Mechanism Study and Combined Process Validation for Oilfield Central Treatment Station Compatibility

Abstract

Fracturing flowback fluid is a major byproduct of unconventional oil and gas development, and its efficient pretreatment is critical to ensuring the overall operational efficiency of oilfield central wastewater treatment facilities. Aiming at the high colloidal stability and heavy pollution load of fracturing flowback fluids, this study systematically analyzed the pollution characteristics and stabilization mechanisms of slickwater and guar gum flowback fluids, and proposed a combined “demulsification-destabilization + Fenton oxidation” pretreatment process. The composite coagulation system of polyaluminum chloride (PAC) and polyacrylamide (PAM) combined with pyrite-catalyzed Fenton oxidation effectively disrupted colloidal stability. For slickwater flowback fluid, the removal rates of SS and COD reached 91.13% and 92.52% respectively. For guar gum flowback fluid with a stable network structure formed by natural polysaccharides, the SS and COD removal rates reached 84.89% and 69.87% respectively. Organic composition and stability analysis confirmed that the pretreatment process reconstructed system stability through interfacial electrochemical regulation and organic degradation. A simulated “air flotation – sand filtration” system of oilfield joint stations was adopted to treat the 1:1 volume mixture of pretreated flowback fluid and produced water. Results showed that the mixture of pretreated slickwater flowback fluid and produced water achieved treatment performance comparable to that of produced water alone. For the guar gum flowback fluid mixture, SS and COD were significantly removed, but effluent SS remained relatively high; treatment performance could be further improved by reducing the blending ratio of guar gum flowback fluid in the mixed influent.

1. Introduction

Hydraulic fracturing is the core technology for unconventional oil and gas reservoir development. By injecting water, proppants and chemical additives to create complex fracture networks in target formations, it significantly improves reservoir permeability and enables efficient hydrocarbon recovery. Fracturing flowback fluid, the primary byproduct of fracturing operations, is formed by the mixture of residual fracturing fluid and formation fluid. It has complex components including suspended solids, dissolved salts, chemical additives and hydrocarbon components, with variable physicochemical properties, and typically accounts for 30%–50% of the total fracturing fluid volume.

Due to the uncertainty of flowback volume and intermittent production, oilfields usually do not build dedicated treatment facilities for fracturing flowback fluid. Instead, they mix it with produced water and treat it through the integrated “oil separation – air flotation – sand filtration” process of existing central treatment stations. However, the high colloidal stability of fracturing flowback fluid significantly reduces the efficiency of oil-water separation units, causing operational disturbances to the entire treatment system. Therefore, necessary pretreatment is required before mixing to minimize its negative impact on subsequent processes.

Existing single pretreatment technologies have limitations in addressing both high stability and complex multi-component pollution of fracturing flowback fluids. In this study, a combined “coagulation destabilization + catalytic oxidation” pretreatment process was developed for two typical types of flowback fluids (slickwater and guar gum). Through the synergistic effect of interfacial property regulation and organic degradation, comprehensive destabilization of the colloidal system was achieved, providing theoretical and technical support for the collaborative treatment of fracturing flowback fluid and produced water in oilfield central stations.

2. Wastewater Characteristics and Colloidal Stability Analysis

2.1 Sampling and Analytical Methods

All flowback fluid and produced water samples were collected from the Jilantai Central Station of Huabei Oilfield, Bayannur, China. Slickwater and guar gum flowback fluid samples were mixed from the early, middle and late stages of respective flowback processes.

Pollution load was characterized by total suspended solids (SS), chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD₅), total organic carbon (TOC), petroleum hydrocarbons and total nitrogen. System stability was characterized by pH, Zeta potential, particle size distribution, polydispersity index (PDI) and instability index. The instability index ranges from 0 to 1, with values closer to 0 indicating stronger colloidal stability.

2.2 Pollution Characteristics of Two Types of Flowback Fluids

The key water quality parameters of the two flowback fluids are summarized as follows:

Petroleum hydrocarbon concentrations are at low levels for both types, indicating that oil content is not the main factor affecting pollution load and stability. The significant difference in SS is attributed to different polymer components: slickwater contains polyacrylamide-based drag reducers forming fine suspended particles, while guar gum polysaccharides form large aggregated particles and high-concentration suspensions.

2.3 Colloidal Stabilization Mechanisms

Both flowback fluids exhibit strong colloidal stability, but their stabilization mechanisms are fundamentally different:

• Slickwater flowback fluid: Stability is dominated by physical dispersion of polyacrylamide polymers. The high absolute Zeta potential (-20.73 mV) creates strong electrostatic repulsion between particles, and the low surface tension (31.7 mN/m) reflects high concentrations of surfactants that enhance emulsification and dispersion. The low instability index (0.252) and PDI (0.759) confirm a highly stable, uniformly dispersed colloidal system.
• Guar gum flowback fluid: Stability is dominated by chemical crosslinking of natural polysaccharides. The β-1,4 glycosidic bonds of guar gum form a stable three-dimensional network structure, resulting in a wide particle size distribution and high PDI of 1.000. The low absolute Zeta potential indicates weak electrostatic repulsion, but the spatial network structure prevents particle aggregation, maintaining the system in a stable colloidal state.

3. Pretreatment Process Optimization

3.1 Coagulation and Destabilization Optimization

PAC and PAM composite coagulation was adopted for colloidal destabilization. For slickwater flowback fluid, single PAC treatment only reduced COD to approximately 2300 mg/L with loose, easily re-dispersed flocs. With the addition of PAM, flocs became denser and more stable. The optimal dosage was determined as 3.0 g/L PAC and 0.030 g/L PAM, achieving 56.33% COD removal with effluent COD of 1454.8 mg/L.

For guar gum flowback fluid, the optimal dosage was 1.5 g/L PAC and 0.015 g/L PAM, achieving 44.33% COD removal with effluent COD of 1598 mg/L. The lower removal efficiency is attributed to the stable polysaccharide network structure, which is less susceptible to charge neutralization and flocculation.

3.2 Advanced Oxidation Process Screening

Three advanced oxidation processes (persulfate oxidation, catalytic ozonation and Fenton oxidation) were compared for further treatment of coagulated effluent.

• Persulfate oxidation: Both pyrite activation and thermal activation achieved limited COD removal (less than 6%), insufficient for deep treatment requirements.
• Catalytic ozonation: Showed limited degradation efficiency for slickwater flowback fluid, and caused severe foaming in guar gum flowback fluid with fluctuating COD levels, along with high equipment investment and operating costs.
• Fenton oxidation: Delivered significantly better performance in removal efficiency, reaction rate and process stability. Under optimized conditions (2 g/L pyrite catalyst, 100 mmol/L H₂O₂, pH = 3), COD removal reached 85.53% for slickwater flowback fluid and 59.58% for guar gum flowback fluid. Pyrite catalyst achieved comparable treatment performance to traditional ferrous salts, with lower chemical costs.

Fenton oxidation was selected as the optimal advanced oxidation unit for its high efficiency, simple equipment configuration and good cost-effectiveness.

3.3 Overall Performance of Combined Pretreatment

The complete “coagulation destabilization + Fenton oxidation” process achieved the following overall treatment performance:

For slickwater flowback fluid, both SS and COD were efficiently removed through the combined process. For guar gum flowback fluid, most suspended solids were removed in the coagulation stage, while the oxidation stage was limited by the stable polysaccharide structure, resulting in lower overall COD removal efficiency.

4. Mechanism of Colloidal Stability Reconstruction

4.1 Evolution of Organic Components

Coagulation primarily removes particulate and colloidal organic matter through charge neutralization and bridging adsorption, leaving refractory components such as halogenated hydrocarbons and aromatic compounds. Fenton oxidation further degrades refractory macromolecular organics through hydroxyl radical (·OH, oxidation-reduction potential 2.80 V) oxidation.

Three-dimensional fluorescence spectroscopy confirmed that aromatic proteins and humic-like substances in slickwater flowback fluid were almost completely removed after oxidation, due to the efficient ring-opening and chain-breaking effect of ·OH on aromatic and conjugated structures. For guar gum flowback fluid, residual humic-like fluorescence peaks remained after treatment, related to its stable molecular structure and oxidation intermediates.

BOD₅ and TOC analysis further verified the degradation effect. Total BOD₅ removal exceeded 90% for both types of flowback fluid, while TOC removal reached 88.1% for slickwater and 26.1% for guar gum, reflecting differences in organic biodegradability and mineralization difficulty.

4.2 Changes in Interfacial Properties

The evolution of stability indicators during treatment is shown below:

In the coagulation stage, aluminum-based coagulants compress the electric double layer and neutralize surface charges, significantly reducing the absolute Zeta potential and weakening electrostatic repulsion between particles. Surface tension increases as surfactants are adsorbed and removed by flocs.

In the Fenton oxidation stage, hydroxyl radicals degrade polymer and surfactant molecules, further increasing surface tension to near pure water levels. The instability index decreases as residual colloidal substances are mineralized, and the system transitions from a stable dispersed state to a low-stability state with minimal residual colloidal components.

4.3 Particle Size Distribution Evolution

Raw flowback fluids show broad particle size distributions and high scattering intensity, indicating abundant suspended and colloidal particles. Slickwater has a dominant particle size below 2000 nm, while guar gum has a wider distribution with partial particles exceeding 6000 nm.

After coagulation, small particles aggregate into large flocs that are removed by sedimentation, reducing the overall particle size range. After Fenton oxidation, the particle size distribution curves approach the baseline, indicating that residual colloidal particles and polymer aggregates are further decomposed into small molecular substances.

The combined process achieves stepwise removal of particulate matter through the synergistic effect of charge neutralization, bridging flocculation and free radical oxidation, completing the reconstruction of colloidal system stability.

5. Compatibility with Produced Water Treatment System

5.1 Colloidal Stability of Mixed Water

Untreated flowback fluids mixed with produced water significantly reduce surface tension and increase absolute Zeta potential, enhancing colloidal stability and adversely affecting subsequent oil-water separation units.

After pretreatment, the mixed water of flowback fluid and produced water (1:1 volume ratio) has surface tension restored to a level close to that of pure produced water, with Zeta potential stabilized at 0.028–0.061 mV. The instability index of the mixed system decreases to 0.127 for pretreated slickwater mixture and 0.156 for pretreated guar gum mixture, both lower than that of raw produced water. This confirms that the pretreatment process effectively eliminates the interference of flowback fluid on the colloidal stability of produced water systems.

5.2 Performance of Air Flotation – Sand Filtration Treatment

Simulated central station “air flotation + sand filtration” treatment results show:

• Pure produced water: SS removal of 77.4% (186 mg/L → 42 mg/L), petroleum removal of 54.8% (12.04 mg/L → 5.44 mg/L)
• Pretreated slickwater mixture (1:1): SS removal of 70.8% (136.8 mg/L → 40 mg/L), petroleum removal of 51.3% (10.1 mg/L → 4.9 mg/L), with treatment performance comparable to pure produced water
• Pretreated guar gum mixture (1:1): SS removal of 81.7% (579 mg/L → 106 mg/L), petroleum removal of 48.2% (10.8 mg/L → 5.6 mg/L). The removal rate is comparable to pure produced water, but effluent SS is significantly higher due to residual SS in pretreated guar gum flowback fluid.

For guar gum flowback fluid, reducing the blending ratio in the mixed influent can effectively lower the effluent SS and minimize impact on the produced water treatment system. In actual field applications, the blending ratio is usually controlled below 15% to ensure stable operation of the central station.

6. Engineering Application Insights

6.1 Applicable Scenarios

This pretreatment technology is suitable for the following scenarios:

• Pretreatment of fracturing flowback fluid in unconventional oil and gas fields before entering the central produced water treatment system
• On-site treatment and reuse of fracturing flowback fluid for subsequent fracturing operations
• Emergency treatment of intermittent high-stability oilfield wastewater
• Upgrading and retrofitting of existing oilfield wastewater treatment systems to improve shock load resistance

6.2 Key Engineering Design Considerations

• Modular skid-mounted design: Fracturing operations are typically temporary and mobile, so the pretreatment system should adopt a skid-mounted modular design for easy transportation and deployment at different well sites.
• Stepwise dosing control: Set up multi-point dosing for coagulant and flocculant, with online turbidity and Zeta potential monitoring to achieve precise dosage adjustment according to influent quality fluctuations.
• pH regulation accuracy: Fenton reaction requires strict pH control at 2.5–3.5. Configure automatic pH adjustment systems to ensure stable oxidation efficiency and reduce chemical consumption.
• Catalyst recovery: Install solid-liquid separation devices to recover pyrite catalyst, reduce operating costs and avoid secondary pollution from iron sludge.
• Blending ratio control: Set up online flow ratio control system to dynamically adjust the mixing ratio of flowback fluid and produced water according to the treatment capacity and effluent quality of the central station.

6.3 Operation and Maintenance Best Practices

• Regularly observe floc morphology in the coagulation tank, and adjust PAC and PAM dosage in time to ensure dense floc formation and good sedimentation performance.
• Monitor ORP and residual H₂O₂ in the Fenton reaction tank to maintain optimal oxidation conditions and avoid excessive chemical dosing.
• Periodically activate and regenerate the pyrite catalyst to maintain catalytic activity and extend service life.
• Conduct regular sludge dewatering and disposal, and properly treat iron-containing sludge from Fenton reaction according to hazardous waste identification results.
• Strengthen the operation and maintenance of online monitoring instruments to ensure accurate data collection for automatic control systems.

7. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that efficient, modular pretreatment solutions are the key to solving the collaborative treatment problem of fracturing flowback fluid and produced water in oilfields, supporting both environmental compliance and production cost reduction for oil and gas operators.

Our skid-mounted integrated fracturing flowback fluid pretreatment systems integrate coagulation, Fenton oxidation and solid-liquid separation units, designed to adapt to the harsh field conditions and intermittent operation characteristics of oilfield sites. Equipped with an intelligent dosing control system based on real-time water quality feedback, the system can automatically adjust chemical dosage and reaction parameters, improving chemical utilization efficiency by 15–20% and reducing manual operation requirements. We also provide supporting advanced treatment processes including membrane separation and ion exchange, enabling deep treatment and reuse of flowback fluid for fracturing operations, helping oilfields achieve water resource recycling and reduce freshwater withdrawal. For large oilfield gathering stations, we offer full-process water treatment system upgrading solutions to improve the system’s shock load resistance and treatment efficiency, supporting the green and low-carbon transformation of oilfield development.

8. Conclusion

The combined “coagulation destabilization + pyrite-catalyzed Fenton oxidation” pretreatment process effectively addresses the high colloidal stability of fracturing flowback fluids, with the following core conclusions:

1. Slickwater and guar gum flowback fluids both exhibit strong colloidal stability, with different stabilization mechanisms: slickwater stability is dominated by electrostatic repulsion and spatial dispersion of polyacrylamide, while guar gum stability comes from the cross-linked network structure of natural polysaccharides.
2. The PAC-PAM composite coagulation combined with pyrite-catalyzed Fenton oxidation process achieves 91.13% SS removal and 92.52% COD removal for slickwater flowback fluid, and 84.89% SS removal and 69.87% COD removal for guar gum flowback fluid.
3. The pretreatment process reconstructs colloidal stability through interfacial charge neutralization, floc bridging and hydroxyl radical oxidative degradation, significantly improving the compatibility of flowback fluid with produced water treatment systems.
4. Pretreated slickwater flowback fluid can be mixed with produced water for treatment without obvious impact on the central station system, while guar gum flowback fluid requires control of blending ratio to ensure stable effluent quality.

FAQ

Q1: Why is there a significant difference in treatment efficiency between slickwater and guar gum flowback fluids?
The difference comes from their distinct stabilization mechanisms. Slickwater mainly contains polyacrylamide and surfactants, which are relatively easily removed by coagulation and oxidation. Guar gum has a stable three-dimensional network structure formed by β-1,4 glycosidic bonds, which is resistant to coagulation and oxidative degradation, resulting in lower overall treatment efficiency.

Q2: What are the advantages of pyrite-catalyzed Fenton over traditional ferrous salt Fenton for flowback fluid treatment?
Pyrite provides a sustained release of ferrous ions through surface dissolution, avoiding the rapid iron sludge generation caused by instantaneous high concentration of ferrous ions in traditional Fenton systems. It also reduces chemical costs, and the solid catalyst can be recovered and reused, making it more suitable for field oilfield wastewater treatment.

Q3: Can pretreated fracturing flowback fluid be directly discharged or reused?
The pretreatment process in this study is designed as pre-treatment before entering the central produced water treatment system. The effluent still needs to be further treated by the air flotation, sand filtration and subsequent deep treatment units of the central station to meet discharge or reuse standards. For direct reuse, additional advanced treatment units such as membrane filtration are required.

Q4: How to further improve the treatment efficiency of guar gum flowback fluid?
For guar gum flowback fluid, adding a specific enzyme gel-breaking unit before coagulation can destroy the polysaccharide network structure in advance, significantly improving subsequent coagulation and oxidation efficiency. Increasing Fenton reaction time, adjusting catalyst dosage, or adopting a two-stage oxidation process can also further enhance COD and SS removal performance.