Study of Plasma Treatment of Produced Water from Oil and Gas Exploration
Authors:   , Wright Kamau , Cho Young I. , Fridman Alexander A. 
Published: July 2021

Unconventional gas and hydraulic fracturing is helping to increase natural gas production, which is widely viewed in the U.S. as a key asset to bolstering a clean and energy-independent future. Safe and economical management and treatment of water produced during such processes remain of key importance. With the increase of hydrocarbon production and national shale gas production expected to increase threefold and account for nearly half of all natural gas produced by 2035, advanced water treatment and management processes must be investigated, to ensure water conservation and associated economic prudence. The state of the art of produced water treatment technologies is described including the efficacy of plasma to modulate the contents of such aqueous solutions, meeting target parameters and potentially enabling the operation of other treatment technologies. Among other effects, progress is presented on the enhancement of an arc-in-water system to remove bicarbonate ions and prevent the mineral fouling ability of water which causes formation of CaCO3 in heat exchangers and distillation units. Qualitative and quantitative treatment targets of produced water treatment are discussed. Experimental work is conducted to test theories and identify and reproduce favorable effects useful to treating wastewaters. Plasma arc-in-water systems demonstrated capability of producing bicarbonate-depleted wastewaters, with experiments with gas-field produced waters indicating that generation of H⁺ ions plays a greater role in bicarbonate ion removal than local heating. Tests showed abatement of bicarbonate ions from a range of 684-778 mg/L down to zero. Subsequent scaling/fouling tests with waters ranging from 0 to 500 mg/L bicarbonate ions, in the presence of high calcium ion concentrations, showed that scale thickness, as well as mass on a 1-kW heating element was an order of magnitude less for process water containing 100 mg/L bicarbonate ions compared to process water with 500 mg/L of bicarbonate ions. Water with bicarbonate ion concentration approaching zero resulted in prevention of scale. To enhance this new plasma induced fouling mitigation method, a plasma arc-in-water reactor was re-engineered, using a ground electrode, and two high-voltage electrodes, to stretch the arc discharge in water and increase contact between plasma and water. Results of simultaneous effects were also collected, showing within 5 min, a 4-log reduction in both Sulfate Reducing Bacteria (SRB) and Acid Producing Bacteria (APB), bacteria that are characteristic of oil-field produced waters; as well as oxidation of organics, with degradation of visually observable organics within 3 minutes, and decrease of oil and grease from 40 mg/L to under 10 mg/L within one min. With an arc-in-water system utilizing a stretched arc, simultaneous effects were exhibited on fouling ability of produced water, inactivation of bacteria, and degradation of organics. Plasma discharges in water represent a unique option in the treatment of produced waters from oil and gas production. While the water softening capabilities of arc-in-water systems present a new method for fouling mitigation and remediation of scale in heat exchangers, the simultaneous effects, including oxidation of organics and inactivation of bacteria, may allow application of plasma to water, to satisfy treatment targets that allow for the reuse of such waters in oil and gas operations.