Experimental Study on Carbon Isotopic Composition Changes During the Formation of Gas Hydrates

doi:10.19657/j.geoscience.1000-8527.2018.01.21

Abstract

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

CLC Number:

TE132.2

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.

Figures/Tables 9

References 27

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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

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

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

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

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