Numerical modeling on hydrate formation and evaluating the influencing factors of its heterogeneity in core-scale sandy sediment

• Published in: Journal of Natural Gas Science and Engineering
• Main Authors: Song, Rui, Sun, Shuyu, Liu, Jianjun, Feng, Xiaoyu
• Other Authors: Physical Science and Engineering (PSE) Division, Earth Science and Engineering Program, Computational Transport Phenomena Laboratory (CTPL), King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, KSA, School of Geoscience and Technology, Southwest Petroleum University, Chengdu, 610500, China, State Key Laboratory of Geomechanics and Geotechnical Engineering, Wuhan Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, 430071, China
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier BV 2021
• Subjects: Clayton; Methane hydrate
• Online Access: http://hdl.handle.net/10754/669438; https://doi.org/10.1016/j.jngse.2021.103945

Natural gas hydrate (NGH) has been regarded as a fossil fuel reserve for the future on account of its tremendous potential. The numerical modeling on NGH formation/dissociation mechanism contributes to better understanding its accumulation and distribution feature, and optimizing the development program. This paper aims to develop a new simulator for the NGH formation in the core-scale sandy sediments based on the computational fluids dynamic (CFD) methods. The mathematical model is established based on the kinetic reaction model of hydrate formation, the permeability reduction model by the NGH, model of heat and mass transfer in porous media. The hydrate formation model is programmed by C language, and used as a subroutine for Fluent software which is adopted to solve the governing equations of the multiphase flow. The simulator scheme is verified by comparison with the experiment and numerical simulation in literature. What's more, this study reproduces the same fluctuant tendency of temperature as the experiment during the 1.0 h–2.0 h for the first time. Different reaction surface models of NGH formation/dissociation are evaluated by the developed codes. The effects of the reaction surface of hydrate (RSH) model and the initial fluids distribution on the hydrate formation process are simulated and analyzed. The variation of the RSH in NGH formation/dissociation should be taken into consideration when modeling the hydrate re-formation in the exploitation of NGH. The initial distribution of water and gas has a great impact on the hydrate formation in the sealed reactor. The hydrate distribution is ununiform, even when assuming the water and methane are mixed uniformly in a homogeneous porous media. This study provides new insight for the parametric estimation of the RSH model in the hydrate formation and dissociation modeling. Many scholars tried to generate the NGH using methane of high purity and water in the designed reactor. Some surfaces of the rector are made of transparent materials, e.g., Lucite, which provides visualization of the NGH formation. The formation kinetics of NGH was believed to be dependent on interfacial area, temperature, pressure, as well as the surfactants in the water (Zhong and Rogers, 2000a; Vysniauskas and Bishnoi, 1983). Stern et al., 1996, 1998 injected the cold CH4 gas at 23 MPa and 255 K into the silica tube filled with the ice grains, and investigated the formation process of the NGH which coated on the ice grain surface. However, most natural NGH is stored in the porous sediments, which raised the difficulty in investigating the formation and dissociation process of NGH in the lab (Song et al., 2020a, 2020b). Thus, some scholars have tried to produce methane hydrate in the synthetic cores using natural sandstone or synthetic cores generated by the mineral grains (e.g., quartz glass beads, quartz sand, corundum, and silicon carbide) and cylindrical vessel with different sizes. The high-pressure condition is needed due to the low solubility of methane in water and for the sake of stabilizing methane hydrate (Vlasic et al., 2019a). Some scholars used the immiscible mixture of water and the tetrahydrofuran (THF) or tetra-n-alkyl ammonium halides to generate the THF hydrate as an experimental analogy to natural NGH at a relatively higher temperature and lower pressure (Pearson et al., 1986). However, the accuracy of these substitutions has been contentious in the literature (Vlasic et al., 2019b; Lee et al., 2007). Handa and Stupin (1992) synthesized NGH in the porous silica gel pores, and found that the equilibrium pressures were 20–100% higher than those for the bulk hydrates. They also studied the dissociation process of NGH, and determined the composition of hydrate as well as its dissociation heat. Since that they used excess gas to stabilize the hydrate after the formation process, the proposed workflow was also called as the excess-gas method. Since then, many scholars have conducted many experiments on the synthesized NGH using the excess-gas method. Bagherzadeh et al. (2011) investigated the hydrate formation and dissociation process in different sand particle size ranges and different initial water saturations with nuclear magnetic resonance (NMR) technique. They observed the ununiform distribution of NGH in the porous media, and found that the formation rate increased with the decreasing of the water content and grain size. Linga et al. (2012) found that the rate of NGH formation in the fixed bed column is significantly greater than that in the stirred vessel. Chong et al. (2015) found that the presence of the NaCl inhibits the kinetics of methane hydrate formation in porous media. Madhusudhan et al. (2019) studied the effects of the NGH on the stiffness and strength of the hydrate-bearing sands, which were generated by the excess-gas methods. Besides, some scholars injected more water than that needed for the hydrate formation into the reactor, which is called the excess-water method. Priest et al. (2009) firstly proposed this technique and found that the NGH morphology was mainly frame supporting, and studied the stiffness of sands cemented by the NGH (Clayton et al., 2010). Best et al. (2013) studied the effect of NGH morphology and water saturation on seismic wave attenuation of the synthetic methane hydrate-bearing sand created under excess-water conditions. Chong et al. (2016) generated the hydrate in the packed sandy grains in the cylindrical vessel, and found that the fractional conversion of methane is around 81.5%. They also investigated the gas production of the synthesized hydrate sandy sediments and found that the temperature driving force of about 2.1 K was required to achieve a 90% dissociation within 10 h. Yang et al. (2017) investigated the dissociation process of the NGH by depressurization using magnetic resonance imaging (MRI) technique in the hydrate sediments, which was formed by excess-gas or excess-water methods. The heterogeneous NGH distribution of the monitored panel by MRI was revealed according to the changing of the water saturation. In addition, the Kinetic promoters (e.g., sodium dodecyl sulphate, SDS) were also adopted to improve the gas consumption rate and to decline the induction period of the methane with water to form the hydrate without affecting the hydrate phase equilibrium (Zhong and Rogers, 2000b; Huang and Fan, 2005; Mech et al., 2016). Spangenberg et al. (2005) synthesized the NGH using the dissolved methane in the aqueous solution without injected free gas, which was also called the dissolved-gas method. This method is based on the theory that the solubility of CH4 in the warmer water was higher in the presence of hydrate, which means the seeded hydrate was required initially (Waite and Spangenberg, 2013). The generated hydrate saturation by the dissolved-gas method can reach 95%, far beyond the excess-water or excess-gas method reported in the literature, but the formation rate was slower. However, it was difficult to determine the spatial hydrate distribution of these experiments in the porous media, though most scholars have inferred indirectly the heterogeneity of the NGH distribution by the relevant experimental phenomenon. Kneafsey et al., 2007, 2011b investigated the migration of the mineral grains and water during the formation by different techniques and dissociation of the methane hydrate in the sandy core using X-ray computed tomography (CT). They confirmed that the distribution of the NGH produced by the technique mentioned above was ununiform. However, the resolution of the CT images in their study was too low to distinguish the hydrate in the pores. Some scholars (Wang et al., 2018a; Li et al., 2019) used the micro-CT technology to investigate the spatial distribution of the NGH generated in the lab, and revealed its heterogeneity, but the ice phase and NGH phase in the high-resolution image were difficult to distinguish accurately (Wu et al., 2020; Yang et al., 2015). Thus, the numerical modeling on NGH formation become a relevant and important supplement for the lab experiments, which has been emphasized by many scholars.This work was financially supported by National Natural Science Foundation of China (Grant Number 51909225, 51874262); King Abdullah University of Science and Technology (KAUST) (Grant Number BAS/1/1351-1301); and financial support from China Scholarship Council.