A new hydrate deposition prediction model for gas-dominated systems with free water
Introduction
Gas hydrates are crystalline inclusion compounds in which light hydrocarbon species (e.g., methane, ethane, etc.) are trapped by hydrogen-bonded cages that are formed by water molecules at low temperatures and high pressures (Sloan and Koh, 2008). As oil and gas exploration expands into deep-water regions, where low-temperature and high-pressure environments are conducive to hydrate formation, the risk of hydrate deposition in a pipeline obviously increases (Boxall et al., 2009, Sohn et al., 2015). Once hydrate deposition occurs in a pipeline, the safety of personnel and production equipment is severely threatened (Grasso et al., 2014, Lee et al., 2013, Sloan, 2005). The pipeline may burst due to a sharp rise in pressure, which directly influences hydrocarbon production and offshore transportation. The hydrate plug will behave as a high-speed projectile because of the large pressure difference between the front and back of the plug, which can cause very serious damage to the pipeline and platform (Jassim et al., 2010, Sloan, 2003). Therefore, hydrate formation and deposition in the deep-water region should be paid enough attentions (Ballard, 2006).
Previous studies focused on hydrate formation and deposition in oil-dominated systems. Boxall et al., 2009, Davies et al., 2010 proposed and improved the CSMHyK model, which can predict temporal and spatial hydrate plug formation in oil-dominated flowlines by monitoring changes in pressure and relative viscosity. The prediction results agreed with the experimental results and the model received good industrial application. Using computational fluid dynamics analysis and the population balance approach technique, Balakin et al. (2016) simulated the agglomeration and deposition of hydrates in oil-dominated pipelines, and the results were validated with experimental data. Creek et al., 2011, Creek, 2012 proposed a hydrate management approach that allows hydrates to form when no significant deposition risk exists in the flowlines. This method requires more accurate theoretical models and simulation tools to predict hydrate deposition risk (Di Lorenzo et al., 2014a).
There is also an abundance of research available on hydrate formation and deposition in gas-dominated systems. Bondarev et al. (1982) first established a hydrate layer growth model for gas flow in tubes by considering the heat transfer between the gas and the surrounding environment. The modeling results indicated that the maximal thickness of a hydrate layer at a given time appears at a certain location. Based on the formation and sloughing of hydrate layer on pipe wall, Lingelem et al., 1994, Sloan et al., 2011 proposed a conceptual mechanism for the formation of hydrate plugs in gas-dominated systems (shown in Fig. 1). Di Lorenzo et al. (2014a,b) carried out experimental investigations on hydrate formation and transportation in a gas-dominated flow loop that presents annular-mist flow, and received an abundance of valuable experimental data. Aman et al. (2016) studied the effect of velocity and subcooling on hydrate formation and deposition in a gas-dominated flow loop, and found that liquid droplet entrainment in the gas phase is important for hydrate formation and deposition. Li et al. (2013) developed a high-pressure flow loop with an inner diameter of 25.1 mm and a length of 30 m to research hydrate plug formation in a natural gas pipeline. Wang et al. (2016) developed a coupling model to predict the hydrate deposition rate in gas-dominated systems, which considered hydrate formation in liquid film for the calculation of the thickness of the hydrate layer.
Rao et al., 2011, Rao et al., 2013 demonstrated that the mechanism of hydrate deposition in water-saturated gas systems is similar to frost/ice deposition, which goes through initial formation, growth, and annealing stages, based on experimental results. Aspenes et al. (2010) verified that entrained hydrate particles are more likely to deposit on the water-wet pipeline wall through experimental investigations. Their results suggest that the deposition of hydrate particles (formed by liquid droplets in the gas phase) should be included in the hydrate deposition prediction model for gas-dominated systems with free water.
However, there are very few studies available that explain the deposition principle of hydrate particles formed by liquid droplets entrained in the gas phase, which affects the flow passage of the pipeline. Hence, it is necessary to research the impacts of hydrate particle deposition on hydrate layer growth, rather than consider hydrate formation in liquid film only. This approach is very important for accurately predicting hydrate blockage timing and location, and for preventing hydrate blockages from occurring.
Section snippets
Model development
In gas-dominated systems with free water, where gas is the continuous phase and water is the dispersed phase, the flow pattern is an annular-mist flow when the superficial gas velocity reaches the critical value. This value can be predicted by a relation developed by Taitel et al. (1980). As hydrates form continuously, gas-liquid flow turns into gas-liquid-solid flow in the pipeline, and hydrates begin to deposit on the pipe wall, as shown in Fig. 2a.
A new model is developed via analyzing the
Model performance and comparison with experimental data
The accuracy of the proposed model is verified with experimental data from Di Lorenzo et al. (2014a). The experimental loop is composed of stainless steel with an inner diameter of 0.8 in (20.3 mm) and a length of 131 ft (40 m). The gas used in the experiments is domestic pipeline gas and its approximate composition can be seen in reference (Di Lorenzo et al., 2014b). The gas and liquid flow rates were maintained at 169 L/min and 1.6 L/min in the experiment, where the superficial gas velocity and
Conclusions
In this work, under the comprehensive considerations of the deposition of the hydrates formed in liquid film and liquid droplets, a new model is developed to predict hydrate deposition in gas-dominated systems with free water. In the new model, we proposed a macroscopic method with a parameter of effective deposition ratio (EDR) to estimate the deposition of hydrate particles from the gas phase, including the mass exchange and the interaction between liquid droplets and liquid film.
The value of
Acknowledgments
The work was supported by the National Key Basic Research Program of China (973 Program, 2015CB251200), National Natural Science Foundation–Outstanding Youth Foundation (51622405), the Program for Changjiang Scholars and Innovative Research Team in University (IRT_14R58), and the National High Technology Research and Development Program of China (863 Program, 2013AA09A215).
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