Liquid CO2 Fracturing Technology for Coal Outburst Prevention in Coal Breakthrough Zones A Study Note

Literature Overview and Technical Background

This study note examines the liquid CO2 fracturing and gas drainage acceleration technology applied in coal breakthrough zones, with particular emphasis on the materials science, pressure vessel integrity, and equipment reliability aspects that a cladding and bimetal pressure vessel engineer must understand when supporting such operations. Coal outburst prevention in breakthrough zones is a critical safety challenge in underground mining, and the introduction of liquid CO2 as a fracturing agent represents a paradigm shift from traditional hydraulic fracturing methods. The core concept involves injecting liquid CO2 into the coal seam at controlled pressures, where it undergoes a phase transition upon depressurization, generating substantial expansion forces that fracture the coal matrix and enhance gas permeability. This process creates channels for methane drainage, thereby reducing outburst risk.

From a pressure vessel and equipment engineering perspective, the technology demands rigorous attention to containment systems, high-pressure cylinder design, and the mechanical integrity of injection apparatus operating under cyclic loading conditions. The transition from liquid to gaseous CO2 within confined spaces generates pressures exceeding 70 MPa, which places extraordinary demands on the design, fabrication, and inspection of pressure-containing components. These components frequently require corrosion-resistant overlays or bimetallic construction to withstand the aggressive environment of dissolved CO2 and potential carbonic acid formation.

Core Technical Principles and Process Parameters

The liquid CO2 fracturing process operates on the principle of Joule-Thomson cooling followed by rapid depressurization-induced expansion. When liquid CO2 at approximately 6°C and 6.5 MPa is injected into a pre-drilled borehole in the coal seam, the sudden pressure drop at the borehole face causes the CO2 to transition from liquid to supercritical and then gaseous states. This phase change produces a volumetric expansion ratio of approximately 500 to 1000 times, generating fracture pressures that exceed the tensile strength of coal. The resulting fractures extend radially from the borehole, creating a network of enhanced permeability pathways.

The key process parameters governing effective fracturing include injection pressure, injection rate, liquid CO2 volume per borehole, borehole diameter, and borehole depth. Typical operating parameters are summarized in the following table:

The critical engineering challenge lies in managing the thermal effects associated with CO2 phase transition. The Joule-Thomson effect causes significant local cooling, potentially dropping temperatures below -50°C at the borehole face. This cryogenic environment poses risks of coal embrittlement, equipment material low-temperature toughness degradation, and seal failure. For pressure vessels and high-pressure cylinders used in the injection system, materials must exhibit adequate Charpy V-notch impact energy at temperatures as low as -60°C, which typically necessitates the use of austenitic stainless steels or nickel-based alloys rather than standard carbon or low-alloy steels.

Equipment Integrity and Materials Considerations

The high-pressure injection system comprises several critical components that demand careful materials selection and fabrication quality control. The liquid CO2 storage cylinders, high-pressure pumps, injection valves, and delivery hoses all operate under conditions that challenge conventional materials. Dissolved CO2 in the presence of any moisture forms carbonic acid, which can cause corrosion of carbon steel components. For long-term service in mining environments where humidity control is difficult, corrosion-resistant cladding or overlay is essential.

From a cladding perspective, the high-pressure cylinders and pump housings may benefit from weld overlay protection using materials such as 316L stainless steel or Inconel 625 on carbon steel substrates. The overlay layer must maintain ductility at cryogenic temperatures and resist stress corrosion cracking in the carbonic acid environment. Electroslag welding (ESW) overlay or submerged arc welding (SAW) overlay are suitable methods for applying thick protective layers to cylindrical pressure vessels, while gas tungsten arc welding (GTAW) overlay is preferred for thinner, more precise coatings on valve seats and seal surfaces.

The inspection regime for these components must include ultrasonic testing (UT) for overlay bond strength verification, magnetic particle testing (MT) for surface crack detection, and hydrostatic pressure testing at 1.5 times the maximum working pressure. Non-destructive testing of the weld overlay layers should follow NB/T 47014 qualification procedures to ensure that the overlay process produces metallurgically sound interfaces without cracking, porosity, or lack of fusion defects.

Engineering Practice and Safety Implications

In practical implementation, the liquid CO2 fracturing technology has demonstrated significant improvements in gas drainage efficiency compared to conventional hydraulic fracturing. Field trials have reported gas drainage volume increases of 30 to 60 percent and drainage time reductions of 40 to 70 percent. The technology is particularly effective in low-permeability coal seams where traditional drainage methods are insufficient to reduce gas concentrations below outburst thresholds.

However, the technology introduces new safety challenges that must be addressed through rigorous engineering controls. The primary risks include high-pressure fluid injection failures, CO2 asphyxiation in confined spaces, thermal shock damage to equipment, and potential ignition of methane released during fracturing. A comprehensive failure mode and effects analysis (FMEA) of the injection system should identify critical failure modes such as cylinder rupture, hose burst, valve malfunction, and seal degradation, and assign risk priority numbers based on severity, occurrence, and detectability ratings.

The pressure relief devices on CO2 cylinders must be calibrated and tested at intervals not exceeding six months, and all high-pressure connections should incorporate redundant safety features including rupture discs and pressure relief valves. Personnel operating the equipment must be trained in CO2 hazard recognition and equipped with personal gas detection instruments capable of detecting concentrations above 0.5 percent by volume.

Study Insights and Recommendations

The liquid CO2 fracturing technology represents a significant advancement in coal outburst prevention, but its successful deployment depends heavily on the reliability of high-pressure equipment and the integrity of materials exposed to cryogenic and corrosive conditions. Engineers specializing in cladding and pressure vessel fabrication should engage early in the design phase to ensure that containment components are appropriately specified, fabricated, and inspected. The use of corrosion-resistant overlays on carbon steel components offers an economical solution that extends equipment life while maintaining safety margins.

Future development should focus on optimizing the injection parameters for specific coal seam geologies, improving the fatigue resistance of high-pressure components through advanced overlay materials, and developing predictive maintenance strategies based on real-time monitoring of equipment condition. The integration of bimetallic construction principles into the design of injection equipment will be critical for achieving the reliability required for safe and effective coal outburst prevention in challenging mining environments.