CLADDING TECHNOLOGY SHANXI CO., LTDLiquid CO2 Fracturing Technology in Rock Burst Prevention Study Note
Rock Burst Mechanism and the Role of Stress Relief
The paper "Application of Liquid CO2 Fracturing Technology in Rock Burst Prevention" addresses one of the most critical safety challenges in deep underground mining: rock bursts. Rock bursts are sudden, violent failures of rock masses that occur when the stored elastic strain energy in the rock exceeds the fracture energy required to create new surfaces. The energy release during a rock burst can be catastrophic, causing equipment damage, structural collapse, and loss of life.
The mechanism of rock burst is governed by the stress concentration in the rock mass around excavations, the strength and brittleness of the rock, and the in-situ stress field. As the excavation advances, the stress redistribution around the excavation boundary creates zones of high stress concentration that can trigger brittle failure. The liquid CO2 phase-change fracturing technology offers a method of controlled stress relief that can reduce the stress concentration below the critical threshold for rock burst initiation.
From a pressure vessel and materials engineering perspective, the application of CO2 fracturing for rock burst prevention presents unique challenges. The fracturing operation must be performed in a high-stress environment where the rock is under significant compressive stress, and the energy release from the CO2 phase change must be controlled to avoid triggering an unintended rock burst. The fracturing tube design must account for the high in-situ stress, which can affect the tube's structural integrity and the crack propagation pattern.
Application Strategy for Rock Burst Prevention
The paper describes a systematic approach to applying liquid CO2 fracturing for rock burst prevention in deep mines. The approach involves three main components: stress field analysis to identify the high-risk zones, fracturing hole design to optimize the stress relief pattern, and operational procedures to ensure safe and effective implementation.
The stress field analysis is performed using numerical modeling techniques, such as finite element analysis or boundary element analysis, to determine the stress distribution around the excavation boundary. The analysis identifies the zones of high stress concentration, known as the stress shadow zones, where the stress exceeds the rock's strength and the risk of rock burst is highest. The fracturing holes are then designed to intersect these high-risk zones, creating a network of fractures that redistributes the stress and reduces the stress concentration.
The fracturing hole design includes the selection of the hole depth, the hole spacing, and the hole orientation. The hole depth is determined by the depth of the high-risk zone, which is typically 3 to 8 meters from the excavation boundary. The hole spacing is optimized to ensure that the fracture zones from adjacent holes overlap, creating a continuous stress relief zone. The hole orientation is aligned with the principal stress direction to maximize the fracture propagation in the desired direction.
| Design Parameter | Typical Value | Design Rationale |
|---|---|---|
| Hole depth | 3–8 m | Intersects the high-stress zone |
| Hole spacing | 1.5–3.0 m | Ensures fracture zone overlap |
| Hole orientation | Along principal stress | Maximizes fracture propagation |
| CO2 mass per hole | 1.5–3.0 kg | Provides adequate fracture energy |
| Tube length | 1.0–2.0 m | Matches hole depth and fracture radius |
| Number of holes per face | 20–60 | Covers the entire high-risk zone |
Field Performance and Stress Relief Effectiveness
The field results presented in the paper demonstrate the effectiveness of liquid CO2 fracturing in reducing rock burst risk. The stress relief effectiveness was evaluated using multiple methods, including microseismic monitoring, borehole stress measurement, and the observation of rock burst frequency and severity.
The microseismic monitoring data showed that the CO2 fracturing operation induced a significant number of microseismic events in the high-risk zone, indicating that the stress was being released in a controlled manner. The energy of the microseismic events was significantly lower than the energy of rock burst events, confirming that the CO2 fracturing was relieving the stress before it reached the critical threshold for rock burst initiation.
The borehole stress measurement data showed a reduction in the stress concentration factor from 3.0 to 1.5 after the CO2 fracturing operation, indicating that the stress relief was effective. The stress relief zone extended 3 to 5 meters from the fracturing holes, which was consistent with the theoretical predictions based on the fracture mechanics analysis.
The observation of rock burst frequency and severity showed a significant reduction after the implementation of the CO2 fracturing program. The number of rock burst events per month decreased by 70 to 90 percent, and the severity of the remaining events was reduced from major to minor. The economic impact of the rock burst prevention program was substantial, with the reduction in equipment damage, production downtime, and safety incidents providing a clear return on investment.
Materials and Equipment Considerations in High-Stress Environments
The application of liquid CO2 fracturing in high-stress environments requires special attention to the materials and equipment used in the fracturing system. The high in-situ stress can affect the structural integrity of the fracturing tube, the performance of the initiation system, and the reliability of the charging and handling equipment.
The fracturing tube material must have adequate yield strength and ductility to withstand the combined loading of the internal pressure from the CO2 phase change and the external stress from the surrounding rock. The material selection should consider the stress state of the rock, the temperature conditions, and the potential for corrosion from the CO2-water interaction. The tube design should include a safety factor that accounts for the variability in the in-situ stress and the potential for stress concentration at the tube ends.
The initiation system must be reliable in the high-stress environment, where the rock may be under significant compressive stress and may have a low fracture toughness. The initiation charge must be designed to provide a consistent energy input that triggers the phase transition without causing premature tube failure or incomplete energy release. The initiation system should include safety features that prevent accidental initiation and that allow for the safe disposal of uninitiated charges.
The charging and handling equipment must be designed for operation in the high-stress environment, where the rock may be unstable and the working conditions may be challenging. The equipment should be lightweight and portable, with adequate safety features to protect the operators from rock falls and other hazards. The equipment should also be designed for easy maintenance and repair in the field, with readily available spare parts and clear operating procedures.
Key Insights and Engineering Reflections
The application of liquid CO2 fracturing technology in rock burst prevention represents a significant advancement in the field of mine safety. The technology provides a controlled and repeatable method of stress relief that is safer and more predictable than conventional methods such as blasting or hydraulic fracturing. The field results demonstrate that the technology is effective in reducing rock burst risk and improving the safety of deep mining operations.
From a pressure vessel and materials engineering perspective, the application of CO2 fracturing in high-stress environments highlights the importance of understanding the interaction between the fracturing system and the surrounding rock mass. The in-situ stress field affects the fracture propagation pattern, the energy efficiency of the fracturing operation, and the structural integrity of the fracturing tube. A thorough understanding of the stress field is essential for the design and operation of the fracturing system.
The field performance data also reveal that the effectiveness of the CO2 fracturing is highly dependent on the accuracy of the stress field analysis and the quality of the hole preparation. The stress field analysis must be performed with sufficient accuracy to identify the high-risk zones, and the holes must be drilled to the correct depth and orientation to intersect these zones. Any deviation from the design can result in reduced stress relief effectiveness and increased risk of unintended fractures.
The continued development of the liquid CO2 fracturing technology for rock burst prevention will require ongoing research into the fundamental mechanisms of stress relief, improvements in the fracturing system design, and the development of standardized procedures for stress field analysis and hole design. The integration of real-time monitoring and feedback systems into the fracturing operation would further enhance the safety and effectiveness of the technology, enabling adaptive stress relief that responds to changing stress conditions in real time.