Optimization of Treatment Processes for Produced Water Containing Fracturing Flowback Fluid

In the development of tight, low-permeability gas fields, the mixing of fracturing flowback fluid with produced water leads to increased water quality complexity, a drastic rise in COD and suspended solids, and an overburdening of existing treatment systems. Addressing the specific operational challenges of a certain gas field, this paper presents an economical and efficient treatment solution—developed through a combination of oxidative gel-breaking, chemical optimization, and process modification—that ensures the treated effluent consistently meets water reinjection standards.

I. Field Pain Points: The Impact of Fracturing Flowback Fluid on Produced Water Treatment Systems

The gas field in question utilizes a guar gum-based fracturing system. During the concentrated commissioning phase of new wells, large volumes of fracturing flowback fluid mixed with the produced water, giving rise to three major issues:

Water Quality Deterioration: COD levels peaked at over 3,000 mg/L, suspended solids (SS) exceeded 200 mg/L, and oil content rose significantly;

Treatment Failure: The existing 'oil removal + air flotation + filtration' process proved almost entirely ineffective against the high-molecular-weight guar gum, resulting in a failure of flocculation and sedimentation;

Operational Difficulties: Chemical dosages doubled, backwashing frequency increased significantly, and the rate of compliance with discharge standards dropped to 97%.

The water samples used for testing were categorized into three types:

Standard Produced Water (Station #10): Stable water quality, relatively easy to treat;

Pure Fracturing Flowback Fluid (Well DS-462): High COD, high viscosity, and difficult to degrade;

Mixed Produced Water (Station #16): Containing flowback fluid; representative of the typical complex water quality encountered at the field site.

II. Verification of Original Process: Incapable of Handling Complex Water Quality

Comparative treatment using the site's original chemical agents (coagulant + flocculant):

Standard Produced Water: Treatment results were excellent, meeting standards for reinjection.

Fracturing Flowback Fluid: Virtually no phase separation occurred, no flocculation was observed, and the effluent failed to meet standards in every respect.

Mixed Produced Water: Flocs were fine and remained suspended; oil content and Suspended Solids (SS) levels remained above permissible limits.

Conclusion: An oxidative gel-breaking step must be added to the process to first degrade high-molecular-weight organic compounds, followed by flocculation and sedimentation.

III. Process Optimization: Oxidative Gel-Breaking + Precise Chemical Dosing—A Comprehensive, One-Step Solution

1. Selection of Optimal Oxidant: Sodium Hypochlorite is Best Suited for On-Site Application

Comparison of four oxidants (at equivalent cost):

Fenton's Reagent: Achieved the highest Chemical Oxygen Demand (COD) removal rate, but required stringent management protocols and incurred high costs.

Sodium Hypochlorite: Demonstrated effective performance, was readily available, cost-effective, and safe to handle; ultimately selected as the preferred oxidant.

Function: To cleave the long-chain structures of guar gum, thereby reducing viscosity and COD levels, and enabling subsequent flocculation to effectively 'capture and settle' suspended particles.

2. Optimal Chemical Formulation (for typical mixed produced water)

Oxidant (Sodium Hypochlorite): 700 mg/L

pH Adjuster (NaOH): 400 mg/L

Coagulant: 450 mg/L

Organic Flocculant: 0.6 mg/L

3. Key Operating Parameters

Oxidation Reaction Time: 2 hours

Agitation and Mixing: Ensure thorough contact between the chemicals and the fluid to facilitate effective gel-breaking.

Neutralization: Adjust pH to a neutral range prior to entering the flocculation and sedimentation stage.

IV. Treatment Results: Consistently Meets Standards and Satisfies Reinjection Requirements

Treatment of mixed produced water from Station #16 using the optimized process yielded the following effluent parameters:

SS (Suspended Solids): 4.67 mg/L (Limit: ≤ 15 mg/L)

Oil Content: 3.81 mg/L (Limit: ≤ 30 mg/L)

Turbidity, COD, and Corrosion Rate all met the applicable standards.

Regardless of the proportion of fracturing flowback fluid present in the mixture, the treated effluent consistently met the water quality standards required for reinjection into the gas field.

V. Engineering Solution: Dual Adaptability for Existing Plant Upgrades and New Construction Workflows

Solution 1: Upgrading the Existing Water Treatment Plant (Recommended; Low Investment)

Modification Point: Introduce sodium hypochlorite into the oil removal tank.

Advantages: Leverages existing hydraulic retention time (>2 hours); requires no major modifications to the workflow.

New Equipment: Chemical dosing skid, metering pumps, agitators.

Effectiveness: Substantially meets the treatment requirements for complex produced water.

Solution 2: Constructing a New Water Treatment Workflow

New Equipment: Skid-mounted oxidation reactor.

Workflow: Oil Removal & Sedimentation → Oxidative Gel Breaking (2 hours) → Coagulation & Air Flotation → Filtration → Re-injection (Meeting Standards).

Advantages: Highly adaptable, mobile, and capable of accommodating increased gas field production rates and fluctuations in water quality.

VI. Technical Highlights and Commercial Value

Targeted Gel Breaking: Specifically addresses contamination issues caused by guar-based fracturing flowback fluids.

Economical and Efficient: Precise control over chemical dosage, low operating costs, and easy field implementation.

Minimal System Disruption: Existing plants require only the addition of chemical dosing points, resulting in minimal downtime.

High Versatility: Applicable to high-water-cut production scenarios involving tight gas, shale gas, coalbed methane, and similar resources.

Conclusion

By employing a process combining sodium hypochlorite oxidation for gel breaking with optimized flocculation and sedimentation, the challenging problem of treating produced water contaminated by fracturing flowback fluids can be effectively resolved, ensuring that the effluent consistently meets regulatory standards for reinjection. This process is suitable for both the retrofitting of existing facilities and new construction projects; it features low capital investment, rapid results, and stable operation, making it the preferred solution for enhancing the quality and efficiency of water treatment in low-permeability gas fields.