Shale gas has emerged as a potential resource to transform the global energy market. Nevertheless, gas extraction from tight shale formations is only possible after horizontal drilling and hydraulic fracturing, which generally demand large amounts of water. Part of the ejected fracturing fluid returns to the surface as flowback water, containing a variety of pollutants. For this reason, water reuse and water recycling technologies have received further interest for enhancing overall shale gas process efficiency and sustainability. Water pretreatment systems (WPSs) can play an important role for achieving this goal. This paper introduces a new optimization model for WPS simultaneous synthesis, especially developed for flowback water from shale gas production. A multistage superstructure is proposed for the optimal WPS design, including several water pretreatment alternatives. The mathematical model is formulated via generalized disjunctive programming (GDP) and solved by re-formulation as a mixed-integer nonlinear programming (MINLP) problem, to minimize the total annualized cost. Hence, the superstructure allows identifying the optimal pretreatment sequence with minimum cost, according to inlet water composition and wastewater-desired destination (i.e., water reuse as fracking fluid or recycling). Three case studies are performed to illustrate the applicability of the proposed approach under specific composition constraints. Thus, four distinct flowback water compositions are evaluated for the different target conditions. The results highlight the ability of the developed model for the cost-effective WPS synthesis, by reaching the required water compositions for each specified destination.
To address water planning decisions in shale gas operations, we present a novel water management optimization model that explicitly takes into account the effect of high concentration of total dissolved solids (TDS), and its temporal variation in the impaired water. The model comprises different water management strategies: a) direct wastewater reuse, which is possible due to the new additives tolerant to high TDS concentration but at the expense of increasing the costs; b) wastewater treatment, taking separately into account pre-treatments, softening and desalination technologies and c) send to Class II disposal sites.The objective is to maximize the "sustainability profit" determining flowback destination (reuse, degree of treatment or disposal), the fracturing schedule, fracturing fluid composition and the number of water storage tanks needed at each period of time.Due to the rigorous determination of TDS in all water streams, the model is a non-convex MINLP model that is tackled in two steps: first, an MILP model is solved based on McCormick relaxations; next, the binary variables that determine the fracturing schedule are fixed, and a smaller MINLP is solved.
Thermal membrane distillation (MD) is an emerging technology to desalinate highsalinity wastewaters, including shale gas produced water to reduce the corresponding water footprint of fracturing operations. In this work, we introduce a rigorous optimization model with energy recovery for the synthesis of multistage direct contact membrane distillation (DCMD) system. The mathematical model (implemented in GAMS software) is formulated via generalized disjunctive programming (GDP) and mixed-integer nonlinear programming (MINLP). To maximize the total amount of water recovered, the outflow brine is fixed close to salt saturation conditions (300 g·kg -1 water) approaching zero liquid discharge (ZLD).A sensitivity analysis is performed to evaluate the system's behavior under different uncertainty sources such as the heat source availability and inlet salinity conditions. The results emphasize the applicability of this promising technology, especially with low steam cost or waste heat, and reveal variable costs and system configurations depending on inlet conditions. For a produced water salinity ranging from 150 g·kg -1 water to 250 g·kg -1 water based on Marcellus play, an optimal treating cost are between 11.5 and 4.4 US$ m -3 is obtained when using low-cost steam. This cost can decrease to 2.8 US$ m -3 when waste heat from shale gas operations is used.
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