CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment and Application of Underground High-Pressure Liquid CO2 Fracturing Equipment for Coal and Rock Permeability Enhancement
Literature Overview and Technical Context
This study note addresses the development and application of a complete set of underground high-pressure liquid CO2 fracturing equipment designed for enhancing the permeability of coal and rock masses. The equipment system is intended for use in underground coal mines to pre-fragment coal seams and surrounding rock, thereby improving gas drainage efficiency, reducing coal and gas outburst risks, and facilitating subsequent excavation operations.
The development of this equipment represents a significant advancement in underground mining technology, as it provides a self-contained, portable system that can be deployed in confined underground environments. Unlike surface-based fracturing systems, this equipment must operate under the constraints of limited space, poor ventilation, and the presence of combustible gases, which impose strict requirements on equipment design, safety systems, and operational procedures.
Equipment System Configuration and Technical Specifications
The complete equipment set consists of several integrated subsystems, each designed to perform a specific function in the fracturing process:
| Subsystem | Key Components | Technical Specifications |
|---|---|---|
| CO2 storage and supply | High-pressure CO2 cylinders, manifold | 15–25 MPa working pressure, 40–60 L cylinder volume |
| High-pressure pump | Triplex plunger pump, pressure regulator | 15–20 MPa output pressure, 0.5–2.0 L/min flow rate |
| Fracturing device | Steel sleeve, seal plug, initiation mechanism | 75–95 mm borehole diameter, 15–30 kg CO2 capacity |
| Control system | PLC controller, pressure sensors, temperature sensors | Real-time monitoring, automatic shutdown |
| Safety system | Pressure relief valve, gas detection, emergency vent | Overpressure protection, gas concentration alarm |
| Transport and deployment | Mobile cart, lifting equipment, tool kit | Designed for underground transport, 500–800 kg total weight |
The high-pressure pump is the core component of the system, responsible for pressurizing liquid CO2 from storage cylinders to the target injection pressure. The pump must be designed for continuous operation under high differential pressure conditions and must be compatible with liquid CO2, which has unique lubrication and thermal characteristics. Materials selection for pump components must account for CO2's tendency to form dry ice at low temperatures and its potential for stress corrosion cracking in certain alloys.
The fracturing device is a reusable component that is inserted into the drilled borehole and sealed with a borehole plug. The device contains a chamber for receiving pressurized liquid CO2 and an initiation mechanism that triggers the phase-change reaction at the appropriate time. The design of the fracturing device must balance the need for high-pressure containment with the requirement for reliable and repeatable initiation.
Operational Procedure and Process Control
The operational procedure for the underground CO2 fracturing equipment follows a structured sequence:
- Site preparation and borehole drilling: Boreholes are drilled in the coal or rock mass ahead of the excavation face, following a predetermined pattern based on the coal seam geometry and gas distribution. Borehole depth and angle are determined by geological and geomechanical analysis.
- Equipment deployment: The fracturing equipment is transported to the borehole location and connected to the borehole. The fracturing device is inserted into the borehole and sealed with the borehole plug. The high-pressure pump is connected to the fracturing device via high-pressure hoses.
- CO2 injection: Liquid CO2 is injected from the storage cylinders through the high-pressure pump into the fracturing device. The injection pressure is monitored in real time by pressure sensors, and the injection is stopped when the target pressure is reached.
- Initiation and fracturing: The initiation mechanism is triggered, causing the liquid CO2 to undergo rapid phase change. The resulting pressure wave propagates through the borehole and fractures the surrounding coal and rock mass. The initiation time is controlled to ensure that the borehole plug remains sealed during the pressure buildup.
- Post-fracture verification: After the fracturing event, the borehole is monitored for gas emission and pressure changes. Gas concentration measurements and borehole pressure readings are recorded to verify the effectiveness of the fracturing operation.
- Equipment recovery and reset: The fracturing device is retrieved from the borehole, inspected for damage, and prepared for the next cycle. The borehole is sealed with a temporary plug if gas drainage is not immediately required.
Performance Evaluation and Application Results
The application of this equipment in underground coal mines has demonstrated significant improvements in gas drainage efficiency and excavation safety. The following table summarizes the key performance indicators:
| Performance Indicator | Baseline (No Fracturing) | With CO2 Fracturing | Improvement |
|---|---|---|---|
| Coal seam permeability | 0.1–0.5 mD | 5–15 mD | 10–30× increase |
| Gas drainage rate | 200–500 m³/day | 800–2000 m³/day | 3–5× increase |
| Gas concentration at face | 1.0–2.0% | 0.3–0.6% | 60–80% reduction |
| Excavation advance rate | 1.0–2.0 m/day | 2.5–4.0 m/day | 100–150% increase |
| Outburst incidents | Occasional | None recorded | Eliminated |
| Equipment availability | N/A | 95%+ | High reliability |
The permeability enhancement achieved by the CO2 fracturing process is the primary mechanism through which gas drainage efficiency is improved. The fractures created by the phase-change pressure wave provide interconnected pathways for gas migration, significantly increasing the effective drainage radius from the boreholes. This enhancement is sustained over the service life of the drainage system, providing long-term benefits to mine safety.
Safety Considerations and Risk Management
The use of high-pressure liquid CO2 in underground environments introduces specific safety risks that must be managed through engineering controls and procedural safeguards:
- High-pressure hazards: The high-pressure pump and fracturing device operate at pressures up to 20 MPa, posing a risk of equipment failure and high-velocity fluid release. Pressure relief valves and burst discs are installed as redundant protection measures, and all high-pressure components are designed to exceed the maximum operating pressure by a safety factor of at least 2.0.
- Asphyxiation risk: CO2 is an asphyxiant gas that can displace oxygen in enclosed spaces. Gas detection systems are installed in the operating area, and ventilation is maintained to ensure oxygen levels remain above 19.5%. Emergency evacuation procedures are established and rehearsed regularly.
- Cold injury risk: Liquid CO2 can cause severe cold burns upon contact with skin. Personnel handling liquid CO2 must wear appropriate personal protective equipment, including insulated gloves and face protection. Equipment surfaces that may contact liquid CO2 are designed to prevent ice formation and are insulated where practical.
- Fire and explosion risk: Although CO2 itself is non-flammable, the presence of methane in underground coal mines creates a potential explosion hazard. All electrical equipment in the fracturing area is certified for the applicable gas group and temperature class, and ignition sources are strictly controlled.
Study Insights and Development Recommendations
The development of underground high-pressure liquid CO2 fracturing equipment represents a significant achievement in mining technology, providing a practical solution to the persistent challenge of gas management in underground coal mines. The equipment's design successfully addresses the unique constraints of underground operation, including limited space, hazardous atmosphere, and the need for portability and rapid deployment.
One key insight from this study is the importance of system integration. The performance of the fracturing equipment depends not only on the individual components but also on the seamless integration of the CO2 supply, high-pressure pump, fracturing device, control system, and safety systems. Any weakness in the integrated system can compromise the overall performance and safety of the operation.
Another important observation is the role of operational training and procedural discipline. The equipment's safety and effectiveness depend on the competence of the operating personnel and the consistent application of established procedures. Investment in training and qualification programs is therefore essential for the successful deployment of this technology.
Future development efforts should focus on several areas: improving the equipment's automation level to reduce operator workload and minimize human error; developing more efficient fracturing devices with higher energy utilization; expanding the range of applicable geological conditions; and integrating the fracturing equipment with intelligent gas drainage management systems for predictive maintenance and optimized operation.
In conclusion, the underground high-pressure liquid CO2 fracturing equipment provides a powerful tool for enhancing coal and rock permeability and improving mine safety. Its continued development and wider adoption will contribute significantly to the safe and efficient extraction of coal resources in underground mining environments.