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Sensitivity assessment of strontium isotope as indicator of polluted groundwater for hydraulic fracturing flowback fluids produced in the Dameigou Shale of Qaidam Basin
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Abstract: Hydrogeochemical processes that would occur in polluted groundwater and aquifer system, may reduce the sensitivity of Sr isotope being the indicator of hydraulic fracturing flowback fluids (HFFF) in groundwater. In this paper, the Dameigou shale gas field in the northern Qaidam Basin was taken as the study area, where the hydrogeochemical processes affecting Sr isotope was analysed. Then, the model for Sr isotope in HFFF-polluted groundwater was constructed to assess the sensitivity of Sr isotope as HFFF indicator. The results show that the dissolution can release little Sr to polluted groundwater and cannot affect the εSr (the deviation of the 87Sr/86Sr ratio) of polluted groundwater. In the meantime, cation exchange can considerably affect Sr composition in the polluted groundwater. The Sr with low εSr is constantly released to groundwater from the solid phase of aquifer media by cation exchange with pollution of Quaternary groundwater by the HFFF and it accounts for 4.6% and 11.0% of Sr in polluted groundwater when the HFFF flux reaches 10% and 30% of the polluted groundwater, respectively. However, the Sr from cation exchange has limited impact on Sr isotope in polluted groundwater. Addition of Sr from cation exchange would only cause a 0.2% and 1.2% decrease in εSr of the polluted groundwater when the HFFF flux reaches 10% and 30% of the polluted groundwater, respectively. These results demonstrate that hydrogeochemical processes have little effect on the sensitivity of Sr isotope being the HFFF indicator in groundwater of the study area. For the scenario of groundwater pollution by HFFF, when the HFFF accounts for 5% (in volume percentage) of the polluted groundwater, the HFFF can result in detectable shifts of εSr (ΔεSr=0.86) in natural groundwater. Therefore, after consideration of hydrogeochemical processes occurred in aquifer with input of the HFFF, Sr isotope is still a sensitive indicator of the Quaternary groundwater pollution by the HFFF produced in the Dameigou shale of Qaidam Basin.
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Figure 1. (a) The topography of the study area, and locations of the CY1 well and Quaternary spring (b) Geologic map of the study area
Figure 2. Mineral stability diagrams in weathering systems of (a) k-feldspar, (b) albite, (c) anorthite for the HFFF (CY1-3d), Quaternary groundwater (QG1), and simulated polluted-groundwater.
Note: The mixing volume ratio of the HFFF in simulated polluted-groundwater is labeled on the figure. The equilibrium phase boundary was plotted at 15°C and 0.1 MP.
Figure 3. Response curve of Sr chemistry and isotope in groundwater under different pollution intensity.
Note: The labeled tick marks on the curve indicate the volume proportion of HFFF in polluted groundwater.
Table 1. Hydrochemical and isotopic results of HFFF and Quaternary groundwater
Sample name mg/L εSr pH T (°C) Na Ca Mg Cl SO4 HCO3 TDS Sr CY1-1d 1 683 375 239 3 486 412 415 6 413 5.77 55.64 6.3 29.1 CY1-2d 2 815 410 170 4 783 90 1 368 8 973 8.21 60.92 7.0 38.5 CY1-3d 2 897 359 162 4 662 89 1 602 8 990 8.24 61.82 7.0 40.1 CY1-5d 3 263 316 129 5 042 41 1 688 9 658 8.57 62.71 7.7 43.6 CY1-7d 3 467 294 113 5 228 46 1 764 10 056 9.14 62.57 7.9 46.1 QG1 1 531 221 377 1 949 2 199 341 6 500 8.00 43.77 8.0 14.2 QG2 1 796 224 329 2 017 2 052 370 6 791 9.43 41.89 8.1 14.6 
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Table 2. Cation exchange coefficients relative to Na+, cation molar concentrations and activities in QG1 groundwater, and exchange capacity for individual cation in Quaternary aquifer
Exchangeable cation Na+ K+ Mg2+ Ca2+ Sr2+ KNa\I / 0.25 0.60 0.50 0.40 m (mmol/L) 66.57 1.30 15.71 5.52 0.09 γ 0.78 0.75 0.55 0.54 0.55 meqI-Xi (meq/kg) 6.43E+01 4.85E+00 5.31E-01 2.63E-01 6.80E-03 Note: KNa\I is following the Gaines-Thomas convention; / in the table means that the value does not need to acquire.
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Table 3. Summary of cation exchange process for calculating 87Sr/86Sr at different mixing volume ratios of HFFF proportions in polluted QG1 groundwater
7-day HFFF mixing ratio in mixing water (%) 0.10 1.00 5.00 10.00 30.00 50.00 70.00 90.00 95.00 100.00 mmix Na+ (mM) 66.63 67.16 69.54 72.51 84.39 96.27 108.10 120.00 123.00 126.00 mmix Sr2+ (mM) 0.091 0.091 0.091 0.091 0.092 0.093 0.093 0.094 0.094 0.094 γmix Na+ 0.78 0.78 0.78 0.77 0.77 0.76 0.76 0.75 0.75 0.75 γmix Sr2+ 0.54 0.54 0.53 0.53 0.50 0.47 0.45 0.42 0.42 0.41 87Sr/86Srmix 0.712274 0.712286 0.712336 0.712398 0.712646 0.712891 0.713132 0.713371 0.713430 0.713489 εSr mix 43.79 43.95 44.65 45.53 49.03 52.48 55.89 59.25 60.09 60.92 ΔmSr2+ (mM) 4.69E-05 4.78E-04 2.30E-03 4.43E-03 1.14E-02 1.64E-02 2.02E-02 2.29E-02 2.35E-02 2.40E-02 mSr2+ in polluted groundwater (mM) 0.091 0.092 0.093 0.096 0.103 0.109 0.113 0.117 0.117 0.118 Proportion of ΔmSr2+ in
mSr2+ in polluted groundwater0.1% 0.5% 2.5% 4.6% 11.0% 15.1% 17.8% 19.6% 20.0% 20.3% 87Sr/86Srpolluted groundwater 0.712274 0.712285 0.712334 0.712392 0.712605 0.712798 0.712979 0.713155 0.713199 0.713242 εSr polluted groundwater 43.79 43.95 44.63 45.45 48.45 51.17 53.73 56.21 56.82 57.43 Note: Positive number of ΔmSr2+ indicates the Sr2+ is released from solid phase by cation exchange.
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