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Experimental Study of Methane Hydrate Formation in Water-continuous Flow Loop Weiqi Fu, Zhi-yuan Wang, Xinjian Yue, jianbo zhang, and baojiang sun Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00132 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019
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Experimental Study of Methane Hydrate Formation in Water-continuous Flow Loop Weiqi Fu1, Zhiyuan Wang3*, Xinjian Yue2, Jianbo Zhang1, Baojiang Sun1* 1. School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, China 2. Institute of Experiment and Test, Xinjiang Oil Field Company, PetroChina, Karamay 834000, China 3. Institute of Offshore Oil & Gas and Hydrate Research, China University of Petroleum (East China), Qingdao 266580, China * Corresponding author Email address:
[email protected] (B. Sun);
[email protected] (Z. Wang) Abstract As the offshore oil and gas fields are maturing, the water production rate from the reservoir is increasing progressively year by year. The methane hydrate formation in water-continuous system become a significant flow assurance issue for offshore oil and gas production. In this study, a group of methane hydrate formation experiments are designed to study characteristics of the hydrate formation in the water-continuous flow loop which performed under void fractions from 2.6% to 5.0%, flow velocities from 1.24 to 1.57m/s, subcooling temperatures from 4.5 to 7.2°C and hydrate particle concentration from 0 to 0.14kg/kg. The methane hydrate formation process is considered as a mass transfer process and the multiple influencing factors on the hydrate formation are analyzed experimentally, such as flow velocity, subcooling temperature and hydrate particle concentration. Results show that higher flow velocity induces the higher hydrate formation rate. Higher hydrate particle concentration results in the lower hydrate formation rate. Thus, an integrated mass transfer coefficient is proposed including the effect of the hydrate particle concentration and the flow velocity. In this work, the effect of subcoolings on the integrated mass transfer coefficient is found to be negligible. A corresponding mass-transfer-limited hydrate formation model is proposed to predict methane hydrate formation in the water-continuous system, which is a function of the proposed integrated mass transfer coefficient, flow velocity, hydrate particle concentration, subcooling and gas-liquid interfacial area. After comparing with experimental data, the proposed hydrate formation model shows its good agreement with experimental data. 1. Introduction Gas hydrates are crystalline compounds which form from hydrocarbon gas and water under high pressure and low temperature condition [1]. Gas hydrate formation in deep-water gas and oil transportation pipeline is considered as a serious flow assurance problem where causes impermeable plug in pipeline [2,3]. Previous researchers focus on the hydrate formation in the water-in-oil emulsion system [4,5] and the gasdominated system [6-11]. In recent years, the hydrate formation in the water-continuous system began to attract some attentions from researchers because the large water production happened in the maturing oil and gas field [12] and the development of gas
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hydrates in deep water also accompanied with the large water production [13]. Therefore, understanding the mechanism of hydrate formation in water-continuous system becomes important in developments of crude oil, natural gas and gas hydrates. The mechanism of gas hydrates is initially studied by Vysniauskas et al. [14], Topham [15] and Englezos et al. [16,17] through performing experiments of gas hydrate formation in the stirred reactor and the mechanisms of hydrate formation can be generally concluded as the intrinsic kinetic, the mass transfer or the heat transfer. Although numbers of hydrate formation models were developed based on the experiments in stirred reactor empirically, mechanisms of hydrate formation in industrial conditions, such as pipeline, are quietly different with the ideal experimental conditions. The models developed based on the stirred reactor shows the poor performance on the industrial application. Therefore, considering the industrial application, numbers of researchers began to investigate the mechanisms of hydrate formation in simulated industrial environments, such as the water-in-oil emulsion, the gas-dominated system and the water-continuous system. The varied hydrate formation models are proposed based on different hydrate formation mechanisms, including the intrinsic kinetic, the mass transfer and the heat transfer. The water-in-oil emulsion happened in the oil-gas mixed transportation pipeline and the hydrate formation is treated as an important flow assurance problem. Boxall et al. [9] used the PVM and FBRM to study the hydrate formation and dissociation in the water-in-oil emulsion and developed a strategy to find out when and where the hydrate plug occurring in water-in-oil emulsion pipeline. Thinking that entrained water droplets in oil phase is the main resource contributing to the hydrate formation, Turner et al. [21] developed an inward growth hydrate shell model for water droplets. Gong et al. [19] and Shi et al. [20] developed an inwards and outwards hydrate shell formation model sequentially to describe hydrate shell formation on water droplets in the waterin-oil emulsion considering the intrinsic kinetic and the heat-mass transfer. The Center for Hydrate Research in Colorado School of Mines developed a transient hydrate formation model called CSMHyK model through decades of efforts [2,3]. The CSMHyK model describes the procedure of hydrate plug in the water-in-oil emulsion comprehensively and has been extended to the commercial application. The majority of works on hydrate formation and deposition in the gas-dominated system are just began after 2010. Di Lorenzo et al. [6,7] studied the hydrate formation and deposition in the gas-dominated system experimentally and a hydrate formation model for the gas-dominated system is developed based on Turner et al. model [18]. Through experimental realizations, the hydrate formation in liquid film dominated the hydrate formation and deposition in the gas-dominated system. Amen et al. [10] studied the influence of flow velocity and subcooling temperature on hydrate formation and deposition in the gas-dominated system experimentally, and provided a new insight for developing the next-generation hydrate formation and deposition model, which the deposited hydrate layers are removed effectively under the subcooling temperature from 3.5°C to 6°C. Combing with the multiphase flow concept of the annular flow, Wang et al. [8] believed that the water droplets entrained in gas phase of annular flow also contributed to the hydrate formation and deposition in the gas-dominated system
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and modeled the hydrate deposition behavior in the gas-dominated system with free water. For hydrate formation in water-continuous system, Tang et al. [21] performed natural gas hydrate formation experiments in the ejected-type loop reactor and studied hydrate formation rates during large amounts of gas bubbles entrained in the liquid phase. Joshi et al. [12] performed gas hydrate formation experiments under high water cut flow condition in the flow loop where the slug flow is verified in all experiments. The effect of mixture velocity from 1 to 2.5m/s and liquid loading from 50% to 90% are investigated and a hydrate deposition mechanism in 100% water cut system is proposed. Shimizu et al. [13] observed the process of hydrate formation in the bubbly flow experimentally and concluded that the hydrate slurry after the end of hydrate formation becomes a hydrophilic crystal colloid where fine particles of methane hydrates suspends in water homogenously. Fu et al. [25,43] have performed the experiments to investigate the methane hydrate formation in the water-continuous system and pointed out that the hydrate formation is controlled by the mass transfer process. Although the hydrate formation in water-continuous system has been attracted some attentions, only a few researches have just been started in recent years [12,13,21]. More works are still required to study the phenomenon and characteristics of hydrate formation in the water-continuous system further. Besides, the multiphase flow system transfers from methane-water two-phase flow system to methane-water-hydrate particle three-phase flow system after hydrate particles forming, but the influence of hydrate particles on hydrate formation is ignored in the past researches. Obviously, the mass transfer phenomenon in three-phase flow are more complex than that in the two-phase flow. Ignoring the effect of hydrate particles during hydrate formation leads to the large discrepancies during modelling the hydrate formation in the water-continuous system. In this work, groups of hydrate formation experiments are designed under the void fraction of about 5.0% and flow velocities from 1.24 to 1.57m/s. In experiments, the HPC increases from 0 to 0.14kg/kg and the influence of hydrate particles on the hydrate formation in the water-continuous system is firstly elaborated. A corresponding empirical correlation of the mass transfer coefficient is developed based on experimental data as a function of the hydrate particle concentration (HPC) and the flow velocity. A mass transfer hydrate formation model is also built.
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2. Experimental Experimental facility. The main test section of the multiphase flow system is a total length of 826cm pipe (Pipe1, Pipe2 and Pipe3) with a transparent PVC pipe of 50cm, as shown in Figure.1. All pipes have the pipe-in-pipe structure and made by stainless steel with inter diameter (ID) of 25.4mm and outer diameter (OD) of 63.5mm, which have pressure tolerant of 23.5MPa. Because the pressure tolerant pressure of the transparent PVC pipe is 13.5MPa, the maximum pressure of the multiphase system is 13.5MPa. In our experiments, all experimental pressures are below 10MPa considering the safety issue. The detailed instruction of the facility has been given in our previous works [25, 43].
Figure.1 Schematic of multiphase flow hydrate formation experimental facility: TM: Thermometer; PT: pressure meter; ΔPT: Differential pressure meter. [25,43] Experimental procedure. The pure methane with the purity of 99.99% and the deionized water with density of 1g/cm3 are used to form methane hydrates in experiments. The multiphase flow in the experimental flow loop is verified by pictures taken by the high-speed camera as methane-water-hydrate particles three-phase flow system. The experimental procedure is depicted as follows: 1. The whole experimental facility is clean by circulating deionized water. The total volume of experimental facility is measured as 9702cm3. 2. The experimental facility is dying by blowing nitrogen into the system and make sure that no residual water remains in the system. After vacuuming the flow system by vacuum pump, the deionized water is injected into the experimental facility and an electric balance is used to detect the weight of deionized water injected in the system. 3. After injecting required amount of deionized water, the gas cylinder with 13MPa supplies to the flow loop from gas cylinder slowly until the system pressure is 3MPa (lower than hydrate equilibrium condition). 4. Start the circulation of the flow loop and Chiller, as the system temperature decreases, the system pressure decreases gradually with no hydrate formation in this stage, because the methane dissolves in deionized water slowly. When the system temperature and the system pressure keep almost the constant, the deionized water in the flow loop is fully saturated with pure methane. 5. Reinject pure methane into the flow loop until the desired system pressure (higher
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than hydrate equilibrium condition). Circulate the flow loop again and methane hydrates start to form in the flow loop. 6. As methane hydrates are forming in the flow loop, the real-time system pressures, system temperatures, flow pressure drops and flow rates are monitored and collected by computer at each 5s. 7. Since water is incompressible, the system pressure of flow loop will decrease during the hydrate formation is due to the methane consumption for the hydrate formation. Thus, the amount of methane hydrates formed in the flow loop can be determined based on the difference of the system pressures at the moment of hydrate formation onset and the moment of hydrate formation end via the gas equation of states. 8. When the system pressures arrive at a constant value, it means that the methane hydrate equilibrium condition is reached. 9. After the hydrate formation stopping, calculate the present amount of the hydrate particles in the flow loop and inject pure methane into the flow loop again to the desired system pressure. 10. Start the circulation pump and the hydrate formation starts again inside the flow loop with the presence of hydrate particles. Thus, the HPC in the flow loop will ascend accompanying with numbers of gas injections. In this study, groups of experiments are performed to investigate the hydrate formation in the multiphase flow system under varied the hydrate particle concentration (HPC) condition. The range of HPC is from 0 to 0.14kg/kg (0 to 11vol.%). The hydrate formation rates are investigated under mixture velocity of 1.24m/s, 1.35m/s and 1.47m/s. The initial void fraction starts at 5% approximately. 3. Results and discussion Experimental results. Figure.2 gives two hydrate formation experiments under HPC of 1.54% and 8.92%, and void fractions of 4.49% and 3.15%. Blue line, green line, orange line and yellow line are the experimental system pressure, the methane hydrate equilibrium temperature, the experimental system temperature, the subcooling temperature, correspondingly. The hydrate formation in each experiment experiences three different steps: Hydrate formation onset, Hydrate formation end, Hydrate slurry velocity increasing. All steps are marked by red circle in Figure.2. In the step of hydrate formation onset, the decreasing of system pressures stands for the hydrate formation in the flow loop because the methane and water are consumed for forming methane hydrates. In Figure.2(a), a clear angle of system pressure curve (blue line) is found when the hydrate formation stops. Meanwhile, the hydrate equilibrium temperatures (green line) coincide with the system temperatures (orange line) perfectly and the subcooling temperatures are zero. However, no clear angle is found for the end of hydrate formation in Figure.2(b). The system pressures have obviously smoother and slower decreasing rates in Figure.2(b) than that in Figure.2(a). When the hydrate formation ends in Figure.2(b), the system pressures keep almost the constant of 4.3MPa and the subcooling temperatures are kept at 1°C (apparently larger than 0). The curve of methane hydrate equilibrium temperatures are still away from the system temperatures. In the current experiment, both the subcooling condition and the void fraction condition satisfy the requirement of the hydrate formation, but the hydrate formation is inhibited.
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(a) 1st Injection, αhydr,f=1.54%; αgas,f=4.49%
(b) 7th Injection, αhydr,f=8.92%; αgas,f=3.15%
Figure.2 Hydrate formation under HPC of 1.54% and 11.67% After the hydrate formation stops, the hydrate slurry is accelerated to 2.2m/s and two distinct performances of the system pressures are shown in Figure.2(a) and (b). In Figure.2(a), all the system pressures, the hydrate equilibrium temperatures and the system temperatures increase mildly, because extra heat is released due to extra friction caused by the acceleration of the hydrate slurry. However, in Figure.2(b), as hydrate slurry velocity increasing, the system pressure decreases sharply and extra hydrates are formed again in the system. Since the deionized water is used in the experiments, the hydrate formation cannot be affected by the salinity of water. Therefore, according to comparisons between Figure.2(a) and Figure.2(b), the hydrate formation may be affected by the HPC in the hydrate slurry and the flow velocity of the hydrate slurry. One possibility is that the existence of large quantities of suspended hydrate particles around methane bubbles lowers the mass transfer rate between methane phase and water phase during the hydrate and hampers the hydrate formation. When the flow velocity of the hydrate slurry increases, the mass transfer phenomenon is further enhanced and gas hydrates start to form again until reaching the new equilibrium
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condition, as shown in Figure.2(b). The detailed explanation for the influence of HPC on the mass transfer phenomenon is given in the following section. Figure.3 shows morphologies of hydrate slurry at the end of the Hydrate formation corresponding to Figure.2(a) and (b). All pictures are taken under the same degree of backlight by the high-speed camera. Thus, decreasing brightness of photos means the increasing of HPC in the flow loop. From Figure.3(a) and (b), methane bubbles still exist in the flow loop. It demonstrated that the void fraction condition meets demands of the hydrate formation and all methane bubbles are not consumed for the methane hydrates at the end of the Hydrate formation.
(a) αhydr=1.54%; αgas=4.49%; (b) αhydr=8.92%; αgas=3.15%; vL=1.24m/s vL=1.24m/s Figure.3 Morphology of hydrate slurry under varied void fractions and hydrate concentrations The influence of hydrate particle concentration (HPC). The hydrate formation in methane-water bubbly flow induces the flow regime transferring to the methane-waterhydrate particle flow. The gas-liquid-hydrate particle three-phase flow is quietly similar with the gas-liquid-solid particle flow system. Although methane hydrates prefer to crystalize on the interface between gas phase and liquid phase, and form as the hydrate shells covering methane bubbles at the initial stage [22-24], the hydrate shells may be soon sloughed and broken up into small hydrate particles by the high motion of liquid phase [25]. Considering that the hydrate formation is a mass transfer limited process [16,17,26] and the solid particles also play an important role in the mass transfer of gasliquid-solid system [27-29], the effect of hydrate particles should be discussed when we study the methane hydrate formation in the methane-water hydrate slurry. The behavior of solid particles suspended in liquid phase will suppress the mass transfer process as the solid particle concentration continuously increasing in the multiphase flow system [29-31]. Roman and Tudose [31] experimentally studied the mass transfer coefficient in gas-liquid-CaCO3 particles system and indicated that the mass transfer coefficient increase slightly until the CaCO3 particle concentration of 0.05kg/kg under the constant stirring rate. However, when the CaCO3 particle concentration increases beyond 0.05kg/kg, the mass transfer coefficient decrease significantly. Ozkan et al. [30], Littlejohns and Daugulis [32], Kherbeche et al. [33] and Wongwailikhit et al. [34] had the similar experimental observations with Roman and Tudose [31] where the mass transfer coefficient for different types of gas-liquid systems
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would reach the peak firstly at varied solid concentrations for varied kinds of solid particles and then decrease gradually with the solid concentrations increasing. Mena et al. [35] studied the mass transfer phenomenon between gas and liquid influenced by large diameter particles (590
