Development of Porous Silicon Membranes with Hydrophobic Vapor Traps to Desalt Hydraulic Fracturing Flowback Water

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2019-05

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The Ohio State University

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Abstract

In 2017 the United States consumed 98 quadrillion BTUs of energy, but only produced 88 quadrillion BTUs of energy domestically. This has amplified the United States’ reliance on foreign entities for energy imports and induced high volatility in the energy market. Consequently, there is a need for localized, cost-effective energy sources, which has resulted in numerous unconventional oil and gas production methods, including hydraulic fracturing. During the hydraulic fracturing process, an estimated 3-5 million of gallons of water per well are pumped into the rock bed to generate fissures in the surrounding rock in order to extract the underlying hydrocarbons. Approximately 20-40% of this water flows back to the surface in 60 days with a hyper-saline composition that can be 3-10 times saltier than seawater. Moreover, flowback water, depending on the geographical location, contains heavy metals, organic contamination, and possible radionuclides. Currently, flowback water disposal occurs commonly through deep well injection, as no reliable flowback water treatment methods presently exist. One possible method to desalinate hydraulic fracturing flowback water is through the use of hydrophobic vapor traps. Proof-of-concept experiments with three nanochannels in silica of length 32 μm and at a pressure of 48 bar, containing hydrophobic sections have been able to achieve an average desalting of 95% in 20 minutes using a 5 M NaCl draw solution. This project outlines the development of porous silicon membranes with hydrophobic vapor traps, as a possible strategy to scale-up the proof-of-concept demonstration to viable laboratory evaluation of desalination of flowback water using forward osmosis. The main deliverables of this project include the successful development and validation of a porous silicon recipe process. Additionally, the design and fabrication of a porous silicon membrane for transport testing is also documented. The fabricated porous silicon membranes had 10 ± 2.2 nm diameter pores and a pore surface density of 23.05%. The pore size and pore distribution were based on image analysis of scanning electron microscopy (SEM) imaging. The porous silicon membranes were tested in the custom designed and built permeation set-up to show that membranes of diameter 1.2 cm and thickness 10 ± 1.3 μm were able to sustain osmotic gradients of 4.825 atm and demonstrate transport of water across the nanopores. It was determined that non-functionalized membranes, during a 24-hour period, were able to desalt a 100 mM NaCl solution by, 24.25 ± 1.63%.

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Porous Silicon, Desalination, Hydraulic Fracturing, Fracking, Microsystems and Nanosystems Lab

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