Successful Execution of World’s First ZLD Pilot with Extreme Salinity Unconventional Frac Flowback Produced Water

A significant portion of the water used for the hydraulic fracturing of unconventional hydrocarbon resources is later recovered when subject wells are flowed back post fracturing. Treating fracturing flow-back produced water from unconventional resources unlocks its full value and provide alternative supplies of low salinity water to seawater desalination and ground water consumption, while promoting a circular water economy approach through 4Rs (Reduce, Reuse, Recycle, and Recovery). The main objective of this work is to highlight the design and deployment of a zero-liquid discharge (ZLD) flowback produced water management solution to treat quite challenging extreme salinity unconventional fracturing flow back flowback produced water (up to 250,000 ppm Total Dissolved Solids, TDS), and generate the desired high-quality low-salinity water suited for fracking applications. The designed ZLD unit comprised of a unique combination of a pretreatment system to remove residual oil/ H2S and dynamic vapor compression technology for salt removal from hypersaline flowback produced water. The pretreatment system relied on chemical scavengers to lower the dissolved H2S from flowback produced water, while induced gas floatation with nitrogen gas was used to remove dispersed hydrocarbons. The pretreated flowback produced water is then processed in a dynamic vapor compression unit to effectively remove salts and generate low salinity "fresh water". The ZLD pilot was successfully executed at one of the unconventional well pad site locations, wherein a primary separator separated the three flowing phases (water, oil, and gas) from producing wells. The separated flowback produced water contained more than 500 ppm H2S, about 1,000 ppm of residual hydrocarbons, and an extremely high salinity (up to 250,000 ppm TDS). The pilot unit was sized to process about 220 barrels per day (BPD) of flowback produced water obtained from the separator to generate distillate (fresh water) and remove salts as concentrated brine. The results obtained from ZLD pilot were very promising. The system was able to remove residual hydrocarbons and dissolved H2S to less than 1 ppm, and most importantly, lower the salinity of flowback produced water to less than 200 ppm TDS, generating fresh water at an average 70-80% recovery efficiency. The pilot was operated at a range of salinities from 50,000 ppm to 250,000 ppm TDS (with different blending ratios of flowback produced water and freshwater) to develop the ZLD operating envelope for a spectrum of flowback produced water salinities. In summary, ZLD is a breakthrough industry-first pilot in the world that can promote flowback produced water recycle/reuse and circular water economy in unconventional resources operations. The impressive performance and engineering excellence achieved during the execution of this pilot presents a step-change in flowback produced water desalination and reuse, which has huge potential to revolutionize water treatment and desalination practices followed in the oil and gas industry.