Advanced Electrochemical System Desalts Produced Water, Saves Polymer

Abstract

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 190272, “An Advanced Electrochemical System for EOR Produced-Water Desalination and Reduced Polymer Consumption,” by Ben Sparrow, Adrian Ebsary, Derek Mandel, and Malcolm Man, Saltworks Technologies, prepared for the 2018 SPE Improved Oil Recovery Conference, Tulsa, 14–18 April. The paper has not been peer reviewed. This paper presents pilot-testing results and economics from a novel electrochemical desalination technology for enhanced oil recovery (EOR) produced water. The pilot objectives were to (1) economically desalt produced water to improve hydrocarbon recovery and lower polymer consumption costs for chemical-flood EOR; (2) inform full-scale plant development with a field pilot; and (3) optimize prefiltration, chemical consumption, and energy use to realize greater than 20% return on investment through reduced polymer consumption. Background and Pilot Summary An electrodialysis-reversal (EDR) system that requires minimal pretreatment with proprietary hydrocarbon antifouling ion-exchange membranes packaged in rugged skids with advanced process controls was used. The EDR plant can (1) desalt EOR produced water from up to 20 000 mg/L total dissolved solids (TDS) down to 500–5000 mg/L for reinjection and (2) reduce polymer requirements to decrease chemical costs in polymer-flood operations. EDR desalts by use of an electric field; dissolved ions in the produced water are moved across ion-exchange membranes, desalting one stream while concentrating a smaller volume discharge. The TDS in the produced water can be desalted to any concentration that provides optimal performance specific to the reservoir. Depending on the presence of scaling ions, the concentrated stream can achieve TDS concentrations of 80,000–150,000 mg/L, or, in the case of offshore operations, the concentrated stream can be eliminated through a novel process. EOR operators may add polymers and other chemicals to increase viscosity of injected water and enable increased oil recovery. Tests completed for this work proved that EOR polymers present in the produced water that returns to surface do not foul or pass through the EDR membranes, instead remaining in the desalted water for reuse. In fact, some polymers are reactivated in the desalted output, with viscosity increasing by almost double during desalination, thereby enabling some recycling of original polymer. More importantly, results proved that lower TDS of the injected water can result in up to 65% polymer cost savings, which is a major contributor to operating costs. Polymer consumption increases with TDS in order to reach a target viscosity goal; therefore, desalting the injected water can result in net savings if desalination costs are lower than the incremental polymer-consumption savings. This work showed a 30–40% rate of return on investment. To test the theory, an offsite pilot was completed in advance to prepare for onsite work. Produced water up to 20 000 mg/L TDS with oil in water present up to 600 ppm were tested. Prefiltration consisted of proprietary media filtration to remove suspended solids to less than 20-µm particle size. No pretreatment for the oil was required. This is because of the resiliency of the antifouling ion-exchange membranes, EDR stack design to prevent plugging, and intelligent cleaning systems that detect and react to remove fouling or partial plugging before irreversible events occur.