Safety Management Countermeasures for CO2 Fracturing Borehole Technology in Underground Coal Bins

Overview of the Topic

The study of CO2 fracturing borehole technology in underground coal bins addresses a critical intersection between high-pressure gas systems, containment vessel integrity, and mining safety management. Underground coal bins (bottom coal bunkers) serve as temporary storage and transfer points in coal mining operations, where accumulated coal can self-ignite, producing carbon monoxide and carbon dioxide under confined conditions. The application of CO2-based fracturing technology to create boreholes for pressure relief and gas extraction introduces complex metallurgical and pressure vessel considerations that demand rigorous safety management protocols.

This literature review examines the technical framework for managing CO2 fracturing operations in confined underground spaces, with emphasis on the mechanical integrity of pressure-containing equipment, material selection for high-pressure CO2 service, and the systematic safety management strategies that govern the entire operation.

Core Technical Principles

CO2 Fracturing Mechanism and Pressure Parameters

CO2 fracturing borehole technology relies on the rapid phase transition of liquid carbon dioxide to create controlled fractures in rock or coal mass. The phase transition from liquid CO2 to supercritical or gaseous state produces enormous volumetric expansion — approximately 500 to 600 times the original liquid volume — generating fracture pressures in the range of 15 to 35 MPa depending on the charge configuration and geological conditions.

Material Selection for High-Pressure CO2 Service

The materials used in CO2 fracturing equipment face unique challenges. Liquid CO2 at elevated pressures creates a corrosive environment, particularly when trace moisture is present, forming carbonic acid that accelerates corrosion of carbon steel components. The following material selection criteria are essential:

• Pressure vessels and cylinders: Carbon steel grades such as Q345R or 16MnR (per GB/T 150) with minimum wall thickness calculated per pressure vessel design codes. For higher purity requirements or aggressive environments, austenitic stainless steel 304 or 316L may be specified for critical components.
• Valve bodies and fittings: Duplex stainless steel (e.g., 2205) or nickel-based alloys for valve internals exposed to high-pressure liquid CO2.
• Seal materials: PTFE or Viton compounds rated for low-temperature service, as CO2 phase transitions can cause local temperature drops below -50°C.
• Surface treatment: Hard chrome plating or thermal spray coatings (e.g., WC-Co) on valve stems and piston surfaces to resist CO2-induced corrosion and wear.

Safety Management Framework

Hazard Identification and Risk Assessment

A systematic hazard identification process following the PDCA cycle is fundamental to managing CO2 fracturing operations in underground coal bins. The primary hazards include:

1. Overpressure and vessel rupture: The most catastrophic failure mode, capable of releasing stored energy equivalent to several kilograms of TNT. Pressure vessel design must comply with GB/T 150 or ASME VIII Div.1 requirements, with safety valves set at 110% of maximum working pressure.
2. CO2 asphyxiation: In confined underground spaces, displaced oxygen levels can drop below 19.5% rapidly. Continuous gas monitoring with alarm thresholds at 23.5% O2 (warning) and 19.5% O2 (evacuation) is mandatory.
3. Thermal shock and brittle fracture: Rapid CO2 expansion can cool metal surfaces to cryogenic temperatures, potentially inducing brittle fracture in carbon steel components below their minimum ductile-brittle transition temperature (DBTT).
4. Explosive gas accumulation: CO2 injection into coal seams can displace methane, creating flammable gas mixtures in adjacent areas.
5. Residual charge instability: Unreacted or partially reacted CO2 charges can remain dangerous for extended periods, requiring controlled disposal procedures.

Engineering Control Measures

The engineering controls for CO2 fracturing safety are organized into a hierarchy:

Management Control Measures

Beyond engineering controls, organizational and procedural measures form the second line of defense:

• Permit-to-work system: All CO2 fracturing operations require a written permit documenting risk assessment, gas monitoring results, personnel accountability, and emergency response arrangements.
• Competency management: Operators handling high-pressure CO2 systems must demonstrate proficiency in pressure vessel operation, gas detection interpretation, and emergency shutdown procedures, with recertification at intervals not exceeding 24 months.
• Equipment integrity management: Pressure vessels and high-pressure cylinders must undergo periodic inspection per NB/T 47002 schedules, including internal examination, thickness measurement, and non-destructive testing of welds.
• Change management: Any modification to the fracturing procedure, equipment, or operating parameters must undergo formal management of change (MOC) review with updated risk assessment.

Integration with Pressure Vessel and Cladding Technology

The safety of CO2 fracturing equipment depends fundamentally on the integrity of pressure-containing components. Several cladding and bimetal technology considerations are directly relevant:

Weld Overlay for Corrosion Protection

High-pressure CO2 cylinders and charge delivery equipment frequently employ weld overlay cladding to protect carbon steel substrates from CO2 corrosion. Typical overlay configurations include:

• Single-layer overlay: 304L or 316L stainless steel overlay, 3–5 mm thick, applied by GTAW or SAW processes. The dilution ratio must be controlled below 30% to maintain the corrosion resistance of the overlay layer.
• Multi-layer overlay: A transition layer of 309L followed by 304L or 316L finishing layers, providing enhanced crack resistance at the base metal/overlay interface.
• Nickel-based overlay: Inconel 625 or Hastelloy C-276 overlay for severely aggressive environments, providing superior resistance to carbonic acid corrosion.

Bond Strength Verification

The bond strength of overlay layers on pressure vessel components is critical. Per NB/T 47014 and ASME IX requirements, bond strength testing of the overlay weld must demonstrate:

• Minimum bond strength ≥ 90% of the lower tensile strength of the base metal or overlay material (whichever is lower).
• No crack initiation at the base metal/overlay interface under shear or peel loading.
• Metallographic examination confirming full fusion and absence of lack-of-bond defects.

Hydrogen-Induced Cracking (HIC) Considerations

Carbon steel components exposed to liquid CO2 containing moisture are susceptible to hydrogen-induced cracking. The HIC resistance of the base material must be verified per NACE MR0175/ISO 15156 or equivalent standards. For pressure vessel applications, the following criteria apply:

Key Questions and Reflections

The study of CO2 fracturing borehole technology reveals several areas where pressure vessel and cladding engineering knowledge is directly applicable:

1. Low-temperature toughness of clad components: When overlay layers are applied to carbon steel pressure vessels for CO2 service, the thermal cycling during overlay welding can affect the toughness of the heat-affected zone (HAZ) in the base metal. Post-weld heat treatment (PWHT) must be carefully controlled to restore HAZ toughness while maintaining overlay layer integrity.
2. Fatigue life under cyclic pressure loading: Repeated charging and discharge cycles of CO2 fracturing equipment subject pressure vessels to cyclic loading. The fatigue design margin must account for the full spectrum of operating cycles, including transient overpressure events during fracture initiation.
3. Inspection intervals for clad pressure vessels: The presence of a weld overlay layer complicates the non-destructive examination of the underlying base metal. Ultrasonic testing (UT) through the overlay layer requires calibrated reference standards that account for the acoustic impedance mismatch at the overlay/base metal interface.

Study Insights and Implications

The literature on CO2 fracturing borehole technology safety management underscores a fundamental principle: the safety of any high-pressure gas operation is only as strong as the weakest link in the chain — from material selection and design, through fabrication and inspection, to operation and maintenance. The metallurgical aspects of CO2 service — corrosion, hydrogen embrittlement, low-temperature brittleness — demand the same rigor of engineering analysis as any other pressure vessel application.

For engineers involved in the design and fabrication of equipment for CO2 fracturing applications, the key takeaway is that material selection cannot be treated as an afterthought. The interaction between CO2, moisture, and carbon steel is a well-documented degradation mechanism that must be addressed at the design stage through appropriate material selection, corrosion protection (including weld overlay cladding where applicable), and rigorous inspection protocols. The safety management framework described in the literature provides a comprehensive organizational structure, but its effectiveness ultimately depends on the technical integrity of the equipment it governs.