DOE OSTI2023
Executive Summary This Final Report is composed of two major sections. The first section presents experimental results obtained in the laboratory during Budget Period 1. The second section presents results from the field test conducted during Budget Period 2. Laboratory Results This research investigated a novel electrochemical process for producing a ferric iron coagulant for use in treating flowback and produced water from hydraulic fracturing and oil production operations. The treatment system improves the effectiveness and lowers the cost of coagulation processes using Fe3+ as the coagulant. The electrolytic coagulant generation (ECG) system uses an electrochemical cell to produce acid and base from oilfield brine solutions. The acid is used to dissolve scrap iron to provide a Fe3+ coagulating agent. The base is used to neutralize the treated water. Compared to conventional electrocoagulation (EC), the main advantage of the ECG system is an order of magnitude lower cost for the source of iron. The second advantage over conventional EC is that it can deliver Fe3+ doses greater than 1 mM, since it is not limited by the amount of dissolved oxygen in the water required to oxidize ferrous to ferric iron. The capital costs for conventional EC and the ECG system are similar, but the operational costs for the ECG system are an order of magnitude lower than conventional EC. The combined costs for iron and electrical energy for treating 1 m3 of FPW with 1 mM Fe3+ is estimated to be $0.87 for conventional EC, and $0.087 for the ECG system. The estimated all-in cost for treating FPW with a 2 mM Fe3+ dose is $0.73/m3 ($0.12/bbl). Field Test Results The field test was conducted at the Paul Foster Central Tank Battery (CTB) in Lea County, New Mexico from November 10, 2022 through December 15, 2022. The feed water to the system was produced water from the Tatanka 1H formation. After approximately two weeks of testing, the initial batch of produced water had been treated and no untreated produced water was available. Thus, after this time, the feed water to the system consisted of previously treated water (i.e., recycled water). The recycled water had nearly all colloidal particles removed, and had a much lower alkalinity due to precipitation of carbonate minerals during the first pass through the system. Although the recycled water was not an ideal test solution due to its low particulate concentrations, its lower alkalinity did allow us to identify the main problem with the treatment system. The main problem with the treatment system was caused by the high alkalinity of the initial feed water (5.4 meq/L) that consumed a significant fraction of the electrochemically generated acid. This resulted in pH values exiting the iron contact tank that were too high to dissolve enough iron to effectively treat the produced water. Tests performed with recycled water with lower alkalinity did not have this issue, and dissolved iron concentrations greater than 20 mM could be achieved. One consistent observation was that effluent water from the iron contact tank was always free of particulates, even when fed with circumneutral solutions. This suggests that there is no need to dissolve high concentrations of iron if all the water to be treated is passed through the scrap iron canister. In this case, dissolved O2 and hypochlorous acid can promote sufficient iron corrosion to provide an effective coagulating agent – even in neutral pH water. This solves the problem resulting from highly alkaline produced water. Modifications to the design of the treatment system were made based on the field test results. These modifications will add minimal additional cost, and were tested in bench-scale laboratory experiments. In short-term testing, the modified treatment process was able to remove colloidal FeS particulates to levels below detection. Long-term, steady state testing will be required to determine whether the modified process is suitable for commercial treatment systems.
54 ENVIRONMENTAL SCIENCES↗