finding implies that

the hydrate slurry system investigated in this work is not an emulsion but a solid


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dispersion system. When the pump was restarted, the hydrate slurry returned to the

uniform dispersion state. From the analysis of the flow characteristics and

morphologies of hydrate slurry, it can be concluded that the oil and gas can be safely

transported by forming stable and flowable hydrate slurries.

3.3. Rheological properties of gas hydrate slurry

After the system stabilised after hydrate formation, rheological studies on the

hydrate slurries, which were composed of hydrate particles dispersed in a

hydrocarbon liquid phase, were performed using the flow loop and the capillary

(Ostwald) viscosimeter method. Several assumptions must be made before the hydrate

slurry rheology is evaluated using the capillary viscosimeter method. Hydrate slurries

must be considered as pseudo-homogeneous fluids circulating in a laminar regime in a

cylindrical pipe without any wall slip. The assumption of wall slip was not checked in

this work because it requires the use of different pipe sizes. The assumption of a

laminar regime will be discussed later. After introducing these assumptions, the flow

rate, shear stress, and shear rate can be represented at the wall by the Rabinowitsch

and Mooney equation (Metzner and Reed, 1955):

2

3 3 0

1 ww

w

Qd

R

= (1)

wherew

is the shear stress at the wall, which is related to the pressure drop by the

following expression:

4w

D P

L

= (2)

where D is the inside diameter of pipeline, L is the pipe length, and P is the

pressure drop.


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Differentiation of the Rabinowitsch and Mooney equation yields the following

expression of the shear rate at the wall:

8 3 1

4w

u n

D n

+

= (3)

where nis the behaviour index, defined as

ln

8ln

wd

nu

dD

= (4)

Combining eqs 2 and 3, the experimental measurements of the pressure drop and flow

rate can allow the rheological behaviour of the hydrate slurry to be established:

( )ww f

= (5)

According to eq 5, the relationship between the shear stress and shear rate allows

various fluid classes to be distinguished, such as Newtonian fluids ( is proportional

to

) and non-Newtonian fluids ( is not proportional to

).

For a given hydrate volume fraction system, the behaviour index n, consistency

index k, and yield stress 0 are identified based on the general Herschel-Bulkley (HB)

equation:

0

n

ww k

= + (6)

As noted by Anderson and Gudmundsson (2000), the apparent viscosity of the

hydrate slurry can be defined as the ratio between the shear stress and the shear rate at

the wall:

wapp

w

= (7)

To systematically investigate the rheological behaviour of hydrate slurry, broader

flow rates were examined in this work than in Peng et al. (2012), and the


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corresponding pressure drops were recorded simultaneously after the hydrate slurry

reached the stable state. Figure 7 shows an example of the rheological measurements

for a 5 vol% water cut after the slurry reaches the stable state, showing the various

flow rate plateaus and corresponding pressure drops.

According to eqs 2 and 4, the logarithmic relationships between8u

D and

4

D P

L

at different hydrate volume fractions are shown in Figure 8. The data could be

approximately regressed by a linear curve with a slope corresponding to the behaviour

index, n, denoting the difference from Newtonian behaviour. Figure 9 shows the

variation of the behaviour index with the hydrate volume fraction s . It can be

observed that n decreases with increasing s from 6.17 vol% to 34.88 vol%. The

following correlation for nas a function of s was established:

21.0000 0.4352 3.2395s sn = (8)

From Figure 9, it can be clearly seen that the behaviour index is always less than one

and decreases with increasing hydrate volume fraction, meaning that the hydrate

slurry exhibits a more typical non-Newtonian behaviour with increasing hydrate

volume fraction.

Figure 10 represents the relationship between shear stress w , obtained from eq

2, andn

w , deduced from eqs 3 and 4. The shear stress tends to zero when the value

of nw decreases to zero at different hydrate volume fractions from 6.17 to 34.88

vol%. According to the HB model (eq 6), the experimental points can be modelled by

a linear curve, where the consistency index kis the slope and the yield stress 0 is

the ordinate at the origin. Based on the regression results, it can be found that the


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yield stress is negligibly different from zero:

0 0 = (9)

Figure 11 represents the evolution of the consistency index k as a function of

hydrate volume fraction obtained from the experimental data. The correlation of the

consistency index k as a function of the solid volume fraction obtained from the

experimental data is given by the following expression:

2exp( 4.7798 0.2777 24.3751 )s s

k = + + (10)

As shown in Figure 11, the consistency index k grows exponentially with the hydrate

volume fraction to a given extent. This index increases rapidly when the hydrate

volume fraction is higher than 18.07 vol%, which means that the apparent viscosity of

the hydrate slurry increases significantly under the same conditions. This

phenomenon is similar to that observed by Peng et al. (2012) and Clain et al. (2012).

Furthermore, the apparent viscosity of the hydrate slurry can be expressed as

follows, based on eqs 6 to 10,

20.4352 3.2395

2exp( 4.7798 0.2777 24.3751 )s s

app s s w

= + + (11)

The experimental apparent viscosities of the hydrate slurry at different hydrate

volume fractions and shear rates determined from eqs 2, 3, 4, and 6 and the apparent

viscosity predictions obtained from eq 11 were compared and are presented in Figure

12. Figure 12 clearly shows that there is a good agreement between the experimental

data and model predictions for all hydrate volume fractions. In general, the apparent

viscosities of the hydrate slurry decrease with increasing shear rate, meaning that the

hydrate slurry fluid is a pseudoplastic fluid with a shear-thinning behaviour in this


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work. However, for hydrate volume fractions of 6.17 vol% and 12.2 vol%, the

apparent viscosities are always lower than 8.0 mPas and decrease slightly with

increasing shear rate from 120 to 350 s-1, meaning that the shear-thinning behaviour is

not obvious for these two hydrate volume fractions. This phenomenon can be

attributed to the behaviour indexes (see Figure 9) of these two hydrate volume

fractions being so close to unity that the non-Newtonian behaviour (shear-thinning) is

not obvious.

3.4. Shutting-down/restarting tests

It is well known that hydrate plugs can occur easily in deep-sea multi-phase

transmission pipelines in the shutting-down and restarting stages. Therefore, tests

using a long shutting-down period are essential for investigating the flow

characteristics of hydrate slurry. In this work, three shutting-down times, 2 h, 4 h, and

8 h, were applied for each run, which are much longer than the shutting-down times

investigated by Peng et al. (2012).Table 3 lists the flow rates and pressure drops at

seven different stages for different initial water cuts: stable flow before shutting down,

restarting after shutting down for 2 h, stable flow after shutting down for 2 h,

restarting after shutting down for 4 h, stable flow after shutting down for 4 h,

restarting after shutting down for 8 h, and stable flow after shutting down for 8 h.

Table 3 clearly shows that hydrate slurry in the presence of anti-agglomerant can be

easily and safely restarted after spending a long time in the static state in all

experiments. The restarting effect on the hydrate slurry becomes more obvious as the

shutting-down time increases from 2 h to 8 h for a given hydrate volume fraction


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system. For instance, the pressure drop and flow rate of the hydrate slurry at the

restarting stage after shutting down for 2 h for a hydrate volume fraction of 12.2% are

10.51 kPa and 1.30 m3/h, respectively, while the pressure drop and flow rate increase

to 11.12 kPa and 1.32 m3/h at the restarting stage after shutting down for 8 h. In

addition, the restarting effect also becomes more obvious with increasing hydrate

volume fraction under the same shutting-down time. Compared with the hydrate

volume fraction of 12.2% mentioned above, the pressure drop and flow rate at the

restarting stage after shutting down for 2 h and 8 h for a hydrate volume fraction of

34.88% are 11.56 kPa and 1.26 m3/h and 14.13 kPa and 1.37 m3/h, respectively.

Therefore, the hydrate slurry studied in this work presents a typical thixotropic

behaviour, which becomes more obvious with the increase of the hydrate volume

fraction. This phenomenon can be attributed to the microscopic structure of the

hydrate slurry. When the hydrate slurry is subjected to a long shutting-down period, a

netted texture may form due to the adhesive force between hydrate particles. When

the pump is restarted, the netted structure will be destroyed. However, it may take

some time to build the new stable inner structure because of the adhesive force

between the hydrate particles. This time can be regarded as the reason that the hydrate

slurry investigated in this work exhibits thixotropic behaviour after a long

shutting-down time.

As discussed in Section 3.3, the results mentioned in this work are valid when

two assumptions are satisfied: laminar flow and no wall slip. According to the method

used by Clain et al. (2012), in which the TBPB hydrate slurry was verified to be in the


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laminar regime by the determination of the Metzner-Reed Reynolds number ReMR

(Metzner and Reed, 1955) and the Fanning friction factor f, the first assumption can

be validated. The Metzner-Reed Reynolds number is determined from the behaviour

index nand consistency index k:

2

1

Re1 3

( ) 84

n n

HSMR

n n

D U

nk

n

=+

(12)

If the fluids are in laminar regime, the Fanning factor and the Metzner-Reed Reynolds

number can be correlated using the Hagen-Poiseuille equation regardless of whether

the fluid exhibits Newtonian or non-Newtonian behaviour:

16

ReMR

f = (13)

The classical expression of the regular Fanning factor contributions as a function of

the fluid velocity and pressure drop can be written as follows:

22HS

D P

f LU

= (14)

Figure 13 presents the relationship between the Fanning factor and the

Metzner-Reed Reynolds number for hydrate slurry with different hydrate volume

fractions. The experimental data obtained from eq 14 are in good agreement with the

prediction of the Hagen-Poiseuille equation, which indicates that the hydrate slurries

investigated in this work are all in the laminar regime.

4. Conclusions

A dynamic loop was adopted to investigate the flow characteristics and

rheological properties of gas hydrate slurry in the presence of anti-agglomerant, where


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the initial water cuts range from 5.0 to 30.0 vol%, and the shear rates range from 50 to

350 s-1. The experimental results demonstrate that the hydrate slurry exhibits a

shear-thinning behaviour and is a pseudoplastic fluid. The slurrys non-Newtonian

behaviour becomes more obvious with increasing hydrate volume fraction. An

empirical HB-type equation that included solid fraction dependency was used to

describe the rheological behaviour of the gas hydrate slurry. The apparent viscosities

of the hydrate slurry with different hydrate volume fractions were determined by the

new model and were in good agreement with the experimental data. The

shutting-down/restarting tests indicate that the hydrate slurry exhibits obvious

thixotropic behaviour. Based on the analysis of the flow characteristics and

morphologies of the hydrate slurry, the oil and gas can be safely transported by

forming stable and flowable hydrate slurries.

Acknowledgements

The financial support received from National 973 Project of China (No.

2012CB215005) and National Natural Science Foundation of China (Nos. 20925623,

U1162205, 51376195) are gratefully acknowledged.

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Table 1. Composition of the diesel oil used

component mol% wt%

heptanes 0.50 1.05

octanes 0.50 0.92

nonanes 2.81 4.60

decanes 7.74 11.40

undecanes 8.74 11.73

dodecanes 9.95 12.24

tridecanes 8.74 9.94

tetradecanes 6.53 6.90

pentadecanes 4.92 4.86

hexadecanes 4.72 4.37

heptadecanes 5.33 4.64

octadecanes 6.83 5.63

eicosanes 14.47 10.74

tetracosanes 15.78 9.77

octacosanes plus 2.41 1.28

total 100.00 100.00


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Table 2. Composition of the original natural gas used

component mol%

methane 85.41

ethane 6.01

propane 5.79

i-butane 0.16

n-butane 0.05

i-pentane 0.01

n-pentane 0.02

carbon dioxide 0.02

nitrogen 2.53

Total 100.00


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Table 3. Variation of the flow rate and pressure drop during shutting-down/restarting

tests for different initial water cuts

stage hydrate volume fraction ( vol%)

6.17 12.2 18.07 23.81 29.4 34.88stable flow before

shutting down

pressure drop (kPa) 7.97 8.32 8.56 9.25 9.91 9.98

flow rate (m3/h) 1.25 1.23 1.23 1.21 1.19 1.15

restart after 2h

shutting down


flow rate (m3/h) 1.29 1.30 1.26 1.30 1.25 1.26

stable flow after

restarting


flow rate (m3/h) 1.28 1.23 1.20 1.19 1.16 1.14

restart after 4h

shutting down


flow rate (m3/h) 1.26 1.25 1.26 1.34 1.31 1.32

stable flow after

restarting


flow rate (m

3

/h)1.25 1.21 1.22 1.21 1.15 1.15

restart after 8h

shutting down


flow rate (m3/h) 1.42 1.32 1.36 1.42 1.35 1.32

stable flow after

restarting


flow rate (m3/h) 1.27 1.22 1.21 1.23 1.16 1.13


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Figure Captions

Figure 1. Schematic of hydrate flow-loop system.

Figure 2. Morphologies of natural gas hydrate slurry formed in the high pressure

sapphire cell with six different initial water cuts when at 274.2 K and 7.50

MPa.

Figure 3. Variation of the fluid flow rate and pressure drop of the hydrate slurry with

the elapsed time at 5 vol% water cut.

Figure 4. Variation of the fluid flow rate and pressure drop of the hydrate slurry within

the first 5 h at 5 vol% water cut.

Figure 5. Variation of temperature of the hydrate slurry with time at different initial

water cut.

Figure 6. Morphologies of the emulsion or the hydrate slurry at different water cuts

and stages.

Figure 7. Variation of pressure drop by adjusting the flow rate during the

measurement of the rheological behaviour of the hydrate slurry when at 5

vol% water cut.

Figure 8. Logarithmic relationship between 8 avu D

and 4D P L for the hydrate

slurry at different hydrate volume fractions.

Figure 9. Behaviour index as a function of the hydrate volume fraction.

Figure 10. Shear stress w as a function of shear raten

w

for hydrate volume

fractions from 6.17 to 34.88 vol%.

Figure 11. Variation of consistency index with the hydrate volume fraction.

Figure 12. Comparison of apparent viscosity of the hydrate slurry between

experimental data and model prediction for different hydrate volume fractions.Figure 13. Variation of friction factor of the hydrate slurry as a function of Reynolds

number for different hydrate volume fractions.


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mixingtank

Data AcquistionSystem

ScanningThermometer

Circulating Bath

Hydrate Tube Swagelok Union

U-bend

Flowmeter

Differential

Pressure Transducer

Pump

Cooling System Gas Cylinder

Recirculation Tube

P

Resistance thermocouple detector

Visual Window

Figure 1. Schematic of hydrate flow-loop system.


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5 vol% water cut 10 vol% water cut 15 vol% water cut

20 vol% water cut 25 vol% water cut 30 vol% water cut

Figure 2. Morphologies of natural gas hydrate slurry formed in the high pressure

sapphire cell with six different initial water cuts when at 274.2 K and 7.50

MPa.


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0 5 10 15 20 250.0

0.3

0.6

0.9

1.2

1.5

1.8

Flow

rate(m3/h)

Time (h)

hydrate formation flow rate

Pressure drop

Pressuredrop(kPa)

2

4

6

8

10

12

Figure 3. Variation of the fluid flow rate and pressure drop of the hydrate slurry with

the elapsed time at 5 vol% water cut.


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0 1 2 3 40.0

0.3

0.6

0.9

1.2

1.5

1.8

Time (h)

hydrate formationflow rate

Pressure drop

Pressuredrop(kPa)

Flow

rate(m3/h)

3

6

9

Figure 4. Variation of the fluid flow rate and pressure drop of the hydrate slurry within

the first 5 h at 5 vol% water cut.


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0 1 2 3 4

274

275

276

277

278

hydrate formation

5 vol%

10 vol%

15 vol%

20 vol%

25 vol%

30 vol%

Time (h)

Temperature(K)

initial water cut

Figure 5. Variation of temperature of the hydrate slurry with the time at different

initial water cut.


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5 vol% water cut

10 vol% water cut

15 vol% water cut

20 vol% water cut

25 vol% water cut

30 vol% water cut

Before the formation Stable flow Shutting down After restarting

Figure 6. Morphologies of the emulsion or the hydrate slurry at different water cuts

and stages.


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35

19.6 20.0 20.4 20.8 21.2

0.5

1.0

1.5

flow rate

Pressure drop

Pressuredrop(kPa)

Flowrate(m3/h)

Time (h)

3

6

9

12

Figure 7. Variation of pressure drop by adjusting the flow rate during the

measurement of the rheological behaviour of the hydrate slurry when at 5

vol% water cut.


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3.3 3.6 3.9 4.2 4.5 4.8 5.1 5.4 5.70.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

ln(DP/4/L)

Hydrate volum fraction6.17 vol%

12.20 vol%

18.07 vol%

23.81 vol%

29.41 vol%

34.88 vol%

Regressed

ln(8uav/D)

Figure 8. Logarithmic relationship between 8 avu D

and 4D P L for the hydrate

slurry at different hydrate volume fractions.


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37

5 10 15 20 25 30 35 400.4

0.5

0.6

0.7

0.8

0.9

1.0

Hydrate volume fraction (vol%)

Experimental dataRegressed line

Behaviourindex

Figure 9. Behaviour index as a function of the hydrate volume fraction.


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0 50 100 150 200 250 300 350

0

1

2

3

4

5

66.17 vol%

12.20 vol%

18.07 vol%23.81 vol%

29.41 vol%

34.88 vol%

Regressed line

Hydrate volume fraction

w

(Pa)

nw

(s-n

)

Figure 10. Shear stress w as a function of shear rate

n

w

for hydrate volume

fraction from 6.17 to 34.88 vol%.


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39

5 10 15 20 25 30 35 40

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

Experimental DataRegressed line

Consistencyindexk(Pa.s

n)

H drate volume fraction (vol%

Figure 11. Variation of consistency index with the hydrate volume fraction.


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40

40 80 120 160 200 240 280 320 360

6

8

10

12

14

16

18

20

22

w

app(

mPas)

(s-1)

Hydrate volum fraction

6.17 vol%

12.20 vol%

18.07 vol%

23.81 vol%

29.41 vol%

34.88 vol%

Model line

Figure 12. Comparison of apparent viscosity of the hydrate slurry between

experimental data and model prediction for different hydrate volume fractions.


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41

1 10 100 10001E-3

0.01

0.1

1

10

100

6.17 vol%

12.20 vol%

18.07 vol%

23.81 vol%

29.40 vol%

34.88 vol%

16/ReMR

Fanningfactor

ReMR

Hydrate volume fraction

Figure 13. Variation of friction factor of the hydrate slurry as a function of Reynolds

number for different hydrate volume fractions.


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42

Higlights

Hydrate slurry presents obvious shearthinning behaviour with increase ofhydrateratio.

HydrateslurryistransportedasasoliddispersionsystemwithadditionofAAs. AHerschelBulkley typeequationwasbuiltby considering thehydratevolume

fraction. Shuttingdown/restarting testsshow that thehydrate slurry iseasilyandsafely

restarted. Hydrate slurry exhibits obvious thixotropic behaviour with increasing

shutting

down

time.

Documents

Flow Characteristics and Rheological Properties of Natural Gas Hydrate Slurry in the Presence of Anti Agglomerant in a Flow Loop Apparatus