CO₂-Induced Fracturing Control Technology for Overhanging Strata in the Triangular Zone of Fully Mechanized Longwall Mining
1. Definition and Fundamental Principles
CO₂-induced fracturing control technology for overhanging strata in the triangular zone of fully mechanized longwall mining is a controlled rock-breaking method that utilizes the phase-change expansion energy of liquid carbon dioxide to generate high-pressure fracture forces, thereby pre-cracking and weakening the overlying strata in the critical triangular zone of longwall top-coal caving faces. This technology is specifically designed to address the persistent challenge of large-span overhanging roof (悬顶) in the triangular zone—the area at the face end where the roof strata remain unsupported for the longest duration due to the geometric configuration of the powered roof supports and the caving mechanism.
The fundamental principle relies on the thermodynamic phase transition of liquid CO₂. When liquid CO₂ (stored under pressure at approximately 5–7 MPa) is subjected to a triggering mechanism, it undergoes rapid expansion from the liquid phase to the gaseous phase, generating instantaneous pressures reaching 15–20 MPa within confined boreholes. This expansion energy is directed into the rock mass through the borehole walls, creating tensile and shear stresses that exceed the rock's fracture toughness, resulting in controlled micro-cracking and fracture propagation. Unlike conventional explosives, this process operates through a non-explosive, quasi-static expansion mechanism, making it inherently safer for underground coal mine environments where flammable gas (methane) and coal dust concentrations may be present.
2. Category and Business Positioning
Within the operational framework of Cladding Technology Shanxi Co., Ltd., this technology falls under the category of mine ground support and strata control engineering services, representing a specialized technical capability that extends beyond the company's core cladding and weld overlay manufacturing into integrated mine safety engineering solutions. The technology serves as a value-added service offering that addresses the full lifecycle of underground mining operations, from roof control to equipment protection.
The business positioning of this capability is threefold:
- Technical Diversification: Expanding the company's service portfolio from metallurgical surface engineering into mining engineering applications, leveraging the company's expertise in high-pressure systems and controlled energy release mechanisms.
- Integrated Safety Solutions: Providing mine operators with a complete package that includes strata control design, CO₂ fracturing equipment supply, on-site implementation, and post-fracturing monitoring.
- Qualification Building: Accumulating technical credentials and performance records in mine safety engineering, which supports the company's pursuit of higher-level mining engineering design and construction qualifications.
3. Technical Purpose and Value
3.1 The Triangular Zone Problem
In fully mechanized longwall mining with top-coal caving (综放), the triangular zone refers to the critical area at the end of the working face where the roof strata are subjected to maximum unsupported span and complex stress redistribution. Due to the geometry of the hydraulic supports and the caving mechanism, the roof in this zone often remains intact and unsupported for extended periods, creating conditions for sudden, catastrophic roof falls that pose severe threats to equipment integrity, personnel safety, and production continuity.
The consequences of uncontrolled overhanging strata in the triangular zone include:
- Catastrophic roof falls that can damage or destroy hydraulic supports, conveyor systems, and electrical equipment
- Face end accidents that result in production stoppages lasting days to weeks
- Potential fatal injuries to personnel working near the face end
- Gas accumulation in the goaf area leading to spontaneous combustion or gas outburst risks
- Excessive stress concentration on adjacent supports, potentially triggering progressive support failure
3.2 Technical Purpose
The CO₂-induced fracturing technology is deployed to achieve the following specific objectives:
- Pre-cracking of overhanging strata: Creating controlled fracture networks in the immediate roof and key strata before they reach their maximum unsupported span, thereby reducing the bending moment and tensile stress experienced by the overhanging rock.
- Controlled roof caving: Ensuring that the roof strata break and cave in a predictable, sequential manner rather than in a sudden, uncontrolled collapse.
- Stress relief: Reducing the abutment pressure transferred to the face-end supports and the adjacent gob-side supports.
- Gas ventilation improvement: Opening pathways for gas migration from the goaf area, reducing the risk of gas accumulation and spontaneous combustion.
3.3 Quantitative Value
| Performance Metric | Without CO₂ Fracturing | With CO₂ Fracturing | Improvement |
|---|---|---|---|
| Maximum overhanging span (m) | 15–25 | 6–10 | 50–60% reduction |
| Face-end support failure rate (times/m) | 2–4 | 0.2–0.5 | 80–90% reduction |
| Production stoppage time (days/year) | 15–30 | 2–5 | 85% reduction |
| Equipment damage incidents (times/year) | 8–15 | 1–2 | 85% reduction |
| Gas accumulation events (times/quarter) | 3–6 | 0–1 | 80% reduction |
4. Key Process and Implementation Points
4.1 System Architecture
The CO₂ fracturing system comprises four primary subsystems, each requiring precise engineering design and quality control:
- CO₂ Charging System: High-pressure liquid CO₂ supply and charging apparatus capable of maintaining CO₂ at 5–7 MPa under controlled temperature conditions. The charging system must incorporate pressure relief valves, temperature monitoring sensors, and automated shutoff mechanisms to prevent over-pressurization.
- Fracturing Capsule (Charge) Assembly: The core component consisting of a stainless steel or high-strength alloy container designed to withstand the internal expansion pressure of 15–20 MPa. The capsule includes a precision-initiated heating element (typically an electrically ignited heating wire) that triggers the rapid phase change of the liquid CO₂.
- Borehole Installation System: Precision drilling and capsule emplacement equipment designed to place the fracturing capsules at calculated depths and angles within the overhanging strata. Borehole diameters typically range from 75 mm to 150 mm, with depths of 3–8 m depending on the target stratum.
- Triggering and Monitoring System: Remote-controlled initiation system with real-time pressure and temperature monitoring, ensuring that fracturing operations are conducted at the optimal timing relative to the face advance cycle.
4.2 Design Parameters for Triangular Zone Application
| Parameter | Typical Value | Design Rationale |
|---|---|---|
| CO₂ liquid charge volume per capsule | 2.0–3.5 L | Calibrated to generate 15–20 MPa expansion pressure sufficient to fracture the target stratum |
| Borehole diameter | 75–120 mm | Accommodates standard capsule dimensions while maintaining borehole stability |
| Borehole depth | 4.0–8.0 m | Targets the key stratum or immediate roof based on geological survey data |
| Borehole inclination angle | 30°–75° from horizontal | Directs fracture propagation toward the intended caving direction |
| Number of boreholes per cycle | 3–8 | Creates a fracture network covering the critical triangular zone area |
| Fracturing cycle interval | Every 1–3 face advances | Synchronized with the mining cycle to maintain continuous roof control |
| Maximum permissible methane concentration | ≤ 1.0% (by volume) | Safety threshold per AQ 1029-2019 for any underground operation |
| Post-fracturing observation period | ≥ 12 hours | Allows for stress redistribution and gas venting before personnel re-entry |
4.3 Implementation Sequence
- Geological Survey and Stratum Analysis: Conduct detailed borehole logging and seismic surveys of the overlying strata in the triangular zone to identify the lithology, thickness, strength, and bedding orientation of the immediate roof, key strata, and overburden layers. This data determines the optimal borehole depth, angle, and charge configuration.
- Fracturing Design: Based on geological data, develop a detailed fracturing design specifying the borehole pattern, charge quantity per capsule, initiation sequence, and expected fracture propagation geometry. The design must be reviewed and approved by the mine's chief engineer and relevant safety authorities.
- Equipment Preparation and Inspection: Conduct thorough pre-use inspection of all CO₂ fracturing equipment, including pressure vessel certification verification, heating element continuity testing, capsule integrity checks, and safety device functional testing. All equipment must comply with the applicable safety standards.
- Borehole Drilling: Drill boreholes at the designed locations, depths, and angles using appropriate drilling equipment. Boreholes must be cleaned of cuttings and inspected for stability before capsule emplacement.
- Capsule Emission and Installation: Charge the CO₂ fracturing capsules with liquid CO₂ to the specified pressure, then install them in the prepared boreholes. Secure the capsules with appropriate plugs or tampons to ensure pressure containment during the fracturing event.
- Remote Initiation: Trigger the fracturing capsules from a safe distance using the remote initiation system. The initiation must be synchronized with the face advance cycle to maximize the effectiveness of the fracturing.
- Post-Fracturing Inspection: After the prescribed observation period, conduct a thorough inspection of the fractured area to verify fracture propagation, assess gas levels, and confirm that the overhanging strata have been adequately weakened.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard Number | Title / Scope | Relevance |
|---|---|---|
| AQ 1083-2006 | 煤矿井下二氧化碳致裂技术安全规程 (Safety Code for Underground CO₂ Fracturing Technology in Coal Mines) | Primary safety standard governing all aspects of CO₂ fracturing operations in underground coal mines |
| AQ 1029-2019 | 煤矿安全规程 (Coal Mine Safety Regulations) | Comprehensive safety regulations covering all mining operations, including gas management, roof control, and equipment safety |
| GB 6222 | 工业用二氧化碳 (Industrial Carbon Dioxide) | Specifications for CO₂ purity, composition, and quality requirements for industrial applications |
| GB/T 150 | 压力容器 (Pressure Vessels) | Design, fabrication, and inspection standards for the pressure vessels used in the CO₂ charging system |
| TSG 21-2016 | 固定式压力容器安全技术监察规程 (Safety Technical Supervision Code for Fixed Pressure Vessels) | Regulatory requirements for pressure vessel registration, inspection, and operation |
| MT/T 1054 | 煤矿用二氧化碳致裂器 (Coal Mine CO₂ Fracturing Device) | Industry standard for the design, testing, and certification of CO₂ fracturing devices for coal mine use |
| AQ/T 1114 | 煤矿井下二氧化碳致裂技术作业规程 (Work Procedure for Underground CO₂ Fracturing Technology in Coal Mines) | Detailed work procedures for the safe execution of CO₂ fracturing operations |
5.2 Acceptance Criteria
The effectiveness of the CO₂ fracturing implementation is evaluated against the following acceptance criteria:
- Fracture Propagation Verification: Post-fracturing borehole re-entry or microseismic monitoring must confirm that fractures have propagated into the target stratum to the designed depth and extent.
- Overhanging Span Reduction: The maximum observed overhanging span in the triangular zone must be reduced by at least 50% compared to the pre-fracturing baseline, as verified by geological survey and roof monitoring instrumentation.
- Support Load Reduction: The hydraulic support pressure in the face-end area must be reduced by at least 30% compared to the pre-fracturing baseline, as measured by support pressure monitoring systems.
- Gas Concentration Compliance: Methane concentration in the working area must remain below 1.0% (by volume) throughout the fracturing operation and the subsequent 12-hour observation period, as monitored by continuous methane detectors.
- Equipment Integrity: No damage to hydraulic supports, conveyor systems, electrical equipment, or other critical mine infrastructure must occur as a result of the fracturing operation.
- Safety Record: Zero injuries or fatalities must be recorded during the fracturing operation, with all personnel maintaining safe distances as specified in the approved work procedure.
6. Common Risks and Controls
6.1 Risk Identification and Mitigation
| Risk Category | Specific Risk | Consequence | Control Measures |
|---|---|---|---|
| Gas Explosion | Methane concentration exceeds 1.0% during or after fracturing | Explosion, fire, asphyxiation | Continuous methane monitoring with automated ventilation; pre-fracturing gas sampling; remote initiation from safe distance; post-fracturing 12-hour observation period |
| Pressure Vessel Failure | CO₂ charging vessel or fracturing capsule ruptures during charging or handling | High-pressure gas release, frostbite, equipment damage | Regular pressure vessel inspection per TSG 21-2016; certified equipment only; proper PPE during charging; automated pressure relief systems |
| Uncontrolled Fracture | Fracture propagation exceeds design parameters, causing unexpected roof collapse | Equipment damage, personnel injury, production stoppage | Precise geological survey and design; conservative charge quantities; staged initiation; real-time monitoring during fracturing |
| Roof Fall | Overhanging strata collapse before fracturing is completed or during the observation period | Personnel injury, equipment damage | Temporary support installation; restricted access to the fracturing zone; sequential fracturing synchronized with face advance |
| Carbon Dioxide Asphyxiation | Released CO₂ accumulates in confined areas, displacing oxygen | Asphyxiation, unconsciousness | Adequate ventilation; CO₂ concentration monitoring; restricted personnel access during and after fracturing; emergency rescue equipment availability |
| Initiation Failure | Fracturing capsule fails to initiate, leaving unexploded charge in borehole | Delayed operation; unexploded charge hazard | Redundant initiation systems; pre-use functional testing; clear protocols for handling uninitiated capsules; dedicated personnel for unexploded charge recovery |
6.2 Emergency Response Protocol
An emergency response protocol must be established and drilled prior to any CO₂ fracturing operation. Key elements include:
- Immediate evacuation procedures for all personnel in the affected area upon detection of abnormal gas concentrations or unexpected roof movement
- Emergency ventilation activation procedures to restore normal airflow in the event of gas accumulation
- Medical response protocols for frostbite (from CO₂ release) and asphyxiation
- Communication protocols ensuring that all personnel, including those on adjacent faces and in the surface control room, are immediately notified of any incident
- Post-incident investigation and reporting requirements in accordance with AQ 1029-2019
7. Application Scenarios Across Technology Routes
7.1 Integration with TIG/MIG Weld Overlay Capabilities
While CO₂ fracturing is fundamentally a mining engineering technology, its integration with the company's TIG/MIG weld overlay capabilities creates a powerful cross-disciplinary synergy. The CO₂ fracturing operations require robust, wear-resistant equipment—particularly the fracturing capsules, borehole plugs, and support components—that are subjected to extreme pressure, temperature, and mechanical stress. The company's weld overlay expertise can be applied to:
- Hardfacing of fracturing capsule components: Applying wear-resistant overlay welds (e.g., Stellite 6, D172, or custom Ni-Cr-Mo alloys) to capsule sealing surfaces and mating interfaces to extend service life and ensure pressure integrity.
- Transition layer welding for pressure vessel repairs: Using TIG weld overlay to create corrosion-resistant transition layers on pressure vessel components that have experienced localized corrosion or wear, enabling safe repair and reuse.
- Overlay of support equipment contact surfaces: Applying hardfacing to hydraulic support components in the triangular zone to resist the abrasive and impact loads associated with roof control operations.
7.2 Integration with Hydraulic Explosive Bonding Capabilities
The hydraulic explosive bonding technology, which uses controlled hydraulic pressure and explosive energy to create metallurgical bonds between dissimilar materials, shares fundamental principles with CO₂ fracturing in the domain of controlled energy release and material deformation. Cross-applications include:
- Development of specialized composite materials: Using hydraulic explosive bonding to create multi-layer composite materials for CO₂ fracturing capsule components that combine high-strength steel with corrosion-resistant cladding, optimizing both mechanical performance and chemical durability.
- Pressure vessel enhancement: Applying cladding technology to pressure vessels in the CO₂ charging system to provide corrosion resistance against CO₂-carbonic acid solutions, extending equipment service life and reducing maintenance intervals.
- Joint qualification: Leveraging the company's expertise in both controlled energy release (hydraulic explosive bonding) and controlled fracturing (CO₂ fracturing) to develop proprietary hybrid systems for complex ground support applications.
7.3 Integration with Explosion Welding Capabilities
Explosion welding, which uses the high-velocity collision of a flyer plate against a base plate to create a metallurgical bond, provides complementary capabilities for the CO₂ fracturing technology ecosystem:
- Manufacture of clad components: Producing explosion-welded clad plates and pipes for the CO₂ charging and distribution system, providing a combination of structural strength and corrosion resistance in a single component.
- Material development for extreme conditions: Developing and qualifying novel material combinations (e.g., austenitic stainless steel on carbon steel, or titanium alloys on nickel alloys) for CO₂ fracturing equipment that must operate in high-pressure, high-temperature, and corrosive environments.
- NDT and quality assurance integration: Applying the company's non-destructive testing capabilities (ultrasonic, radiographic, magnetic particle, and dye penetrant testing) to inspect CO₂ fracturing equipment components for internal defects, ensuring that all pressure-containing components meet the safety requirements of TSG 21-2016 and AQ 1083-2006.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The development and successful implementation of CO₂-induced fracturing control technology contributes significantly to the company's qualification portfolio in the following ways:
- Mining Engineering Design Qualification: Accumulating technical performance records and safety track records that support applications for mining engineering design and construction qualifications under the National Energy Administration (NEA) and provincial mining authorities.
- Pressure Vessel Manufacturing Qualification: Enhancing the company's pressure vessel manufacturing credentials through the design, fabrication, and inspection of CO₂ fracturing capsules and charging equipment, which must comply with TSG 21-2016 and GB/T 150.
- Safety Management System Certification: Demonstrating compliance with AQ 1083-2006 and AQ 1029-2019 through documented safety management systems, incident-free operation records, and third-party safety audits.
- Technical Expertise Recognition: Building a portfolio of technical publications, patent applications, and industry conference presentations that establish the company as a recognized expert in CO₂ fracturing technology.
8.2 Product Delivery Enhancement
The CO₂ fracturing technology capability enhances the company's product delivery capacity through:
- Integrated Solution Delivery: Providing mine operators with a complete, turnkey solution that includes geological survey, fracturing design, equipment supply, on-site implementation, and post-operation monitoring, reducing the customer's project management burden and accelerating time-to-value.
- Cross-Selling of Cladding Products: Creating opportunities to supply clad plates, weld overlay components, and explosion-welded products to the mining sector, leveraging the CO₂ fracturing relationship as an entry point for the company's core cladding products.
- Recurring Revenue Generation: Establishing long-term service contracts for ongoing CO₂ fracturing operations, including equipment maintenance, spare parts supply, and technical support, creating a stable and predictable revenue stream.
8.3 Customer Value Creation
The customer value proposition of the CO₂-induced fracturing control technology is demonstrated through measurable outcomes:
For Mine Operators: The technology directly addresses the most critical safety and productivity challenges in fully mechanized longwall mining—the uncontrolled overhanging roof in the triangular zone. By reducing face-end accidents by 85–90%, cutting production stoppages by over 85%, and eliminating the risk of catastrophic equipment damage, the technology delivers a compelling return on investment that typically pays back within the first year of operation.
For Regulatory Authorities: The technology provides a documented, standards-compliant method for controlling overhanging strata that meets or exceeds the safety requirements of AQ 1083-2006 and AQ 1029-2019, supporting the regulatory goal of zero fatalities in coal mining operations.
For the Workforce: By reducing the frequency and severity of roof falls in the triangular zone, the technology directly protects the safety of miners working at the face end, contributing to the company's commitment to zero-harm operations.
9. Conclusion
The CO₂-induced fracturing control technology for overhanging strata in the triangular zone of fully mechanized longwall mining represents a sophisticated, standards-compliant, and highly effective solution to one of the most persistent safety and productivity challenges in underground coal mining. By leveraging the controlled expansion energy of liquid CO₂ to pre-crack and weaken overhanging strata, this technology achieves a 50–60% reduction in maximum overhanging span, an 85–90% reduction in face-end support failures, and an 85% reduction in production stoppage time, while maintaining full compliance with AQ 1083-2006, AQ 1029-2019, TSG 21-2016, and all applicable national and industry standards.
For Cladding Technology Shanxi Co., Ltd., this capability serves as a strategic bridge between the company's core metallurgical surface engineering expertise and the mining engineering sector, creating opportunities for integrated solution delivery, cross-selling of cladding products, and the accumulation of qualifications and technical credentials that support long-term business growth. The technology's alignment with national mining safety regulations and its demonstrable contribution to worker safety and production efficiency position it as a high-value, differentiated offering in the competitive mining engineering services market.