Circulated Reinjection Design for Oilfield Produced Water Treatment Process Scheme

I. Design Basis and Core Objectives

1.1 Characteristics of Oilfield Produced Water and Reinjection Challenges

Oilfield produced water, the formation water extracted alongside crude oil, has complex water quality. Its effective treatment and reinjection into the formation are crucial for energy conservation, environmental protection, and sustainable development in oilfields. Its core characteristics and reinjection challenges are as follows:

• High Oil Content: Contains free oil, dispersed oil, emulsified oil, and dissolved oil. Oil content typically ranges from hundreds to thousands of mg/L and must be deeply removed to prevent formation plugging.

• High Suspended Solids (SS): Contains sand, corrosion products, bacterial metabolites, colloids, etc., with a wide particle size distribution. This is a major factor causing formation and injection pipeline network plugging.

• High Salinity and Scaling Tendency: High salinity (TDS can reach tens of thousands to hundreds of thousands of mg/L), containing ions such as Ca²⁺, Mg²⁺, Ba²⁺, Sr²⁺, SO₄²⁻, HCO₃⁻, CO₃²⁻, which easily form scales like calcium carbonate, barium/strontium sulfate, and calcium sulfate, plugging pores and pipelines.

• High Corrosivity: Contains dissolved oxygen, H₂S, CO₂, SRB (Sulfate-Reducing Bacteria), etc., causing severe corrosion to pipelines and equipment.

• Bacterial Growth: Presence of SRB, TGB (Total General Bacteria), IB (Iron Bacteria), etc., producing biological slime, exacerbating corrosion and plugging.

• Fluctuations in Water Quality and Quantity: Varies with the production stage, reservoir characteristics, and stimulation operations.

1.2 Circulated Reinjection Design Objectives and Principles

• Core Objective: To stably treat the produced water to meet the injection water quality standards of the specific oilfield block and reinject it into the target formation, achieving "zero discharge" and effective water resource utilization.

• Reinjection Water Quality Standards: Comply with "SY/T 5329-2022 Recommended Indexes and Analysis Methods for Water Quality of Injection Water in Clastic Reservoirs"or stricter internal corporate standards. Key control indicators include:

  • Suspended Solids Content and Median Particle Size: Controlled in grades based on formation permeability (e.g., SS ≤5.0 mg/L, median particle size ≤3.0 μm).

  • Oil Content: Controlled in grades based on formation permeability (e.g., ≤15.0 mg/L).

  • Average Corrosion Rate: <0.076 mm/a.

  • SRB, TGB, IB Counts: Meet control index limits.

  • Dissolved Oxygen Content: <0.05 mg/L (especially for anaerobic systems).

• Design Principles:

  1. Differential Treatment, Graded Control: Determine differentiated treatment precision based on formation reinjection requirements.

  2. Oil and SS Removal as the Core: Employ multi-stage combined processes to ensure both SS and oil content meet standards.

  3. Integrated Corrosion Inhibition, Bactericide, and Scale Inhibition: Control corrosion, bacteria, and scaling risks throughout the entire process, from treatment to system operation.

  4. Reliable Technology, Stable Operation: Prioritize mature processes and equipment that adapt to oilfield environments and withstand shock loads.

  5. Energy Saving, Consumption Reduction, Intelligent Management: Optimize processes, reduce energy consumption, and establish an intelligent monitoring and early warning system.

II. Full Process Design for Circulated Reinjection

Addressing the core needs of oilfield produced water—"oil removal, SS removal, corrosion inhibition, scale inhibition, and disinfection"—this scheme adopts a main process route of "Oil Removal Pretreatment + Fine Suspended Solids Removal + Water Quality Stabilization & Disinfection + High-Pressure Reinjection". The core lies in constructing multiple barriers for stepwise purification and ultimately providing stable protection for the reinjection water.

2.1 Full Process Circulated Reinjection Process Design Flowchart

III. Key Treatment Unit Technical Design Points

3.1 Oil Removal Pretreatment Unit

• Buffer/Equalization: Designed with sufficient volume to homogenize water quality and quantity, and accomplish preliminary natural settling and separation.

• Hydrocyclone/Gravity Separation: Efficiently removes larger free oil droplets and solid particles using density difference, reducing load on subsequent stages.

• Flotation: Employs Dissolved Air Flotation (DAF) or Cavitation Air Flotation (CAF) to generate micro-bubbles that adhere to oil droplets and fine suspended solids, achieving efficient three-phase (oil, water, solids) separation. It is a key step for demulsification and emulsified oil removal. A small amount of demulsifier and/or coagulant can be dosed to enhance performance.

3.2 Fine Suspended Solids Removal Unit (Core)

• Walnut Shell Filter: Oleophilic and hydrophobic, effectively adsorbs and captures residual oil and suspended solids. It has strong backwash regeneration capability and is less prone to fouling, making it a widely used deep oil removal filter in oilfields.

• Dual-Media/Multi-Layer Filter: Achieves depth filtration through filter media of different particle sizes and densities (e.g., anthracite, quartz sand, magnetite), effectively reducing effluent SS content and median particle size.

• Membrane Filtration Unit:

  • Ultrafiltration/Microfiltration Membranes: For ultra-low permeability reservoirs or high-standard reinjection, Ceramic Membranes or Fouling-Resistant Hollow Fiber Ultrafiltration Membranes are employed. They can almost completely retain bacteria, suspended solids, and oil droplets larger than the membrane pore size, providing extremely stable effluent quality. This is the core technology for ensuring high standards. Requires a complete backwash and chemical cleaning-in-place (CIP) system.

3.3 Water Quality Stabilization and Disinfection Unit

• Deaeration:

  • Vacuum Deaerator: Physical deaeration, highly efficient with low operating costs. Can reduce dissolved oxygen to below 0.05 mg/L. It is the preferred method for controlling oxygen-induced corrosion.

  • Chemical Oxygen Scavenging: Serves as an auxiliary or backup, dosing agents like sodium sulfite.

• Biocide Treatment:

  • Oxidizing Biocides: Periodically shock-dose biocides like sodium hypochlorite or chlorine dioxide to kill various bacteria.

  • Non-Oxidizing Biocides: Continuously or intermittently dose agents like quaternary ammonium compounds or glutaraldehyde to prevent bacterial resistance. Alternating use is recommended, and bacterial counts should be monitored.

• Scale and Corrosion Inhibition:

  • Scale Inhibitors: Based on scaling tendency analysis, dose specialized scale inhibitor/dispersants to prevent carbonate and sulfate scale formation.

  • Corrosion Inhibitors: Dose film-forming or adsorption-type corrosion inhibitors to form a protective film on pipe walls, reducing corrosion rates.

3.4 High-Pressure Reinjection Unit

• Reinjection Water Buffer Tank: Stores qualified water, provides a stable water source for high-pressure pumps, and also functions as a final settling stage. Uses natural gas or nitrogen blanketing to prevent air ingress.

• High-Pressure Water Injection System: Selects plunger pumps or multi-stage centrifugal pumps based on injection pressure requirements. Equipped with automatic flow, pressure regulation, and protection systems.

IV. Key Design Parameters and Intelligent Control

4.1 Key Unit Design Parameters (Example: Treatment Capacity 10000 m³/d)

4.2 Intelligent Monitoring and Management System

• Online Monitoring Network: Install online instruments at key nodes, including oil-in-water analyzers, suspended solids/laser particle counters, dissolved oxygen analyzers, pH/ORP meters, online corrosion monitors, rapid bacteria detectors, etc.

• Intelligent Chemical Dosing System: Automatically and precisely adjusts the dosage of corrosion inhibitors, scale inhibitors, biocides, and oxygen scavengers based on online water quality data and flow rate.

• Early Warning and Automatic Control: Establish warning mechanisms for water quality exceedances, equipment failures, and abnormal corrosion rates. Link these with interlock controls for key equipment (e.g., backwash systems, chemical dosing pumps, main process pumps).

V. Economic Analysis

• Capital Expenditure (CAPEX) Estimate: Unit investment varies significantly with the process route and treatment standard. Conventional route (de-oiling + filtration) is approximately 10-20 million RMB; a high-standard route incorporating membrane treatment is approximately 30-50 million RMB.

• Operating Cost: 3 - 8 RMB/ton of water.

  • Power Consumption: 1.0-2.5 RMB/ton (mainly for lift pumps, filter backwash pumps, high-pressure injection pumps, vacuum pumps).

  • Chemical Costs: 1.0-2.5 RMB/ton (corrosion/scale/bio inhibitors, demulsifier, etc.).

  • Membrane Replacement & Maintenance: 0.5-2.0 RMB/ton (if membrane system is used).

  • Labor & Others: 0.5-1.0 RMB/ton.

• Benefits:

  • Direct Benefits: Reinjection water replaces fresh water, saving purchase and treatment costs for fresh water; avoids the high environmental costs and risks associated with produced water discharge.

  • Enhanced Oil Recovery (EOR) Benefits: Qualified reinjection water effectively maintains reservoir pressure and improves oil recovery, yielding significant economic benefits.

  • Environmental & Social Benefits: Achieves wastewater resource utilization, protects the regional water environment, and aligns with green oilfield development requirements.

VI. Conclusion and Operational Recommendations

This circulated reinjection process design scheme of "Graded De-oiling & Filtration + Polishing + Water Stabilization" is a systematic solution tailored to the characteristics of oilfield produced water and the water quality requirements for injection into clastic reservoirs. The combination of walnut shell and multi-media filtration is an economical and reliable choice, while membrane technology is a powerful tool for meeting the challenges of low-permeability zones. Vacuum deaeration combined with comprehensive chemical treatment is key to controlling system corrosion and scaling.

Keys to Successful Operation:

1. Upstream Influent Stability: Ensuring relatively stable influent from the dehydration station is fundamental for the efficient operation of subsequent treatment systems.

2. Scientific Backwashing of Filtration Systems: Developing and strictly implementing a multi-factor backwash strategy based on pressure differential, time, and water quality is core to maintaining filtration performance and equipment lifespan.

3. Chemical Compatibility and Targeting: All chemicals to be dosed must undergo compatibility testing. The most effective formulations should be selected based on specific water quality (especially ion composition, bacteria types).

4. Comprehensive Data Monitoring and Trend Analysis: Establish a robust monitoring database, regularly analyze corrosion coupon data and water quality trends for early warning of potential issues, enabling predictive maintenance.

5. Strict Final Safeguard at Wellhead: The wellhead polishing filter is the final barrier protecting the formation and must be inspected and replaced regularly.

Through technology integration and detailed design, this scheme provides solid technical support for oilfields to achieve "zero discharge" of produced water, conserve energy, reduce emissions, and enhance oil recovery. It is an essential choice for the sustainable development of oilfields.