Natural CO2 leakage and responses of shallow aquifers in the southern Xining Basin
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摘要:
CO2地质封存是重要的CO2减排技术之一, 其中泄漏风险评价是该技术实施的关键, 而天然CO2泄漏研究是获取泄漏评价关键信息的重要手段。通过野外调查, 现场测量以及水样、气样和岩样的采集和测试, 分析了西宁盆地南部CO2的来源、泄漏特征和其泄漏到浅部含水层后相关的响应规律。结果显示西宁盆地南部发现了多处天然CO2泄漏, 包括高含CO2的泉、废弃钻孔的间歇水气喷发和CO2井喷等多种形式的泄漏显示, 以及与之伴随的较大范围的钙华。气体中CO2占绝对含量, CO2碳同位素介于-2.5‰~-0.4‰, 指示泄漏的CO2来源于深部无机成因, 并通过深部断层泄漏进入浅部承压含水层, 与地下水一起径流、排泄或在浅部二次聚集。CO2泄漏区域土壤222Rn浓度异常(超过9 000 Bq/m3), 这可作为识别隐伏泄漏通道的重要方法。地下水对CO2的泄漏产生了明显的物理化学响应, 包括现象独特的井筒间歇喷发(喷发200 s和停止130 s), 地下水水化学特征的变化(比如pH值的降低(小于7), 电导、HCO3-、Ca2+的升高和氧同位素的漂移), 以及地表大面积特征明显(比如气泡构造)和成分以方解石为主的钙华。该场地天然CO2泄漏特征与美国Utah场地具有很高的相似性。研究成果不仅能为CO2地质封存的泄漏风险评价提供天然类比知识, 而且还有助于了解地球深部地质活动。
Abstract:Objective Geological storage of CO2 is an important technology for reducing CO2 emissions, and the assessment of CO2 leakage risk is the key to its implementation. Research on natural CO2 leakage is an important means to obtain key information for leakage assessment.
Methods The source of CO2, the characteristics of CO2 leakage, and the relevant response of shallow aquifers to the CO2 leakage have been analysed through field investigations, on-site measurements, and sampling and testing of water, gas and rock.
Results A number of natural CO2 leakages have been discovered in the southern Xining Basin, including CO2-rich springs, CO2-driven cold-water geysers from abandoned wells, and CO2 blowouts, as well as large-scale travertine associated with them. CO2 is the dominant component in the gas phase, and the abundance of carbon isotope of CO2 is between -2.5‰ and -0.4‰, indicating that the leaked CO2 comes from a deep inorganic origin, leaks into shallow confined aquifers through deep faults, and flows and discharges with the groundwater or accumulates secondarily in shallow formations. The concentration of soil 222Rn in areas of CO2 leakage is abnormal (over 9 000 Bq/m3), which can be used as an important method of identifying hidden leakage channels. The groundwater has a pronounced response to CO2 leakage, including a unique phenomenon of intermittent eruptions (eruption for 200 s and incubation for 130 s), changes in groundwater hydrochemical characteristics (e.g., a decrease in pH, an increase in conductance and HCO3- and Ca2+ concentrations, and a drift in oxygen isotopes), and travertine composed mainly of calcite with a bubble structure at the surface. The natural CO2 leakage characteristics at this site are highly similar to those in Utah, USA.
Conclusion The results of this study not only provide knowledge of natural analogous for leakage risk assessment of CO2 geological storage, but also contribute to the understanding of geological activity in the deep earth.
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Key words:
- natural CO2 leakage /
- shallow aquifer /
- response /
- CO2 geological storage /
- Xining Basin
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图 1 西宁盆地南部基底埋深及构造纲要图(据文献[18]修改)
Figure 1. Burial depth and tectonic outline map of the southern Xining Basin
图 2 研究区地质图及CO2泄漏点位置分布
1.第四系风积物、冲积物及石灰华沉积物；2, 3.新近系及古近系泥岩、粉砂岩；4.白垩系砂岩及砂砾岩；5.侏罗系泥岩及砂岩；6~12.泥盆系、奥陶系、志留系、寒武系、震旦系及下元古界岩浆岩、变质岩；13.资料记载的富CO2泉点；14.现场勘察确定的CO2泄漏泉点；15.揭露CO2的钻孔点；16.断层；17.河流
Figure 2. Geological map of the study area and CO2 leakage locations
图 5 上洛麻CO2矿泉
a.古钙华; b.现代钙华; c.古钙华中的气泡构造
Figure 5. Mineral springs containing CO2 in Shangluoma
图 9 ZK10井间歇喷发周期测量
Figure 9. Period measurement of intermittent eruption in the Well ZK10
图 10 CO2间歇喷泉形成机理示意图
Figure 10. Schematic diagram of the formation mechanism of CO2 geyser
图 11 典型CO2泄漏点钙华样品
①上洛麻附近(黄色钙华)；②上洛麻附近(红色钙华)；③上洛麻附近(结晶钙华)；④药水滩(黄色钙华)；⑤药水滩(白色钙华)；⑥ZK10井(黄色钙华)；⑦ZK10井(树枝状钙华)；⑧尧庄；⑨上洛麻
Figure 11. Travertine samples from typical CO2 leakage locations
表 1 气体组分和同位素测试结果
Table 1. Test results for gas composition and isotopeic abundance
取样点 N2 O2 H2S Ar CO2 CO2同位素
13δCCO2/ ‰土壤氡
222Rn冰凌山 - - - - - -2.5 - 尧庄 2.45 0.62 0.01 0.03 96.88 -1.3 - 上洛麻 26.72 7.15 0.01 0.30 65.82 -0.6 20 000 ZK10井 2.58 0.63 0.02 0.03 96.74 -0.4 10 600 XCDCSZ-3-1钻孔 72.50 19.20 0.00 0.84 7.44 -0.9 9 590 注：气体组分单位为体积百分比(%)，土壤氡的单位Bq/m3 
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表 2 水样测试结果
Table 2. Test results of water samples
项目 冰岭山 尧庄 上洛麻 ZK10井 高程/m 2 285 2 607 2 633 2 392 温度/℃ 15.6 16.2 0.8 7.1 电导/(mS·cm-1) 6.13 3.77 3.57 4.06 pH值 6.42 6.62 6.52 6.75 Na+ 763.60 92.91 123.85 135.98 K+ 212.46 25.81 26.75 67.61 Ca2+ 502.70 655.03 724.50 432.42 Mg2+ 149.37 137.41 133.76 131.14 Cl- 318.85 58.65 96.08 60.23 SO42- 1653.15 981.06 789.92 469.83 HCO3-a ρB/(mg·L-1) 1 433.5
(1 993.4)1 063
(1 634.1)2 397
(2 088.6)1 894
(1 749.8)Li 1.08 0.80 0.78 0.82 F 0.60 1.19 1.40 1.16 Sr 2.20 1.93 1.95 1.30 Br 1.13 0.86 4.42 0.92 TDS 4 596.83 2 767.92 2 939.16 2 172.11 δDVSMOW/‰ -77.80 -76.60 -77.80 -78.40 δ18OVSMOW/‰ -12.20 -11.40 -11.70 -11.60 矿物饱和指数 方解石 0.37 0.69 0.52 0.60 文石 0.22 0.54 0.35 0.44 白云石 0.43 0.93 0.22 0.75 石膏 -0.18 -0.21 -0.19 -0.57 地下水类型 SO4·HCO3-
Na·CaHCO3·
SO4-CaHCO3·
SO4-CaHCO3-
Ca·Mg注：括号中数值为根据电荷平衡计算
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表 3 典型CO2泄漏点钙华矿物组分XRD测试结果
Table 3. Mineral composition of the travertine in typical CO2 leakage locations tested by XRD
样品编号 方解石 白云石 文石 石英 钠长石 斜长石 ① 98.70 0 0 1.30 0 0 ② 99.07 0 0 0.93 0 0 ③ 99.30 0 0 0.70 0 0 ④ 90.72 5.06 0 4.22 0 0 ⑤ 100.00 0 0 0 0 0 ⑥ 98.92 0 0 1.08 0 0 ⑦ 73.44 0 0 18.39 8.17 0 ⑧ 48.41 1.65 0 35.59 0 14.35 ⑨ 92.83 0 5.57 1.60 0 0 注：表中各值表示各矿物组分的体积百分比(%)
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表 4 西宁和美国Utah天然CO2泄漏场地特征对比
Table 4. Comparison of the characteristics of natural CO2 leakage between Xining, China and Utah, USA
主要特征 西宁 美国Utah 规模 分散在整个盆地南部，范围大，但主要受断层控制泄漏 集中在Little Grand Wash和Salt Wash Graben断层附近，范围小 CO2同位素(13δCCO2)和来源 -0.4‰~-2.5‰，深部壳源 -6.61‰~-7.55‰，深部幔源 CO2地表通量/(g·m-2·d-1) 最大53 200 最大36 259 地表钙化 泄漏点普遍存在 泄漏点普遍存在 地表含CO2水化学特征 温度0.7~20℃，TDS介于2 000~5 000 mg/L，地下水类型HCO3·SO4-Ca，pH值介于6.4~6.8 温度13~18℃，地下水类型Cl·HCO3-Na·Ca，pH值介于6.2~6.5 井筒动力学 间歇喷发，喷发持续时间200 s，相对停止130 s 间歇喷发，喷发持续时间25~30 h，相对静止80~90 h
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