Plasma Discharges in Produced Water and Its Applications to Large Scale Flow
Authors:   , Kim HyoungSup , Cho Young I. 
Published: July 2021

Natural gas from shale is a relatively clean energy source and is widely viewed as a key asset for economic growth in the U.S. Extraction of shale gas and oil through horizontal drilling and hydraulic fracturing is expected to continue to grow. Likewise, volumes of produced water, which is wastewater from oil and gas production, are also expected increase in the future. Currently, a number of different methods are used to treat produced water for surface discharge, reuse in the shale formation, and other beneficial use. The objective of the present study was to utilize plasma discharges to treat a large volume of produced water for the purpose of recycling it for subsequent fracking. To recycle produced water, both bacterial inactivation and water softening are required, which are the two main objectives of the present plasma water treatment study. The present study consists of two parts: (1) a bacterial inactivation study using both gliding arc discharge (GAD) and high frequency spark discharge and (2) the study of a spark-assisted self-cleaning (SASC) filtration technology in produced water. Note that the SASC filtration technology is a key component for plasma-assisted water softening. Microorganisms, particularly acid-producing bacteria (APB) and sulfate-reducing bacteria (SRB) in produced water, cause microbiologically influenced corrosion such as pipeline corrosion, reservoir souring, and biofouling, which are persistent problems in oil and gas production. E-coli contaminated water was used for the bacterial inactivation study, and the effects of H₂O₂ and low pH generated by GAD on the inactivation of E-coli bacteria were investigated at a range of water injection rates and air flow rates within GAD system. Furthermore, it was shown that the magnitude of bacterial inactivation and its energy efficiency in a large volume of water could be increased by the use of a microbubble generator to re-inject plasma byproducts in gas form to plasma-treated water enabling the residual effects of plasma water treatment to be captured and used. A new co-axial electrode system was developed for the generation of spark discharges in high-conductivity produced water. This new technology was used in both the biodecontamination and water softening parts of the present research. In static batch tests (no flow), 10-min treatment of semi-transparent produced water by spark plasma showed a 3-log reduction of APB and 2-log reduction of SRB based on the most probable number method in cfu/mL units. In once-through flow tests at water flow rate between 1 and 5 gpm, 2-log reduction of APB was observed in the case of semi-transparent produced water, whereas 1-log to 1.5-log reductions were observed in dark produced water. The energy efficiency (D-value) for APB inactivation in once-through flow tests of dark produced water was 1.7 kJ/L per 1-log reduction. The validation study for the SASC filtration technology was conducted using 10-in cartridge filters having 3- and 5-micron pores in synthetic (high conductivity) produced water and actual produced water. Tests of SASC filtration were performed at different levels of total dissolved solids (TDS) and total suspended solids (TSS) and at a range of flow rates. The present study demonstrated the validity of the SASC filtration concept with water samples having TDS levels [less than or equal to] 50,000 mg/L and TSS levels [less than or equal to] 2,500 mg/L. Pressure drops across the filter cartridges obtained with spark discharges were significantly less than baseline pressure drops (i.e., without spark discharges). Pressure drops increased consistently over time in baseline tests resulting in pump failure due to excessive pressure buildup. In summary, the present study demonstrated the feasibility of plasma treatment of wastewater from shale-oil and shale-gas exploration. In particular, the present study validated that the plasma water treatment could be done in an energy-efficient manner in a large enough scale that can be utilized by the wastewater treatment industry.