Optimal Pretreatment System of Flowback Water from Shale Gas Production

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.