Study Notes on CO2 Slug-Assisted Fracturing Technology Application in Zhuangxi Oilfield

Field Application Context

The application of CO2 slug-assisted fracturing technology in the Zhuangxi Oilfield represents a practical demonstration of CO2-based fracturing technology in a real-world operational environment. The Zhuangxi Oilfield, located in the Dongying Depression of the Bohai Bay Basin, is characterized by tight oil reservoirs with low permeability and porosity, making conventional water-based fracturing less effective. The CO2 slug-assisted approach offers a promising alternative by utilizing the unique properties of CO2 to enhance fracture creation and proppant placement in these challenging reservoirs.

Technical Description of CO2 Slug-Assisted Fracturing

Operational Principle

The CO2 slug-assisted fracturing technology involves the injection of CO2 slugs (discrete volumes of CO2) into the wellbore, followed by the injection of proppant-laden fluid. The CO2 slugs serve multiple functions:

• Fracture initiation: The high compressibility and low viscosity of CO2 enable effective fracture initiation in tight formations with low fracture gradients.
• Proppant transport: CO2 slugs help transport proppant through the wellbore and into the fracture network, improving proppant placement efficiency.
• Fracture complexity: The phase change of CO2 from supercritical to gaseous states during fracture propagation creates additional fracture complexity, enhancing reservoir connectivity.
• Formation protection: CO2 reduces water imbibition damage to the formation, preserving reservoir permeability and flow capacity.

Operational Parameters

Pressure Equipment Requirements

The CO2 slug-assisted fracturing operations in the Zhuangxi Oilfield require specialized pressure equipment that must be designed, fabricated, and inspected to meet stringent requirements:

Equipment Inventory and Specifications

Design and Fabrication Considerations

The design and fabrication of pressure equipment for CO2 slug-assisted fracturing operations must address several critical considerations:

• Corrosion resistance: CO2 corrosion is a primary concern, requiring material selection and overlay protection per NACE MR0175 guidelines. The overlay thickness must be sufficient to provide long-term protection against CO2 corrosion, with periodic inspection to monitor overlay integrity.
• Low-temperature service: CO2 can reach very low temperatures during depressurization events, requiring materials with adequate low-temperature toughness. Impact testing per ASTM E192 or equivalent is mandatory for all carbon steel and low-alloy steel components.
• Cyclic loading: The repeated pressurization and depressurization cycles inherent in fracturing operations impose fatigue loading on pressure equipment. Fatigue analysis per ASME Section VIII Div.2 or API 934 is recommended for critical components.
• Safety systems: Emergency depressurization systems, pressure relief valves, and rupture discs must be designed for CO2 service, accounting for the unique expansion characteristics of CO2 during rapid depressurization. The safety systems must be qualified for sour service if applicable.

Quality Control and Inspection

The quality control and inspection requirements for CO2 slug-assisted fracturing equipment are stringent, reflecting the critical nature of the application:

NDT Requirements

Mechanical Property Testing

• Tensile testing: Performed on overlay welds to verify mechanical properties and bond strength.
• Hardness testing: Hardness profile across the overlay/base metal interface to verify adequate mixing and dilution.
• Metallographic examination: Cross-sectional examination of overlay welds to verify microstructure, dilution, and absence of defects.
• Corrosion testing: HIC and SSC testing per NACE MR0175 for sour service qualification.

Engineering Practice Observations

The application of CO2 slug-assisted fracturing technology in the Zhuangxi Oilfield provides valuable practical insights for pressure equipment engineers:

1. Real-world performance data: The field application generates performance data on CO2 corrosion rates, overlay integrity, and equipment reliability that can inform future design and material selection decisions.
2. Operational experience: The operational experience gained from CO2 slug-assisted fracturing in the Zhuangxi Oilfield highlights specific challenges and solutions that can be applied to similar operations in other fields.
3. Maintenance practices: The maintenance and inspection practices developed for CO2 slug-assisted fracturing equipment can serve as a model for similar operations elsewhere.

Study Insights and Implications

The application of CO2 slug-assisted fracturing technology in the Zhuangxi Oilfield demonstrates the practical viability of CO2-based fracturing methods in tight oil reservoirs. For pressure equipment engineers, this application underscores the importance of proper material selection, overlay protection, and quality control in CO2 service. The success of CO2 slug-assisted fracturing in the Zhuangxi Oilfield will likely drive the adoption of similar technologies in other fields, creating increased demand for specialized pressure equipment designed for CO2 service. Engineers must stay informed of field application experiences to continuously improve design practices, material specifications, and inspection protocols for CO2 fracturing equipment. The practical lessons learned from the Zhuangxi Oilfield application will be invaluable in advancing the technology and ensuring safe, reliable, and cost-effective operations in the future.