Huang Tianming et al.-JH: Effect of adsorption-desorption on fluid solute isotope tracing
Isotopes such as boron (δ11B), strontium (87Sr/86Sr), and lithium (δ7Li) are often used to trace the origin and evolution of water-soluble components. For example, seawater and its derived brine have a higher δ11B value (typically 40‰–60‰), compared to rocks (usually 11B). Similarly, marine carbonate 87Sr/86Sr has a lower value (usually less than 0.709), while continental silicate dissolved 87Sr/86Sr has a higher value (the dissolution process 87Sr/86Sr fractionation is not obvious). In aqueous-rock systems containing clay minerals, adsorption-desorption (adsorption-desorption and ion exchange have many common features that are difficult to occur in nature alone and are therefore often considered together) may affect the tracing of these solute isotopes. However, the adsorption-desorption effect is fast (minutes to several hours), while the groundwater flow rate is generally slower, and the adsorption-desorption effect is not easy to observe or quantify in field samples. In addition, the adsorption (exchangeable state) characteristics of clay minerals are unclear, resulting in unclear understanding of the effect mechanism of adsorption-desorption on these solute isotopes, and cannot be considered in practical applications.
In view of the above problems, Huang Tianming, Dr. Li Zhenbin (graduated), Dr. Li Zhenbin, Dr. Pang Zhonghe, and other researchers of the Institute of Geology and Geophysics of the Chinese Academy of Sciences used the short-term changes in the composition of solute isotopes in the effluent of shale gas hydraulic fracturing to explore the effect of clay adsorption-desorption on several specific isotopes. The advantages of the ejection liquid are: (1) the clay mineral content is high, and the adsorption-analysis effect may be more significant; (2) the lithology of the fracturing section is relatively uniform, and the adsorption composition is easy to determine; (3) due to the short hydraulic fracturing time, the interference of silicate dissolution can be excluded. For the morphological composition of shale that may affect the interaction between water and rock, the relevant element content and isotopic composition of the three forms of shale samples in the fracturing section were determined by sequential extraction method (shale culture), and the effect of adsorption-desorption on the solute content and isotope composition of the effluent liquid was studied by combining the matching indoor water rock reaction experiment and the hydraulic fracturing test of clean water without additives in the field, and the effect of adsorption-desorption on the solute content and isotope composition of the effluent liquid was studied.
Fig. 1 Different morphologies (water soluble, exchangeable and carbonate) B, Sr, Li content and isotopic composition in shale samples
The ejection liquid is composed of fracturing water and shale formation water in different proportions, mainly composed of fracturing water in the early stage of the repatriation, and mainly composed of formation water in the later stage. The results of the study showed that adsorption-desorption was the key factor in controlling the content of B, Sr, Li and isotope composition of the initial return solution, while the dissolution contribution rate of carbonate was less than 16%. Due to the mixing between the fracturing water and the formation water, as shown in the mixing line shown in Figure 2, B in the clay mineral is desorbed into the egress, so that the B content of the egress solution increases, and its isotope δ11B value decreases, representing a relatively low commutative B of δ11B. For Sr, the content shows a decreasing trend, which is mainly due to the adsorption (that is, Sr, Ca, etc. in the water are adsorbed to the surface of clay minerals, and the original adsorbed Na, K, Li, etc. are desorbed into the water). However, the indoor water rock reaction experiment showed that the value of 87Sr/86Sr was almost unchanged during the adsorption process, which meant that the adsorption only reduced the Sr content of the ejection liquid. The increase in the value of 87Sr/86Sr in the return solution (Figure 3) is caused by the isotope realimation of the dissolved Sr and shale adsorption Sr (87Sr/86Sr=0.71803) in the return solution. Matching indoor water rock experiments also confirmed this.
Fig. 2 Characteristics and mechanism of B content and isotope change of return solution (the mechanism of Li and B is similar)
Fig. 3 Characteristics and mechanism of Sr content and isotope change of back discharge solution
This study shows that adsorption-desorption significantly affects the tracing of isotopes such as B and Sr and the analysis of Na sources in water. The increase in the Sodium content of the return liquid and the increase in the ratio of 87Sr/86Sr are often considered to be the dissolution of silicates (87Sr/86Sr=0.75402 in the shale samples of this study), but in fact, it is caused by adsorption-desorption, so ignoring this effect will overestimate the dissolution of silicates. With about 33% of the rocks exposed from the continental surface of the world being shale and 16% sandstone, and most aquifers contain clay minerals, this study is important for isotopic tracer water-rock interactions and to resolve the sources of specific ions.
The research results were published in the international academic journal Journal of Hydrology (Huang Tianming, Li Zhenbin, Long Yin, Zhang Fen, Pang Zhonghe. Role of desorption-adsorption and ion exchange in isotopic and chemical (Li, B, and Sr) evolution of water following water–rock interaction [J]. Journal of Hydrology, 2022, 610: 127800. DOI: 10.1016/j.jhydrol.2022.127800）。 The achievements have been co-funded by the National Natural Science Foundation of China (41877207, 42141009, 42172277), the Second Qinghai-Tibet Scientific Expedition (2019QZKK0904), the Strategic Pilot Science and Technology Special Project of the Chinese Academy of Sciences (XDB10030603) and the Youth Innovation Promotion Association of the Chinese Academy of Sciences.
Further reading: Huang Tianming et al. - Geophys Res Lett: A fine depiction of geochemical processes in hydraulic fracturing