Performance of Polymer Hydrogels Incorporating Thermodynamic

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Performance of Polymer Hydrogels incorporating Thermodynamic and Kinetic Hydrate Inhibitors Juwoon Park, Huen Lee, Yutaek Seo, Wendy Tian, and Colin D. Wood Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02978 • Publication Date (Web): 21 Mar 2016 Downloaded from http://pubs.acs.org on March 26, 2016

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Performance of Polymer Hydrogels incorporating Thermodynamic and Kinetic Hydrate Inhibitors Juwoon Park,2Huen Lee,1 Yutaek Seo,2* 1

Graduate School of EEWS and Department of Chemical and Biomolecular Engineering (BK21+ Program), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejon 305-701, Republic of Korea 2

Department of Naval Architecture and Ocean Engineering, Collage of Engineering, Seoul National University, 1, Gwanak-ro, Gwanak-gu, Seoul, 151 - 744, Korea

Wendy Tian,2 and Colin D. Wood 2*

2

CSIRO Manufacturing Flagship, Clayton, VIC, 3168, Australia

AUTHOR EMAIL ADDRESSES: [email protected] and [email protected] CORRESPONDING AUTHORS: Yutaek Seo and Colin D. Wood

ACS Paragon Plus Environment

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ABSTRACT This study investigates the hydrate formation characteristics of hydrogel particles containing thermodynamic or kinetic hydrate inhibitors. Hydrate onset time, hydrate fraction in the liquid phase, and torque changes were determined from the experiments using a high pressure autoclave. To simulate the mixture of water and liquid hydrocarbon encountered in subsea pipelines, decane was added corresponding to an initial 60% water-cut. In the liquid phase a high hydrate fraction was observed in the pure water + decane mixture with a local maximum in torque. For the hydrogel decane mixture a lower hydrate fraction was observed and the torque remained stable during the hydrate formation. The addition of 0.5 wt% of a commercial kinetic hydrate inhibitor (Luvicap) to the aqueous phase (with no hydrogel) delayed the hydrate onset time, but several spikes in the torque were observed during the hydrate formation which suggests that segregation and deposition of hydrate particles is occurring in the liquid phase. When Luvicap (0.5 wt%) was incorporated into the hydrogel particles at the same concentration there was an increase in the hydrate onset time and the torque remained stable during the hydrate formation. This study is the first attempt to achieve both kinetic hydrate inhibition and antiagglomeration using polymer hydrogels. Moreover, the synthesized hydrogel is compatible with monoethylene glycol (MEG) solution and the resulting MEG-hydrogel particles showed the hydrate formation characteristics of an under-inhibited system. These results suggest that the hydrogel particles can be utilized as a hybrid hydrate inhibitor because thermodynamic and kinetic hydrate inhibition performance can be coupled with the anti-agglomeration performance of the base hydrogel particles. KEYWORD


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Hydrate, synergistic inhibition, polymer hydrogels, monoethylene glycol, kinetic hydrate inhibitor.


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Introduction Gas hydrates are non-stoichiometric crystalline compounds that form when guest molecules are incorporated into host frameworks that are formed by water molecules through hydrogen bonding at high pressure and low temperatures.1 Guests include methane, ethane, propane, and carbon dioxide, thus there have been many attempts to develop gas storage2-4 and separation5, 6 technologies based on hydrate formation. They have also attracted significant attention over recent decades as an energy source as methane gas deposited in marine sediments and permafrost regions across the globe.7-10 However, hydrate formation has been an inherent risk for operators of offshore oil and gas fields as hydrates can form in offshore flowlines transporting hydrocarbon fluids at high pressure and low temperatures which results in costly remediation of hydrate blockages.1, 11 Among the solid deposition issues encountered in offshore flowlines, gas hydrates are the major flow assurance concern as their formation and deposition occurs on a shorter timeframe than other issues such as wax and asphaltene. There are several options available to industry for avoiding hydrate blockages including thermal management of the hydrocarbon fluids via insulation of flowlines and/or injection of thermodynamic hydrate inhibitors (THIs) such as methanol or mono-ethylene glycol (MEG). The concentration of THI in the aqueous phase must be sufficiently high to avoid hydrate formation. This is calculated from the flowline pressure and temperature, however, the injection rate of THI increases depending on the volume of the aqueous phase. The handling of THI in offshore fields significantly increases the operating expenditure due to the large injection volume, and it was estimated that the cost of methanol injected into a flowline in offshore oil fields producing 50,000 bbl/day water can reach 50% of the operating costs.14 Although this is the conventional strategy it is now facing challenges such as limited space on floating structures and bulk logistics,


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as the exploration and production of offshore oil and gas fields moves to deeper and remote regions. As such hydrate prevention strategies are now moving toward hydrate risk management, where the hydrates are allowed to form, but the formation is delayed or the agglomeration is prevented before blocking flowlines.1,

12-15

The nucleation and growth of hydrate crystals is

delayed significantly in the presence of kinetic hydrate inhibitors (KHIs), which are watersoluble polymers including homo- and co-polymers of the N-vinyl pyrolidone and N-vinyl caprolactam.1,

16-19

Although they have been applied in offshore fields successfully, their

performance can be affected by the presence of other chemicals such as corrosion inhibitors and may not be effective at subcoolings larger than 14 oC.16 Alternatively the agglomeration of hydrate particles can be prevented in the liquid phase by using anti-agglomerates (AAs), which cause the water phase to be dispersed in the hydrocarbon phase as fine droplets inducing their formation into dry hydrate particles under hydrate formation conditions. 14, 16, 20, 21 AAs based on quaternary ammonium surfactant have been deployed in a number of oil fields,16, 21, 22 however they are considered to be ineffective at high water volume fractions and they are also affected by the composition of the fluids. Recently, it has been suggested that delaying the hydrate formation and reducing the risk of hydrate blockage can be achieved in the presence of THI where the THI concentration is lower than the value required to fully prevent the formation of hydrates, i.e. under-inhibition. The amount of hydrate can be controlled by changing the concentration of THI due to a self-inhibition effect that is observed with this strategy, thus avoiding the formation of hydrate blockages.23 In our previous work

24

, we investigated the potential to use polyacrylamide-based hydrogel

particles for simultaneously inhibiting the formation of gas hydrates and preventing their agglomeration. The particles were synthesized specifically for the application and swell to a


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controlled degree in water whilst remaining discrete. Stable torque was observed for the polymer hydrogels during the hydrate formation while there were sharp increases in torque before the stirrer stops for a water and decane mixture. The experimental results showed that the hydrogel particles were well dispersed in the decane phase even after the hydrate formation and do not deposit under stirring because the particles remain discrete due to the polymer network. As such polymer hydrogel particles have potential to prevent hydrate bed formation and deposition without using surfactants. Moreover the hydrogel particles can be further optimized to include thermodynamic or kinetic hydrate inhibitors. In the present study, polymer hydrogel particles were synthesized that incorporate MEG or a KHI, and their hydrate inhibition performance was tested by measuring the hydrate onset time, initial growth rate, hydrate fraction, and torque changes. Control experiments for MEG and KHI solutions (without polymer hydrogels) were carried out to investigate the effect of the polymer hydrogels. We report the results from this extensive set of experiments and suggest a mechanism for hydrate particles deposition in the presence of both hydrate inhibitor and the hydrogel particle. The obtained results suggests that the polymer hydrogel can be utilized as a versatile base material for hydrate inhibitors to be coupled with either thermodynamic hydrate inhibitors or kinetic hydrate inhibitors.


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Experiments Materials and hydrogel synthesis The distilled water used for hydrate experiments was purchased from OCI and decane was from Sigma-Aldrich. The simulated natural gas (CH4: 90mol%, C2H6: 6 mol%, C3H8: 3 mol%, and C4H10: 1 mol%) was provided by Special gas (Korea). All of the chemicals for the hydrogel polymer synthesis were purchased from Sigma-Aldrich and were used as received including: polyacrylamide-co-acrylic acid partial sodium salt (PAM-coAA), Mw 520 000, Mn 150 000, typical acrylamide level 80%; N-(3-(dimethylamino)propyl)-N′ethylcarbodiimide hydrochloride (EDC, commercial grade); N-hydroxysuccinimide (NHS, 98%); and 1,2-diamino ethane (EDA >99%).

Tris(2-aminoethyl)amine (TREN);, technical grade;

heptane (HPLC grade, >99.5%). The synthesis of the hydrogel particles has been described previously and involves the carbodiimide-mediated cross-linking (CMC) reaction of PAM-co-AA in an inverse suspension.24 For MEG swollen hydrogels: the hydrogel particles containing water were dried by precipitating the polymer in ethanol to remove the water. The resulting dried polymer was swollen in a 20% w/w% solution of MEG and the final polymer concentration was 13%. For Hydrogels incorporating KHIs: Hydrogel blocks can be freeze dried and the resulting porous network can be ground and sieved to form particles. The hydrogel in this case was formed by dissolving EDC (0.3993g) in 0.5 ml of deionised water and adding this solution to 5ml of an aqueous PAM-co-AA (15 wt%) solution. The resulting highly viscous solution was mixed and after 3 mins, 0.24g of NHS dissolved in 0.5 ml deionised water was added. At this stage, the viscosity of the solution decreased and after 3 mins, the requisite crosslinker, dissolved


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in 1 ml of deionised water, was added to the above solution, and the mixture was shaken. The total volume of hydrogel formed was approximately 15 ml. Luvicap-hydrogel (0.5 wt%) is formed by adding luvicap to the aqueous solution PAM-co-AA (15 wt%) before addition of the EDC, the weight ratio of Luvicap and polymer is 0.5 to 100. After formation of the hydrogel the water was removed by freeze drying for approximately 4 hrs. These polymers were ground using a mortar and pestal. To assist the grounding, dry ice was applied to enhance the brittleness as needed. The dried particles were swelled by suspending in heptane and adding water in a dropwise manner while stirring vigorously to ensure the particles were separated in the jar. The particles became transparent and the weight ratio of water to particles was approximately 3:1. The heptane was removed by decanting and the particles were left to air dry resulting in a bead-like gel. Microscopy images of the MEG hydrogel are shown in Figure 1. The images prove that the hydrogel particles are stable even after the cycles of hydrate formation and dissociation.

Figure 1. Microscopy images of MEG hydrogel after hydrate formation and dissociation. (left): 10x, (right):20x


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Experimental apparatus and procedures A high pressure autoclave equipped with a magnetic stirrer coupling and a four-blade impeller was used in this study. A total liquid volume of 80mL was loaded into the autoclave which had an internal volume of 360 mL. The cell was immersed in a temperature-controlled liquid bath connected to an external refrigerated heater. A platinum resistance thermometer monitored the temperature of the liquid phase inside of the autoclave with an uncertainty of 0.15 oC . The pressure was measured by a pressure transducer with an uncertainty of 0.1 bar in a range of 0 200 bar. To provide vigorous mixing of the liquid phase, an anchor type impeller on a solid shaft coupled with the motor (BLDC 90) was used. The impeller was located on the base of the shaft and the stirring rate is maintained at 600 rpm for all experiments. A torque sensor (TRD-10KC) with platinum coated connector measured the torque of continuously rotating shaft with an uncertainty of 0.3%. It used a strain gauge applied to a rotating shaft and a slip ring that provides the power to excite the strain gauge bridge and transfer the torque signal. Temperature, pressure and torque data were recorded using a data acquisition system. The experiment was commenced by loading the 80mL of liquid phase into the autoclave. After purging the cell three times with natural gas, the autoclave was pressurized to 120 bar at 24 °C while stirring at 600 rpm to saturate the liquid phase with gas. The Reynolds number at this mixing speed was about 32,000 indicating the fluid is in a fully developed turbulent regime. Once the pressure and temperature reached steady-state, the cell was cooled to 4 oC within two hours and for the temperature was maintained for 10 hours. During this time, torque, pressure and temperature were continuously monitored. Ten experiments were carried out for each system to determine the average hydrate onset time, subcooling temperature, and the amount of gas consumed. The repeat experiments improved the statistics regarding the trends in hydrate


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formation and transportability. The dissociation of the hydrates was carried out at 24 oC for three hours to remove the residual hydrate structures. A total of 30 experiments were carried out for a water + decane mixture, 20 wt% MEG solution + decane mixture and 0.5 wt% Luvicap solution + decane mixture. Another 30 experiments were performed to investigate the effects of polymer hydrogels on hydrate inhibition across three systems including hydrogel + decane mixture, MEG-hydrogel (20 wt%) + decane mixture, and Luvicap-hydrogel (0.5wt%) + decane mixture. The water-cut was maintained at 60 % for all experiments. In order to investigate the performance of hydrate inhibitors continuous cooling is widely used to measure the hydrate onset time and resistance-to-flow.25-34The present study also adopted an isochoric continuous cooling method to investigate the effect of polymer hydrogels on the hydrate inhibition performance. Figure 2 shows an example of pressure, temperature, and torque changes during the continuous cooling of a water + decane mixture with natural gas from 24 to 4 o

C at 600 rpm. Time zero is defined as the point where the cooling process is initiated. Hydrate

formation can be identified at about 34 min by a temperature increase which is accompanied by a decrease in pressure due to the exothermic formation of hydrates consuming gas molecules (Figure 2 (a) and (b)). As the hydrate particles grow further, the decrease in pressure becomes significant and the torque starts to rise at 58 min, Figure 2 (c) as the hydrate particles suspended in the liquid phase agglomerate and/or deposit on the wall. As seen in Figure 2, there is a time difference between the hydrate onset moment and the equilibrium condition, which is presented as tonset indicating how long the hydrate formation is delayed. Similarly the subcooling temperature, ∆Tsub, is calculated by the temperature difference between the hydrate onset moment and the equilibrium condition. The hydrate fraction in the total liquid phase, φhyd, is estimated from the decrease of pressure during the hydrate formation using the compressibility


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factors of natural gas, then water conversion to hydrate, xhyd, was calculated from the ratio of consumed water to the amount of water loaded into the cell initially. The impact of segregation and deposition of hydrate particles in the liquid phase was assessed from torque changes as a function of time and hydrate fraction. The consumed gas mol% was calculated from the pressure difference between the experimental pressure and the postulated pressure with no hydrate formation.11, 31-36 Thus, Δ, = − where Δ, is the consumed gas moles for hydrate formation at a certain time, Pcal is the calculation pressure with postulation of no hydrate formation, Pexp is the observed pressure, Vcell is the volume of gas, z is the compressibility factor value from calculation of the Cubic Plus Assocition equation of state26, R is the ideal gas low constant, and T is the gas temperature. The hydrate fraction in the liquid phase is obtained by calculation of following equation: =

+ + ( − , )

where Φhyd is he hydrate volume fraction in the liquid phase, Vhyd is the hydrate volume that is calculated from the density of hydrate and molecular weight, Vw is the water volume, and Vw,conv is the converted water volume to hydrate. The hydration number 6.5 was used for calculation, which was calculated from cage occupancy of small (512) and large (51264) cages of structure II hydrate of pure water and natural gas.5, 25


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Figure 2. Pressure, temperature, torque change during first cooling cycle of water + decane mixture with natural gas. The onset means difference between hydrate equilibrium condition (temperature and pressure) and hydrate formation condition (temperature and pressure).


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Results and Discussion Performance of MEG and KHI in aqueous solution The deposition of hydrate particles increases the resistance-to-flow inside the autoclave resulting in an increase in the torque required to maintain a constant mixing rate. Sohn et al.

37

indicated

that the highest resistance-to-flow was observed for systems with around 60% water-cut, where severe local spikes in the torque were observed. The present study extends the work to investigate the effect of adding thermodynamic or kinetic hydrate inhibitors on the deposition of hydrate particles. Table 1 presents the mean value and standard deviation over ten repeat trials for hydrate onset time, subcooling temperature, hydrate volume fraction at which torque increased, hydrate volume fraction at the end of the experiment, and water conversion. The torque difference between the hydrate onset and the highest peak was presented in Table 1 as well. Kinetic inhibition performance can be assessed with the hydrate onset time and subcooling temperature while the segregation and deposition of hydrate particles are discussed based on hydrate fraction, water conversion, and torque values. The average hydrate onset time was 20.4 min and the average subcooling temperature was 4.7 oC for water +decane mixture at 60% water-cut. Addition of 0.5 wt% Luvicap increased the onset time to 83.8 min as well as the subcooling temperature to 11.6 oC, which indicates the nucleation and growth of hydrate crystals was delayed significantly in the presence of Luvicap. The onset time increased to 57 min by adding 20 wt% MEG possibly due to the shift of the hydrate equilibrium condition which reduced the thermal driving force for hydrate formation. The subcooling temperature was 8.8 oC. These results suggest that the addition of Luvicap and MEG in the aqueous phase affect the nucleation and growth of the hydrate which increases the hydrate onset time and subcooling temperature.


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Table 1. Experimental results for water + decane systems with and without hydrate inhibitor at water-cut 60%. The standard deviation of ten repeat trials is shown in brackets for each reported value. Hydrate onset time for each system during 10 cycles also showed in Table S1.

Systems

tonset (min)

∆Tsub (oC)

φhyd, tran

φhyd, final

xhyd (%)

Δτ

Water + Decane

20.4 (2.1)

4.7 (0.6)

0.13

0.50 (0.04)

74.0 (3.9)

9.4

Luvicap 0.5 wt% solution + Decane

83.8 (5.2)

11.6 (0.18)

0.035

0.40 (0.06)

58.9 (9.85)

1.5

MEG 20 wt% solution + Decane

57.0 (2.7)

8.8 (0.6)

0.035

0.28 (0.02)

40.0 (3.6)

1.1

Hydrate growth with and without hydrate inhibitors are described in Figure 3 – 5. The torque changes and hydrate fraction data for water + decane mixture are shown in Figure 3 as a function of time after the onset. There was no distinct increase of torque upon hydrate onset, however when the hydrate fraction reached 13.5% the torque started to rise gradually. It is worth noting that the torque reached a maximum value of 13.9 N cm at water conversion of 33%, then the torque drops sharply to 7.4 N cm while water conversion becomes 54.5 % after 50 min since the hydrate onset. The formation of hydrates proceeds to water conversion of 74% in a time of 1000 min, suggesting that most hydrate formation and growth occurred in initial stage. Figure 3 (a) also shows that the water-cut changed from 60% to 35% when the torque showed a local spike, suggesting the dominant phase (by volume) may change from water to decane due to consumption of water during hydrate formation.


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Figure 3. Water + Decane + Natural gas mixture at 60% water cut. (a) The change of hydrate fraction and water-cut during the cycle since hydrate onset: (red), water-cut; (black), hydrate volume fraction with time (b) Torque trend during hydrate formation

Figure 4 shows the hydrate fraction and torque change as a function of time after the onset in Luvicap 0.5 wt% solution + decane system. As shown in Table 1 and Figure 3, the addition of 0.5 wt% Luvicap delays the hydrate onset time significantly and the growth rate in the initial stage decreases as well. However water conversion to hydrate reached 58.9% at the end of experiment and the torque rises earlier when the hydrate fraction reaches 0.04. The torque rises


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gradually leading to the high torque values of 10.5 N cm with an instant maximum value of 10.7 N cm. The Luvicap is an effective KHI as seen from the delayed hydrate onset time, however it cannot limit the hydrate fraction and the deposition of hydrate particles once hydrate growth proceeds.

Figure 4. Luvicap 0.5wt% solution + Decane + Natural gas mixture under water-cut 60 system. (a) The change of hydrate fraction and water-cut during the cycle since hydrate onset: (green), water-cut; (black), hydrate volume fraction with time (b) Torque trend during hydrate formation.

The under-inhibition experiment with MEG 20 wt% solution + decane mixture was performed at 60 % water-cut and the obtained results are shown in Table 1 and Figure 5. Considering the


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target temperature of 4 oC and the initial pressure of 120 bar, the MEG concentration needs to be maintained above 43.0 wt% to completely avoid the hydrate formation. The addition of 20 wt% MEG in the aqueous phase shifts the hydrate equilibrium curve (Supplementary Information, Figure S1.) and reduces the thermal driving force for hydrate formation. The average hydrate onset time was delayed by 57 min, which is close to the value obtained by adding 0.5 wt% Luvicap. Moreover the final hydrate fraction in Figure 5, 0.28, is also less than that of water + decane and Luvicap 0.5wt% + decane mixtures. No significant torque change was observed in this work and it is likely that soft hydrate particles were formed in the presence of 20 wt% MEG in the aqueous phase. Further studies are required to investigate the mechanism of hydrate particles formation with MEG present in the aqueous phase. The above results suggested that the hydrate formation in water + decane mixture at water-cut 60% accompanied the segregation of hydrate particles from a continuous liquid phase and deposition onto the autoclave wall. The addition of Luvicap 0.5 wt% delayed the hydrate onset time about 4 times longer, however its growth and deposition process was similar to that of water + decane mixture. The presence of MEG 20 wt% showed the best inhibition performance, i.e. significantly delayed hydrate onset time, less hydrate fraction in the liquid phase, and stable torque during the hydrate formation. However other literature suggests that the under-inhibited fluid with MEG showed the hydrate deposition and spikes of pressure drop signals although the deposits tend to slough more readily with increasing MEG concentration.23 It seems the size distribution of hydrate particles and their interaction with liquid phase increases complexity in terms of the deposition mechanism of hydrate particles, suggesting better approaches are required for controlling the formation and growth of hydrate particles.


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Figure 5. MEG 20wt% solution + Decane + Natural gas mixture under water-cut 60 system (a) The change of hydrate fraction and water-cut during the cycle since hydrate onset: (blue), watercut; (black), hydrate volume fraction with time (b) Torque trend during hydrate formation.


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Hydrate management with polymer hydrogels containing hydrate inhibitors The polymer hydrogel particles synthesized in the Materials and hydrogel synthesis section were tested as hydrate inhibitor using the standard cooling method. Table 2 presents the obtained experimental results. The amount of water loaded in the form of hydrogels matched an initial water-cut of 60%. The average hydrate onset time was 18.5 min and the average subcooling temperature was 4.4 oC for hydrogel + decane mixture which is broadly similar to the water only case (20.4 min and 4.7°C) which indicates that the presence of the hydrogel network does not significantly inhibit hydrate formation. Addition of 0.5 wt% Luvicap into hydrogels increased the onset time to 58.5min as well as the subcooling temperature to 11 oC. The hydrate onset was delayed three times longer than without Luvicap, however the KHI performance was slightly less than Luvicap 0.5 wt% solution + decane mixture. The onset time also increased to 60 min by adding 20 wt% MEG into hydrogel, which indicates the KHI performance was less significant for hydrogels containing thermodynamic or kinetic hydrate inhibitors. A possible explanation is that the initial dispersion of hydrogel particles results in a high surface area for contacting hydrocarbon phase, thus the nucleation and growth of hydrate can occur on the surface of hydrogel particles with enhanced mass transfer as well as reduced mobility of inhibitors in the presence of polymer network. A comparison of the hydrate onset time and subcooling temperature for hydrogel particles with the solutions indicate the kinetic inhibition performance of Luvicap and MEG was diminished when dispersing the aqueous phase in the form of hydrogel particles. However dramatic differences were observed in hydrate fraction and torque changes.


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Table 2. Experimental results for hydrogel + decane systems with and without hydrate inhibitor at water-cut 60%. The standard deviation of ten repeat trials is shown in brackets for each reported value. Hydrate onset time for each system during 10 cycle also showed (Table S2.).

Systems

tonset (min)

∆Tsub (oC)

φhyd, tran

φhyd, final

xhyd (%)

Δτ

Hydrogel + Decane

18.5 (1.9)

4.4 (0.5)

0.07

0.22 (0.02)

31.6 (2.4)

0.5

Luvicap hydrogel (0.5 wt%) + Decane

58.5 (5.2)

11 (0.5)

0.01

0.13 (0.01)

38.7 (2.6)

1.4

MEG hydrogel (20 wt%) + Decane

66.9 (8.9)

8.8 (3.9)

0.01

0.15 (0.03)

20.9 (4.6)

0.4

Figure 6 shows the hydrate fraction and torque changes over time since the hydrate onset in a mixture of hydrogel + decane. The hydrate fraction reaches 0.22 at the end of the experiment, which is much lower than that of water + decane mixture (0.50). The volumetric ratio of water to decane was 6:4, thus the hydrogel particles are dispersed while decane remained between hydrogel particles. Without the hydrogels and mixing, there would be clear separation of water from the decane phase, however the presence of a hydrogel polymer network absorbing the aqueous phase allows the particle to remain separate. As seen in Figure 6, the hydrate fraction increases at a greater rate in early stage of hydrate formation than in water + decane mixture (Figure 3) as the hydrate formation occurs on the surface of dispersed hydrogel particles that have a higher contact surface area with the gas. However, the formation rate quickly slows after only 10 min and is further reduced at 90 min after the onset. It is likely the hydrate shell is rapidly formed on the surface of hydrogel particles, resulting in a mass transfer limitation during the inward growth of the hydrate shell. The hydrate fraction reaches 0.16 at 90 min after the


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onset and increases further to 0.22 for the remainder of the experiment (600 min). The torque remained stable during the hydrate onset and growth, suggesting the hydrate shell covered hydrogel particles did not aggregate or deposit inside the autoclave. It is noted that although water was consumed during the formation, the hydrogel particles maintain their shape and no severe deposition of the particles was observed. For water + decane mixture, the water-cut changed during the formation of hydrate and hydrate particles segregated from the liquid phase resulting in an instant increase of torque. However for hydrogel + decane mixture the hydrate formation was restricted to the surface of hydrogel particles and there was no clear segregation of hydrate from liquid phase due to the absorbent properties of the hydrogel. Aman et al.38 suggested that cohesion and sintering of hydrate particles would dominate the formation of hydrate blockages. In their work the cohesion force becomes higher in the presence of an aqueous phase between hydrate particles in a cyclopentane phase, which enhances the sintering of hydrate particles by inducing formation of a hydrate-bridge between particles. However the presence of a polymer hydrogel network holds the water molecules inside the hydrogel particle and prevents the outbreak of free water from the particle. The hydrate shellcovered hydrogel particles are likely to become similar to the annealed hydrate particles, where the cohesion force between particles is reduced significantly.


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Figure 6. Hydrogel + Decane + Natural gas mixture under water-cut 60 system (a) The change of hydrate fraction and water-cut during the cycle since hydrate onset: (pink), water-cut; (black), hydrate volume fraction with time (b) Torque trend during hydrate formation.

Figure 7 shows the hydrate fraction and torque changes over time in Luvicap-hydrogel + decane mixture. The concentration of Luvicap was 0.5 wt%. As discussed in Table 2, the hydrate onset time for Luvicap-hydrogel was delayed three times longer than hydrogel + decane, indicating the Luvicap remains as a kinetic hydrate inhibitor even inside hydrogel particle structured with polymer hydrogel network. However the growth curve for the hydrate fraction in Figure 6 suggests the initial growth rate of hydrate in Luvicap-hydrogel particle was similar to that in


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hydrogel particle. The final hydrate fraction for Luvicap-hydrogel, 0.13, was lower than the pure hydrogel case (0.22). Although the hydrate fraction increases readily, that the torque remains stable during the hydrate formation, Figure 7 (b). The torque spike was observed in Figure 4 (b) for Luvicap 0.5 wt% + decane mixture, however it was not observed in Luvicap-hydrogel + decane mixture. The stable torque clearly suggests the hydrate formation occurs only on the surface of Luvicap-hydrogel and the particles remain separate without bedding or deposition of the particles. The mechanism for avoiding deposition of Luvicap-hydrogel particles is different with the conventional anti-agglomerant as it does not involve a surfactant which is advantageous in terms of effective water-cut and sensitivity to environment as is observed with surfactantbased AAs. Therefore, by incorporating Luvicap into hydrogel particles a hybrid inhibition strategy can be developed for simultaneously delaying hydrate onset and preventing agglomeration of hydrate particles.


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Figure 7. Luvicap hydrogel (0.5 wt%) + Decane + Natural gas mixture under water-cut 60 system (a) The change of hydrate fraction and water-cut during the cycle since hydrate onset: (dark red), water-cut; (black), hydrate volume fraction with time (b) Torque trend during hydrate formation

The hydrate formation characteristics in MEG-hydrogel + decane mixture was also studied. Figure 8 shows the changes of hydrate fraction and torque over time after the onset. When hydrates form in under-inhibited systems, the maximum hydrate fraction in the liquid phase can be estimated from the hydrate equilibrium condition whilst considering the self-inhibition effect. It is noted that the MEG molecules cannot be accommodated into hydrate cages during the


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hydrate formation, thus the MEG concentration in the remaining aqueous phase increases as the hydrate fraction increases. If the concentration is sufficient to inhibit the hydrate formation under corresponding pressure and temperature, further hydrate formation would be prevented. From the equilibrium conditions and the P-T trace during the cooling of the MEG-hydrogel + decane system, hydrate formation would cease due to thermodynamic constraint once the MEG concentration in aqueous phase reaches 43.0 wt%. For 20.0 wt% MEG-hydrogel + decane mixture, the theoretical maximum value for water conversion would be 60%, however the Table 2 and Figure 8 present the water conversion from the experiments varies from 12% to 26%, leading to the average water conversion of 21%. Therefore the hydrate fraction was reduced substantially in hydrogels with MEG at under-inhibited concentration. Considering the total amount of water at the end of experiment, the MEG concentration would increases from 20 wt% to 25 wt%. During the formation of a hydrate shell on the hydrogel surface, MEG molecules would diffuse into the hydrogel core as they were expelled from the growing hydrate structures. Local increase of MEG concentration would reduce the driving force for hydrate formation further, thus preventing it at an earlier stage. It is noted that the hydrate fraction in the MEGhydrogel + decane system is less than that of MEG solution + decane mixture, possibly due to the local increase of MEG concentration. Further studies will be carried out to investigate the role of MEG during the hydrate formation in under-inhibition condition. The torque remains stable as seen in Figure 8 during the entire experiment, indicating the negligible deposition of MEG-hydrogel particles covered with hydrates. As discussed for hydrogel + decane and Luvicap + decane mixtures, the feature of the hydrogel particles would be the restricted formation of a hydrate shell on the surface of the particles and this solid hydrate phase does not segregate from the liquid phase due to the presence of polymer network holding


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the particle format. Unlike the polymer hydrogels, the formed solid hydrate phase is segregated from the liquid phase and induces bedding and/or deposition with increasing hydrate fraction in the liquid phase. Flow parameters such as flow velocity, water-cut, and gas-liquid ratio would affect the conditions of transitioning from the homogeneous dispersion of hydrate particles into the bedding/deposition of the particles.39-42 However, the addition of a polymer network in the aqueous phase maintains the integration between the hydrate shell and the hydrogel core particles.

Figure 8. MEG hydrogel (20 wt%) + Decane + Natural gas mixture under water-cut 60 system (a) The change of hydrate fraction and water-cut during the cycle since hydrate onset: (dark green), water-cut; (black), hydrate volume fraction with time (b) Torque trend during hydrate formation.


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When a hydrate shell is formed on water droplets dispersed in a hydrocarbon phase, a thick hydrate shell is desirable because a thin hydrate shell may fracture upon contacting with other hydrate particles. This fracture results in an outbreak of free water from inside and sintering of the two hydrate particles. However when a hydrate shell is formed on hydrogel particles, the polymer network holds the water inside the particle and minimizes release of free water into the surrounding decane phase. After completing the cycles of hydrate formation and dissociation, there was no free water phase released from the hydrogel particles, suggesting the synthesized polymer structure was effective at retaining the water inside the network. Previous works

38, 43

suggested that both cohesion and sintering of hydrate particles might be the reason for forming hydrate blockages, however their effect was minimized when forming hydrate in MEG-hydrogel particles. Figure 9 shows torque changes as a function of hydrate fraction during hydrate formation with and without polymer hydrogels in aqueous phase. Figure 9 (a) shows a rapid increase in torque for a water + decane mixture when the hydrate fraction reaches 0.25. Homogeneous distribution of hydrate particles was transformed to heterogeneous segregation from the liquid phase. However the hydrogel + decane mixture shows stable torque until the hydrate fraction reaches 0.22 as the polymer network maintains the particle shape even after hydrate formation on the surface of the hydrogel particles. This is a different mechanism for preventing the agglomeration of hydrate particles from the conventional anti-agglomerant. Figure 9 (b) presents the similar behavior of torque spike in Luvicap 0.5 wt% solution + decane mixture when hydrate fraction reaches 0.035, suggesting the Luvicap has limited capability of suppressing the growth and deposition of hydrate particles in the liquid phase. Once again, the torque remains stable when


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hydrates formed in Luvicap-hydrogel (0.5 wt%) + decane mixture. It is noted that the Luvicap still delayed the hydrate onset time significantly and the PAM-co-AA polymer network plays major role to prevent the agglomeration of hydrate particles after the hydrate onset with the mechanism discussed in the above. To the best of our knowledge, this is the first work suggesting a hybrid inhibition performance of KHI and AA by incorporating Luvicap into hydrogel particles. There was no adverse effect by dissolving Luvicap in hydrogel particles. Figure 9 (c) presents the torque changes in under-inhibition systems with and without polymer hydrogels. For both cases, torque remains stable during the hydrate formation, suggesting the hydrate particles are likely to be less sticky in the presence of MEG. However adding polymer hydrogels to make MEG-hydrogel can provide the control over the distribution of particle size, thus increasing flexibility for transporting the aqueous phase with hydrocarbon fluid.


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14

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6 Luvicap 0.5wt% solution Luvicap 0.5wt% hydrogel 5 0.0

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6.0 5.5 5.0 4.5 4.0 3.5 0.00

MEG 20wt% solution MEG 20wt% hydrogel 0.05

0.10

0.15

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Hydrate volume fraction

Figure 9. Comparison between the both systems with and without hydrogel. (a) water + decane + natural gas mixture and PAM-hydrogel + decane + natural gas mixture (b) Luvicap 0.5wt% solution + decane + natural gas mixture and Luvicap 0.5wt% hydrogel + decane + natural gas mixture (c) MEG 20wt% solution + decane + natural gas mixture and MEG 20wt% hydrogel + decane + natural gas mixture.


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The obtained results in this work suggest a novel concept for managing the hydrate risk in offshore oil and gas fields with polymer hydrogels synthesized from PAM-co-AA. The hydrogel particles with minimum diameter can be injected into a subsea flowline, then swelling to a controlled-size by absorbing free water and turning into separate hydrogel particles. Depending on the field location and available infrastructure, an optimized management strategy can be developed using the hydrogels as a versatile base. The anti-agglomeration performance of the hydrogel particles is the basic strategy as the hydrogel particles do not agglomerate when hydrate forms in a hydrocarbon phase, then a kinetic inhibitor can be incorporated for the fields with relatively less subcooling and short travel duration of fluids. However for the high subcooling and long transformation duration, such as long distance tieback for deepwater gas fields, an under-inhibition concept can be coupled with hydrogels to minimize the infrastructure for THI as well as to manage the hydrate blockage risks for both steady-state and transient operations. The particle size of hydrogels can be controlled by adjusting the synthesis process of the polymer, suggesting the hydrate formation process in hydrogel particles can be optimized further. The hydrogel polymer would easily precipitate through heating or changing chemical environment, thus its recovery process can be developed. This is a new and promising alternative to the conventional hydrate management strategy.


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Conclusions From the experimental measurement of hydrate onset conditions, hydrate fraction, and torque changes, polymer hydrogels can prevent the heterogeneous segregation of hydrate from a liquid phase and could be effective as an anti-agglomerant, via a new mechanism. The decane phase was added in the liquid phase to achieve the water-cut 60% initially. Thus the hydrate formation in the mixture of water and decane induces the segregation and deposition of hydrate particles due to cohesion and sintering of hydrate particles in liquid phase. The local maximum torque was observed in water + decane mixture when the hydrate fraction reached 0.25. The addition of 0.5 wt% Luvicap into the aqueous phase increased the average hydrate onset time from 20.4 to 83.8 min, however there were several torque spikes during the hydrate formation suggesting the segregation and deposition of hydrate particles in liquid phase. Instead of Luvicap, the addition of 20 wt% MEG into the aqueous phase showed typical behavior of hydrate particles in an under-inhibited system featuring low hydrate fraction and stable torque during the hydrate formation. However the addition of hydrogel particles in the aqueous phase resulted in antiagglomeration in all hydrogel + decane, Luvicap-hydrogel + decnae, and MEG-hydrogel + decane mixtures. It is noted that the water conversion ratio was reduced substantially in the presence of hydrogels, suggesting that the hydrate shell forms on the surface of hydrogel particles and was not segregated from the liquid phase due to the polymer hydrogel networks. For Luvicap-hydrogel + decane mixture, the anti-agglomeration performance of hydrogel particles were coupled with the kinetic inhibition performance of Luvicap. The under-inhibition with MEG was also possible as MEG-hydrogel can be synthesized readily. To the best of our knowledge this is the first attempt to present a base platform to incorporate different hydrate inhibition strategies. The polymer hydrogel would provide a flexible option to manage the


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hydrate formation risks by coupling its anti-agglomeration performance with thermodynamic or kinetic hydrate inhibition performance considering the specific aspects of offshore oil and gas fields.

ACKNOWLEDGMENT This work was supported by the Technology Innovation Program (10038605) funded by the Ministry of Trade industry & Energy (MI, Korea) and also supported by the Global Leading Technology Program of the Office of Strategic R&D Planning (OSP) funded by the Ministry of Knowledge Economy, Republic of Korea (10042424).


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Performance of Polymer Hydrogels Incorporating Thermodynamic

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