Investigating Gas Hydrate Formation in Moderate to High Water Cut

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Investigating gas hydrate formation in moderate to high water cut crude oil containing Arquad and salt using differential scanning calorimetry Chenwei Liu, Mingzhong Li, Vishal K Srivastava, and Carolyn A. Koh Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02433 • Publication Date (Web): 31 Mar 2016 Downloaded from http://pubs.acs.org on April 5, 2016

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Investigating gas hydrate formation in moderate to high water cut crude oil containing Arquad and salt using differential scanning calorimetry Chenwei Liu1,2, Mingzhong Li1*, Vishal K Srivastava2, Carolyn A. Koh2* 1

College of Petroleum Engineering, China University of Petroleum, Qingdao 266580, China 2

Center for Hydrate Research, Chemical and Biological Engineering Department, Colorado School of Mines, Golden, CO 80401, USA

ABSTRACT Gas hydrate formation can result in blockage of deepwater flowlines, leading to severe economic and safety risks. As oil and gas production moves to greater water depths, the operating conditions of high pressure and low temperature can lead to greater risks of hydrate formation in the flowlines. Additionally, as the fields mature, the continuous increase of water cut further enhances the potential problems associated with hydrate formation. With these more challenging production conditions, anti-agglomerants could offer an economical and environmentally attractive alternative for preventing hydrate plug formation. However, reported work on hydrate anti-agglomerant behavior for moderate to high water content systems is quite limited. The work reported in this paper investigates the effects of Arquad 2HT-75 (used as a model anti-agglomerant) and salt (NaCl) on water-in-oil emulsions, and explores the gas hydrate formation characteristics for moderate to high water content (50 and 75 vol.%) crude oil using a high pressure differential scanning calorimeter (DSC). The results indicate that the formation and dissociation of both ice and hydrate could lead to destabilization of the water-in-oil emulsion at certain conditions. At high water cut (75 vol.%), hydrate conversion is much lower due to mass transfer limitations for hydrate formation. Furthermore, the DSC and bottle tests also suggest that, at the concentrations and conditions used in 1

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this work, the addition of salt to Arquad 2HT-75 can help to form stable water-in-oil emulsions.

Keywords: gas hydrate, anti-agglomerant, differential scanning calorimeter, hydrate conversion, water-in-oil emulsion

1. INTRODUCTION Gas hydrates are crystalline clathrate compounds comprised of hydrogen-bonded water cavities, which encapsulate suitably sized gas molecules at high pressures and low temperatures. The high pressure and low temperature conditions in deep water oil production flowlines provide favorable conditions for gas hydrate formation. The accumulation and aggregation of hydrates in flowlines can lead to flowline blockage, resulting in potential operational failures. As gas and oil exploration and production move to ultra-deep water (>8000 ft. of sea water), the risk of hydrate blockage increases significantly [1]. Under these conditions, traditional thermodynamic prevention methods of hydrate avoidance become economically unacceptable. In contrast, new low-dosage hydrate inhibitors (LDHIs) [1-4] provide a more economical and environment friendly alternative.

In the oil-dominated system, due to the turbulence in flow in flowlines, water can be dispersed into the oil phase, creating a water-in-oil (W/O) emulsion [5]. Over the past decade, a conceptual model that depicts hydrate plug formation in W/O emulsions has been developed. The model suggests that hydrate agglomeration is a critical factor in plug formation [6-8]. Anti-agglomerants (AAs) represent one type of LDHI, and are surface active agents that allow small hydrate particles to form, but keep them dispersed in the oil [2,9-10]. As a consequence, preventing hydrate agglomeration is gradually becoming one attractive engineering strategy to reduce the risks of hydrate plugging in the deep offshore fields [11]. 2


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In less profitable and/or older wells, the amount of produced water increases significantly, in some cases up to 50-80 vol.% water cut [12-14]. In these cases, the operational/economic feasibility of using thermodynamic hydrate inhibitors (e.g. methanol, glycol) to avoid hydrate formation is reduced further, and AAs are expected to offer a more economical alternative

[15,16]

. However, the performance of AAs in high water cut systems is still

underexplored [16,17]. In previous work, we studied gas hydrate formation from moderate to high water cut systems containing Arquad 2HT-75 surfactant and salt using a high pressure autoclave cell [17]. The purpose of this study is to investigate hydrate formation in moderate to high water cut under quiescent (cf. shut-in) conditions. Hydrate formation under shut-in conditions is of critical importance, but not well understood. The effects of Arquad 2HT-75, NaCl and their combination on water-in-oil emulsions were evaluated and the gas hydrate formation characteristics were investigated using a high pressure differential scanning calorimeter.

2. EXPERIMENTAL METHODS and MATERIALS 2.1 Apparatus The majority of experiments were performed in a high pressure micro-differential scanning calorimeter (µ-DSC VIIa, Setaram Inc.). The µ-DSC could be used to measure the thermal properties of ice and hydrate in a water-in-oil (W/O) emulsified system at both atmospheric and pressurized conditions

[18]

. In a high pressure

µ-DSC, the pressure ranges from 1 to 400 bar[19], and is controlled by a gas pressure panel which is connected to a hand pump. The thermocouples in the calorimeter furnace measure the temperature difference between the reference and sample cells, and the necessary heat to achieve a zero temperature difference between the cells is recorded [20].

In this work, a Cyclone IQ2 Microprocessor Controlled Homogenizer (VirTis Co.) was used for preparing 3


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emulsions

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[21]

. The homogenizer consists of a control box and a mixing unit that provides a mixing range from

5000 to 30,000 RPM. The homogenizer shaft comprises a 10 mm rotor/stator shaft assembly.

2.2 Materials The crude oil used is a low API gravity oil with a viscosity of 100 ±1.5 cP at 20 ºC and 100 s-1 and a density of 0.908 ± 0.003 g/cm [24]. The interfacial tension value for the crude oil-water is 28.9 ± 0.4 mN/m under atmospheric conditions. The oil has a saturated, asphaltene, resin, and aromatic (SARA) content of 49.1 wt.%, 1.7 wt.%, 20.2 wt.%, and 29 wt.%, respectively.

The liquid phase consists of deionized water or brine solution containing 3.5 wt.% sodium chloride pre-dissolved in

deionized

water.

The

model

anti-agglomerant,

Arquad

2HT-75

(75%

dehydrogenated

tallow

dimethylammonium chloride in water/isopropyl alcohol, Sigma-Aldrich®, 88-92% purity) was selected due to its similarity with commonly used commercial anti-agglomerants, which are typically quaternary ammonium salts [23] (Figure 1).

Figure 1. Structure of model anti-agglomerant Arquad 2HT-75 (n = 12-18)

2.3 Procedures Emulsion preparation method Crude oil (15 or 30 ml) was firstly put in a glass beaker. If Arquad 2HT-75 was used in the emulsion, then the Arquad 2HT-75 (2 wt. %) was measured and added to the oil. The Arquad 2HT-75 was dissolved in the crude oil using a magnetic stirrer. The oil and Arquad solution was heated to about 50°C to ensure full dissolution of the 4


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surfactant in the crude oil. The beaker with the oil was then placed under the homogenizer and the impeller was set at 8000 RPM. In this study, 75 vol.% of water was considered a high water cut (i.e. WC), while 50 vol.% was regarded as a moderate water cut. For 50 vol.% WC emulsions, the mixtures were stirred for 3 min, and water/ brine solution was added dropwise using a syringe in the first minute. For 75 vol.% WC emulsions, the mixtures were stirred for 6 min, while the water/ brine solution was added during the first 4 min (a longer stirring time was required to avoid phase inversion). In all cases, 60 ml emulsion samples are prepared.

Bottle tests on emulsion stability Phase separation was monitored daily by classic bottle tests for a period of 6 days

[5]

. The emulsion was

considered stable if no phase separation was observed at about 24 hours. If phase separation occurred, then the emulsion was considered unstable. Bottle tests were performed at room temperature (~22 °C).

Differential Scanning Calorimetry (DSC) tests Two different modes were used in the DSC tests: ambient pressure and high pressure.

For ambient pressure DSC tests, ~15 mg of an emulsion sample was measured with a precision electronic balance (TB-215D, DENVER INSTRUMENT; accuracy: 0.01 mg) and then placed in an ambient sample vessel. The sample was cooled from 30 °C to -45 °C at a rate of 0.3 °C/min. The sample was then heated back to 30 °C at the same rate and this cooling-heating cycle was repeated three times. The ambient pressure mode designed for ice formation had two purposes. First, the mean droplet size of the emulsion can be approximately determined through the nucleation temperature of the ice

[24]

, and the second, the effect of ice formation/dissociation on the

emulsion stability could be compared with the effects of hydrate formation/dissociation on emulsion stability.

5


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In the high pressure tests, a similar amount of emulsion sample is added into the high pressure sample vessel. Methane gas (99.99% ultra high purity, General Air) was then introduced into the vessel at a pressure of 125 bar. The emulsion sample was held at 125 bar and 30 °C for 3 h to allow saturation of the emulsion with methane [5]. Next, the sample was cooled from 30 °C to -20 °C at a rate of 0.3 °C/min and then heated at a rate of 0.3 °C/min to 30 °C. Similarly, the cooling-heating cycle was repeated three times to determine the effect of hydrate formation/dissociation on emulsion stability, by quantifying any changes in the integrated area (under the exothermic/endothermic peak) as compared to the first trial

[21]

. Due to the temperature change and the hydrate

formation/dissociation, the fluctuations in cell pressure were observed within ±1 bar.

For each sample, the DSC test was conducted at least two times. Although the randomness of hydrate/ice nucleation onset makes the thermograms show certain differences, the changes in the endothermic /exothermic peak with each testing cycle show good consistency. The average relative variation of initial hydrate/ice conversion ratio (corresponding to the first cycle) was within 10%.

3. RESULTS AND DISCUSSION 3.1 Emulsion stability by bottle tests Table 1 shows the results of the bottle tests (see Figure S2), which suggest that NaCl, Arquad 2HT-75 and their combination can help the mixture to form a more stable water-in-oil emulsion. However, the ability of Arquad 2HT-75 alone to stabilize the water-in-oil emulsion is limited, similar to some other reported quaternary anti-agglomerants[9]. By contrast, the natural surfactants (e.g. asphaltenes, acids, and resins) present in crude oils can have a significant effect on the stability of the emulsion (see reference [17]). In addition, it is worth noting that the 75 vol.% WC, 3.5 wt.% salt + 2 wt.% Arquad 2HT-75 emulsion appeared solid-like and difficult to transport. 6


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Table 1 Summary of bottle test results Sample

50 vol.% WC

Bottle test result

Blank

Stable (free water was observed 2 days later)

+3.5 wt.% salt

Stable (no free water was observed within 6 days)

+2 wt.% Arquad 2HT-75

Stable (free water was observed 5 days later)

+3.5 wt.% salt+2 wt.% Arquad 2HT-75

75 vol.% WC


Blank

Unstable (free water was observed just after emulsification)

+3.5 wt.% salt


+2 wt.% Arquad 2HT-75

Unstable (free water was observed within several hours after emulsification)

+3.5 wt.% salt+2 wt.% Arquad 2HT-75


(Note: the interfacial tensions of the samples are shown in Figure S1.)

3.2 Differential Scanning Calorimetry (DSC) tests Ice formation/ dissociation

Droplet size is important for the characterization of an emulsion and can affect various properties (e.g. emulsion stability, rheological properties, optical properties etc.) Dalmazzone et al.

[24]

and Montengro et al.

[26]

[25]

. From nucleation theory and experimental studies,

showed that a lower nucleation temperature is required for a

smaller mean water droplet size in an emulsion system. This further suggests that the knowledge of the droplet freezing temperatures is a simple method to obtain information on the droplet sizes. Therefore, DSC could present a useful and simple tool to determine approximate mean emulsion droplet size, and shows unique advantages for the concentrated and opaque emulsions, as compared with other commonly used techniques such as optical 7


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microscopy [27].

According to the ambient pressure thermograms of the emulsion systems (see Figure S3), the changes in the nucleation temperatures of water droplets with repeat cycles indicate that ice formation/dissociation could destabilize water in oil emulsions. This phenomenon was also observed by visual bottle tests with ice formation/dissociation (see Figure S4). For emulsions with 75 vol.% WC, as mentioned in section 3.1, the blank emulsion and the emulsion with 2 wt.% Arquad 2HT-75 were observed to be very unstable, so both samples could not be used for these DSC tests [28].

Hydrate formation/ dissociation

50 vol.%WC

Figure 2(a) shows the high-pressure thermogram for the 50 vol.% WC blank emulsion. In the three cycles, one exothermic peak appeared upon cooling and another smaller exothermic peak appeared upon the beginning of heating corresponding to hydrate formation. In the heating portion, there is only one hydrate dissociation peak at about 15 °C, which is close to the reported dissociation temperature for methane hydrate at 125 bar (14.82 °C) [5]. It is seen that the intensity of the hydrate dissociation peak decreases progressively with each cycle, i.e. the amount of hydrate formation decreases with each subsequent cycle. This has been previously attributed to possible agglomeration of hydrate particles upon hydrate formation/dissociation, resulting in an increase in larger droplets and consequently a decrease in the total water droplet surface area (see Figure 3), which would be available for subsequent hydrate formation [21].

Figure 2(b) represents the high-pressure thermogram for a 50 vol.% WC emulsion with 3.5 wt.% salt. In cycle 1, hydrate begins to form upon cooling and continues to form during the heating portion, and is then followed by a large endothermic peak, corresponding to hydrate melting. While in cycle 2 and cycle 3, hydrate only formed 8


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15

(a)

0

-20

-18

Heat flux(mW)

Heat flux(mW)

2

5

(b)

cycle 1 cycle 2 cycle 3

4

10

-16

0


0

(onset) 2

-5 -5

-10

1

-20

-15

-20

-10

0 10 Temperature(℃)

20

-20

30

-10

-12

0

10

20

30

15

(c)


(d)

cycle 1 cycle 2 cycle 3 cycle 4 cycle 5 cycle 6

10 Heat flux(mW)

10 Heat flux(mW)

-16

Temperature(℃)

15

5 0

-5

5 0

-5

-10 -10

-15

10

-20

-10


20

-20

30


(e)

-10


20

30


(f) 5 Heat flux(mW)

5 Heat flux(mW)

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0 2

0

1

-5

0

-10 -15

-1 -2

-10

0

5

0

10

-5

15

10

20

30

-20

-10

Temperature(℃ )


20

30

Figure 2. High pressure thermograms (125 bar) during hydrate formation and dissociation for 50 vol.% WC emulsions with (a) blank, (b) 3.5 wt.% salt, (c-e) 2 wt.% Arquad 2HT-75 and (f) 3.5 wt.% salt + 2 wt.% Arquad 2HT-75. 9


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Hydrate

Water droplet Yes Agglomeration No

Water Emulsion

Emulsion after

Hydrate formation

hydrate dissociation Figure 3. Mechanism for hydrate agglomeration in the DSC after successive cycles (redrawn from Lachance et al.[ 21]).

during the cooling portion and dissociated upon heating. As compared with high-pressure thermograms for the blank sample, upon increasing cycles, the intensity of the peak decreases during hydrate melting, and the onset of hydrate melting shifts to higher temperature. Karanjkar et al.[29] investigated the formation of cyclopentane (CP) hydrates in water-oil-emulsions with or without surfactant (span 80). By direct observations in the single-drop experiment, they found that the event of CP hydrate formation in the absence of surfactants can be described in three steps: (i) hydrate nucleation, nucleation occurs randomly at the water-oil interface, (ii) lateral growth, hydrate crystals grow along the interface leading to a thin faceted crust/shell around the water droplet, (iii) radial growth, once a complete layer of hydrate covers the drop, diffusion of CP through the hydrate shell towards the bulk water will determine the rate of radial growth. In the presence of surfactants (span 80), it was observed that radial hydrate growth follows lateral growth without a complete hydrate shell formation and a ‘mushy’ porous structure with needle-like crystals is formed in the interface. It is evident that a higher hydrate conversion leads to 10


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a higher salt concentration in the inside residual water. According to the methane hydrate equilibrium temperature curve shown in Figure 4 (CSMGem[1]), a higher salt concentration leads to a lower hydrate equilibrium temperature. Therefore, during the heating period, the dissociation temperature of hydrate in contact with the interior brine solution would be much lower than that of the hydrate layer at the outside surface. So the dissociation could begin from the hydrate in contact with interior brine solution at higher salt concentration. For the 50 vol.% WC emulsion with salt, possibly due to the agglomeration of water droplets/particles during hydrate formation/dissociation, the hydrate conversion (i.e. integrated area of peak) decreases with each cycle. Using the integrated peak area from the thermograms, the hydrate conversions for the three cycles are estimated at about 72%, 12% and 8%. The salt concentrations in the residual water could be estimated at 11.5%, 4.0% and 3.8%, and the corresponding hydrate dissociation temperatures are determined at ~9.0°C, 13.0°C and 13.2°C (using CSMGem

[1]

) respectively (see Figure 4). These show relatively good agreement with the hydrate dissociation

temperatures in Figure 2(b). This could support the hypothesis that hydrate dissociation may initiate from the hydrate layer in contact with the interior brine solution at higher salt concentration.

Phenomenon in Figure 2(b) has important applications in hydrate research for salt samples using DSC. For the salt system, the original salt concentration increases due to hydrate formation. Consequently, this procedure cannot be used to accurately obtain the hydrate dissociation temperature for the original salt concentration using direct measurements. It is better to use a step-wise procedure to directly measure the hydrate dissociation temperature for these salt-containing systems cyclopentane emulsion

[30]

. Zylyftari et al. have studied the effect of aqueous phase salinity on a

[31]

. With a similar temperature ramp (0.2 °C/min) and DSC apparatus, accurate

dissociation temperatures for CP hydrate in the presence of brine without surfactants (span 80) were obtained by direct measurement. The reason can be explained in Zylyftari’s work, the water conversions for the brine solution without surfactant are quite low, most of the water conversions are lower than 2.0%. Therefore, the concentrations 11


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of salt and cyclopentane show insignificant changes and the corresponding hydrate dissociation peaks are quite small and narrow, allowing accurate dissociation temperatures to be obtained by direct visualization of the dissociation peaks. The step-wise procedure allows the salt concentration to decrease gradually back to the original concentration and enables the determination of hydrate dissociation temperature more precisely for systems containing salts.

CH4 hydrate equilibrium temperature,℃

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20 Hydrate

16 12 Cycle 3 Cycle 2

8

Cycle 1

4 0 -4 -8 0

5

10

15

20

25

NaCl % mass

Figure 4. Methane hydrate equilibrium temperatures with different NaCl mass concentration at 125 bar. The red stars present the hydrate equilibrium temperature for the three cycles in Figure 2(b).

Figure 2(c) presents the high-pressure thermogram for 50 vol.% WC emulsion with 2 wt.% Arquad 2HT-75. This is a unique case among all reported here, in which hydrates formed again before melting. In order to obtain further information for this anomaly, the other two tests were conducted. In the former test, a high pressure DSC mode was used and the cooling-heating cycle was repeated for six times (Figure 2(d)). While in the latter test, the isothermal mode was used (Figure 2(e)), during which the sample was first cooled from 30 °C to -10 °C at 12


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0.3 °C/min, and then the sample was held at -10 °C for 8 h. Upon hydrate formation, the sample was heated to dissociate the hydrate. The cycle was repeated three times. Similar to Figure 2(c), hydrate also formed before melting in the two tests. The same phenomenon was also observed in a lower water content system (e.g. 30 vol.% WC, Figure S5(a)) with 2 wt.% Arquad 2HT-75.

As shown in Figures 2(c), (d) and (e), the final hydrate conversion included two independent parts: hydrate formation during cooling and before melting periods. By quantitatively comparing the intensities of these independent parts, the hydrate conversion during the cooling portion was only about 20 - 25 % (i.e. thinner hydrate film), and was found to be much less than that of all the other 50 vol.% WC emulsion systems (~50 -75 %, cycle 1) in the cooling portion. As suggested by many researchers

[1,29]

, the hydrate shell is porous and the

permeability of the hydrate crust/shell determines the transportation of hydrate former and consequently the hydrate conversion [1]. Based on the quantitative analysis, one possible explanation for the difference may be that a more dense (low permeability) hydrate shell seems to be formed during the cooling portion due to the addition of Arquad 2HT-75 which prevents the further formation of hydrate. During the heating portion, due to the temperature gradually increasing to the hydrate melting point, and due to a lower thickness of the hydrate film, hydrate shell cracking could take place. This could facilitate a continued interaction of methane and water, thus permitting the hydrate formation to be sustained until the dissociation point is reached. This phenomenon will be investigated further in our future work. Furthermore, the intensities of the hydrate dissociation peaks show no significant changes with cycles. This is in contrast to the tests without Arquad 2HT-75. It suggests that the emulsion in the presence of Arquad 2HT-75 is stable during hydrate formation/dissociation under certain conditions.

Similarly, in Figure 2(b) and 2(f), the high pressure thermograms exhibit the same pattern for the 50 vol.% WC + 3.5 wt.% salt, and 50 vol.% WC + 3.5 wt.% salt + 2 wt.%. Arquad 2HT-75, as only hydrate formation/dissociation 13


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takes place. While the intensity of the hydrate dissociation peak decreases progressively for each of the cases depicted in 2(a), 2(b), 2(f), the decreasing peak intensity is more subtle for 2(f). It suggests that the emulsion with a combination of salt and Arquad 2HT-75 shows a relatively higher stability during hydrate formation/dissociation. Compared with the emulsion with 3.5 wt.% salt and emulsion with 3.5 wt.% salt + 2wt.% Arquad 2HT-75, which show higher initial stability in the bottle tests, the emulsion with 2 wt.% Arquad 2HT-75 shows lower hydrate agglomeration tendency (similar to that at low water cut, see Figure S5). It suggests that Arquad 2HT-75 is not creating a more stable initial water-in-oil emulsion to prevent hydrate agglomeration, which corresponds to its mediocre ability to stabilize emulsions. Similar conclusions were suggested for other quaternary anti-agglomerants[9,28]. DSC tests for the 50 vol.% water cut system with salt (3.5 wt.%) and Arquad 2HT-75 (2 wt.%) exhibited some hydrate agglomeration. York et al [28] have reported that Arquad 2C-75 becomes ineffective at forming hydrate slurries at low salt and relatively high surfactant concentration; these concentration ranges are 1-3 wt.% up to 6 wt.% NaCl and 1.0-1.5 wt.% Arquad 2C-75. One proposed hypothesis suggests the salt concentration (3.5 wt.%) may reduce the solubility of the Arquad surfactant in the aqueous phase to such an extent, that not enough is available to adsorb to the propagating hydrate crystals to form a steric barrier around the hydrate crystals[2,32], which then could result in agglomeration.

Recently，Karanjkar et al. reported that on a similar experimental design with cyclopentane hydrate-forming emulsions, upon cooling the emulsion, ice formation predominantly occurs rather than hydrate formation[29]. The peak for ice formation in their experiment occurred at ~ -15 °C to -20 °C, which is similar to the peaks observed upon cooling in Figure 2. There could be a couple of reasons that the exothermic peak observed upon cooling corresponds to hydrate formation. Firstly, in Figure 2, there is no endothermic peak at ~0 °C, this is the most common evidence to determine no ice formed in the test. In some cases, hydrate will form during the melting of ice and can offset some heat flux, which will decrease the intensity of ice endothermic peak. However, (i) hydrate 14


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formation from the melting of ice will lag behind the melting of ice and (ii) there is a relatively large difference in the heat of dissociation of ice and hydrate (ice: ~334.0 J/gm water, methane hydrate: ~504.07 J/gm water)[21]. Therefore, entirely offsetting any observable heat flux in all of our experiments is unlikely. In Karanjkar’s and Zylyftari’s work[31], initially ice formed upon cooling and hydrate formed upon melting ice, however, an ice endothermic peak was also observed in both reports. Similar observations were made in our previous work

[19]

.

Hydrate formation is limited by methane mass transfer while ice is not, which will induce a different endothermic peak shape. Usually, hydrate formation peaks are wider and shorter, and the intensity of the formation/dissociation peaks depend on the water-oil interface area. For an unstable emulsion, the intensity of the endothermic/dissociation peak (i.e. hydrate conversion) will decrease with cycles [21]. While, for ice formation, the formation peaks are usually narrower and with a high peak value and the intensity of the formation /dissociation peaks are almost constant with cycles, whether or not the emulsion is stable or unstable. In Figure 2, the formation peaks are shorter and wider (compared Figure 2(c), (d) with Figure S3(c)) and for unstable emulsions, the endothermic peaks all decrease with cycles (compared Figure 2(a) with Figure S3(a), Figure 2(b) with Figure S3(b) and Figure 2(f) with Figure S3(d)). The possible reasons for the difference between our work and Karanjkar’s work could be due to the different emulsion system and hydrate former. According to the ambient pressure thermograms for our emulsions, almost all of the ice nucleation temperatures of the initial emulsions (i.e. first cycle) are lower -20 °C，which means our emulsion systems have smaller droplet size, resulting in a lower ice nucleation temperature. Furthermore, in Karanjkar’s work, cyclopentane was used as the hydrate former. At ambient pressure, the dissociation temperature for pure cyclopentane hydrate is about 7.7°C[1]. At the temperature of ice formation (-12°C–-15°C), the subcooling for CP hydrate is about -20 °C. However, in our experiment, the hydrate dissociation temperature is about 15°C. Thus, within the temperature range of -12°C to -15°C, the subcooling for methane hydrate is about -30°C, which may lead to hydrate forming predominantly or earlier than 15


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ice formation. As evidenced in our experiments, where both hydrate and ice formed in the cycle, hydrate always formed earlier than ice formation.

75 vol.% WC

Similar to the ambient pressure test, only 75 vol.% WC emulsions with 3.5 wt.% salt and 3.5 wt.% salt + 2 wt.% Arquad 2HT-75 are presented in Figure 5.

As shown in Figure 5(a), the high-pressure thermograms for 75 vol.% WC + 3.5 wt.% salt emulsion are similar to that of 50 vol.% WC +3.5 wt.% salt emulsion. The hydrate forms/dissociates during the cooling/ heating portion and the intensity of hydrate melting peak decreases progressively with each successive cycle.

Figures 5(b-d) show the high-pressure thermograms for 75 vol.% WC + 3.5 wt.% salt + 2 wt.% Arquad 2HT-75 emulsion systems during hydrate formation and hydrate dissociation using both scanning (b and c) and isothermal mode (d) methods. The results suggest that the intensity of the dissociation peak increases and/or does not change significantly with each successive cycle. The high-pressure thermograms for 75 vol.% WC +3.5 wt.% salt + 2 wt.% Arquad 2HT-75 emulsion are different from that for 50 vol.% WC + 3.5 wt.% salt + 2 wt.% Arquad 2HT-75 emulsion. The hydrate conversion of the former sample (~2 days) and initially have good dispersion. On this premise, during hydrate formation/dissociation, the emulsion with 2 wt.% Arquad 2HT-75 shows much lower agglomeration tendency than the emulsions without Arquad 2HT-75 (see Table 2). Further suggesting that Arquad 2HT-75 can prevent the agglomeration between hydrate particles and unconverted water droplets, which has been confirmed in previous reported literature [34]. McCulfor et al. investigated the effect of Arquad 2HT-75 on the flow properties of a glass particle suspension in mineral oil. It was found that Arquad 2HT-75 can greatly reduce the ability of water to wet the glass particles, essentially preventing liquid bridges from connecting the particles. This 18


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could prevent strong adhesion between hydrate particles and the formation of hydrate plug. However, for the 75 vol.% WC dataset, the emulsion with 2 wt.% Arquad 2HT-75 is so unstable (bulk water was observed within hours after emulsification) and exhibited higher hydrate plug tendency in our previous autoclave experiment [17]. It implies that in the presence of bulk water due to water coalescence before/during hydrate formation, Arquad 2HT-75 cannot behave as some reported anti-agglomerants, which can disperse the hydrate particles effectively formed in the bulk water

[35]

. Furthermore, for the 75 vol.% WC emulsion with 3.5 wt.% salt + 2 wt.% Arquad

2HT-75, the initial emulsion is very stable and shows quite low agglomeration/plug tendency [17](see Table 2). The addition of salt can supply a stable and dispersible emulsion environment to allow Arquad 2HT-75 (for a high water content system) to prevent agglomeration between hydrate particles and droplets (see Figure 5). Based on the above discussion, it can be concluded that Arquad 2HT-75 can prevent the agglomeration between hydrate particles and unconverted water droplets, but its ability to prevent water droplet coalesce is limited due to its mediocre ability at stabilizing emulsions. If bulk water formed due to water coalescence (e.g. at high water cut), hydrate aggregates might form and also lead to plugging.

4. Conclusion High-pressure differential scanning calorimetry was used to investigate the effects of Arquad 2HT-75 and NaCl on hydrate/ice formation in moderate to high water cut crude oil systems under quiescent conditions. Bottle emulsion stability tests were performed to supplement the observations from the DSC results. Overall, the following conclusions can be drawn from this study.

For the moderate to high water cut systems, the emulsions were found to be very stable when Arquad 2HT-75 and NaCl were combined. Both ice and hydrate formation/dissociation can induce the destabilization of emulsions 19


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under certain conditions. Compared to the moderate water cut (50 vol.%) systems, the hydrate conversions of the higher water cut systems (75vol.%) are relatively low due to mass transfer limitations. At the concentrations used in this work, the addition of NaCl to Arquad 2HT-75 can help form stable water-in-oil emulsions.

Acknowledgements

The authors acknowledge support from the CSM Hydrate Consortium. Chenwei Liu also acknowledges the support from the Program for Changjiang Scholars and Innovative Research Team in University (IRT1294), China Postdoctoral Science Foundation funded project (Grant No. F1502081B) and “the Fundamental Research Funds for the Central Universities” (Grant No. 14CX06026A).

Supporting information

The photos of the bottle tests, the interfacial tension data, and ambient pressure thermograms for samples are supplied in the supporting information. In addition, the high pressure thermograms for 30 vol.% WC emulsions and hydrate conversion procedure are also included. This material is available free of charge via the internet at http://pubs.as.org.

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Investigating Gas Hydrate Formation in Moderate to High Water Cut

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