Macroscopic Observations of Catastrophic Gas Hydrate Growth

Article pubs.acs.org/crystal

Macroscopic Observations of Catastrophic Gas Hydrate Growth during Pipeline Operating Conditions with or without a Kinetic Hydrate Inhibitor Jega Divan Sundramoorthy,*,† Paul Hammonds,‡ Khalik M. Sabil,# Khor Siak Foo,⊥ and Bhajan Lal§ †

Baker Hughes (M) Sdn. Bhd, 207 Jalan Tun Razak, 50400 Kuala Lumpur, Federal Territory of Kuala Lumpur, Malaysia Cairn India Ltd., Jacaranda Marg, DLF City, Gurgaon 122002, Haryana, India # Institute of Petroleum Engineering, School of Energy, Geoscience, Infrastructure and Society, Heriot-Watt University Malaysia, No. 1 Jalan Venna P5/2, Precinct 5, 62200, Putrajaya, Malaysia ⊥ Murphy Oil Corporation, Level 26, Tower 2, Petronas Twin Towers, Kuala Lumpur City Centre, 50088, Kuala Lumpur, Federal Territory of Kuala Lumpur, Malaysia § Chemical Engineering Department, Universiti Teknologi Petronas, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia ‡

ABSTRACT: This paper presents the macroscopic observation of catastrophic gas hydrate growth during a shut-in, cold start-up and flowing conditions, simulating a natural gas transmission line operation. All experiments are conducted with a fixed simulated natural gas composition to form structure II gas hydrate with 11 K subcooling in the isochoric rocking cells. In order to simulate inhibited test systems, a formulated copolymer of vinylpyrolidone and vinylcaprolactam (PVP/PVCap) is included in some of the cells studied. Detailed macroscopic images and interpretation of pressure (P) and temperature (T) data are used to present our findings. It is found that production profiles such as different shut-in time and the mechanism of mass transfer of water from the bulk water phase to gas hydrate phase influence the gas hydrate growth in distinctive ways. Moreover, the capillary force in the gas hydrate structure may provide a greater driving force to promote gas hydrate growth than the diffusion rate of gases into the bulk water phase under shut-in and cold-start up conditions. Additionally, the number of critical nuclei formed during the initial stage of gas hydrate growth may influence the type of bulk gas hydrate present in the system at a later stage, i.e., finely dispersed hydrates or a slush type of gas hydrate. interfere with hydrate nucleation and/or growth.11−15 Therefore, hydrate nucleation and/or growth can be delayed during the residence time of fluids exposed to conditions vulnerable to gas hydrate formation.16−18 Unlike THIs, treatment dosage for KHIs cannot be determined with commercially available thermodynamic software. Therefore, KHIs are normally selected through a tedious and time-consuming laboratory performance testing program.19 Laboratory tests are conducted to determine the induction time using brine and gas composition to mimic the P−T conditions in the field. Induction time is defined as the formation of first stable hydrate structure under field simulated conditions.19,20 KHIs that can prolong the induction time beyond the fluids residence time in the system under hydrate formation conditions are considered for field trial. However, researchers have pointed out that selection of a KHIs based on the induction time alone can be very risky.21 For example, some KHIs may function as excellent nucleation inhibitors in a particular system, but as

1. INTRODUCTION Clathrate hydrates are nonstoichiometric solids inclusion compounds that consist of polyhedral structures formed by hydrogen-bonded water molecules and stabilized by encaged guest molecules through van der Waals forces.1,2 On the basis of guest molecule size, a gas hydrate can take the form of cubic structure I (sI), cubic structure II (sII), and hexagonal structure H (sH).3−6 Recently, gas hydrates have been identified to be of great relevance in diverse areas including energy, environmental deposition, astrophysics, geology, and marine ecosystems.7−9 However, the formation of gas hydrates in petroleum transmission lines, production wells, and process equipment may lead to blockage. This is a major concern to the industry as gas hydrate blockages result in safety hazards, ecological risks, and eventually economic losses.6−8 The key to manage the risk of gas hydrate formation in natural gas transmission line is chemical inhibition.5,6,10 It is reported that concentration of a kinetic hydrate inhibitor (KHI) equal to or less than 1.0 wt % based on water cut can effectively prevent gas hydrate blockage.5−7 However, these water-soluble polymers do not prevent hydrate formation like the thermodynamic hydrate inhibitors (THIs) but rather © 2015 American Chemical Society

Received: September 24, 2015 Revised: October 26, 2015 Published: October 27, 2015 5919

DOI: 10.1021/acs.cgd.5b01376 Cryst. Growth Des. 2015, 15, 5919−5929

Crystal Growth & Design

Article

Figure 1. Schematic diagaram of the rocking cell.37

contrast, there is a potential for the bulk water phase to be already seeded with gas hydrate crystals or hydrate nuclei under a cold start-up condition.28 Therefore, a sudden increase in the gas concentration will facilitate a catastrophic gas hydrate event as the mass-transfer barrier is now removed.28 There are a number of publications where gas hydrate formation has been studied under shut-in (static)9,16,23,24 and continuous flowing (rocking/stirred)3,11,13−15,29−33 conditions. However, to the best of our knowledge, published work to evaluate the risk of gas hydrate formation under cold start-up conditions is still scarce.21 Researchers are also reporting that the fundamental knowledge gained from gas hydrate growth observation is significant for industrial process optimization, where prediction of macroscopic flow or transport characteristics may be possible.2,34−36 Nevertheless, visual observations of sI and sII crystal growth of mixed hydrates are reported to be extremely limited.16,37 With intense interest to understand the basic fundamental nature of how gas hydrates may grow in simulated petroleum transmission lines, researchers have also started to describe crystal growth behavior in flowing liquid water presaturated with simulated natural gas with the aid of growth images.34−36 However, to the best of our knowledge, there is still no published microscopic and macroscopic observational work that describes the impact of different shut-in periods on catastrophic gas hydrate growth during a cold restart, with or without a KHI. In a shut-in condition, it is found and described that capillary aided gas hydrate growth may result in a catastrophic gas hydrate growth in an uninhibited system.37 Researchers have also hypothesized that capillary aided water transport may result in a catastrophic gas hydrate growth in a KHI inhibited system.38 But there is still very limited research with explicit experimental growth images that describes how gas hydrates could grow with capillary aided water transport for a KHI treated system under a natural gas pipe-line condition.16,38

soon as stable hydrate nuclei are formed, gas hydrates may grow rapidly and cause pipeline blockages.16 Therefore, indepth understanding of the KHIs inhibition properties will be an advantage. This knowledge can be crucial in deep-water production when subcooled production fluids are subjected to shut-down and cold start-up conditions. In petroleum transmission lines, fluids are normally under a steady-state flowing condition, especially during stable production.22 However, it is very likely that during operation, natural gas transmission lines go through unplanned or planned shutdown conditions. An unplanned shutdown is considered as a loss of hydrocarbon volume produced due to a production shutdown (PSD), emergency shutdown (ESD), or other events that is not predicted in advance. An unplanned shutdown will result in shutting down of production at the wellhead or a number of wells in the field. As this type of shutdown is not included in any production forecast, it poses a greater risk to flow-assurance. Meanwhile, a planned shutdown is forecasted in advance, and the process is controlled with the risk on flow assurance issues minimized. Regardless of an unplanned or planned shut-in, during start-up of a petroleum transmission line, the cold fluid, which is initially stagnant in a pipeline will go through a transient cold flow phase, before being displaced by the hotter flowing well fluids from the reservoir. This period of transient cold flow is termed a cold start-up. It is highlighted that a cold start-up condition poses greater risk for catastrophic gas hydrate growth compared to a shut-in (static) condition.21 Under a static condition, only a thin gas hydrate layer might form at the gas−water interface once the formation condition is met.23−28 Once this thin layer of gas hydrate is formed, it becomes a barrier at the gas−water interfaces, restricting mass-transfer of gas molecules from the gas phase toward the bulk water.21,25,28 As a result, growth of gas hydrates in bulk water will be retarded once the dissolved gas concentration is insufficient to support further growth. In 5920



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concentration is also added to the cell. The premixed gas is pumped into the cells until a stable pressure of 40 bar is achieved. Once filled, the cells are mounted on a custom design rocking mechanism as illustrated in Figure 1. Table 1 shows the chemical composition of the cell contents. There is a total of five experimental cycles for each cell under predetermined shut-in period which is followed by a cold startup and flowing condition.39 On the basis of CSMGem,39 only structure II gas hydrates are predicted to form under experimental conditions. Pressure (P), temperature (T), and gas phase volume (V) data are continuously used to calculate the number of moles gas consumed during gas hydrate formation by using an equation that has been described elsewhere.22 The compressibility factor (z) of methane and propane mixture is calculated from the Peng−Robinson equation of state.40 The following sections will better describe the shut-in condition and start-up and flowing condition tests. 2.3.1. Shut-In Test (3 and 24 h). Two different shut-in times are used in this experimental work. For long-term shut-in, a 24 h shut-in period time is used similar to previous work on an uninhibited system.37 A 3 h period is used to represent a short shut-in period. It is the fastest time (3 h) a field operator can recover a production deepwater field in Malaysia from a shut-in condition (planned/unplanned) to a cold start-up and flowing condition. The cell composition is rapidly cooled at a rate of 0.24 K/min under static condition. The utilized temperature gradient is similar to one of the selected subsea pipeline shut-in cooling gradient. The final temperature is maintained to be equivalent to a subcooling temperature (ΔTsub) of 11.0 K. The subcooling temperature is defined as the temperature difference between temperature of the system (277.0 K) and the gas hydrate equilibrium temperature as predicted by CSMGem. Visual observations are provided by continuous video monitoring of the cells which are positioned at 30° inclination. This is a standard position of the cell when the rocking mechanism is stopped. In this part of the work, the period of catastrophic gas hydrate growth is defined as a time taken from the initiation of capillary aided hydrate growth, until the exhaustion of free brine solution. Once the shut-in time is completed, experimental cells in which no gas hydrates form will be monitored for the start-up and flowing test stage. 2.3.2. Start-Up and Flowing Test. The start of rocking mechanism mimics the cold start-up and flowing condition. The rocking speed used in this present study is 18 counts/min. The temperature in the environmental chamber is maintained at 277.0 K throughout the duration of the cycle. Following the researchers’ suggestion,20 the catastrophic growth in this section is measured from induction time until the stoppage of the moving glass ball. The cells are heated to 308 K for 4 h to destroy the memory effect before they are being used for another test cycle.28,37,41−43

With limited understanding of hydrate growth mechanisms in a field application of kinetic inhibitors, more research work is being suggested.38 This work aims to observe and describe catastrophic gas hydrate growth with the aid of induction time and hydrate growth measurement under different pipe-line shut-in periods followed by a cold start-up condition. Using simulated natural gas mixture, all observations are carried out with 11.0 K subcooling (ΔTsub). This is done to replicate a catastrophic gas hydrate growth that has occurred in a gas transmission line that has been shut-in (short-term and long-term) and restarted at seabed temperature (277.0 K). A formulated copolymer (PVP/ PVCap) is also included to represent a KHI in this work.

2. EXPERIMENTAL APPARATUS AND PROCEDURES 2.1. Materials. A premix gas mixture of 90 mol % methane and 10 mol % propane is used in this work. The gas is purchased from

Table 1. Fluid Composition in Experimental Cells experimental cell

gas pressure in the cell (bar) (%)

NaCl conc (wt %)

KHI conc (vol %)

total volume of brine in cell (mL)

1 and 2 3 and 4 5 and 6

40 ± 2 40 ± 2 40 ± 2

3.0 3.0 3.0

0 0.5 1.0

10.0 10.0 10.0

National Oxygen Pte Ltd. and used without any further purification. Sodium chloride with >99% purity for brine preparation is purchased from Sigma-Aldrich and used to prepare brine solutions. The KHI used in this work is a blend of PVP and PVCap and is supplied by Baker Hughes. The KHI has a molecular mass of (5000−8000) g/mol in butyl glycol ether (BGE) solution. 2.2. Apparatus. Schematic of the experimental apparatus used in this present work is illustrated in Figure 1. The apparatus has six units of identical high pressure rocking cell placed inside a temperature controlled chamber. A custom designed rocking mechanism and ball count mechanism is also part of the chamber. All the functions and measurements are controlled by a PLC controller, which is integrated with data processing software. The glass ball used in the cells has a diameter of 1.8 cm ± 3%. The detailed description of this apparatus has been previously discussed elsewhere.37 2.3. Experimental Procedure. Prior to assembly, all cell parts are washed with soap solution (1% Alconox’s Citranox). Then, hot tap water is used to rinse the parts several times to remove any impurities from the soap solution. This is followed by a distilled water rinse and ultrapure water rinse. The cells are then allowed to dry in the oven at 60 °C. In this work, 10 mL of 3 wt % of brine solution is carefully injected into the cell. The solution occupies 50% of the total cell volume of 20 mL. In some cells, KHI at a predetermined

3. RESULTS AND DISCUSSION 3.1. Shuti-in Condition. 3.1.1. Induction Time Measurement under a 24 and 3 h Shut-In Condition. Figure 2 illustrates the number of cells where gas hydrate formations are

Figure 2. Illustration shows hydrate detection results during five experimental cycles: (a) 24 h and (b) 3 h shut-in period. 5921



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formation is detected under the 24 h shut-in, the induction time is represented in Figure 3. In Figure 2a, it is clearly shown that the inclusion of KHI in the system reduces the risk of hydrate formation. However, it should be noted that KHI does not completely eliminate the possibility of gas hydrate formation under this condition (2 out of 10 cells). Furthermore, the trending of the induction time for hydrate formation under shut-in condition could not be clearly distinguished as represented in Figure 3. 3.1.2. Macroscopic Observation of Catastrophic Gas Hydrate Growth under a 24 and 3 h Shut-In Condition. Utilizing detailed macroscopic hydrate growth observation, and correlating the images with induction time, the following will discuss how a catastrophic hydrate growth could be triggered in three separate cases: Case 1: uninhibited cell, Case 2: 0.5 wt % KHI cell, and Case 3: 1.0 wt % KHI cell. Similar types of growth are observed in all respective cells that form hydrates during a 24 h shut-in period (Figure 2a).

Figure 3. Induction time of gas hydrate formation under 24 h shut-in period.

detected during shut-in test condition (24 and 3 h). In this present work, no hydrate formation is detected in all the cells under a 3-h shut-in period. For cells where gas hydrate

Figure 4. (a−f) Gas hydrates growth in the uninhibited system under a 24 h shut-in condition at 277 K. 5922



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Figure 5. (a−d) Gas hydrates growth in the 0.5 wt % KHI system under a 24 h shut-in condition at 277 K.

Case 1: Catastrophic Gas Hydrate Growth in an Uninhibited System under a 24 h Shut-In Condition. This part of the work is an extension of the work that has been recently published.37 Results from this section are used to compare the results of KHI inhibited systems. Figure 4 shows an example of the macroscopic observation of a cell at selected time intervals. In Figure 4a, an isolated hydrate mass on a wetted metal surface is observed at a distance of 2.6 cm from the bulk water. The presence of this hydrate mass does not trigger any immediate gas hydrate growth in the bulk water phase. Furthermore, it is also noted that the isolated hydrate mass at the metal end-cap (Figure 4a) does not appreciably grow in size even after 235 min (Figure 4d). Therefore, condensation of water from the gas vapor alone is insufficient to grow gas hydrate catastrophically. In contrast to the observation of the isolated hydrate mass, later a fibrous gas hydrate structure is observed in the bulk water phase at 755 min as shown in Figure 4b. Then, this tangled fibrous structure slowly grows in size and forms branches of a fibrous tree rootlike structure as depicted in Figure 4c. It is also shown that this fibrous structure does not grow to become a diffusion barrier at the gas−water interphase. Instead, rapid growth of hydrate crystals at the wet sapphire surface above the gas−water interphase is observed. It seems that the crystals growing on the sapphire surface are supplied with water from the bulk water phase by a capillary action. It is also observed that the volume of the bulk water reduced significantly during the growth process. The newly forming hydrate crystals are observed to displace the “older” and denser appearing hydrate crystals away from the bulk water phase along the sapphire glass surface (Figure 4d−e). The catastrophic capillary aided water transport continues until all free water is exhausted (Figure 4f).

Case 2: Catastrophic Gas Hydrate Growth in 0.5 wt % KHI System under a 24 h Shut-In Condition. Figure 5 is a series of hydrate growth images representing a system with 0.5 wt % KHI solution. No gas hydrates are detected in the cell after 360 min at 277 K. As depicted in Figure 5b, gas hydrates are detected at the metal end-cap and sapphire glass surface after 398 min. During this period, no hydrate structures are detected in the bulk water phase indicating an effective hydrate inhibition. Similar to the observation made in Figure 4a−d, insufficient gas hydrates are formed from the water condensed from the vapor phase (Figure 5b). However, if these hydrate grows near the bulk water phase (Figure 5b), it triggers a capillary aided mass transport (Figure 5c) that utilizes all free water from the bulk water phase for a catastrophic hydrate growth (Figure 5d). Case 3: Catastrophic Gas Hydrate Growth in 1.0 wt % KHI System under a 24 h Shut-In Condition. Figure 6 shows the sequential images of capillary-aided catastrophic gas hydrate growth in close vicinity to a glass ball. A meniscus formed earlier due to the surface tension between bulk water and metal wall (end-cap of the cell) is disturbed with the formation of hydrates as shown in Figure 6a. Researchers have earlier described that any disturbance in surface tension, and a surface energy difference between a water wet solid and bulk water may result a meniscus to break, leading to a capillary action.44−46 Similarly, when a gas hydrate mass is observed to form on the surface water film, slightly above the gas−water interface, a capillary aided mass-transfer is initiated. After 10 min, it is clearly visible that the meniscus and receding bulk water level correspond directly to the growing hydrate structure above the bulk water along the sapphire glass surface as depicted in Figure 6b. As depicted in Figure 6c−e, it appears that the hydrate mass and bulk water have no direct contact. However, the wetted 5923



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Figure 6. (a−f) Gas hydrates growth in the 1.0 wt % KHI system under a 24 h shut-in condition at 277 K.

Figure 8. Induction time during cold start-up and flowing conditions for systems that do not form gas hydrates during the 24 h shut-in period.

Figure 7. Average gas hydrate growth rate (mmol/min) from nucleation (induction time) until catastrophic hydrate growth is detected under shut-in condition.

system until the exhaustion of free water as shown in Figure 6f. It is also noted that there is no fibrous tree root-like structure seen in the inhibited bulk water phase during hydrate growth.

sapphire wall and the ball surfaces provide the means by which water is transported from the bulk water phase to the hydrate structure. This action supports the gas hydrate growth in the 5924



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concentration of KHI, the capillary aided catastrophic gas hydrate growth rate becomes relatively slower. KHI used here functioned as a crystal growth inhibitor during a capillary aided water-transport. Additionally, unlike growth observed in uninhibited cells (Figure 4), it has been shown that a denser appearing hydrate barrier is mainly present in between the vicinity of inhibited bulk water and the location of newly forming hydrate crystals (Figures 5 and 6). This barrier could have provided some mass transfer restriction for bulk water transport to the growing hydrate crystals, resulting in a slower catastrophic growth. For future work, it is suggested that a shutin period longer than 24 h, and/or a higher number of experimental cycles, are conducted. This may generate more kinetic data for a better comparison. 3.2. Cold Start-Up and Flowing Condition. 3.2.1. Induction Time Measurent during Cold Start-up and Flowing Conditions Post 3 and 24 h Shut-in Period. In Figures 8 and 9, the induction time measurement can be seen to be scattered between experimental cycles (cycle 1−5). However, unlike the induction time observed during the shut-in condition (Figure 3), there is a clear and distinctive pattern between the uninhibited and inhibited systems (0.5 wt % KHI and 1.0 wt % KHI) during a rocking motion. Therefore, a more consistent inhibition can be obtained if the internal surface area above the bulk water phase is continuously renewed with KHI solution during the rocking motion. The induction time for each system is averaged and presented in Figure 10. From Figure 10, it appears that with a shorter shut-in time, hydrate formation is less susceptible during cell restart (Figure 10). However, the limited experimental cycle in this present work may limit the results to within the experimental error of the studied systems. 3.2.2. Macroscopic Observation of Catastrophic Gas Hydrate Growth during Cold Start-Up and Flowing Conditions. In Figure 11, the images represent gas hydrate growth exactly 1 min after induction time is detected. The images when the movement of the glass ball has ceased, representing a catastrophic hydrate growth, are also presented in Figure 12. The following sections describe three separate cases of a catastrophic gas hydrate growth from the point of induction time: Case 4: uninhibited cells, Case 5: 0.5 wt % KHI cells and Case 6: 1.0 wt % KHIs during cold start-up and flowing conditions. Similar gas hydrate growths (Figure 11 and Figure 12) are observed for all the cells during cold-restart. Case 4: Catastrophic Gas Hydrate Growth in an Uninhibited System during Cold Start-Up and Flowing Conditions. In Figure 11a,b, the changes of water appearance from clear to cloudy indicate formation of gas hydrates.28 Relatively, it is observed that more hydrate crystals (cloudier appearance) form in the systems with a prior 24 h shut-in period (Figure 11b). On the basis of hydrate nucleation theory,28 with a longer shut in period, it can be hypothesize that more hydrate nuclei may form. A sudden mix of gas during cold start-up enriches the water phase and possibly promotes these nuclei to achieve critical size for a rapid growth. This may suggest why more gas hydrates appear in cells from a longer shut-in period (Figure 11b). For unknown reasons, the gas hydrate growth with a relatively higher number of nuclei is observed to result in a more dispersed and finer appearing gas hydrates as depicted in Figure 12b. Case 5: Catastrophic Gas Hydrate Growth in 0.5 wt % KHI Inhibited System during Cold Start-Up and Flowing Conditions. Gas hydrates are observed to form and grow in

Figure 9. Induction time during cold start-up and flowing conditions for systems that do not form gas hydrates during the 3 h shut-in period.

Figure 10. Average induction time during cold start-up and flowing conditions post 24 and 3 h shut-in conditions at ΔTsub of 11.0 K.

Moreover, the dense hydrate mass remained close to the vicinity of bulk water during hydrate growth. This is unlike the observations made in the uninhibited cells where the dense hydrate structure moves away from the bulk water interphase along the sapphire glass surface (Figure 4). Lee42,47 has suggested that gas hydrates formed in the presence of KHI may be very porous and may allow a capillary action to transport water. These observations (Figures 5 and 6) of capillary aided mass-transport in KHI inhibited system substantiates researchers’ hypotheses.38,42,47 The formation of stable hydrate crystals above the inhibited bulk water phase as depicted in Figures 5 and 6 suggests that this area is vulnerable for gas hydrate nucleation. This observation has bearing on the actual pipeline shut-in conditions. Similar to a real pipeline shut-in condition, the experimental cells in this work are cooled down from a warm temperature (308 K) to seabed temperature (277 K). Water which is more volatile at a warmer temperature (308 K) condenses at a colder seabed condition (277 K). Since KHIs and salts are nonvolatile, the condensed material is solely water. Therefore, the presence of uninhibited condensing water at the sapphire glass surface or metal surface above inhibited bulk water may have resulted in the indistinguishable induction time measurements observed in Figure 3 during a shut-in condition. 3.1.3. Catastrophic Gas Hydrate Growth Rate Measurements during 24 h Shut-In Condition. In this work, as described earlier (Figures 4−6), catastrophic hydrate growth only occurs when gas hydrates form close to the bulk water phase; to trigger a capillary aided mass transport. Figure 7 represents the average gas hydrate growth of cells from the moment a capillary aided water transport is visually detected until all the free water are exhausted (catastrophic growth). As depicted in Figure 7, it appears that by increasing the 5925



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Figure 11. (a−f) Nucleation and growth after 1 min of induction time during cold start-up and flowing conditions.

24 h shut-in as depicted in Figure 11e,f. In both figures, hydrate crystals are observed to form only above the inhibited bulk water phase. Later, these crystals appear to grow to a catastrophic structure with the aid of mass transfer of water through capillary action (Figure 12e,f). Explicit information on hydrate growth with time-scale images from the early growth stage (Figure 11f) to become a catastrophic structure (Figure 12f) are presented and described in Figure 13. These crystals (Figure 11f) above the inhibited bulk water phase grow, while the bulk water volume recedes (Figure 13a− d). A meniscus is formed each time the bulk water phase comes in contact with the hydrate mass. Furthermore, it is observed that water is seeping out of the dense hydrate mass (on the opposite end) from the gas−water interphase (Figure 13d,e). The seeping water provides rapid new hydrate crystal formation and is part of the bulk water phase volume reduction (Figure 13d,e). These observations are also similar to capillary aided hydrate growth observed during a shut-in period of inhibited cells (Figures 5 and 6). In Figure 13e (24th minute), the moving glass ball is temporarily stuck in the hydrate mass grown above. With the ball movement ceased, only the bulk water phase is moving during the cell rocking motion (Figure 13e). Capillary contact between the hydrate mass and the traces

the inhibited bulk water phase as depicted in Figure 11c,d. These observations are in contrast to the observations made during a shut-in condition as depicted in Figure 5. The rocking motion to mimic the cold start-up and flowing conditions increases the rate at which gas molecules dissolve in the bulk water phase.20 Therefore, a higher driving force is provided to facilitate mass transfer between the two phases, resulting in smaller but stable critical nuclei formation.48 Like uninhibited cells, relatively higher numbers of gas hydrates are also observed in the system with a longer (24 h) shut-in history, depicted in Figure 11d. Furthermore, relatively higher numbers of crystals are also observed along the sapphire glass surface above the inhibited bulk-water interphase. Similar to observations made in uninhibited cells (Figure 11b), gas hydrate growth with a relatively higher number of crystals are observed to result in a more dispersed and finer appearing gas hydrates as depicted in Figure 12d. In contrast, gas hydrate grown from a relatively fewer hydrate crystals produces a slush type of gas hydrates as depicted in Figure 12c. Case 6: Capillary aided Catastrophic Gas Hydrate Growth in 1.0 wt % KHI Inhibited System under a Cold Start-Up and Flowing Conditions. On the basis of visual observations, no difference could be distinguished in cells growing from a 3 or 5926



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Figure 12. (a−f) Catastrophic gas hydrate growth during during cold start-up ad flowing conditions.

4. CONCLUSION This paper presents the macroscopic observation of gas hydrate growth in uninhibited and inhibited systems during shut-in, cold start-up and flowing conditions. It has been shown that the process of catastrophic gas hydrate growth is dependent on the compositions of the fluids present, the length of shut-in period, and the locations of the initial hydrate nuclei. The results can be summarized as follows: (1) For capillary aided gas hydrate growth under a shut-in condition, KHIs may successfully interrupt the formation of stable hydrate structures in the bulk water phase, even under a high subcooling temperature (ΔTsub of 11 K). However, hydrate nucleation occurs within condensed water vapors at the sapphire glass surface. This water film is free from KHI and is deposited during the cool down phase. If gas hydrate crystals form in the close proximity to the gas−bulk water interphase, a capillary force can be initiated. The capillary force provides a rapid supply of water (inhibited/uninhibited) to grow gas hydrate catastrophically until the exhaustion of brine. (2) During cold start-up of an uninhibited or poorly inhibited (KHI) system, the flow motion (rocking) increases gas supply to the bulk phase. Therefore, gas hydrate structures are observed to form and grow in the bulk water phase. No capillary force is observed here. (3) During cold start-up of a system that has a high dosage of KHI, the

of water at the sapphire glass bottom are also clearly visible (Figure 13e). These observations (Figure 13a−f) of surface wetting with reducing water volume are characteristics of capillary aided mass transport of water.37,45,46 Particles separated from the hydrate mass above the bulk water phase that sink into the bulk water phase do not grow in this phase (Figure 13e). Later in Figure 13f (27th minute), the glass ball is observed to be stuck permanently during the rocking motion. This did not stop the capillary transport of water toward the growing hydrate mass above (Figure 13f). The reduced volume of free water in the cell is exhausted later (Figure 12f). 3.2.3. Catastrophic Gas Hydrate Growth Rate Measurements during Cold Start-Up and Flowing Conditions. The average gas hydrate growth rate (mmol/min) for all cells is presented in Figure 14. It is clear that under a cold start-up condition (Figure 14), the KHI used in this present work (PVP/PVCap) can inhibit gas hydrate growth upon nucleation. The results indicate that similar to induction time, gas hydrate growth rate is affected by the length of the shut-in period. However, with limited experimental cycle in this present work may limit the results to within the experimental error of the studied systems. 5927



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Figure 13. (a−f) Capillary aided gas hydrate growth under rocking condition between nucleation (Figure 11f) and a catastrophic growth (Figure 12f).

can be generalized to other types of KHIs as the phenomenon is initiated in an inhibitor free region of condensed water.

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AUTHOR INFORMATION

Corresponding Author

*(J.D.S.) Phone: +60327304100. Fax: +60327304314. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Baker Hughes (M) Sdn. Bhd and Universiti Teknologi Petronas for providing financial support and facilities. Authors would also like to thank Praveena Divan for her assistance in providing laboratory support during experimental work. Thanks also Greg Phillips (Murphy Oil Corp − Sabah Production) and Karthigasen Moodely (SBM − Deepwater Production) for providing assistance in this work.

Figure 14. Average gas hydrate growth rate (mmol/min) from nucleation (induction time) until catastrophic hydrate growth is detected.

facilitated mass transfer of gas to the bulk water phase (rocking motion) is still insufficient to form or grow hydrate crystals in the bulk water phase. However, if a capillary force is initiated from a hydrate structure formed above the bulk water phase, this structure above the inhibited bulk water grows and becomes a catastrophic hydrate mass with a distinctive appearance. (4) Shut-in periods are observed to result in a relatively different number of hydrate crystals. This is observed during the initial stage of hydrate growth during cold start-up. For unknown reasons, the difference in the number of hydrate crystals grown during the early stages are observed to result in either a finely dispersed hydrate mass or a slush-like hydrate mass later. (5) The framework reported in this present work

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Macroscopic Observations of Catastrophic Gas Hydrate Growth

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