Characteristics, Impacts, and Treatment Considerations of Oilfield Produced Water

I. Definition

Oilfield produced water refers to the water that is brought to the surface together with crude oil during oil production. It originates from underground water that is displaced and dissolved due to water injection and enhanced oil recovery (EOR) operations.

II. Sources

1. Natural groundwater production

During oilfield development, drilling activities and changes in reservoir pressure drive groundwater from aquifers to the surface along with crude oil. This groundwater becomes part of the produced water. 

2. Waterflooding operations

To improve production efficiency and recovery factor, water injection is widely applied. Injected water—typically treated freshwater or groundwater—mixes with crude oil in the reservoir and is produced back to the surface as produced water.

3. Operational wastewater from oilfield production

Additional wastewater is generated during oilfield operations, such as well washing wastewater and equipment cleaning water. These streams also contain oil and contaminants and are included in produced water. 

III. Characteristics

Produced water has distinct characteristics that directly determine treatment strategies and reuse potential: 

- High oil content: Contains crude oil and petroleum hydrocarbons, resulting in obvious oil pollution. 

- High suspended solids (SS): Includes sand, clay particles, and mineral impurities, which may cause equipment wear and blockage.

- High salinity: Contains high levels of dissolved inorganic salts (e.g., chlorides, sulfates, carbonates), increasing treatment difficulty and environmental risk.

- High organic content: Includes hydrocarbons, phenols, aldehydes, ketones, and other organics, making water quality complex. 

- Large water quality fluctuations: Variations in well production and operating conditions lead to unstable influent quality. 

- Strong corrosivity: May contain H₂S, CO₂, and corrosive salts or acids, accelerating corrosion of equipment and pipelines. 

- Poor biodegradability: Some organics, such as polycyclic aromatic hydrocar (PAHs), are resistant to biological degradation and pose long-term environmental risks. 

Given these characteristics, produced water must be treated using appropriate combinations of physical, chemical, and biological methods to reduce pollution, enable reuse, or achieve compliant discharge. Continuous monitoring and management are essential to prevent environmental impacts. 

IV. Composition

- Suspended solids: Typically with particle sizes of 1–100 μm.

- Crude oil: Usually 1–2 kg of oil per cubic meter of produced water.

- Dissolved substances: Mainly soluble salts, reflecting high mineralization. 

- Microorganisms: Including total general bacteria (TGB), sulfate-reducing bacteria (SRB), iron bacteria, etc. 

- Organic additives: Such as flocculants, demulsifiers, and oil-displacement agents, contributing to organic diversity.

- Ionic components: Highly complex, including inorganic ions, gases, organic components, and trace elements, with strong regional variability (e.g., sulfate vs. chloride dominance). 

In addition, surfactants and other additives are often present, altering surface tension and oil wettability to facilitate oil–water separation and lifting. 

V. Impacts and Hazards

- Reduced oil recovery: Water and impurities decrease reservoir productivity and increase production difficulty and cost. 

- Lower waterflood efficiency: Untreated produced water may clog injection pipelines and reservoir pores.

- Severe water and sand production: Excessive produced water can intensify sand production and disrupt normal well operation. 

- Increased load on dehydration and wastewater treatment systems: Higher treatment volumes and operating costs.

- Reservoir energy loss and higher production costs: Produced water accelerates reservoir energy depletion. 

- Damage to production equipment: Corrosion and fouling shorten equipment life and increase maintenance costs.

VI. Environmental Impacts

- Free oil pollution: Oil films block oxygen transfer at the water surface.

- Emulsified and dissolved oil pollution: Oxygen consumption during degradation leads to hypoxia, increased CO₂, lower pH, and harm to aquatic life.

- Soil contamination: Oil films inhibit soil microorganisms and damage soil structure.

- Impact on biological treatment systems: High oil concentrations (generally >30 mg/L) inhibit activated sludge and biofilm activity. 

VII. Conclusion

Oilfield produced water poses significant challenges to oilfield operations and serious risks to the environment and ecosystems. Proper treatment and reuse of produced water are therefore essential for environmental protection, cost control, and improved oilfield production efficiency.