**6.6 Impact of geochemistry of shale-HFF reactions on toxicity of produced water**

Flowback water is the fluid produced immediately following treatment of a well. This fluid is generally made up of mixed compositions of fracturing fluids, products of reactions between fracturing fluids and formation, and formation fluids. High salinity and concentrations of dissolved metals have been reported in flowback waters [104, 105]. Potential toxicity of produced water to humans and the environment remains high and of great concern; therefore, researchers have been studying these fluids to assess their risks to the environment. Most studies of this nature have largely been undertaken in producing unconventional hydrocarbon fields in the USA as well as black shales from Germany and Denmark [106, 107]. A summary of these works is presented below:

Chapman et al. [104] sought to characterise the flowback waters from Marcellus Formation in the Appalachian Basin with strontium isotopes in order to help detect and trace contamination of surface and ground waters by flowback water. The geochemical characterisation of the flowback fluids showed elevated levels of Bromide, Calcium, Strontium (up to 5200 mg/L), Sodium, Chloride (up to 12000 mg/L) and Barium. They also reported high total dissolved solids in the fluids in excess of 200000 mg/L. They concluded that high elemental concentrations were the result of interactions between fracturing fluids, formation minerals and formation fluids.

For their part, Wilke et al. [107] studied the rate of release of metals, salts anions and organic compounds from shale rocks in Denmark and Germany. They reported that, concentration of ions in solution largely depended on the composition of the black shale and did not show dependency on pH of the fluids used in experiments. There was however a correlation between the buffering capacity of specific mineral components of the rock, such as pyrite and carbonate on amount of dissolved ions. They also reported decrease in ionic concentrations over time due to precipitation

*Review of Geochemical and Geo-Mechanical Impact of Clay-Fluid Interactions Relevant… DOI: http://dx.doi.org/10.5772/intechopen.98881*

of new minerals. Findings from this research provided an understanding of possible flowback water composition and toxicity from these unconventional fields in Denmark and Germany respectively.

Experimental studies by Macron et al. [106] on geochemical interactions between fracturing fluid and formation showed evidence of clay and carbonate dissolution as well as precipitation of new minerals. Dissolution leads to elevated elemental concentrations in fluids some of which show a drastic reduction when precipitation of new minerals begins. Results from this experimental work were validated using measured concentrations of sodium chloride (NaCl) in Marcellus shale which showed similar trends. They concluded that results from their work can be used to form a basis of assessing controls of geochemical reactions in the reservoir, in other words, flowback water compositions.
