NETL advances water treatment

Flowback is a water-based solution that flows back to the surface during, and after, the completion of hydraulic fracturing. The fluid contains clays, chemical additives, dissolved metal ions and total dissolved solids (TDS). Often, the water has a murky appearance from high levels of suspended particles. Most of the flowback occurs in the first seven to 10 days, but it can continue to occur over the following three to four weeks. In the Marcellus shale, for example, the recovery volume is between 20% and 40% of the volume initially injected into the well. The rest of the fluid remains absorbed in the formation.¹

In contrast, produced water is naturally occurring water, found in formations, that flows to the surface throughout the entire lifespan of the gas or oil well. This water usually has high levels of TDS and leaches out minerals from the formation (particularly shale) including barium, calcium, iron and magnesium. It also contains dissolved hydrocarbons, such as methane, ethane and propane, along with naturally occurring radioactive materials (NORM), such as radium isotopes.

At some point, the water that is recovered from an oil or gas well makes a transition from flowback water to produced water. This transition point can be hard to discern, but can sometimes be identified according to the rate of return measured in barrels per day (bpd) and by looking at the chemical composition. The chemical composition of flowback and produced water is very similar, so a detailed chemical analysis is recommended to distinguish between flowback and produced water.

Fig. 1. Map of U.S. produced water from various sources, United States Geological Survey, <i>Produced Waters Database.</i> Image: USGS.

The volume of produced water in the U.S. is small, compared to total daily water use, but these volumes can be locally significant. Based on the best available data from 2012, the nearly 1 million producing oil and gas wells in the U. S. generated approximately 21.2 Bbbl of produced water each year, Fig. 1. Expressed in other units, this volume equals 58 MMbpd, 890 billion gal/year, 2.4 billion gal/day, or 2.7 million acre-feet/year. Conventional oil and gas wells show little or no produced water initially, with the flowrate increasing over time. Total lifetime water production is typically higher for conventional wells than for unconventional wells.

The U.S. Department of Energy’s Office of Fossil Energy and the National Energy Technology Laboratory (NETL) have selected four projects to receive approximately $4.6 million in federal funding for cost-shared research and development (R&D) in produced water treatment. The projects will accelerate the development and commercialization of treatment technologies that reduce the wastewater being injected into disposal wells and increase water supplies for re-use. These projects are supported through funding opportunity announcement (FOA) DE-FOA-0002004, Low-Cost, Efficient Treatment Technologies for Produced Water.²

This R&D effort supports the Water Security Grand Challenge, a White House-initiated, DOE-led framework to advance transformational technology and innovation to meet the global need for safe, secure and affordable water. In particular, this FOA advances the Grand Challenge’s goal to transform the energy sector’s produced water from a waste to a resource. 

“Water and energy are interdependent resources,” said Assistant Secretary for Fossil Energy Steven Winberg. “That’s why it’s so important to pursue R&D that will transform produced water from a waste to a resource.”

NETL will manage the projects. The selected projects all fall under Area of Interest 1 (Low-Cost, Efficient Treatment Technologies for Produced Water). They are described in the following passages.

Non-fouling, low-cost electrolytic coagulation & disinfection for treating flowback and produced water for reuse. This project is being carried out by The University of Arizona and Water Tectonics.

Goal: The objectives of this project are to develop and test a new method for delivering a Fe³⁺ coagulant and disinfectant for treating flowback and produced water (FPW) at a 25 gal/min., pilot scale. The goal is to treat the water. so that it can be reused for fracing and waterflooding at an overall cost-savings of at least 50%, compared to commercialized processes.

The proposed technology consists of: 1) an electrochemical cell for producing acid, base and disinfectant; 2) scrap iron filings as an inexpensive source of iron coagulating agent; and 3) dissolved air flotation for flocculate removal. The process eliminates the electrode fouling issues associated with electrocoagulation and reduces the cost for providing the Fe3+ coagulant by a factor of ~3 over chemical coagulation, and by a factor of ~10 over traditional electrocoagulation.

Background: FPW contains suspended and colloidal solids, dissolved organic compounds (e.g., naphthenic acids, BTEX), H₂S, microorganisms, salt ions (mostly Na+, Cl-, SO₄2-), and potentially scale-forming cations (e.g., Fe2+, Ba2+, Ca2+, Mg2+ Sr2+). The properties of FPW vary by region of the country, and time, for a given well. In most cases, the total dissolved solids (TDS) concentration is greater than that of sea water (35,000 mg/L) and can be as high as 300,000 mg/L. Treatment for FPW in fracing and secondary oil recovery requires removal of 1) solids; 2) H₂S; 3) dispersed oil; 4) Fe2+; and, sometimes, 5) partial removal of other scale-forming cations (e.g., Ba2+, Ca2+). Disinfection prior to storage is also desirable to reduce the need for organic disinfectants (e.g., glutaraldehyde) during reuse.

A recent publication reviewed 16 commercialized FPW treatment technologies; all but one employed some form of coagulation treatment. Coagulation and flocculation processes remove water contaminants via formation of high surface area, high-porosity flocs that adsorb cations, microorganisms, and hydrophobic organics; and entrained particles and dispersed oil. For solid particle removal, coagulants promote electrical double-layer compression and charge neutralization, thereby encouraging particle aggregation. The coagulated flocs and aggregated particles can be removed from solution via settling and/or DAF. Ferric chloride (FeCl₃) and alum (Al2(SO₄)3•nH₂O) are the most common chemical coagulants used in water treatment. Electrocoagulation is more commonly used than chemical coagulation in FPW treatment, in which a small amount of voltage is applied to a metal sheet or plate in an electrochemical cell.

Impact: The treatment system will remove suspended solids, dispersed oil, H₂S, microorganisms and scale-forming cations from FPW. Technology developed through this research will improve the performance and significantly lower the costs of coagulation processes that have been proven to be effective at treating FPW for reuse.

Resource recovery and environmental protection in Wyoming’s Greater Green River basin, using selective nanostructured membranes. This project is being carried out by University of Wyoming, H₂O Systems and Triton Water Midstream.

Goal: The project’s overall objective is to develop a working prototype of a two-part, affinity-based, membrane separation process for recovering hydrocarbons, and separating organics, from produced water. This effort is focused on produced waters originating from the Greater Green River basin (GGRB) in Wyoming. Achieving the overall objective will be done in two tasks.

Task 2 focuses on the optimization of the separation characteristics of the superhydrophobic/oleophilic and superhydrophilic/oleophobic membranes, to achieve high flux/selectivity for Benzene Toluene Ethylbenzene Xylene (BTEX)/oil. This optimization will be based on existing data and membrane synthesis methodologies for such membranes. Computational Fluid Dynamics (CFD) modeling will be combined
with experimentation to design the membrane spacers and channel geometry for the prototypes to be constructed in Task 3.

As part of Task 3, the research team will execute a techno-economic assessment of the implementation of the proposed membrane process in the GGRB. This will include economic benefits from resource (BTEX/oil) recovery, water savings, and reduced treatment costs.

Background: Produced water generally refers to a mixture of formation water and hydraulic fracturing fluids (if used) that return to the surface during the extraction of oil and natural gas resources. Domestically, approximately 21 Bbbl of produced water are generated each year, with specific production volumes and qualities varying greatly as a function of site-specific characteristics. In 2012, Wyoming ranked as the fourth-highest generator of produced water in the U.S, accounting for 10% of the total volume generated. In the context of being the third most arid state in the U.S., the value of water reuse becomes obvious.

Produced water reuse and resource recovery require some level of treatment to remove particulates, residual (free, dispersed) hydrocarbons, organics and salts. The level of treatment depends on the reuse or resource recovery application requirements. Typical produced water management systems in Wyoming employ reinjection and/or surface impoundments for disposal. Complicating treatment efforts are the relatively high concentration of organics (natural and synthetic), dispersed/free hydrocarbons, BTEX compounds, biologicals, salts, and minerals.

Hydrocarbons (dispersed/dissolved crude oils) and BTEX compounds, as well as synthetic organics, present economic and environmental concerns. The former represents lost revenue, while the latter results in negative environmental impacts like emissions from surface impoundments. Additional concerns arise in the form of performance impacts on downstream filtration and desalination systems, which have played an important role in hindering water reuse and resource extraction from produced water. 

Impact: Expanding produced water treatment and reuse requires that costs associated with treatment, and recovery of resources contained therein, be reduced to be competitive with reinjection or pit evaporation. The proposed project addresses both areas, using nanostructured membranes that take advantage of interfacial chemistry principles to reduce fouling during water filtration and selectively permeate BTEX/oil during resource recovery. As such, this technology improves water quality for reuse (or further treatment) and produces a revenue stream via additional hydrocarbon recovery.

In the GGRB, as well as other basins that use pit evaporation as a management method, BTEX compounds pose air emission concerns. Their recovery, therefore, is expected to reduce management costs via reductions in emissions. The importance of this project lies in its ability to generate a prototype process that may be immediately integrated into existing systems in the GGRB and simultaneously improve the economics and viability of produced water management and reduce the environmental footprint of existing pit storage systems.

A new membrane-based treatment process for reclaiming and reutilization of produced water. This project is being carried out by TDA Research Inc.

Goal: The overall objective of this project is to develop a new membrane-based filtration system for removing organic compounds from produced water (PW). The proposed membrane treatment process integrates the new filter with a series of well-established water treatment technologies, such as mechanical filtration and reverse osmosis (RO) membranes, to remove all suspended and dissolved solids, organic molecules, bacteria and radioactive particles from the PW generated in oil and natural gas production.

The proposed research will focus on the development and demonstration of a unique zeolite-coated ceramic nanofiltration membrane. It can selectively remove the organic compounds to protect a downstream (final-stage) desalination system.

Background: The state-of-the-art RO membranes used to remove dissolved solids are severely fouled by organic compounds in the PW. The proposed ceramic nanofiltration membrane will extend the life of the RO units by removing these impurities prior to desalination. This project will develop and demonstrate a prototype system capable of processing 10 kg/day of PW. A detailed design of the full-scale system, including the design of all auxiliary units supporting operations, will also be developed. Finally, a technoeconomic analysis will be completed to addresses any regulatory issues related to use of reclaimed water and disposal of waste byproducts.

Impact: The novel ceramic nanofiltration membrane offers many benefits over polymer membranes, including stability to chemicals; tolerance to high pressure, temperature, and abrasion; and long lifetimes. Chemical stability allows aggressive chemical cleaning procedures over a wide range of acidity.

Ceramic membranes also offer high flux rates, because they tolerate higher cross-flow. They are easier to operate than polymeric membranes, because they can be drained and removed from service and then restarted (polymer membranes must stay wet to maintain performance). Ceramic membranes are more expensive, but have shown 20 years of operation with minimal loss in permeability.

Fouling-resistant, chlorine-tolerant zwitterionic membranes for the selective removal of oil, organics and heavy metals from produced water in the Permian basin. This project will be implemented by a large group of entities, including ZwitterCo, Advisian (a Worley Company), The Asatekin Laboratory at Tufts University, Heartland Water Technologies, The Brackish Groundwater National Desalination Research Facility (BGNDRF), and Daniel Shannon.

Fig. 2. ZwitterCo’s prototype membrane module. Image: ZwitterCo.

Goal: The project will advance the development of a novel membrane technology based on zwitterionic co-polymers that can reject key constituents from produced water while maintaining immunity to detrimental and irreversible membrane fouling. These membranes can remove nanoscale oils, greases, colloidal material, heavy metals and dissolved organic molecules without removing salts and dissolved solids, making filtration of highly saline waste streams practical and cost-effective. The project will both optimize the membrane technology for the demanding operational parameters of produced water treatment and verify performance with actual samples in a representative environment, and at a commercially significant scale in the Permian basin.

Background: The complexity of the chemical composition of produced water has historically constrained management strategies to reinjection for disposal, or reuse within the oil field. However, disposal restrictions, freshwater scarcity, and high water-cuts amidst a deficit in disposal capacity and water transport infrastructure are forcing operators to evaluate technologies that enable reuse outside of the oil field, a practice that is often considered prohibitively expensive.

Impact: The proposed project will demonstrate that ZwitterCo’s membrane technology (Fig. 2) is an improvement over conventional pre-treatment on the fully burdened cost per barrel. This technology focuses on the economically favorable removal of the constituents that would otherwise handicap further treatment efforts like ion exchange, electrocoagulation, and membrane or thermal desalination.

For desalination, a prerequisite for recycling produced water in most non-oilfield applications, ZwitterCo pre-treatment could enable cost-efficiencies seen in mature zero liquid discharge (ZLD) applications, and accelerate adoption of these technologies in the field and standardization of beneficial reuse. If successful, this project will demonstrate that produced water can be treated to levels supporting economic beneficial reuse of the water.

Water is a critical resource for human health, economic growth, and agricultural productivity. DOE recognizes that water and energy systems are interdependent. To help meet the global need for safe, secure and affordable water, the DOE is advancing transformational technology and innovation through the Water Security Grand Challenge, which features prizes, R&D investments, and other programs. To learn more about opportunities to work with the DOE and its partners to address water security, email: watersecuritychallenge@ee.doe.gov.

The Office of Fossil Energy funds R&D projects to reduce the risk and cost of advanced fossil energy technologies and further the sustainable use of fossil resources. To learn more about the programs within the Office of Fossil Energy, visit the Office of Fossil Energy website (energy.gov/fe/office-fossil-energy) or sign up for FE news announcements (https://public.govdelivery.com/accounts/USDOEOFE/subscriber/new). More information about the National Energy Technology Laboratory is available on the NETL website (https://www.netl.doe.gov/).

REFERENCES

1. What is flowback, and how does it differ from produced water?, Chemical and Energy Consulting, Termine Group, https://www.termine.com/archives/494#:~:text=Flowback%20is%20a%20water%20based,total%20dissolved%20solids%20(TDS).
2. Department of Energy Invests $4.6M In Produced Water Treatment, National Energy Technology Laboratory, September 2019, https://www.netl.doe.gov/node/9231