Horizontal drilling and hydraulic fracturing are technologies designed to increase natural gas flow and to improve productivity in low permeability formations. During this drilling operation, tons of flowback and produced water, which contain several organic compounds, return to the surface with a potential risk of influencing the surrounding environment and human health. In order to conduct predictive risk assessments a mathematical model is needed to evaluate organic compound behaviour along the water transportation process as well as concentration changes over time throughout the operational life cycle. A comprehensive model, which fits the experimental data, combining an Organic Matter Transport Dynamic Model with a Two-Compartment First-order Rate Constant (TFRC) Model has been established to quantify the organic compounds concentrations. This algorithm model incorporates two transportation rates, fast and slow. The results show that the higher the value of the organic carbon partition coefficient (koc) in chemicals, the later the maximum concentration in water will be reached. The maximum concentration percentage would reach up to 90% of the available concentration of each compound in shale formation (whose origin may be associated to drilling fluid, connate water and/or rock matrix) over a sufficiently long period of time. This model could serve as a contribution to enhance monitoring strategy, increase benefits out of optimizing health risk assessment for local residents and provide initial baseline data to further operations.
Keywords: Flowback water; Hydraulic fracturing, chemical concentration; Marcellus shale; Organic compounds; Transport model.
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