Experimental Study on a Novel Way of Methane Hydrates Recovery: Combining CO 2 Replacement and Depressurization

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1 Available online at ScienceDirect Energy Procedia 61 (2014 ) The 6 th International Conference on Applied Energy ICAE2014 Experimental Study on a Novel Way of Methane Hydrates Recovery: Combining CO 2 Replacement and Depressurization Jiafei Zhao, Xiaoqing Chen, Yongchen Song *, Zihao Zhu, Lei Yang, Yunlong Tian, Jiaqi Wang, Mingjun Yang, Yi Zhang Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, school of Energy and Power Engineering, Dalian University of Technology, Dalian , Liaoning, China Abstract A novel way to exploiting natural gas from methane hydrates combining carbon dioxide replacement and depressurization was experimentally studied in a high pressure vessel of mL. Experiment results with or without the procedure of depressurization were compared to reveal the effects of depressurization on final replacement percent. Results show that making methane hydrates melt partially by depressurization will enhance replacement reaction which is restricted by the diffusion and transportation of carbon dioxide. The key factors affecting the replacement percent were investigated systematically for the first time, including: the saturation of methane hydrate, and the pressure and temperature condition. Ten runs of non-depressurization and five runs of depressurization replacement reaction were conducted with hydrate saturations ranging from 0.10 to 0.21, temperature and pressure condition above phase equilibrium line of carbon dioxide hydrates. Results show that higher saturation and temperature, and lower pressure enhance the methane exploiting and CO 2 sequestration. Pressure and temperature condition located between the phase equilibrium lines of methane hydrates and carbon dioxide hydrates is most effective with the optimum methane recovery and carbon dioxide sequestration. The combined method improves the replacement percent by about 10%, while, effects of depressurization on the stability of the sediments remains to be explored. It is supposed that methane hydrates melt once, which provides path for carbon dioxide penetrating into inner layer of hydrates Published The Authors. by Elsevier Published Ltd. This by is Elsevier an open access Ltd. article under the CC BY-NC-ND license ( Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014 Key words: methane hydrates; carbon dioxide; constant volume replacement;depressurization 1. Introduction * Corresponding author. Tel.: ; fax: address: songyc@dlut.edu.cn Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the Organizing Committee of ICAE2014 doi: /j.egypro

2 76 Jiafei Zhao et al. / Energy Procedia 61 ( 2014 ) Clathrate hydrates are of three types of structure-- SI, SII and SH [1]. As is decided by the molecule diameter, both methane and carbon dioxide can only form SI hydrate respectively [2] and will not form stable structure II as binary mixtures [3]. SI methane hydrate constitution can be expressed by the formula 8CH 4 46H 2 O. Note that methane hydrates formed in the lab are probably not ideal, the value of 6 as hydrate number allows for the possibility of empty cages [4]. In the process of exploiting natural gas from methane hydrates, researchers have explored four ways to decompose the crystalline inclusion compounds: depressurization, thermal stimulation, inhibitor injection and CO 2 replacement [5]. Among these four methods, carbon dioxide replacement offers stable long-term storage of a greenhouse gas while benefiting from methane production without requiring heat [6]. Such conception has revealed a tempting prospect and attracted many researches on this issue. The process of methane hydrates swapped by carbon dioxide has been proved feasible kinetically [7] and thermodynamically [8]. Current researches are mainly focused on the field of improving the replacement rate and finding more practical displacer to exchange CH 4. Three types of displacer have been used experimentally: gaseous CO 2, liquid CO 2 and CO 2 emulsion. Xitang Zhou et al. [9] compared the replacement rate as the equilibrium condition changed. Carbon dioxide in emulsion phase in region B of phase equilibrium diagram [7] was found to be the most efficient condition compared with gaseous phase and liquid phase due to better conductivity and diffusibility of the CO 2 emulsion. Qing Yuan et al. [10] compared the replacement percent in the literature using different displacers. CO 2 emulsion is the most effective displacer with the most CH 4 replaced and CO 2 liquid exchanged the second most CH 4 replaced while the gaseous CO 2 the least replacement percent. While in this study, replacement reaction conditions are discussed instead of different kinds of displacer, since different phase of CO 2 is due to certain pressure and temperature condition. Besides, the usage of emulsion is limited by the high cost and complexity of emulsion formation. Our effort is mainly laid on enhancing the recovery of CH 4 with pure CO 2 supply. A novel way is put forward and studied in the procedure of CO 2 replacement by combining depressurization. 2. Experiment apparatus and procedures The adapted schematic diagram of the experimental apparatus is shown in figure 1. It consists of 4 parts: reaction cell (1,2,3), a signal conversion device to record pressure and temperature (4,5,6,7), gas supplying system (8,9,10,11,12), and mass analyzer to analyze the gas composition (13,14,15).The reactor is made of stainless steel and has a pair of quartz windows on either side for direct observation of the process of hydrate forming and dissociation. The inner volume of the cell is ml with diameter of 50 mm and height of 130 mm. The designed maximum pressure is 30MPa. The temperature of the cell during the experiment is controlled by glycol bath. First step: in-situ methane hydrates formation in porous media. Second step: constant volume replacement reaction. Third step: gas composition analysis. The procedure of depressurization combined method was the same for the first and third step except that 2 hours depressurization was added to the second step. Constant volume replacement reaction underwent for 15 hours with gaseous CO 2 at certain temperature (273K) and pressure. Then the temperature was set 267K to reduce hydrates decomposition when lowing pressure to 1.6MPa, which is below methane hydrate phase equilibrium line but above CO 2 hydrate phase equilibrium line. Then the temperature was reset to be 273K. This procedure lasted for 2 hours and then cooled CO 2 was injected to the original pressure before depressurization. As was reported by literatures, the replacement rate dropped dramatically after the first 5 minutes of reaction. The first stage of replacement reaction explained as surface reaction is about 10 hours. [11] The combined method is designed to tackle this problem by breaking through throats to increase the diffusion of CO 2 when replacement rate declines.

3 Jiafei Zhao et al. / Energy Procedia 61 ( 2014 ) Fig. 1. (a)schematic diagram of the experimental apparatus; (b) replacement conditions and phase equilibrium diagram 3. Results and discussion Typical phase diagrams of methane (gas/hydrates) and carbon dioxide (gas/liquid/hydrates) are shown in Fig.1(b)[7]. Target zones of this study are located at zone A(above L_(CO 2 )-V_(CO 2 )), B(between L_(CO 2 )-V_(CO 2 ) and water-h_(ch 4 )-V_(CH 4 )) and C(between water-h_(ch 4 )-V_(CH 4 ) and water- H_(CO 2 )-V_( CO 2 )). It can be seen that zone C is an optimum condition for replacement reaction where CO 2 hydrates exist and CH 4 hydrates decompose. In the process of depressurization, temperature and pressure condition will be changed from zone B to zone C for 2 hours to melt methane hydrates partially. Fig.2 (a) shows the replacement percent of ten runs of non-depressurization reaction. Conditions in zone B where CO 2 is in liquid phase and C where the optimum zone locates have higher replacement percent, while zone C is relatively more effective. There is a tendency that as saturation increases, replacement percent will increase first and decrease when reaching certain degree, which is in consistence with studies in [12]. Five groups of experiments on combined method were conducted with the same duration of 50 hours. Temperature and pressure condition is nearly the same except methane hydrates saturation ranging from 0.11 to Data is shown in table 1 with calculated saturation and replacement percent in. Effective and whole replacement percent is analyzed in Fig.3(b). It is found that the effective replacement percent is much higher than runs without depressurization when replacement condition is located at zone B. Replacement percent of runs only by CO 2 replacement are below 0.2 shown in Fig.3(a), while runs with depressurization in addition are more than 0.3. For higher saturation, amount of methane decomposed by depressurization is larger, while the effective replacement percent is on average level. Results reveal that method with depressurization is effective to enhance methane hydrates recovery. It can be explained that 2 hours depressurization will make certain amount of methane hydrate dissociate and porous sediment loose to support the replacement reaction. Table 1. Data of five runs with depressurization combined method Runs with combined method Run1 Run2 Run3 Run4 Run5 Reaction pressure(mpa) Reaction temperature(k) Methane hydrates saturation Whole replacement percent

4 78 Jiafei Zhao et al. / Energy Procedia 61 ( 2014 ) a Fig. 2. Comparison of replacement percent of runs without (a) and with (b) depressurization (a: diamond: reaction condition in zone A; square points: in zone B; triangle points: in zone C. b: stars: whole replacement percent; square points: effective replacement percent; diamond: percentage of hydrates decomposed not by replacement) b 4. Conclusions Reaction conditions in zone A and C have larger effective replacement percent compared with zone B, where CO 2 is gaseous. When combining depressurization, replacement percent in zone B can reach as high as zone C. Replacement percent climbs up first but declines as methane hydrates saturation increases. It is confirmed that the combined method is effective in enhancing methane hydrate exploitation. References [1] Servio P, Lagers F, Peters C, Englezos P. Gas hydrate phase equilibrium in the system methane carbon dioxide neohexane and water. Fluid phase equilibria 1999; 158: [2] Lee H, Seo Y, Seo YT, Moudrakovski IL, Ripmeester JA. Recovering methane from solid methane hydrate with carbon dioxide. Angewandte Chemie International Edition 2003; 42 (41): [3] Staykova DK, Hansen T, Salamatin AN, Kuhs WF. In Kinetic diffraction experiments on the formation of porous gas hydrates. Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, May 19, 2002, p [4] Sloan Jr, ED, Koh C. Clathrate hydrates of natural gases. 3rd ed. CRC press; [5] Lee J, Park S, Sung W. An experimental study on the productivity of dissociated gas from gas hydrate by depressurization scheme. Energy Conversion and Management 2010; 51 (12): [6] Ersland G, Husebø J, Graue A, Kvamme B. Transport and storage of CO< sub> 2</sub> in natural gas hydrate reservoirs. Energy Procedia 2009; 1 (1): [7] Goel N. In situ methane hydrate dissociation with carbon dioxide sequestration: Current knowledge and issues. Journal of petroleum science and engineering 2006; 51 (3): [8] Ota M, Abe Y, Watanabe M, Smith JrRL, Inomata H. Methane recovery from methane hydrate using pressurized CO< sub> 2</sub>. Fluid Phase Equilibria 2005; 228: [9] Zhou X, Fan S, Liang D, Du J. Replacement of methane from quartz sand-bearing hydrate with carbon dioxide-in-water emulsion. Energy & Fuels 2008; 22 (3): [10] Yuan Q, Sun CY, Yang X, Ma PC, Ma ZW, Liu B, Ma QL, Yang LY, Chen GJ. Recovery of methane from hydrate reservoir with gaseous carbon dioxide using a three-dimensional middle-size reactor. Energy 2012; 40 (1): [11] Ota M, Saito T, Aida T, Watanabe M, Sato Y, Smith RL, Inomata H. Macro and microscopic CH 4 CO 2 replacement in CH 4 hydrate under pressurized CO 2. AIChE Journal 2007; 53 (10): [12] Yuan Q, Sun CY, Liu B, Wang X, Ma ZW, Ma QL, Yang LY, Chen GJ, Li QP, Li S. Methane recovery from natural gas hydrate in porous sediment using pressurized liquid CO< sub> 2</sub>. Energy Conversion and Management 2013; 67:

5 Jiafei Zhao et al. / Energy Procedia 61 ( 2014 ) Biography Xiaoqing, Chen, female, born in 1989, graduate student dedicated to the study of methane hydrates exploitation by CO 2 replacement method in Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, school of Energy and Power Engineering, Dalian University of Technology.