Wastewater Treatment Basics: What Is Produced Water?
Produced water is the oilfield wastewater obtained after the water-laden crude oil (production fluid) lifted from underground has passed through oil, gas and water separation. By origin it usually includes both the formation water originally held in the reservoir and the injected water introduced in mid-to-late field life to maintain reservoir pressure and displace oil toward the wells [1] [3].
In 1992, 15 North Atlantic states and the European Union signed the OSPAR Convention for the Protection of the Marine Environment of the North-East Atlantic, requiring reductions in discharges from the oil and gas industry and setting a limit of 40 mg/L (dispersed oil) for non-land-based oily water. In 1998 Norway issued White Paper No. 58, calling for a substantial cut in discharges of oily waste and hazardous chemicals by 2005 and the achievement of “zero environmentally harmful discharges”; it also required that, from 2006, produced water discharged during offshore oil and gas exploration and production contain less than 30 mg/L of oil (dispersed). Driven by these rules, operators invested in research and development, and by 2003 the average oil content of produced water discharged offshore in the Norwegian sector of the North Sea had fallen to 16.9 mg/L [1] [3].
In 2002 China issued the Integrated Wastewater Discharge Standard (GB 8978-2002), setting the maximum allowable oil concentrations for primary, secondary and tertiary discharge at 10 mg/L, 10 mg/L and 30 mg/L respectively. In 2015 China issued the Code for Design of Oilfield Produced Water Treatment (GB 50428-2015), which explicitly requires produced water treatment facilities to be designed, built and commissioned together with crude oil dehydration facilities, and sets specific requirements for the oil content of raw water entering the treatment station [1] [3].
In recent years produced water treatment technology has continued to advance, with objectives shifting from merely meeting reinjection or discharge standards toward resource recovery, low cost and high efficiency [1] [3]. In 2018, for example, Sinopec completed China’s first produced water resource-recovery station at a high-sulfur gas field in the Yuanba gas field, enabling recycling of produced water [4]. Shengli Oilfield has used high-efficiency biological treatment such as the “biological dual-membrane” process to upgrade treated water beyond tap water quality for thermal-recovery boilers, achieving notable economic and environmental benefits [5-6]. Tuha Oilfield and others have also explored and applied a model in which produced water is blended and reused for fracturing in shale oil development, saving both water and cost [7]. These practices mark a shift of produced water treatment from plain environmental compliance toward water recycling combined with cost reduction and efficiency gains [1] [3].
Oilfield produced water is characterised by large volumes, complex composition and wide fluctuations in quality and quantity. Specifically, the oil content is high — up to 1,000–2,000 mg/L — and occurs as free oil (>100 μm), dispersed oil (10–100 μm), emulsified oil (0.1–10 μm) and dissolved oil (with droplets as small as a few nanometres). Produced water also carries corrosive bacteria such as sulfate-reducing bacteria (SRB), saprophytic bacteria (TGB) and iron bacteria (FB), whose proliferation aggravates equipment corrosion. In addition, enhanced oil recovery methods such as chemical flooding, polymer flooding and alkali-surfactant-polymer flooding leave large amounts of chemicals and polymers in the water, further increasing treatment difficulty [1] [3].
Fracturing flowback fluid from oil and gas production is a particular challenge: it is high in suspended solids, oil content, chemical oxygen demand, salinity and toxicity, is difficult and expensive to treat, and is a key bottleneck for the industry’s green development.
The quality indices of oilfield produced water are affected by the field’s geological conditions, the stage of development and site conditions [1]. Its composition includes suspended solids, crude oil, dissolved matter, microorganisms, organic components (such as flocculants, demulsifiers and oil-displacement agents) and complex ionic species (such as sulfate or chloride), and varies from region to region [12].
To prevent pollution and promote resource recovery, strict regulations and standards apply. GB 50428-2015 requires treatment facilities to be designed and built concurrently [1] [3]; the Technical Policy for Pollution Prevention in the Oil and Gas Extraction Industry requires that water suitable for reinjection be treated to standard before reinjection, and encourages reuse of water from heavy oil production in steam-injection boilers [13]. GB 8978-2002 sets the maximum allowable oil concentration for discharged water [1] [3]. Local environmental authorities, such as the Qingyang Ecological Environment Bureau, also state in their approval opinions that produced water must be treated to standard and reinjected rather than discharged [10-11]. Internationally, instruments such as the EU’s OSPAR Convention and Norway’s “zero environmentally harmful discharge” target impose strict limits on produced water discharge [1] [3].
Of the 780,000 tonnes of produced water generated every day at Shengli Oilfield, 651,000 tonnes are recycled by reinjection, and the remaining 129,000 tonnes are also turned into a resource through new biological technology. Reverse osmosis lowers the salinity of the treated water from 19,000 mg/L to 110 mg/L and its hardness from 1,450 mg/L to 0, with every index better than tap water. [5]
Bin’nan Oil Production Plant, working with the Petroleum Engineering Technology Research Institute, has made progress in research on resource utilisation of produced water. Heavy oil is mainly developed by steam huff-and-puff thermal recovery; using treated produced water instead of fresh water as boiler feed, with the remaining concentrate reinjected into low-permeability reservoirs, closes the produced water loop. [6]
In shale oil development at Jimsar, Tuha Oilfield has already blended and reused 11,294 m³ of produced water, directly saving about RMB 426,000 in water purchase and produced water reinjection fees. [7]
There are essentially 3 ways of handling produced water at home and abroad: reinjection, reuse after simple treatment, and treatment to discharge standard. For shale gas produced water, the choice depends mainly on local regulations and policies, the production environment, the characteristics of the water and the cost of treatment. [8]
Construction has started on a new produced water resource-recovery station at the Yuanba gas field. The project uses a process of “electrocatalytic oxidation + high- and low-pressure reverse osmosis + post-treatment biological treatment”, which shortens the existing resource-recovery flow at Yuanba, lowers the treatment cost per tonne of water and reduces the surface water consumed by the Yuanba gas processing plant. [4]
Application of waste-heat insulation technology to oilfield produced water. Oilfield wastewater contains a certain amount of oil and is fairly warm (50–80 °C); recovering and using that heat is of some significance for controlling air pollution, protecting the environment and building energy-saving modern oilfields. [9]
Produced water has a complex composition; if discharged without treatment it pollutes soil and water sources and may even cause air pollution or start fires [2] [12]. Direct discharge or ineffective disposal also wastes water resources, whereas effective treatment and reuse enables water recycling and crude oil recovery [2].
Reinjecting untreated or inadequately treated produced water can lead to bacterial growth and to mechanical impurities and precipitates plugging the formation, raising injection pressure and impairing waterflood performance and ultimate recovery, while also adding to the load on dehydration and water treatment facilities [2] [12].
The destinations of treated produced water at home and abroad fall into a few categories: discharge to the surrounding environment to standard (surface water bodies or the sea), reinjection into the formation, reuse as feedwater for thermal-recovery boilers, and storage (limited to arid regions such as deserts and gobi). [1]
Effective treatment of produced water is a key step in preventing pollution, protecting the environment, saving water resources, safeguarding oilfield safety and raising crude oil recovery [2].
By contrast, China has seen few significant breakthroughs over the past decade or more in the unit technologies for oil and suspended-solids removal. Under the dual pressure of low-carbon requirements — cost reduction and efficiency gains in oil and gas development, and increasingly strict environmental rules demanding far better separation of oil and suspended solids — there is an urgent need to develop new, high-efficiency produced water treatment technologies and equipment. [1] [3]
In recent years biological treatment has been increasingly applied. [13] Shengli Oilfield has focused on petroleum microbiology, harnessing the properties and functions of microorganisms for green treatment. Researchers screened high-efficiency strains that, through a biological process, degrade organics such as crude oil into carbon dioxide and water; combined with ultrafiltration and reverse osmosis membranes this achieves deep purification of produced water. In 2019 the process was further upgraded into an advanced produced water treatment process — biological dual-membrane technology. The biological plus dual-membrane industrial train at the Dan-14 water injection station of the No. 6 District, Bin’nan Oil Production Plant treats 200 m³ of oilfield produced water per day and has run stably for more than three years. Treated water salinity falls from 19,000 mg/L to 110 mg/L and hardness from 1,450 mg/L to 0, with every index better than tap water and fully meeting boiler feedwater standards. [6] [14]
To address high treatment costs, low-cost fine treatment technology has been developed. Zhongyuan Oilfield independently developed a low-cost treatment technology for produced water in the mature Dongpu area, establishing a “gas-liquid-solid” multiphase water purification mechanism together with a multi-parameter water quality monitoring system and automatic feedback control of chemical dosing. With the new technology, overall treatment cost has fallen by nearly one fifth and residue generation by more than one third while water quality compliance is maintained. [14]
Frontier exploration is also broadening the ways produced water can be used. Dongfu Research Institute and the Oil and Gas Technology Research Institute of Changqing Oilfield jointly carried out a phased field trial of “on-site hydrogen production by electrolysis of gas-field produced water without purification” — the first application of “direct in-situ electrolytic hydrogen production from seawater without desalination” to industrial wastewater. The trial offers a route reference for extending direct seawater hydrogen technology into the energy-saving and environmental field, and for hydrogen production from petrochemical and steelmaking wastewater. [15]
