In order to protect shallow groundwater, many deep aquifers covered by confined formations in the continental United States are used as geological repositories for co-production wastewater from oil and gas exploration, on the assumption that fluids injected in deep aquifers covered by mudstone or shale will not migrate upward to contaminate shallow groundwater. However, so far there has been no systematic testing of this hypothesis.
In view of the above problems, Zhang Yan, associate researcher of the Key Laboratory of the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and Professor Wang Qiyun of the University of California, Berkeley, etc., took the North China Plateau as an example to conduct the first tidal response mixed flow model test on the tidal response mixed flow model test of the mudstone/shale formation with high groundwater level and air pressure observation quality and extremely low pore inlay, calculated the vertical permeability (and barometric pressure coefficient), estimated the cover closure, and analyzed the great earthquake (2008). MW7.9 Wenchuan, 2011 MW9.1 Tohoku) on the impact of closure.
Figure 1 North China platform outline (blue track line), underground fluid station (red triangle), air pressure station (blue dot) and earthquake epicenter location (beach ball) distribution
Studies have shown that aquifers above 3400 m in depth, regardless of depth, can become watertight after a long-range earthquake. The vertical hydraulic conductivity K' of most underground fluid wells is >0, and the size ranking of the barometric pressure coefficient BE is also in line with the calculation result order of the vertical hydraulic conductivity K' (Figure 2), that is, the better the closure of the well (the smaller the K'), the higher the barometric pressure coefficient corresponding to the well. This data suggests that even if the upper cover is a mudstone/shale with very low porosity, most aquifers will experience significant leakage before the great earthquake. Therefore, the closure of deep aquifers may not be as ideal as expected, and the burial of wastewater and waste materials in deep aquifers needs to be used carefully and closely monitored.
Fig. 2 Vertical hydraulic conductivity K' of the inlet from 2008 to 2011 and the storage coefficient T and water conductivity of the aquifer T and the water conductivity S of the aquifer. Vertical dotted lines indicate the moment of the great earthquake (2008 MW7.9 Wenchuan, 2011 MW9.1 Tohoku)
The research results were published in the international academic journal WRR in the field of hydrogeology (Zhang Yan, Wang Qiyun*, Fu Liyun*, Yang Qiuye. Are deep aquifers really confined? Insights from deep groundwater tidal responses in the North China Platform[J]. Water Resources Research, 2021, 57: e2021WR030195. DOI: 10.1029/2021WR030195）。
Editor: Chen Feifei