水合物生成过程中碳同位素组成变化的实验研究

doi:10.19657/j.geoscience.1000-8527.2018.01.21

摘要/Abstract

摘要：

天然气水合物是一种新型的洁净能源。甲烷天然气水合物是储量最丰富的一种类型,常出现在深海中或极地大陆上,其生成的过程中会发生同位素的分馏效应。通过实验室模拟水合物生成的过程,利用天然海水与甲烷或二氧化碳气体反应,以及更接近实际生成环境的甲烷-海水-沉积物动态聚散实验,对甲烷水合物和二氧化碳水合物生成前后δ¹³C值进行测定,研究水合物生成过程中δ¹³C的变化情况。实验证明,水合物反应中碳同位素分馏是存在的,其变化程度明显小于氧同位素和氢同位素。甲烷水合物碳同位素的分馏系数α_C的值为1.000 3~1.000 9。二氧化碳水合物生成反应后气相的碳、氧同位素变轻,重同位素趋向于进入水合物中,二氧化碳水合物碳同位素的分馏系数α_C的值为1.000 7~1.001 2。海水中溶解的CO₂气体在甲烷水合物形成过程中会被水合物捕获,从而使得δ¹³C_DIC值变小,重的碳同位素趋于进入水合物中,而较轻的碳同位素留在海水中。但由于海水中含有的溶解CO₂气体有限,经过多轮水合物动态聚散后δ¹³C_DIC值的变化幅度会越来越小。

关键词: 甲烷水合物, 二氧化碳水合物, 碳同位素, 实验研究

Abstract:

Gas hydrates are a new type of clean energy, and they often appear in the deep sea or in the polar continent. Isotopic fractionation occurs in the gas hydrate formation. In this paper, experiments of methane-seawater hydrates, carbon dioxide-seawater hydrates, and methane-seawater hydrates formed repeatedly in sediments were carried out. Changes of carbon isotopic composition during the formation of gas hydrates were researched. The results prove that the carbon isotope fractionation exists during the gas hydrate formation, and the change of carbon isotope composition is significantly smaller than that of oxygen and hydrogen. The fractionation factor of carbon isotope is 1.000,3 to 1.000,9 in methane-seawater gas hydrate. Carbon and oxygen isotopic composition lightened in gas phase after carbon dioxide hydrate formation, and heavy isotopes tend to enter hydrates. The fractionation factor of carbon isotope is 1.000,7 to 1.001,2 in carbon dioxide-seawater gas hydrates. CO₂ gas dissolved in seawater will be captured in hydrate cage during methane hydrate formation and as a result,δ¹³C_DIC value becomes small and heavy carbon isotope tend to enter hydrates, and lighter carbon isotope is left in the seawater. Because the dissolved CO₂ gas content is limited in seawater, the variation of δ¹³C_DIC value would be smaller with time in the process of methane-seawater hydrates forming repeatedly in sediments.

Key words: methane hydrate, carbon dioxide hydrate, carbon isotope, experimental study

中图分类号:

TE132.2

陈敏, 邓兴波, 刘昌岭, 任宏波, 尹希杰, 李佳宣, 戚洪帅, 张爱梅. 水合物生成过程中碳同位素组成变化的实验研究[J]. 现代地质, 2018, 32(01): 205-212.

CHEN Min, DENG Xingbo, LIU Changling, REN Hongbo, YIN Xijie, LI Jiaxuan, QI Hongshuai, ZHANG Aimei. Experimental Study on Carbon Isotopic Composition Changes During the Formation of Gas Hydrates[J]. Geoscience, 2018, 32(01): 205-212.

图/表 9

图1 实验装置示意图

Fig.1 Schematic diagram of experimental apparatus

图2 实验装置中的高压透明腔组件

Fig.2 Transparent high-pressure chamber in experimentalapparatus

图3 甲烷-海水水合物形成前后气相碳、氢同位素组成变化

Fig.3 The change of carbon and hydrogen isotopic composition in gas phase after methane-seawater hydrate forming

表1 甲烷-海水水合物形成前后气相碳、氢同位素变化

Table 1 The change of carbon and hydrogen isotopic composition in gas phase after methane-seawater hydrate forming

实验编号	P₀/MPa	P_G/MPa	T₀/℃	N/mol	δ¹³C₀/‰	δ¹³C/‰	Δδ¹³C/‰	α_C	ΔδD/‰
A1	8.00	3.77	3.4	1.17	-30.73	-30.66	0.07	—	-0.84
A2	8.00	4.00	5.6	1.11	-30.88	-30.64	0.24	—	-2.44
A3	8.00	3.15	5.1	1.28	-30.81	-31.07	-0.26	1.0003	-1.93
A4	10.00	3.68	5.8	1.66	-30.91	-31.04	-0.14	—	-1.32
A5	10.00	5.47	7.9	1.20	-30.74	-31.23	-0.49	1.0009	-3.02

图4 二氧化碳-海水水合物形成前后气相碳同位素组成变化

Fig.4 The change of carbon isotopic composition in gas phase after carbon dioxide-seawater hydrate forming

图5 二氧化碳-海水水合物形成前后气相氧同位素组成变化(虚线示氧同位素分析精度线)

Fig.5 The change of oxygen isotopic composition in gas phase after carbon dioxide-seawater hydrate forming

表2 二氧化碳-海水水合物形成前后气相碳、氧同位素变化

Table 2 The change of carbon and oxygen isotopic composition in gas phase after carbon dioxide-seawater hydrate forming

实验编号	P₀/ MPa	P_G/ MPa	T₀/ ℃	δ¹³C₀/ ‰	δ¹³C/ ‰	Δδ¹³C/ ‰	α_C	Δδ¹⁸O/ ‰
B1	5.00	3.56	5.9	-28.13	-27.83	-0.29	1.000 7	-2.10
B2	5.00	3.74	3.9	-27.97	-27.82	-0.15	—	-1.20
B3	5.00	3.72	5.6	-28.17	-28.19	0.03	—	1.38
B4	4.00	2.92	5.9	-28.08	-27.47	-0.61	—	2.94
B5	4.00	2.35	5.1	-28.47	-27.85	-0.62	1.001 2	-13.44
B6	4.00	1.98	3.8	-28.10	-28.03	-0.07	—	-17.33

图6 甲烷-海水-沉积物水合物动态聚散实验过程中温度、压力、δ13CDIC随时间的变化图

Fig.6 The change of temperature, pressure and δ13CDIC with time in the processing of methane-seawater hydrate forming repeatedly in sediments

表3 甲烷-海水-沉积物水合物动态聚散实验过程中温度、压力和δ13C的变化情况

Table 3 The change of temperature, pressure and δ13CDIC in the process of methane-seawater hydrate formed repeatedly in sediments

实验轮次	第一次			第二次			第三次
实验轮次	初始	合成	分解	初始	合成	分解	初始	合成	分解
时间/h	0	53	67	68	142	162	170	211	217
液相温度/℃	1.9	1.8	10.9	1.9	1.8	16.3	1.8	1.8	10.2
压力/MPa	8.00	7.54	7.71	7.22	6.57	7.30	6.65	6.18	6.29
δ¹³C_DIC/‰	-5.95	-8.63	-9.05	—	-10.48	-10.29	—	-9.30	-9.88
耗时/h	—	53	14	—	74	20	—	41	6
液相温度变化量/℃	—	-0.1	9.1	—	-0.1	14.5	—	0	8.4
压力变化量/MPa	—	-0.46	0.17	-0.49	-0.65	0.73	-0.65	-0.47	0.11
Δδ¹³C_DIC/‰	—	-2.68	-0.42	—	-1.43	0.19	—	0.99	-0.58

参考文献 27

[1]	KVENVOLDEN K A. Methane hydrate—a major reservoir of carbon in the shallow geosphere[J]. Chemical Geology, 1988, 71(1/3): 41-51. DOI URL
[2]	陈敏, 曹志敏, 龚建明, 等. 海底天然气水合物地球化学勘探新技术[J]. 矿物岩石, 2004, 24(4): 102-107.
[3]	苏新, 陈芳, 张勇, 等. 海洋天然气水合物勘查和识别新技术:地质微生物技术[J]. 现代地质, 2010, 24(3): 409-423.
[4]	BOROWSKI W S, PAULL C K, USSLER W. Global and local variations of interstitial sulfate gradients in deep-water, continental margin sediments: Sensitivity to underlying methane and gas hydrates[J]. Marine Geology, 1999, 159(1): 131-154. DOI URL
[5]	凌洪飞, 蒋少涌, 倪培, 等. 沉积物孔隙水地球化学异常:天然气水合物存在的指标[J]. 海洋地质动态, 2001, 17(7): 34-37.
[6]	TORRES M E, MCMANUS J, HAMMOND D E, et al. Fluid and chemical fluxes in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, I: Hydrological provinces[J]. Earth and Planetary Science Letters, 2002, 201(3): 525-540. DOI URL
[7]	杨涛, 薛紫晨, 杨竞红, 等. 南海北部地区海洋沉积物中孔隙水的氢、氧同位素组成特征[J]. 地球学报, 2003, 24(6): 511-514.
[8]	刘昌岭, 陈敏, 业渝光. 海洋天然气水合物元素地球化学异常的实验研究[J]. 现代地质, 2005, 19(1): 96-100.
[9]	王家生, SUESS E. 天然气水合物伴生的沉积物碳、氧稳定同位素示踪[J]. 科学通报, 2002, 47(15): 1172-1176.
[10]	KVENOLDEN K A. A review of the geochemistry of methane in natural gas hydrate[J]. Organic Geochemistry, 1995, 23(11): 997-1008. DOI URL
[11]	COLLETT T S. Natural gas production from Arctic gas hydrates[J]. AAPG Bulletin, 1993, 77:CONF-9310237.
[12]	WANG S, YAN W, CHEN Z, et al. Carbon isotope evidence of gas hydrate dissociation in South China Sea[J]. Earth Science-Journal of China University of Geosciences, 2010, 35(4): 526-532. DOI URL
[13]	黄霞, 祝有海, 卢振权, 等. 南海北部天然气水合物钻探区烃类气体成因类型研究[J]. 现代地质, 2010, 24(3): 576-580.
[14]	蒋少涌, 凌洪飞, 杨竞红, 等. 海洋浅表层沉积物和孔隙水的天然气水合物地球化学异常识别标志[J]. 海洋地质与第四纪地质, 2003, 23(1): 87-94.
[15]	陆尊礼, 凌洪飞, 蒋少涌, 等. 稳定同位素在天然气水合物地球化学勘查中的应用简介——以“布莱克海隆”海区为例[J]. 海洋地质动态, 2003, 19(2): 28-32.
[16]	YANG T, JIANG S, YANG J, et al. Dissolved inorganic carbon (DIC) and its carbon isotopic composition in sediment pore waters from the Shenhu area, northern South China Sea[J]. Journal of Oceanography, 2008, 64(2): 303-310. DOI URL
[17]	YANG T, JIANG S, GE L, et al. Geochemistry of pore waters from HQ-1PC of the Qiongdongnan Basin, northern South China Sea, and its implications for gas hydrate exploration[J]. Science China Earth Sciences, 2013, 56(4): 521-529. DOI URL
[18]	MAEKAWA T. Experimental study on isotopic fractionation in water during gas hydrate formation[J]. Geochemical Journal, 2004, 38(2): 129-138. DOI URL
[19]	陈敏. 海洋天然气水合物元素地球化学行为模拟实验研究[D]. 青岛: 中国海洋大学, 2005.
[20]	陈敏, 曹志敏, 业渝光, 等. 海洋天然气水合物氢氧同位素分馏初探[J]. 地球化学, 2005, 34(6): 67-73.
[21]	贺行良, 刘昌岭, 王江涛, 等. 气相色谱-同位素比值质谱法测定天然气水合物气体单体碳氢同位素[J]. 岩矿测试, 2012, 31(1): 154-158.
[22]	贺行良, 刘昌岭, 王江涛, 等. 天然气水合物气体组成分析技术[J]. 海洋地质前沿, 2011(6): 65-73.
[23]	LAPHAM L L, WILSON R M, CHANTON J P. Pressurized laboratory experiments show no stable carbon isotope fractionation of methane during gas hydrate dissolution and dissociation[J]. Rapid Communications in Mass Spectrometry, 2012, 26(1): 32-36. DOI PMID
[24]	FRIEDMAN I, ONEIL J R. Data of Geochemistry: Compilation of Stable Isotope Fractionation Factors of Geochemical Interest[M]. Washington: US Government Printing Office, 1977:1-5.
[25]	LAI C A. Effects of gas hydrates on the chemical and physical properties of seawater[J]. Journal of Petroleum Science and Engineering, 2007, 56(1): 47-53. DOI URL
[26]	ENGLEZOS P, KALOGERAKIS N, DHOLABHAI P D, et al. Kinetics of formation of methane and ethane gas hydrates[J]. Chemical Engineering Science, 1987, 42(11): 2647-2658. DOI URL
[27]	SKOVBORG P, RASMUSSEN P. A mass transport limited model for the growth of methane and ethane gas hydrates[J]. Chemical Engineering Science, 1994, 49(8): 1131-1143. DOI URL