flow characteristics and rheological properties of natural gas hydrate slurry in the presence of...
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Flow Characteristics and Rheological Properties of Natural Gas Hydrate Slurry in the Presence of Anti Agglomerant in a Flow Loop Apparatusby Yan Et Al and Li Tao_(2013)_TRANSCRIPT
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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
pressure drop (kPa) 10.20 10.51 10.58 10.91 10.97 11.56
flow rate (m3/h) 1.29 1.30 1.26 1.30 1.25 1.26
stable flow after
restarting
pressure drop (kPa) 7.79 8.35 8.50 9.17 9.55 9.31
flow rate (m3/h) 1.28 1.23 1.20 1.19 1.16 1.14
restart after 4h
shutting down
pressure drop (kPa) 10.11 10.75 11.21 11.33 11.95 12.15
flow rate (m3/h) 1.26 1.25 1.26 1.34 1.31 1.32
stable flow after
restarting
pressure drop (kPa) 7.91 8.34 8.49 9.32 9.47 9.27
flow rate (m
3
/h)1.25 1.21 1.22 1.21 1.15 1.15
restart after 8h
shutting down
pressure drop (kPa) 10.49 11.12 12.63 13.49 13.75 14.13
flow rate (m3/h) 1.42 1.32 1.36 1.42 1.35 1.32
stable flow after
restarting
pressure drop (kPa) 7.98 8.34 8.72 9.28 9.73 9.93
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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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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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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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 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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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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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.