Hydraulic fracturing for shale gas exploration is not free from environmental risk. The environmental concerns related to hydraulic fracturing has been greatly attracted. One of most important environmental concerns is regional water quality which may be contaminated by produced waters through induced and natural fractures and wastewater discharge.
Tianming Huang, Yiman Li, Zhonghe Pang, Yingchun Wang, Shuo Yang
Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
© 2017 The Authors
At present, the baseline water quality must be firstly obtained to identify potential pollution of the activity and monitoring indicators should be studied for better environmental monitoring. We sampled shallow groundwater, produced waters, shale rock and soil in the Jiaoshiba shale-gas region, SW China and measurements have included water chemistry and isotopes. Preliminary results show that the present shallow karst groundwater quality is pretty good with the total dissolved solids (TDS) ranging from 129 to 343mg/L and with water chemistry type of HCO3-Ca, However, some groundwaters have been polluted by agricultural activities.
Produced waters have relatively high salinity with TDS ranging from 2 to 14g/L. Laboratory experiment of fracturing liquid and shale rock interaction at simulated reservoir conditions shows that TDS in the flowback fluid increases 10 times and Ca2+, Na+, Cl− and SO42− make dominant contributions. The main geochemical reactions are inferred to be pyrite oxidation and the dissolution of calcite, dolomite and plagioclase, resulting in increases of major ions in the flowback fluid.
The inorganic geochemical monitoring indicators for shale gas exploration of the Silurian Longmaxi formation has been determined.
Due to rapid development of hydraulic fracturing technique, shale gas has recently emerged as a relatively clean energy source that offers the opportunity for a number of regions around the world to reduce their reliance on energy imports1-3. China has relatively abundant shale gas resource (1/13 of world reserves)4 and since 2011, China has made great advancement on shale gas exploitation, especially in the Jiaoshi region, Chongqing, SW China. However, hydraulic fracturing is not free from environmental risk.
The environmental concerns related to hydraulic fracturing has been greatly attracted and many European countries and American states have prohibited the shale gas exploitation. One of most important environmental concerns is regional water quality which may be contaminated by produced waters through induced and natural fractures and wastewater discharge5. In addition, little information is available on the baseline groundwater quality and hydrogeological conditions associated with unconventional gas production6. The aim of this study is to assess the baseline shallow groundwater quality, hydrogeochemical signatures, and study the water rock interaction during hydraulic fracturing and its implications related to water quality monitoring.
Study area and sampling
The study area is located in the Jiaoshiba district of Chongqing, east of the Sichuan basin, where the lower Silurian Longmaxi formation is one of the most promising shale gas reservoirs in China. The upper formation mainly consists of mudstone, muddy siltstone. The shallow aquifer is the Triassic carbonate karst aquifer. The Sinopec Corp has made great progress in exploitation of Silurian marine Longmaxi group in the Jiaoshi region. We sampled groundwater, produced water, and shale samples. Measurements have included water chemistry and isotopes. We also conducted water-rock interaction experiment.
Results and discussion
Shallow groundwater chemistry
The total dissolved solids (TDS) for shallow groundwater are less than 350 mg/L, and less than 250 mg/L in the upper streams. The pH ranges from 7.04 to 8.25. The water type is HCO3-Ca (Fig. 1). In karst region, the carbonate dissolves as:
Fig. 1 The Piper Diagram of waters: SW-shallow groundwater; WJR-Wujiang River; PW-produced water.
The groundwater δ13C ranges from -16.1‰ to -11.7‰ with the average of -14.1‰, indicating the equilibrium fractionation between the CO2 and the dissolved inorganic carbon (DIC). The water chemistry shows all the monitored indexes is less the threshold of III groundwater standard of China (Fig. 2). However, high NO3 values in some groundwater samples indicate that agriculture contamination occurs in the area. The 14C content ranges from 95.4 to 99.5 pmC, suggesting modern water and strong renewable ability.
Fig. 2 The shallow groundwater chemical composition
The Longmaxi shale formation mainly consists of quartz, plagioclase, calcite, dolomite, pyrite and clay minerals. The sources water for fracturing is Wujiang River (WJR), with pH of 7.2, water type of HCO3.SO4-Ca, TDS of 190.4 mg/L. The concentrations of HCO3– and SO42- are 140.0 mg/L and 46.6 mg/L, respectively. Heavy metal content (e.g., Ni, Pb, Cu, Zn, Mn, Mo, Rb and Ba) is almost below 1 µ g/L. The water -rock interaction experiment between fracturing fluid and shale was conducted under 100 oC for 15 days.
The results indicate that the pH and TDS of water samples after the water-shale interactions are 7.6 and 666.8 mg/L, respectively. The water type is SO4.HCO3-Ca. The concentrations of major ions of Na+, Ca2+, HCO3– and SO42- are 24.6 mg/L, 155.0 mg/L, 175.0 mg/L and 343.0 mg/L. F- and NO3- ions could be ignored due to their very low content. The SiO2 content was analysed to be 27.9 mg/L. The contents of trace elements and heavy metals of Li, Ni, Mo, Rb and Ba are 60.8 µ g/L, 106 µ g/L, 1668 µ g/L, 34.7 µ g/L, and 118 µ g/L, respectively. The δ13CDIC of reacted water enriches from originally -11.0 ‰ to -1.2 ‰ after interactions and a great shift of 9.8 ‰ is observed.
The dominated geochemical reactions during hydraulic fracturing processes includes pyrite oxidation and dissolution of mineral under acid condition (oxidation can produce H+) and carbonate and silicate minerals dissolution based on HCO3– and δ13C increasing. One important difference of water chemistry between reacted water and flowback fluid is concentration changes of Na+ and Cl–.
Great increases of Na+ and Cl– in flowback fluid are also observed in Marcellus gas wells in Pennsylvania, USA and the possible reason is suggested to be that the flowback waters developed from a highly saline brine evaporated from seawater into the stage of halite precipitation, and then diluted and mixed with seawater, fresh water and injected fluids5,7. The Silurian Longmaxi shale in Jiaoshiba County is formed in marine facies depositional environment and is rich in organic carbon. Though mineral analysis indicates that no halite is contained in the shale samples, one possible reason for concentration increases of Na+ and Cl– is suggested to be halite dissolution in the marine formations.
Implication for groundwater monitoring
Previous investigations show that the quality of flowback fluid after circulation is greatly deteriorated2,8. Therefore, long-term geochemical monitoring of flowback fluid, shallow groundwater and surface water in areas of shale gas operation should be carried out for environmental safety concerns. The first step is to determine effective monitoring indicators. For the Silurian Longmaxi shale formation, the TDS of flowback fluid increased by 3 and 10 times compared with WJR water for the experiment and field test, respectively. The main contributions are from Cl–, SO42-, Na+ and Ca2+.
Trace elements of Li, Ni, Pb, Cu, Zn, Mn, Mo, Rb and Ba are greatly increased in the flowback Tianming Huang et al. / Procedia Earth and Planetary Science 17 (2017) 638 – 641 fluid based on the results from the experiment and field test. Based on the analysis above and published papers from other shale gas production projects, TDS; major elements of Cl–, SO42-, Na+, and Ca2+; trace elements of Li, Ni, Pb, Cu, Zn, Mn, Mo and Rb; and stable isotopes of δ13CDIC can be chosen as inorganic geochemical monitoring indicators for shale gas exploration of the Silurian Longmaxi formation.
Preliminary results show that the present shallow groundwater quality is pretty good with TDS ranging from 129 to 343 mg/L and with water chemistry type of HCO3-Ca. However, the produced waters have relatively high salinity with TDS ranging from 2 to 14 g/L. Laboratory experiment of fracturing liquid and shale rock interaction at appointed temperature has carried out to get the geochemical processes during hydraulic fracturing to find the suitable monitoring index. Trace elements are more sensitive for monitoring shallow groundwater pollution.
This work is supported by the “Strategic Priority Research Program (B)” of the Chinese Academy o f Sciences (Grant XDB10030603). The authors wish to express their appreciation to Profs. X. Li and F. Ma for their help during field work.
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Corresponding author (Tianming Huang) Tel.: +86-10-82998276; fax: +86-10-62010846.
E-mail address: tmhu[email protected]
© 2017 The Authors. This is an open access article under the CC BY-NC-ND license
Peer-review under responsibility of the organizing committee of WRI-15