Are Deep Aquifers Really Confined? Insights From Deep Groundwater Tidal Responses in the North China Platform

Abstract

Great amount of wastewaters coproduced from hydrofracking shales for the extraction of natural gas in the mid-continental US has been disposed into deep aquifers (e.g., McGarr et al., 2015;Walsh & Zoback, 2015;Weingarten et al., 2015). The US Environmental Protection Agency that regulates such practices to protect potable water sources from the injected wastewater focuses on well integrity and deep aquifers isolated from shallow groundwater by geologic barrier formations above the injection zone (e.g., Murray, 2014; U.S. Environmental Protection Agency, 2016). Concerns have been raised if the injected fluids may migrate upward and contaminate shallow groundwater (Vidic et al., 2013). Recent studies (Barbour et al., 2019;Wang et al., 2018) also showed that a targeted deep aquifer in Oklahoma (the Arbuckle aquifer) may be leaking near a USGS well, but no systematic evaluation of the confinement of aquifers buried by mudstones or shales has so far been made. The problem is timely and of global significance because the practice of Abstract Many deep aquifers overlain by barrier formations in the continental US are used as geological repositories for wastewaters coproduced from hydrocarbon exploration. This practice is to protect shallow groundwater following the US Environmental Protection Agency's regulation. Implicit in such practice is the assumption that deep aquifers overlain by mudstones or shales are confined so that the injected fluids will not migrate upward to contaminate shallow groundwater. However, no systematic test of this hypothesis has been made. Here we invert the groundwater response to both the M 2 and the O 1 tides and to the barometric pressure across a large (2.046 × 10 6 km 2 ) geologic regime, the North China Platform, to systematically evaluate the hydraulic parameters as functions of depth and time without a priori assumption. Our result, the first of such inversion, shows no depth dependence of aquifer confinement to a depth of 3,400 m and that deep confined aquifers overlain by barrier formations may become leaky after distant earthquakes. It suggests that monitoring of aquifer confinement may be needed to ensure if the targeted deep aquifer for wastewater injection is really confined. The results may be timely and of global significance because the practice of hydrofracking for natural gas and the deep injection for the disposal of the coproduced wastewaters in the US may soon be adopted by other countries, such as China.Plain Language Summary A great amount of wastewaters coproduced from hydrocarbon exploration in the mid-continental US has been injected into deep aquifers. Such practice has been based upon an implicit assumption that deep aquifers confined by aquitards are fully confined so that the injected toxic water will not leak back to contaminate shallow groundwater or the environment. This assumption, however, has never been carefully tested. Here we test this hypothesis using groundwater data from wells over a large geological regime, the North China Platform, as a function of...

Possible Continuous Vertical Water Leakage of Deep Aquifer: Records from a Deep Well in Tianjin Province, North China

“…For well Gaocun, the barometric observation starts from the year 2015. The software Mapsis was used [24] to calculate and analyze the theoretical Earth tides, and it has been tested against BAYTAP-G (e.g., [14]). In order to be uniform, we used the hourly data of water level from January 1st, 1981, to May 9th, 2020, and resampled into the minute data to do calculations in this study.…”

“…Hence, 3 input parameters (A M2 ′ /A O1 ′ , η M2 , and η O1 ) turn out 3 output parameters (S, T, and K ′ ), which will be more stable [14]. Since the least squares fitting relies on the input initial testing values of T, S, and K ′ , the aquifer lithology (each lithol-ogy has value spans for the hydraulic parameters [27]) and the aquitard lithology (reflect the vertical leakage extent K ′) are set as limitation conditions during the fitting processes.…”

“…Since the least squares fitting relies on the input initial testing values of T, S, and K ′ , the aquifer lithology (each lithol-ogy has value spans for the hydraulic parameters [27]) and the aquitard lithology (reflect the vertical leakage extent K ′) are set as limitation conditions during the fitting processes. The detailed calculation processes are also introduced in Zhang et al [14] and Yang et al [12]. Briefly, during the non-linear, least squares inversion (Fsolve, https://www.mathworks.com/ help/optim/ug/fsolve.html), the initial values are set as the constrain conditions for the results, thus the obtained S and T are constrained by the parameter ranges of the aquifer rocks, while K' is the vertical leakage of the fracture, thus need to be >=0 as the constrain, and as the leakance term, K' should be relatively small according to the model setting of Wang et al [9].…”

“…There is no trade-off problem of the parameters in the inversion, and the inversion is independent and without preassumptions. Detailed information and the process of solutions are explained in the previous paper [14].…”

“…That is deduced from the early analytical solution of Hantush and Jacob [10], which is for the leaky and multilayered aquifers. Recently, the derived solutions were applied to study pressure variation in response to water injection/extraction in boreholes (e.g., [11]) or used to calculate hydraulic parameters both for the aquifer and for the aquitard (e.g., [9,[12][13][14]). Meanwhile, for the barometric response of borehole water level in a semiconfined aquifer, several analytical models have been deduced, and the analytical solution of Rojstaczer [15] was widely applied to estimate hydraulic properties (e.g., [13,[15][16][17]).…”