**1. Introduction**

Shale gas, as an unconventional resource, has been widely developed in the United States, Canada, China, and Argentina in the past decades to meet the increasing demands for geo-energy. China is the largest holder of shale gas resources worldwide, with estimates ranging from 12.8 to 31.2 trillion m<sup>3</sup> [1–4]. In addition, more than two thirds of the estimated resources are stored in marine shales, in particular in south China. Therefore, the marine shale reserves have raised much attention.

The lower Cambrian Niutitang shale is a prolific gas play distributed in many places across South China, including Sichuan, Chongqing, Guizhou, Hunan, and Hubei provinces. This formation has been widely analyzed and evaluated over the past decade [5–10]: the Niutitang shale is mainly composed by quartz and clay minerals, which ranges from 35% to 77% and 6.2% to 37%, respectively. Carbonate and feldspar are also common in this formation, with ranges from 0% to 27% and 2% to 22%, respectively. The total organic carbon (TOC) ranges from 0.5% to 10% and the equivalent vitrinite reflectance (equal-Ro) ranges from about 1.5% to 4.6%. The largest thickness of this formation in Hunan Province can reach more than 300 m. Studies have showed that the Niutitang shale contains a very large quantity of shale gas resources. The achievements made in the analysis of pores and mineral composition in shale reservoirs [6,11–13] revealed the favorable pore systems for shale gas storage and a high brittleness that favors fracturing. More recently, wells targeting the Niutitang shale in the western Hubei province have already shown industrial gas flow [14].

However, due to the ultra-low porosity and permeability of unconventional reservoirs (e.g., shale and coal-bed methane reservoirs), horizontal drilling and hydraulic fracturing techniques are needed for e ffective extraction of these unconventional resources [15,16]. During the hydraulic fracturing process, from 7500 to 15,000 cubic metres of water along with specific chemical additives, which are acidic and oxidative, are injected into subsurface shale formations for one typical shale gas well [17,18]. From 5 to 85% of the injected fluid (generally 30–50%) flows back to the near surface and has a salinity three times greater than that of the initial water [19,20]. These flowback waters generally contain lots of toxic and radioactive elements (e.g., U, Pb, Sr), and therefore could lead to contamination of the surface water and shallow aquifers or accumulation of heavy metals in soil when spills and leaks occur [21,22]. There is much research evaluating the influence of shale gas development on groundwater quality in several shale regions in the U.S [23–26] and in the UK [27]. However, when it comes to China, the research is relatively rare. Although some scholars [28,29] have done research about the flowback waters of Longmaxi Shale in the Sichuan Basin, the Niutitang Shale, as a newly developed shale gas reservoir, has not received much attention yet. In addition, most of the research did not explain what caused the high salinity of flowback water. The geochemical interaction between shale and fracturing fluid, which a ffects not only physical properties of the shale but also the composition of the waters produced during shale gas production, is an important controlling factor [13] but rarely has been discussed [30]. Therefore, more studies about the geochemical processes between fracturing fluids and the shale are required to evaluate and minimize the potential environmental impacts.

By conducting simulation experiments and studying the changes of fracturing fluid, mineral, and physical properties during the experiments, key reactions occurring during the fracturing process in the shale reservoirs can be extrapolated. In this study, the Niutitang shale samples from di fferent depths were exposed to and reacted with fracturing fluids. The purposes of this article are: (1) to evaluate the release of major and minor cations and ions as well as trace metal contaminants during hydraulic fracturing, (2) to assess mineralogical and petrophysical changes of the shale during shale gas production, and (3) to investigate the key reactions between the shale samples and fracturing fluids. If the reactions that release metals can be identified, steps to minimize those reactions could be taken, making the flowback waters less toxic, radioactive, and easier to be treated.

#### **2. Materials and Methods**

#### *2.1. Samples and Fracturing Fluid*

Samples were cored from an exploration well drilled in Anhua County, Hunan Province (Figure 1). Two groups of samples from di fferent depths were used in the experiment, one from a depth of about 80 m and the other from a depth of about 795 m, to investigate the influence of the initial shale mineral compositions and physical properties. The information about initial shale samples is presented in Table 1. The fracturing fluid used in our experiments was from a chemical company called Rong Sheng Chemical Co., Ltd. (Shenzhen, China) and this kind of fracturing fluid has been used for shale gas

exploitation in field. It contains 99.7 wt% water, 0.15 wt% guar gum, 0.03 wt% acid (mainly acetic acid and hydrochloric acid), 0.01 wt% pH adjustor (NaOH, NaHCO3), and some other additives like K<sup>+</sup> and Ca2+ salts. In each reactor, approximately 8 g of shale sample was exposed to 50 mL fracturing fluid. Most of the shale samples were powder samples, ground to between 100 to 200 mesh, and others were thin sections, for which the main purpose was to provide a surface for field emission scanning electron microscope (FE-SEM) observation.

**Figure 1.** Location of the shale gas well in Anhua (the red spot), where the samples were obtained. The blue area represents the distribution of Lower Cambrian Niutitang Formation in South China (Modified from [31]).
