Advanced Electrochemical System Desalts Produced Water, Saves Polymer
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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.
After the offsite work passed its technical and economic tests, an onsite pilot was constructed and delivered to a live EOR oil field. It operated for 60 days with 100% reliability, unattended overnight, and achieved all technical goals, including rugged operations in temperatures as low as −14°C and winds up to 100 km/h. The EDR system met the desalination objective of 2000 mg/L TDS with 90% desalted water recovery achieved and was tested subsequently to desalt to lower-TDS targets. Polymer-addition testing also was completed on a range of desalted produced-water TDS concentrations, demonstrating up to 65% polymer reduction and resulting in operating cost savings.
Water-Treatment Technology for Produced-Water Desalination
The most-popular desalination technology on the market is reverse osmosis (RO), which pushes water through semipermeable membranes at pressures up to 1,000 psi and creates low-salinity (less than 500 mg/L) water. The challenge facing conventional RO technology in EOR is the oily nature of produced water that contains hydrocarbons including solvents. RO membranes are susceptible to swelling and damage in oils and solvents, as well as fouling in the presence of other organics. Removing hydrocarbons by pretreatment is possible, yet this can be cost-prohibitive.
Evaporators are another technology available for treating produced water and reach even higher concentrations of brine and equivalent greater quantities of treated water for reuse. However, they generally cost five to 10 times more per unit of water treated when compared with an RO unit. They are also larger, heavier, and more energy-intensive, making them a poor fit for offshore EOR.
The solution explored in this work involves leveraging a 40-year-old, nonpressurized EDR-membrane technology used for desalination. EDR has undergone recent innovations in membrane composition, stack design, and process controls that enable its use on produced waters without extensive pretreatment. EDR avoids the use of a pressure difference across membranes, which is one contributor to fouling in RO systems, in favor of driving desalination by applying electricity across ion-exchange membranes to separate ions by charge (Fig. 1). Because conventional EDR membranes will swell and fail in the presence of oily waste water, the authors used hydrocarbon-resistant ion-exchange membranes that were developed and tested specifically to withstand hydrocarbon and solvents (Fig. 1) through novel highly crosslinked polymer composition, including both ion conductivity and ductility. These membranes also exhibit the added benefit of selectively removing multivalent ions and, therefore, were selected for this work.
When desalinating salt water, operators need to consider what to do with the concentrated, higher-salinity brine left over. Because it has a significantly reduced volume compared with the raw produced-water stream, sending it for disposal may be more economical. Alternatively, pairing any of the treatment options with zero-liquid-discharge technology, such as crystallizers, to produce freshwater and residual solids is also possible.
Project Summary and Equipment Setup
After offsite trials on three produced-water sources ranging in salinity from 5000 to 20 000 mg/L were concluded successfully and economics were confirmed, a fully integrated and automated pilot plant was designed, constructed, and delivered to site. It consisted of full-scale components to be used in future plants and was operated for 60 days, 24 hours a day, including unattended night operation. The plant operated reliably and continuously through major process upsets including oil-in-water levels spiking from 50 to 1,000 ppm and two severe weather events including winds in excess of 100 km/h and freezing conditions.
The plant was outfitted into two 40-ft shipping containers for mobility to future trial sites. One shipping container included a pretreatment system, small wet laboratory, operator desk, and spare-parts storage. The second shipping container consisted of the electrodialysis desalination system, tanks, electrical panels, and controls. A remote operating and alarm-notification system was installed by means of a cellular link.
The proprietary media filtration used for pretreatment could be backwashed and chemically cleaned aggressively without degradation. The pretreatment goal was to reduce particle distribution to less than 20 µm (with a goal of 15 µm), which prevents clogging in inlet manifolds built within the electrodialysis stack. Oil and hydrocarbon products do not need to be removed; however, coagulation of larger hydrocarbons and oil can occur within both the pretreatment and electrodialysis stack. Coagulated oil that may accumulate over long periods of time was removed readily by either a backward flush or a more-aggressive 11-pH wash if necessary. The high-pH wash fluid was recycled and reused multiple times. Backwash fluid was filtered by use of bag filters and sent to the inlet of the plant to be retreated.
The produced water at the field test site averaged 7000 mg/L TDS. It was desalted to the target concentration of 2000 mg/L TDS and then lower, approaching 1000 mg/L TDS, with the midpilot optimal target being 1700 mg/L TDS. Prefiltration improved turbidity and reduced solids distribution to the target particle size of less than 20 µm. Ninety percent desalted water recovery was achieved while treating the produced water in the presence of up to 1,000 ppm oil in water, averaging 500 ppm.
EOR desalination could be economical either where lower salinity, or hardness removal, increases oil recovery in conventional waterflood options or where polymer flood is used to increase injected-water viscosity and oil sweep. This work focuses on the latter.
Polymer savings will vary widely from site to site, depending on desired final viscosity and chemical makeup. For early project-feasibility estimates, data suggest that at least 50% polymer savings can be achieved for chemical-flood operations, possibly much more at higher TDS concentrations. This is because much-higher injected-water viscosities can be achieved at lower polymer concentrations if a lower-TDS water source is used or if produced water is desalted to a lower TDS.
Although all site economics will vary, as a guide, a 1000- to 4000-m3/d polymer-injection site may consume from $2 million to $10 million per year in polymer operating costs. For the sake of this work, and in alignment with the pilot on the 7000-mg/L TDS inlet, polymer costs are assumed to be $5.5/m3 injected without desalination. The 65% polymer cost savings could equal $2.5/m3 in savings.
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Advanced Electrochemical System Desalts Produced Water, Saves Polymer
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