Liquid CO₂ Fracturing and Permeability Enhancement Technology for Gas Outburst Prevention in Coal Reveal Zones
1. Definition and Technical Principles
Liquid CO₂ fracturing and permeability enhancement technology for gas outburst prevention is an advanced coal mine safety engineering method designed to mitigate gas outburst hazards in coal reveal zones — the critical transition areas where crosscuts (石门) intersect with coal seams during underground mining operations. The technology leverages the unique thermodynamic and phase-transition properties of carbon dioxide to create controlled micro-fractures in coal body, thereby increasing coal seam permeability and accelerating gas drainage prior to or during the coal reveal process.
The fundamental principle operates through a multi-stage mechanism:
- Phase Transition Energy Release: Liquid CO₂ is injected into pre-drilled boreholes in the coal body at pressures typically exceeding 10 MPa. Upon depressurization or contact with the warmer coal body (typically 25–40°C at depth), the liquid CO₂ undergoes a rapid phase transition from liquid to gas, expanding by a factor of approximately 500–700 times in volume. This volumetric expansion generates intense localized pressure waves within the coal matrix.
- Thermo-Mechanical Fracture Initiation: The rapid expansion produces a temperature differential between the expanding CO₂ (which cools significantly during phase transition, reaching temperatures as low as −78.5°C at atmospheric pressure) and the surrounding coal body. This thermal shock, combined with the mechanical pressure of the expanding gas, creates tensile stresses exceeding the coal's tensile strength, initiating radial micro-fractures propagating from the borehole wall.
- Permeability Network Formation: The induced fractures connect pre-existing natural cleat systems within the coal body, forming an interconnected permeability network that dramatically enhances gas flow capacity. This is measured by the increase in gas permeability coefficient (K), which can increase by 3–10 times compared to untreated coal.
- Accelerated Gas Drainage: The enhanced permeability allows for significantly faster and more complete gas drainage through existing borehole networks, reducing the residual gas content and pressure in the coal body to below critical outburst thresholds before or during the coal reveal operation.
2. Category and Business Positioning
This technology falls within the domain of Coal Mine Hazard Mitigation Engineering and specifically addresses the category of Gas Outburst Prevention and Control. Within Cladding Technology Shanxi Co., Ltd's diversified capability portfolio, this technology represents the company's commitment to comprehensive industrial safety solutions beyond traditional cladding and metallurgical services. It positions the company as a multidisciplinary engineering partner capable of delivering integrated safety and reliability solutions across heavy industry sectors.
The business positioning encompasses:
- Pre-Engineering Safety Services: Providing hazard assessment, design, and implementation of gas outburst prevention measures for coal mining operations.
- Technical Consulting and Design: Developing site-specific gas outburst prevention plans compliant with national and industry standards.
- Equipment Supply and Installation: Supplying liquid CO₂ fracturing equipment, injection systems, and monitoring instrumentation.
- Training and Knowledge Transfer: Delivering technical training programs (such as the study and learning of the Shimen Mine liquid CO₂ technology) to enhance client operational competency.
- Quality Assurance and Certification Support: Assisting clients in achieving compliance certifications and maintaining safety management system credentials.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The technology addresses the following critical safety challenges in coal mining operations:
- Outburst Hazard Reduction: Reducing the gas outburst risk index (W) and gas content (W_total) in coal reveal zones to below regulatory thresholds, typically targeting W < 0.5 MPa·m³/t and residual gas content < 8 m³/t.
- Permeability Enhancement: Increasing coal body permeability coefficient (K) from baseline values (often 0.01–0.1 mD in tight coal seams) to 0.1–1.0 mD or higher, enabling effective gas drainage through existing borehole networks.
- Drainage Acceleration: Reducing the required gas drainage time by 40–70% compared to conventional borehole-only drainage methods, thereby improving mining progress and reducing the duration of exposure to outburst hazards.
- Stress Relief: Partially relieving in-situ stress concentrations around the coal reveal zone through fracture-induced stress redistribution, complementing mechanical support measures.
3.2 Quantified Value Proposition
| Value Dimension | Conventional Method | Liquid CO₂ Fracturing Method | Improvement |
|---|---|---|---|
| Gas Drainage Time | 30–60 days | 10–20 days | 50–70% reduction |
| Coal Permeability (K) | 0.01–0.1 mD | 0.1–1.0 mD | 3–10× increase |
| Residual Gas Content | 8–15 m³/t | 3–7 m³/t | 40–60% reduction |
| Outburst Risk Index (W) | 0.8–1.5 MPa·m³/t | 0.2–0.5 MPa·m³/t | 60–80% reduction |
| Borehole Utilization Rate | 30–50% | 70–90% | 2× improvement |
| Construction Cost per m³ Gas Drained | Baseline (1.0) | 0.4–0.6 | 40–60% cost savings |
3.3 Strategic Value to Client Organizations
- Regulatory Compliance: Ensures compliance with mandatory safety regulations including AQ 1026-2019 Coal Mine Gas Outburst Prevention Regulations, AQ 1053-2008 Coal Mine Gas Drainage System Safety and Technical Regulations, and AQ 1021-2006 Coal Mine Gas Outburst Prevention and Control Regulations.
- Production Continuity: Reduces unplanned stoppages caused by gas outburst incidents or precautionary shutdowns, protecting annual production targets and revenue.
- Worker Safety: Directly contributes to zero-fatality objectives by reducing outburst risk to acceptable levels, supporting ISO 45001 occupational health and safety management system requirements.
- Environmental Stewardship: CO₂ is a naturally occurring component of coal mine gas; the technology uses liquid CO₂ (which can be captured and recycled), avoiding the use of flammable or toxic gases in the fracturing process.
4. Key Process and Implementation Points
4.1 Pre-Implementation Assessment
Before deploying liquid CO₂ fracturing technology, a comprehensive hazard assessment must be conducted:
- Gas Outburst Prediction: Conduct outburst danger prediction using methods specified in AQ 1026-2019, including gas content measurement, gas pressure determination, and coal strength index (f) evaluation. The outburst danger index (W = p × W_total / f) must be calculated.
- Geological Survey: Map the coal seam thickness, dip angle, lithology of surrounding rock, fault distribution, and aquifer characteristics in the coal reveal zone.
- Stress Field Analysis: Determine in-situ stress magnitudes and orientations using hydraulic fracturing or acoustic emission methods to identify stress concentration zones.
- Existing Infrastructure Audit: Evaluate the condition and capacity of existing gas drainage boreholes, ventilation systems, and monitoring instrumentation.
4.2 Borehole Design and Drilling Parameters
| Parameter | Specification | Notes |
|---|---|---|
| Borehole Diameter | 75–120 mm (drilling), 58–89 mm (final) | Depends on CO₂ injection device size |
| Borehole Length | 20–50 m (into coal body) | Based on outburst prediction results |
| Borehole Inclination | 0°–15° (upward or downward) | Aligned with coal seam dip |
| Borehole Spacing | 2.0–4.0 m | Determined by fracture propagation radius |
| Borehole Arrangement | Radial or parallel pattern | Radial preferred for crosscut faces |
| Drilling Fluid | Water-based or air flush | Avoid oil-based fluids (fire risk) |
| Casing Requirement | Steel or plastic casing in soft coal | Per AQ 1053-2008 |
4.3 Liquid CO₂ Injection Process
- Equipment Preparation: Load the liquid CO₂ fracturing device (typically a sealed steel cylinder with capacity of 5–20 kg liquid CO₂) into the borehole. The device must be inspected for integrity prior to each use, following manufacturer specifications and relevant safety standards.
- Device Placement: Insert the fracturing device to the designed depth using a push rod or conveyance system. Secure the device with a packing plug (wooden or polyurethane foam) to ensure sealing at the borehole mouth.
- Initiation: Detonate the initiation charge (electric detonator or thermal initiator) to trigger the rapid phase transition of liquid CO₂. Personnel must evacuate to a safe distance (minimum 100 m, or as specified by local regulations) before initiation.
- Fracture Formation: The rapid expansion of CO₂ gas (500–700× volume increase) generates pressures of 100–300 MPa at the fracture tip, creating radial fractures extending 3–8 m from the borehole wall in typical coal conditions.
- Post-Fracturing Inspection: After a stabilization period (typically 24–48 hours), inspect the borehole for gas flow, pressure changes, and structural integrity. Conduct acoustic emission monitoring if available.
4.4 Gas Drainage and Monitoring
Following fracturing, the enhanced permeability network is exploited through a systematic gas drainage program:
- Drainage System Activation: Connect fractured boreholes to the mine's main gas drainage system (per AQ 1053-2008 requirements). Ensure drainage negative pressure is maintained at 3–8 kPa depending on system capacity.
- Gas Flow Monitoring: Continuously monitor gas flow rate (Q), gas concentration (CH₄ %), and drainage pressure (P) at each borehole. Record data at intervals not exceeding 4 hours.
- Effectiveness Verification: Conduct periodic gas content measurements (per MT/T 1052-2007) to verify that residual gas content is declining toward target thresholds. Use the gas drainage effect evaluation method specified in AQ 1026-2019.
- Threshold-Based Decision: Proceed with coal reveal operations only when gas content, gas pressure, and outburst danger index all meet the acceptance criteria specified in the approved outburst prevention plan.
4.5 Process Parameter Optimization Matrix
| Coal Condition | CO₂ Charge (kg) | Borehole Spacing (m) | Borehole Length (m) | Expected Fracture Radius (m) | Drainage Time (days) |
|---|---|---|---|---|---|
| Soft coal (f < 0.3) | 5–8 | 2.0–3.0 | 20–30 | 2.0–3.5 | 7–14 |
| Medium coal (0.3 ≤ f < 0.8) | 8–15 | 2.5–3.5 | 25–40 | 3.0–5.0 | 10–20 |
| Hard coal (f ≥ 0.8) | 15–25 | 3.0–4.0 | 30–50 | 4.0–8.0 | 14–28 |
| Fractured coal (high permeability) | 3–6 | 3.0–4.0 | 20–30 | 2.0–4.0 | 5–10 |
5. Applicable Standards and Acceptance Criteria
5.1 Mandatory Standards and Regulations
| Standard/Regulation | Title | Relevance |
|---|---|---|
| AQ 1026-2019 | Coal Mine Gas Outburst Prevention Regulations | Governs outburst prediction, prevention measures, and acceptance criteria |
| AQ 1053-2008 | Coal Mine Gas Drainage System Safety and Technical Regulations | Specifies gas drainage system design, operation, and maintenance requirements |
| AQ 1021-2006 | Coal Mine Gas Outburst Prevention and Control Regulations | Provides general requirements for outburst prevention measures |
| MT/T 1052-2007 | Technical Regulations for Coal Mine Gas Outburst Prediction | Defines prediction methods, measurement procedures, and evaluation criteria |
| GB 50471-2008 | Code for Design of Coal Mine Ventilation | Specifies ventilation system requirements including gas drainage ventilation |
| AQ 2013-2008 | Specification for Safety of Coal Mine Construction | General safety requirements for underground construction operations |
| GB/T 37306-2018 | Technical Requirements for Coal Mine Gas Outburst Prevention | National standard for outburst prevention technical requirements |
5.2 Acceptance Criteria
The following acceptance criteria must be met before coal reveal operations can proceed in a treated zone:
- Gas Content: Residual gas content (W_total) must be reduced to ≤ 8 m³/t (or the threshold specified in the mine's approved outburst prevention plan, which may be more stringent).
- Gas Pressure: Gas pressure (p) must be reduced to ≤ 0.74 MPa, or the gas pressure gradient must be below the critical threshold for the specific coal seam.
- Outburst Danger Index: The outburst danger index (W = p × W_total / f) must be below 0.5 MPa·m³/t (per AQ 1026-2019).
- Drainage Effect Verification: Gas drainage effect must be confirmed through at least two consecutive gas content measurements showing a declining trend, with the final measurement meeting the threshold.
- Borehole Integrity: All drainage boreholes must be verified as intact and connected to the drainage system, with flow rates within design parameters.
- Monitoring System: Continuous gas monitoring (CH₄, CO, temperature, pressure) must be operational with alarm thresholds set per regulatory requirements.
- Documentation: Complete technical documentation including borehole logs, fracturing records, gas measurement data, and effectiveness evaluation reports must be compiled and archived.
5.3 International Standard Alignment
- ISO 45001:2018 — Occupational Health and Safety Management Systems: The technology implementation must be integrated into the mine's OH&S management system, with documented risk assessments, control measures, and emergency procedures.
- ISO 9001:2015 — Quality Management Systems: Process control, documentation, and continuous improvement requirements apply to the fracturing and drainage operations.
- ISO 14001:2015 — Environmental Management Systems: CO₂ usage, waste management, and environmental impact assessment requirements.
6. Common Risks and Controls
6.1 Risk Identification and Mitigation
| Risk Category | Specific Risk | Severity | Mitigation Measures |
|---|---|---|---|
| Gas Outburst During Fracturing | Sudden gas release triggered by fracturing process | Critical | Conduct outburst prediction before fracturing; use staged fracturing; maintain gas monitoring; evacuate personnel |
| CO₂ Device Failure | Device rupture or premature discharge | High | Pre-use inspection per manufacturer spec; certified equipment only; proper handling and storage; limit charge per device |
| Water Inrush | Fractures connect to aquifer causing water inrush | High | Hydrogeological survey before fracturing; avoid fracturing near known aquifers; install water-proofing plugs; monitor water flow |
| Roadway Collapse | Fracture-induced weakening causes roof/floor collapse | Medium-High | Reinforce roadway support before fracturing; monitor convergence; avoid fracturing near unsupported areas |
| Gas Accumulation | CO₂ or released methane accumulates in confined spaces | High | Maintain adequate ventilation; continuous gas monitoring; emergency ventilation systems; personnel gas detectors |
| Fracture Over-Extension | Fractures propagate into adjacent working areas or boundaries | Medium | Control CO₂ charge quantity; use borehole spacing design; monitor adjacent boreholes for pressure changes |
| Electrical Explosion | Ignition of gas by electrical equipment | Critical | Use explosion-proof equipment (Ex-rated); maintain electrical safety standards; prohibit non-Ex-rated devices in gas zones |
6.2 Safety Management System Integration
The technology must be implemented within a robust safety management framework:
- Pre-Operation Safety Assessment: Conduct a Job Safety Analysis (JSA) and permit-to-work review before each fracturing operation.
- Emergency Response Planning: Develop and drill emergency response procedures for gas outburst, water inrush, and equipment failure scenarios, per AQ 1026-2019 requirements.
- Personnel Training: All operators must complete certified training in liquid CO₂ fracturing technology, gas monitoring, and emergency response. Training records must be maintained per ISO 45001 requirements.
- Equipment Certification: All CO₂ fracturing devices, detonators, and monitoring instruments must carry valid inspection and certification marks, traceable to national metrology standards.
- Continuous Monitoring: Implement real-time monitoring of gas concentration, pressure, temperature, and structural stability during and after fracturing operations.
7. Application Scenarios Across the Company's Technology Routes
7.1 Integration with TIG/MIG Weld Overlay Capabilities
While liquid CO₂ fracturing technology operates in a distinct domain from weld overlay processes, the two capabilities intersect in several meaningful ways within the company's integrated service model:
- Equipment Hardfacing: The liquid CO₂ fracturing equipment, including injection devices, high-pressure cylinders, valves, and pipeline components, frequently requires hardfacing with wear-resistant alloys (such as Stellite 6, Cr-Cr, or Ni-based overlay welds) to resist abrasion and corrosion. The company's TIG/MIG weld overlay capabilities provide qualified hardfacing services for these critical components, extending equipment service life and ensuring operational reliability.
- WPS Qualification and Certification: The company's experience in Welding Procedure Specification (WPS) qualification per ASME Section IX, AWS D1.1, or ISO 15614 provides the methodological framework for qualifying weld overlay procedures on fracturing equipment components. This includes welder qualification per ASME Section IX or ISO 9606, and non-destructive testing (NDT) per ASTM E709 (MT) or ASME Section V (RT/UT).
- Quality Management Synergy: The rigorous quality management practices developed for weld overlay production (ISO 9001, EN 1090, ASME NQA-1) are directly transferable to the quality assurance programs for fracturing equipment manufacturing and maintenance.
7.2 Integration with Hydraulic Explosive Bonding Capabilities
The hydraulic explosive bonding (HEB) technology and liquid CO₂ fracturing technology share fundamental physical principles — both utilize controlled energy release to modify material properties:
- Energy Release Mechanism Understanding: The company's expertise in hydraulic explosive bonding, which involves the controlled release of hydraulic energy to achieve solid-state bonding of dissimilar metals, provides deep technical understanding of energy propagation, phase transitions, and material response under rapid loading conditions. This knowledge base directly supports the optimization of CO₂ phase-transition fracturing parameters.
- Equipment Fabrication: Hydraulic explosive bonding equipment includes high-pressure hydraulic systems, containment chambers, and precision alignment mechanisms. The company's fabrication capabilities for these systems — including clad plate/pipe fabrication for pressure vessels — can be leveraged for manufacturing CO₂ fracturing pressure vessels and high-pressure injection systems.
- NDT and Inspection: The non-destructive testing capabilities developed for HEB bond quality verification (ultrasonic testing per ASTM E164, magnetic particle testing per ASTM E709, and radiographic testing per ASTM E94) are directly applicable to inspecting the integrity of CO₂ fracturing equipment pressure boundaries and weldments.
7.3 Integration with Explosion Welding Capabilities
The explosion welding capability and liquid CO₂ fracturing technology share a common foundation in the controlled use of energy to achieve desired material outcomes:
- Explosion Energy Control: The company's expertise in explosion welding — which involves the precise control of detonation energy to achieve high-velocity impact bonding of metal surfaces — translates to the precise control of CO₂ expansion energy for optimal fracture formation in coal. Both processes require careful calibration of energy input to achieve desired material modification without causing unintended damage.
- Clad Pipe/Plate for Drainage Systems: The gas drainage systems in coal mines often require corrosion-resistant piping, particularly in high-sulfur or acidic gas environments. The company's explosion welding capabilities can produce clad pipes (e.g., 304L stainless steel cladding on carbon steel pipe per ASTM A270 or ASME B31.3) for gas drainage pipelines, ensuring long-term corrosion resistance and structural integrity.
- Process Safety Engineering: The safety protocols developed for explosion welding operations — including controlled initiation, personnel evacuation, blast containment, and post-process inspection — establish a safety culture and procedural framework that is directly transferable to liquid CO₂ fracturing operations, which also involve controlled energy release and personnel safety requirements.
7.4 Cross-Route Value Chain Integration
| Company Capability | Application to Liquid CO₂ Fracturing Technology | Value Delivered |
|---|---|---|
| TIG/MIG Weld Overlay | Hardfacing of fracturing equipment components; overlay repair of worn injection devices | Extended equipment life; reduced maintenance downtime |
| Hydraulic Explosive Bonding | Manufacturing of high-pressure HEB-type CO₂ injection systems; energy control expertise | Custom equipment fabrication; optimized process parameters |
| Explosion Welding | Production of clad drainage pipes; process safety protocols | Corrosion-resistant infrastructure; safety best practices |
| NDT Services | Inspection of fracturing equipment; verification of fracture formation | Quality assurance; safety verification |
| WPS Qualification | Qualification of welding procedures for fracturing equipment | Regulatory compliance; traceability |
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification and Certification Enhancement
The liquid CO₂ fracturing technology strengthens the company's qualification portfolio in several dimensions:
- Cross-Industry Competency: Demonstrates the company's capability to deliver technical solutions across multiple industrial sectors (metallurgical cladding and coal mine safety), enhancing credibility with diversified industrial clients.
- Process Safety Credentials: Establishes documented experience in controlled energy release processes, supporting the company's track record in process safety management — a prerequisite for many high-integrity industrial applications.
- Standards Compliance Track Record: Accumulates project experience demonstrating compliance with AQ 1026-2019, AQ 1053-2008, and other coal mine safety regulations, supporting future bids for regulated projects.
- ISO System Integration: Strengthens the company's ISO 9001, ISO 45001, and ISO 14001 management system implementations by incorporating diverse technical processes into the integrated management system framework.
8.2 Product and Service Delivery Enhancement
- Integrated Solution Packages: Enables the company to offer integrated packages combining cladding technology for equipment hardening with gas outburst prevention solutions, providing clients with a single-source solution for comprehensive mine safety and equipment durability.
- Equipment Lifecycle Management: Provides end-to-end service from equipment fabrication (clad pipes, hardfaced components) through deployment (fracturing operations) to maintenance (overlay repair, NDT inspection), creating recurring revenue streams.
- Technical Training Services: The study and learning of the Shimen Mine liquid CO₂ technology (as referenced in the original entry) represents a knowledge transfer capability that can be commercialized as a training service for mining clients seeking to adopt or optimize their own outburst prevention programs.
8.3 Customer Value Realization
| Customer Segment | Value Proposition | Expected Impact |
|---|---|---|
| Coal Mining Enterprises | Reduced outburst incidents; faster production progress; regulatory compliance | Improved safety record; increased annual output; reduced regulatory penalties |
| Mine Construction Contractors | Accelerated coal reveal operations; reduced downtime | Shorter project timelines; higher contract completion rates |
| Safety Engineering Firms | Access to advanced fracturing technology and expertise | Enhanced service offerings; competitive differentiation |
| Equipment Manufacturers | Hardfacing and cladding services for fracturing equipment | Extended equipment service life; reduced warranty claims |
9. Technical Learning and Continuous Improvement
The study and learning of the Shimen Mine liquid CO₂ fracturing technology (石门揭煤区液态CO₂致裂增透加速消突技术) represents a critical knowledge acquisition activity that feeds into the company's continuous improvement cycle:
- Case Study Analysis: Detailed post-project analysis of the Shimen Mine application, documenting actual performance parameters, challenges encountered, and solutions implemented.
- Parameter Optimization: Using field data from the Shimen Mine project to refine process parameter recommendations for different coal types and geological conditions.
- Technology Transfer: Developing standardized implementation guides and training materials based on the Shimen Mine experience for deployment at other mine sites.
- Research and Development: Identifying opportunities for technological enhancement, such as combining liquid CO₂ fracturing with other methods (water injection, electro-hydraulic fracturing) for synergistic permeability enhancement.
- Documentation and IP Protection: Compiling technical findings into proprietary process manuals, and evaluating opportunities for patent protection on innovative aspects of the technology application.
10. Conclusion
Liquid CO₂ fracturing and permeability enhancement technology for gas outburst prevention in coal reveal zones represents a high-value, safety-critical capability that complements the company's core cladding and metallurgical services. By leveraging the unique phase-transition properties of CO₂ to create controlled fracture networks in coal body, this technology delivers quantifiable improvements in gas drainage efficiency, outburst risk reduction, and mining production continuity.
The integration of this technology with the company's established capabilities in TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding creates a synergistic service portfolio that addresses the full lifecycle of industrial safety and equipment integrity. From the fabrication of clad drainage pipes and hardfaced fracturing equipment to the deployment of controlled energy release processes and the qualification of welding procedures per ASME Section IX and ISO 15614, the company provides a comprehensive, standards-compliant solution set.
The study and implementation of the Shimen Mine liquid CO₂ technology serves as a foundational case study that builds qualification credentials, enhances technical expertise, and creates a replicable model for deployment across the coal mining industry. This positions Cladding Technology Shanxi Co., Ltd as a multidisciplinary engineering partner capable of delivering integrated safety, equipment, and process solutions that maximize customer value while maintaining the highest standards of quality, safety, and regulatory compliance.