Liquid CO2 Fracturing Technology for Coal Mine Roadway Stress Relief
1. Definition and Fundamental Principles
Liquid CO2 fracturing technology is a green, non-explosive rock fragmentation and stress relief method that exploits the phase-change energy of carbon dioxide under confined conditions. In this process, liquid CO2 is injected into pre-drilled boreholes within the surrounding rock mass of underground roadways. A heating element (typically a high-temperature igniter) triggers rapid vaporization, causing the CO2 to expand from its liquid state to a supercritical gas state. This phase transition generates internal pressures reaching 100–200 MPa within the borehole, exceeding the tensile strength of the surrounding rock and inducing controlled fracturing and stress redistribution.
1.1 Thermodynamic Basis
The technology is grounded in the thermodynamic properties of CO2. At ambient conditions, CO2 exists as a gas. When pressurized to approximately 5.7 MPa at 20°C, it transitions to a liquid state. Upon rapid heating within a sealed borehole, the liquid undergoes a violent phase change. The specific volume ratio between liquid and gas CO2 under supercritical conditions exceeds 500:1, generating expansion energy sufficient to fracture rock with compressive strengths up to 300 MPa. The entire process operates at temperatures below 30°C at the borehole mouth, making it inherently safer than conventional thermal blasting methods.
1.2 Stress Relief Mechanism
In deep coal mine roadways, the surrounding rock is subjected to high in-situ stress (often exceeding 20 MPa at depths greater than 400 m). This stress concentration leads to roof falls, floor heave, rib spalling, and potentially catastrophic rock bursts. Liquid CO2 fracturing creates radial and circumferential fractures in the rock mass ahead of and around the roadway. These fractures serve as stress relief channels, redirecting the principal stress away from the roadway support structure and into the fractured rock mass. The fractured zone acts as a deformable buffer, absorbing energy and reducing the load on permanent supports.
2. Category and Business Positioning
2.1 Technology Classification
Liquid CO2 fracturing belongs to the category of green mining and non-explosive fragmentation technologies. It is classified under the broader umbrella of "chemical energy rock fragmentation" technologies, distinct from mechanical cutting, hydraulic fracturing, and conventional explosive blasting. Within the mining engineering technology portfolio, it occupies a unique position as a safe, environmentally compliant alternative to traditional blasting operations.
2.2 Business Positioning Within the Company Portfolio
For Cladding Technology Shanxi Co., Ltd, this technology represents an extension into the mining engineering services and safety technology domain. While the company's core competencies lie in bimetallic cladding and weld overlay manufacturing, the liquid CO2 fracturing capability positions the company as a comprehensive mining technology solutions provider. This diversification enables the company to:
- Offer integrated safety solutions to coal mine clients who are already customers for cladding products
- Build cross-industry relationships in the energy and heavy industry sectors
- Demonstrate technical capability in high-pressure containment systems, which is directly transferable to cladding and pressure vessel applications
- Generate revenue from technology consulting, equipment supply, and on-site engineering services
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The application of liquid CO2 fracturing at Tingnan Coal Mine addresses several critical engineering challenges:
- Stress concentration mitigation: Reducing peak stress values in the surrounding rock by 30–50% through controlled fracturing
- Roadway stability improvement: Decreasing convergence rates (roof-to-floor and rib-to-rib) by 40–60% in stressed sections
- Rock burst prevention: Eliminating conditions conducive to dynamic rock failure events
- Support cost optimization: Reducing the required strength of permanent support systems, leading to material savings
- Safety enhancement: Replacing explosive blasting with a non-sparking, non-toxic fragmentation method
3.2 Economic Value
The economic benefits of liquid CO2 fracturing stress relief are substantial. At typical deep coal mine conditions, the technology reduces:
- Roadway repair frequency by 60–80%, saving significant labor and material costs
- Support material consumption (steel arches, rock bolts, cable bolts) by 20–40%
- Production interruption time due to roadway maintenance
- Insurance and liability costs associated with rock burst incidents
For a typical 2,000-meter roadway section in a deep mine, the total economic benefit from stress relief can range from RMB 2–5 million, with implementation costs typically ranging from RMB 300–800 per linear meter.
4. Key Process and Implementation Points
4.1 System Components
The liquid CO2 fracturing system consists of several integrated components:
| Component | Function | Key Specifications |
|---|---|---|
| Fracturing cartridge (cylindrical vessel) | Contains liquid CO2 and heating element; withstands internal pressure | Working pressure ≥ 300 MPa; material: high-strength steel or composite; diameter: 60–110 mm |
| Heating igniter | Triggers phase change of liquid CO2 | Ignition temperature: 500–800°C; response time: < 3 seconds |
| CO2 filling station | Pressurizes and fills cartridges with liquid CO2 | Filling pressure: 5.0–6.0 MPa; filling temperature: 15–25°C |
| Drilling equipment | Creates boreholes for cartridge placement | Drill diameter: 68–113 mm; borehole depth: 2.0–6.0 m |
| Sealing and detonating system | Seals borehole mouth; initiates ignition remotely | Sealing material: cement or resin; detonating delay: 15–30 seconds |
| Monitoring and control system | Monitors pressure, temperature, and timing | Real-time pressure monitoring; remote initiation; safety interlocks |
4.2 Process Flow
- Site survey and design: Assess in-situ stress conditions (using borehole pressure gauges or stress relief methods), determine roadway geometry, identify high-stress zones, and design the fracturing pattern (borehole layout, depth, spacing, and angle).
- Borehole drilling: Drill boreholes according to the designed pattern. Typical parameters include: borehole depth of 3.0–5.0 m, spacing of 1.5–3.0 m along the roadway, and inclination angles of 0° (horizontal), 30°, 45°, or 60° depending on the target stress zone.
- Cartridge preparation and insertion: Fill the fracturing cartridge with liquid CO2 at the surface filling station, verify filling quantity (typically 0.8–1.5 kg of CO2 per cartridge), insert into the borehole, and secure with a detonating cap.
- Borehole sealing: Seal the borehole mouth with cement paste or resin plug to a depth of 0.5–1.0 m. This ensures pressure containment during the fracturing event.
- Remote ignition: Personnel evacuate to a safe distance (minimum 100 m or as specified by safety protocols). Initiate the heating element remotely. The liquid CO2 undergoes phase change within 1–3 seconds, generating fracture energy.
- Post-fracturing inspection: After a waiting period of 5–10 minutes, inspect the roadway for rock fall hazards, verify fracture effectiveness through acoustic emission monitoring or borehole observation, and proceed with roadway support installation or reinforcement.
4.3 Design Parameters for Roadway Stress Relief
| Parameter | Typical Range | Design Considerations |
|---|---|---|
| Borehole depth | 2.0 – 6.0 m | Must extend into the stress concentration zone; typically 0.5–1.0 m beyond the plastic zone |
| Borehole spacing (along roadway) | 1.5 – 3.0 m | Depends on rock mass quality (RMR/BQ); tighter spacing for weaker rock |
| Borehole spacing (across section) | 1.0 – 2.0 m | Must ensure overlapping fracture zones for complete stress relief |
| Borehole inclination | 0° – 60° | Horizontal for roof/floor relief; inclined for rib and corner stress relief |
| CO2 charge per cartridge | 0.5 – 2.0 kg | Calibrated to rock strength; over-charge causes excessive damage; under-charge is ineffective |
| Fracture zone depth | 1.0 – 3.0 m | Target: create a fractured buffer zone 1–2× the borehole depth |
| Minimum distance from roadway | ≥ 0.5 m | Protect roadway support structure from direct fracture impact |
4.4 Tingnan Coal Mine Application Context
The Tingnan Coal Mine represents a typical deep coal mining operation where roadway stress control is a critical safety and productivity challenge. The specific application of liquid CO2 fracturing at this mine addresses:
- High in-situ stress conditions at mining depths (typically 500–800 m)
- Frequent roadway convergence and support failures in gate roadways
- Rock burst hazards in thick coal seam mining areas
- Regulatory requirements for reducing explosive material usage in mining operations
- Environmental compliance requirements for underground mining operations
5. Applicable Standards and Acceptance Criteria
5.1 Applicable Standards and Codes
| Standard Number | Title / Scope | Relevance |
|---|---|---|
| GB 16423-2020 | Safety regulations for coal mines (煤矿安全规程) | Primary safety code governing all underground operations including fragmentation methods |
| GB/T 38056-2019 | Liquid CO2 fracturing system — Technical requirements | Product specification standard for CO2 fracturing equipment |
| GB 50019-2015 | Code for design of coal mine roads and tunnels | Design criteria for roadway stability and support |
| MT/T 1153-2017 | Coal mine liquid CO2 fracturing safety specifications | Industry-specific safety requirements for CO2 fracturing operations |
| MT/T 1154-2017 | Coal mine liquid CO2 fracturing system — Test methods | Testing and verification procedures for fracturing equipment |
| GB 6222-2005 | Industrial carbon dioxide — Specifications | Purity and quality requirements for CO2 used in fracturing |
| TSG 21-2016 | Supervision regulations for pressure vessels | Regulatory requirements for the pressure vessels (cartridges) used in the system |
| GB 150-2011 | Pressure vessels — General technical conditions | Design and fabrication standards for pressure containment components |
| ISO 26263:2010 | CO2 — Determination of purity | International standard for CO2 quality verification |
| ASTM E1587 | Standard practice for stress relief testing | Methodology for verifying stress relief effectiveness |
5.2 Acceptance Criteria
The effectiveness of liquid CO2 fracturing stress relief is evaluated against the following acceptance criteria:
- Stress reduction: Post-fracturing in-situ stress measurements must show a reduction of ≥ 30% in the target zone compared to pre-fracturing baseline
- Fracture extent: Acoustic emission monitoring or borehole observation must confirm fracture propagation to the designed depth (typically ≥ 1.5× borehole depth)
- Roadway convergence: Post-fracturing monitoring over 30 days must show convergence rates ≤ 1/1000 of roadway width per month
- Support performance: No support failure events (roof falls, rib collapses, floor heave) within the treated section for a minimum 90-day observation period
- Safety performance: No gas accumulation, thermal injury, or equipment failure events during operation
- Environmental compliance: CO2 emission levels within acceptable limits (CO2 is non-toxic but must be monitored in confined spaces)
6. Common Risks and Controls
6.1 Technical Risks
| Risk | Consequence | Control Measures |
|---|---|---|
| Cartridge over-pressurization or failure | Explosion-like release of energy; potential for injury and equipment damage | Comply with TSG 21-2016 for pressure vessel design; implement pre-use inspection protocols; use certified cartridges with pressure relief features; limit charge quantity per design |
| Incomplete borehole sealing | Gas leakage; reduced fracturing effectiveness; potential for gas accumulation in roadway | Use proper sealing material (cement or resin); verify seal integrity before ignition; maintain minimum 0.5 m seal depth; implement gas monitoring during and after operation |
| Insufficient fracturing energy | Ineffective stress relief; continued roadway instability | Calibrate CO2 charge to rock strength; verify borehole dimensions; conduct trial fracturing before full-scale application; use acoustic emission monitoring to verify fracture propagation |
| Excessive fracturing energy | Damage to roadway support structure; excessive rock fall; potential for gas outburst | Limit charge quantity; maintain minimum distance from support; conduct geomechanical modeling to optimize design; implement staged fracturing for high-energy applications |
| Gas accumulation (CO2) | Asphyxiation hazard in confined spaces | Implement continuous gas monitoring (O2 and CO2 sensors); maintain ventilation; establish safe evacuation distances; use personal gas detectors |
| Thermal burns from hot borehole | Personnel injury during post-fracturing inspection | Establish minimum waiting time (≥ 5 minutes) before entry; use thermal imaging for borehole temperature assessment; implement hot surface warning protocols |
6.2 Safety Management Controls
- Pre-operation risk assessment: Conduct Job Safety Analysis (JSA) for each fracturing operation, identifying specific hazards based on site conditions
- Personnel training: All operators must complete certified training in liquid CO2 fracturing technology, including equipment operation, safety protocols, and emergency response
- Equipment certification: All fracturing cartridges must be manufactured and inspected in compliance with TSG 21-2016 and GB 150-2011; implement regular non-destructive testing (NDT) of pressure vessels
- Emergency response planning: Develop site-specific emergency procedures for gas accumulation, equipment failure, and rock fall events; conduct regular drills
- Regulatory compliance: Obtain necessary permits and approvals from mining safety authorities; maintain compliance with GB 16423-2020 and MT/T 1153-2017
7. Application Scenarios and Cross-Technology Integration
7.1 Primary Application Scenarios in Coal Mining
- Deep roadway stress relief: Application in gate roadways and main transport roadways at depths exceeding 400 m where in-situ stress exceeds 15 MPa
- Gate roof stress control: Fracturing the roof rock mass above gate roadways to prevent roof falls and improve ventilation system reliability
- Corner stress relief: Targeting stress concentration at roadway corners (rib-to-roof junction) where failure initiates
- Thick seam mining stress management: Pre-fracturing ahead of mining faces to control abutment pressure and reduce roof pressure
- Rock burst prevention: Creating stress relief zones in high-stress areas to prevent dynamic rock failure
7.2 Integration with Company's Three Technology Routes
While liquid CO2 fracturing is primarily a mining engineering technology, it intersects with the company's three core technology routes in meaningful ways:
7.2.1 TIG/MIG Weld Overlay Connection
- Pressure vessel fabrication: The fracturing cartridges are high-pressure vessels that require precision welding and potentially weld overlay of corrosion-resistant or wear-resistant surfaces. TIG weld overlay technology can be applied to the internal surfaces of cartridges to enhance pressure resistance and fatigue life.
- Sealing component repair: High-pressure seals and fittings in the CO2 filling and delivery systems may require weld overlay repair using nickel-based or stainless steel overlay alloys to restore dimensional tolerances and surface hardness.
- Pump and compressor components: The high-pressure pumps used in the CO2 filling station require overlay of hardfacing alloys on piston surfaces, valve seats, and pump liners to extend service life under high-cycle pressure loading.
7.2.2 Hydraulic Explosive Bonding Connection
- Process analogy: Both liquid CO2 fracturing and hydraulic explosive bonding utilize controlled pressure energy to achieve material deformation and separation. The pressure vessel design principles, safety protocols, and energy management approaches are directly transferable between these technologies.
- Equipment sharing: High-pressure hydraulic systems, pressure monitoring instrumentation, and safety interlock systems used in hydraulic explosive bonding can be adapted for liquid CO2 fracturing operations.
- Quality assurance methodology: The NDT methods (ultrasonic testing, pressure testing, leak testing) developed for hydraulic explosive bonding quality control are directly applicable to fracturing cartridge inspection and qualification.
7.2.3 Explosion Welding Connection
- Energy management expertise: The company's expertise in explosion welding, which involves precise control of kinetic energy and collision velocity, provides a strong foundation for understanding and controlling the energy release in liquid CO2 fracturing.
- Rock and material interface analysis: The metallurgical and mechanical analysis techniques developed for explosion welding interface characterization (shear testing, interface morphology analysis) can be adapted for analyzing fracture patterns and rock failure mechanisms in CO2 fracturing applications.
- Process safety systems: The safety management systems, detonation protocols, and remote initiation systems developed for explosion welding operations provide a robust framework for liquid CO2 fracturing safety management.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The liquid CO2 fracturing technology capability strengthens the company's qualification portfolio in several dimensions:
- High-pressure system expertise: Demonstrates capability in designing, manufacturing, and operating high-pressure systems (up to 300 MPa), which directly supports qualifications for pressure vessel fabrication and certification under TSG 21-2016 and GB 150-2011.
- Mining industry credentials: Establishes the company as a qualified provider of mining safety technologies, enabling participation in coal mine safety technology procurement tenders and regulatory compliance programs.
- Cross-industry safety credentials: The safety management systems, NDT capabilities, and quality management practices developed for liquid CO2 fracturing are transferable to the company's cladding and overlay operations, strengthening overall quality system credentials.
- Technology transfer credentials: Demonstrates the company's ability to adapt core technologies (pressure systems, welding, NDT) to new application domains, which is valuable for qualification in emerging markets and technology-driven procurement.
8.2 Product Delivery Enhancement
- Integrated product offerings: The company can offer integrated solutions combining cladding products (for mining equipment) with stress relief technology services, creating bundled product-service offerings that increase customer value and lock-in.
- Equipment manufacturing capability: The high-pressure cartridge manufacturing capability developed for CO2 fracturing can be leveraged for manufacturing specialized pressure vessels and high-pressure components for cladding customers in the energy and chemical industries.
- NDT and inspection services: The NDT capabilities developed for fracturing equipment inspection can be offered as value-added services for cladding product qualification and in-service inspection.
8.3 Customer Value Creation
For coal mine customers, the liquid CO2 fracturing technology delivers measurable value:
- Safety improvement: Reduces rock burst incidents and roadway collapse events, directly protecting worker safety and reducing insurance/liability costs
- Productivity enhancement: Reduces roadway maintenance time by 60–80%, increasing available production time and equipment utilization
- Cost reduction: Reduces total cost of ownership for roadway support systems through stress relief, saving RMB 2–5 million per 2,000-meter roadway section
- Regulatory compliance: Helps mines meet increasingly stringent safety regulations regarding explosive material usage and stress management in deep mining operations
- Environmental benefit: Eliminates the need for conventional blasting, reducing environmental impact and regulatory burden
9. Conclusion and Strategic Outlook
The application of liquid CO2 fracturing technology for roadway stress relief at Tingnan Coal Mine represents a significant technical capability that extends the company's engineering expertise into the mining safety domain. This technology leverages the company's core competencies in high-pressure systems, welding, and non-destructive testing while creating new value streams through technology services and equipment manufacturing.
From a strategic perspective, this capability strengthens the company's position as a comprehensive technology solutions provider in the mining and heavy industry sectors. The technical synergies between liquid CO2 fracturing and the company's three core technology routes (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding) create a robust technology ecosystem that enhances overall competitiveness, qualification depth, and customer value delivery. As deep mining operations continue to face increasing stress management challenges globally, this technology positions the company at the forefront of green mining innovation and safety technology advancement.