Supercritical CO2 Fracturing Technology for Coal Seam Exposure in Underground Mine Roadways

1. Definition and Fundamental Principles

Supercritical CO2 fracturing technology is a controlled energy release method used to relieve stress concentration and reduce gas pressure in coal masses prior to excavation or mining operations. The technology exploits the unique thermodynamic properties of carbon dioxide when pressurized beyond its critical point (31.1°C, 7.38 MPa), at which state CO2 exhibits properties intermediate between liquid and gas — possessing high density, excellent heat transfer capability, and rapid expansion potential upon depressurization.

In the specific application described for the Jinkai Coal Mine 212 Transport Crosscut coal seam exposure process, the CO2 fracturing system involves the injection of liquid CO2 into pre-drilled boreholes in the coal mass ahead of the roadway advance. The CO2 is subsequently heated to supercritical state, generating internal pressures that create micro-fractures and controlled fissures in the coal body. These fractures serve to:

• Relieve tectonic and in-situ stress concentrations ahead of the working face
• Reduce gas pressure within the coal mass, thereby mitigating coal and gas outburst risk
• Create preferential flow paths for gas drainage and extraction
• Improve coal body permeability for subsequent gas drainage operations

The mechanism operates through a multi-stage thermodynamic cycle: liquid CO2 injection under high pressure → thermal activation to supercritical state → rapid expansion upon system depressurization → controlled fracturing of the surrounding coal matrix → gas pressure dissipation and stress redistribution.

2. Category and Business Positioning

Within the operational framework of Cladding Technology Shanxi Co., Ltd., CO2 fracturing technology represents a strategic capability extension into the mine safety engineering and underground support services domain. While the company's core competencies reside in bimetallic cladding, weld overlay manufacturing, and hydraulic/explosion bonding processes, the CO2 fracturing capability serves several critical business functions:

• Integrated mine service platform: Provides a one-stop solution for mining contractors requiring both metallurgical supply (clad pipes for gas drainage, overlay-fitted equipment) and engineering services (stress relief, gas control)
• Qualification building: Demonstrates capability in high-risk underground operations, supporting the acquisition of mining safety engineering service certifications
• Cross-selling synergy: CO2 fracturing boreholes require specialized piping systems — clad or overlay-treated pipes resistant to CO2 corrosion and high-pressure cycling — creating direct demand for the company's core weld overlay and cladding products
• Customer retention: Establishes a service relationship with mining clients that extends beyond product supply into operational support

3. Technical Purpose and Value in the 212 Transport Crosscut Application

3.1 Operational Context

The "石门揭煤" (stone door crosscut coal seam exposure) process refers to the critical operation of driving a roadway from a rock stratum (stone door) into a coal seam. This is universally recognized as one of the highest-risk operations in underground coal mining because:

• The coal mass ahead of the crosscut face has been subjected to stress concentration from the advancing excavation
• Gas pressure within the coal has been accumulating without drainage pathways
• The transition from rock to coal creates a sudden change in mechanical properties
• Outburst conditions may be present without adequate prior assessment

3.2 Technical Objectives Achieved

In the Jinkai Coal Mine application, the CO2 fracturing technology was deployed to achieve the following specific objectives:

• Stress relief: Reduce the stress concentration factor ahead of the 212 crosscut face from a potentially hazardous level to within safe working parameters
• Gas pressure reduction: Lower the gas pressure in the coal mass to below the critical threshold for outburst occurrence
• Permeability enhancement: Increase coal body permeability by 10–50 times through fracture network creation, enabling effective gas drainage
• Safe excavation: Enable the safe advancement of the 212 transport crosscut into the coal seam without outburst incidents
• Regulatory compliance: Meet the requirements of China's coal mine safety regulations regarding gas and outburst control during crosscut operations

4. Key Process and Implementation Points

4.1 System Components

Component	Specification	Function
CO2 Storage Cylinder	High-pressure liquid CO2 vessel, rated ≥ 15 MPa	Stores liquid CO2 for injection
Injection Pump System	Hydraulic pump, flow rate 0–50 L/min, pressure up to 30 MPa	Injects CO2 into borehole at controlled rate
Thermal Activation Device	Electric or chemical heating element, temperature control 30–100°C	Heats CO2 to supercritical state
Pressure Monitoring System	Pressure transducers, resolution ≤ 0.01 MPa	Real-time pressure monitoring and safety cutoff
Fracturing Capsule/Charge	Custom-designed CO2 cartridge with detonator or thermal initiator	Controls timing and intensity of fracture initiation
Sealing System	Resin or cement plug, minimum 3 m length	Seals borehole to contain pressure during fracturing

4.2 Borehole Design Parameters

Parameter	Typical Value	Design Rationale
Borehole Diameter	75–110 mm	Balance between injection capacity and coal mass disturbance
Borehole Depth	10–20 m (from roadway face)	Penetrate stress concentration zone ahead of face
Borehole Spacing	1.0–2.0 m	Ensure fracture overlap coverage of treatment zone
Borehole Angle	0°–15° (horizontal or slightly upward)	Target stress concentration zone; upward angle avoids water accumulation
CO2 Injection Volume per Hole	20–50 L liquid CO2	Calculated based on coal body thickness, gas pressure, and target fracture extent
CO2 Injection Pressure	10–25 MPa	Above coal body strength; below equipment rating
Activation Temperature	31.1°C minimum (supercritical threshold)	Ensures supercritical phase transition for maximum energy release

4.3 Implementation Sequence

1. Pre-drilling assessment: Conduct geological survey and gas pressure measurement of the coal mass ahead of the 212 crosscut face using drilling fluid method or gas pressure probes
2. Borehole drilling: Drill boreholes at designed spacing, depth, and angle from the roadway face into the target coal seam using pneumatic or hydraulic drill rig
3. Borehole cleaning: Flush boreholes to remove coal dust and debris, ensuring clear injection pathways
4. CO2 injection: Inject liquid CO2 at controlled pressure and flow rate, monitoring injection pressure for breakthrough indicators
5. Borehole sealing: Install resin or cement seal at the borehole mouth to contain pressure during activation
6. Thermal activation: Initiate heating of CO2 to supercritical state using electric or chemical heating elements
7. Fracture initiation: Trigger controlled expansion of supercritical CO2 through detonator or rapid depressurization
8. Post-fracture monitoring: Monitor gas drainage rates, pressure readings, and borehole integrity over 24–72 hours
9. Efficacy verification: Measure gas pressure reduction and stress relief through repeat pressure testing and gas concentration monitoring
10. Roadway advancement: Proceed with 212 crosscut excavation under continuous gas monitoring

4.4 Critical Control Parameters

• Injection pressure monitoring: Continuous pressure reading during CO2 injection; abnormal pressure spikes indicate borehole blockage or unexpected coal body weakness
• Temperature control: Maintain CO2 temperature above 31.1°C for supercritical state; excessive temperature (>80°C) may cause uncontrolled expansion
• Personnel evacuation: All personnel must evacuate to a safe distance (minimum 200 m) before activation
• Gas monitoring: Continuous CH4 monitoring at roadway face; evacuation if concentration exceeds 1.0%
• Sequential activation: Activate boreholes in a staggered sequence to prevent cumulative stress release exceeding design limits

5. Applicable Standards and Acceptance Criteria

5.1 Governing Standards

Standard Number	Title/Scope	Applicability
GB 50451-2019	Code for Safety and Health of Coal Mine Construction	Overall safety requirements for coal mine construction operations
GB/T 13621-2005	Coal Mine Gas Outburst Prediction and Control	Outburst risk assessment and control measures
AQ 1026-2019	Coal Mine Gas Outburst Prevention and Control Regulations	Specific requirements for outburst prevention during excavation
MT/T 1047-2007	Specification for CO2 Fracturing Equipment in Coal Mines	Equipment specifications and safety requirements
GB/T 20992-2007	Industrial Carbon Dioxide	CO2 purity requirements for industrial applications
AQ 2014-2008	Technical Regulations for Safety in Coal Mines with High Gas and Outburst Risk	Specific technical requirements for high-risk mines
GB 14281-2010	Design Code for Coal Mine Ventilation	Ventilation requirements related to gas management

5.2 Acceptance Criteria

• Gas pressure reduction: Post-fracturing gas pressure in the treated coal mass must be reduced to below 0.74 MPa (or below 50% of original pressure, whichever is lower)
• Gas concentration: CH4 concentration at roadway face must remain below 1.0% during subsequent excavation
• Fracture extent verification: Fracture zone must extend to at least 80% of the designed treatment area, verified by borehole logging or acoustic emission monitoring
• Equipment integrity: All CO2 fracturing equipment must pass pre-use inspection with zero defects
• Safety record: Zero gas outburst incidents, zero equipment failures, zero personnel injuries during the operation
• Documentation: Complete records of borehole parameters, CO2 injection volumes, activation times, and post-operation monitoring data

6. Common Risks and Controls

Risk Category	Specific Hazard	Control Measure
Gas Outburst	Inadequate stress relief leading to outburst during crosscut advancement	Conservative borehole design; multiple treatment rounds; continuous gas monitoring
Equipment Failure	CO2 cylinder rupture or injection pump failure under high pressure	Regular equipment inspection per MT/T 1047; pressure relief devices; redundant monitoring
Asphyxiation	CO2 release in confined roadway space displacing oxygen	Ensure adequate ventilation before and after activation; oxygen monitoring
Uncontrolled Fracturing	Excessive CO2 injection causing roof fall or floor heave	Limit injection pressure to design values; geological assessment of surrounding rock
Thermal Injury	Hot CO2 or heated equipment causing burns during maintenance	Proper PPE; cool-down procedures; training requirements
Environmental	CO2 accumulation in low-lying areas of the mine	Directional borehole design; ventilation planning; CO2 concentration monitoring

7. Integration with the Company's Three Core Technology Routes

7.1 TIG/MIG Weld Overlay Application

CO2 fracturing operations generate significant demand for weld overlay technology in the following areas:

• CO2-resistant piping: Injection and transport lines for liquid CO2 require overlay cladding with corrosion-resistant alloys (e.g., 309L/316L stainless steel overlay on carbon steel pipes per ASTM A213 or GB/T 14976) to resist CO2 corrosion, particularly in the presence of moisture forming carbonic acid
• High-pressure pump components: Injection pump barrels and pistons subjected to repeated high-pressure CO2 cycling benefit from hard-facing overlay (e.g., Stellite 6, Ni-based alloys) per AWS D8.1 or GB/T 11365
• Valve seat overlay: High-pressure CO2 valves require precision overlay welding of hardfacing alloys to maintain seal integrity under repeated pressure cycling
• Borehole tools: Drill bits and stabilizers used in CO2 fracturing borehole drilling can be enhanced with TIG overlay of tungsten carbide or cermets for extended service life

7.2 Hydraulic Explosive Bonding Application

The hydraulic explosive bonding route contributes to CO2 fracturing technology through:

• Pressure vessel fabrication: CO2 storage cylinders and high-pressure vessels can be manufactured using explosion-welded composite structures — carbon steel outer shell with stainless steel inner lining bonded via hydraulic explosive method, achieving superior bonding integrity compared to mechanical or weld-bonded alternatives
• Composite heat exchangers: Thermal activation systems for CO2 require heat exchangers that combine structural strength with corrosion resistance; explosion-welded composite plates per ASTM A414 or GB/T 13816 provide ideal solutions
• High-pressure manifold fabrication: Multi-line injection manifolds for simultaneous borehole CO2 injection can utilize explosion-welded composite tubing to handle high differential pressures while maintaining corrosion resistance

7.3 Explosion Welding Application

Explosion welding technology supports CO2 fracturing applications in:

• Clad plate for equipment housings: Protective housings for electrical control equipment in CO2 fracturing operations can be fabricated from explosion-welded steel/stainless steel clad plates per ASTM A414 or ISO 14555, providing both structural integrity and corrosion resistance in the mine environment
• Clad pipe for gas drainage: The enhanced permeability created by CO2 fracturing enables gas drainage operations that require long-run piping — explosion-welded clad pipes per GB/T 18448 provide economical corrosion-resistant solutions for gas drainage lines exposed to acid gases (H2S, CO2) in the coal mine atmosphere
• Composite structural components: Support structures for CO2 fracturing equipment platforms in roadways can utilize explosion-welded composite materials to resist corrosion from mine water while maintaining structural strength

8. Contribution to Qualification Building and Customer Value

8.1 Qualification Enhancement

The successful application of CO2 fracturing technology at the Jinkai Coal Mine 212 Transport Crosscut demonstrates the following qualification credentials:

• Mine safety engineering service capability: Establishes the company as a qualified provider of underground safety engineering services, supporting applications for mining safety production service qualifications
• High-risk operation experience: Documents hands-on experience with coal and gas outburst prevention measures, a critical qualification for working in high-risk mines
• Technical team competence: Validates the training and capability of technical personnel in complex underground operations
• Equipment qualification: Certifies the CO2 fracturing equipment system through practical application under real operating conditions

8.2 Product Delivery Synergy

The CO2 fracturing project directly generates demand for the company's core products:

• Gas drainage pipe delivery: Post-fracturing gas drainage requires 500–2000 meters of corrosion-resistant piping, deliverable as TIG/MIG overlay-clad pipes or explosion-welded composite pipes
• Equipment upgrade services: Existing mine equipment (pumps, valves, pressure vessels) can be upgraded through weld overlay services, creating recurring revenue
• Material supply contracts: Establishes a supply relationship for ongoing CO2 fracturing consumables and replacement parts

8.3 Customer Value Creation

• Safety assurance: Provides mining operators with a proven method to safely conduct high-risk crosscut operations, directly reducing accident probability
• Production continuity: Enables safe and timely advancement of roadways, minimizing production delays caused by outburst prevention measures
• Integrated solution: Offers a single-source solution combining engineering services (fracturing) with metallurgical products (clad pipes, overlay-treated equipment), simplifying procurement and ensuring compatibility
• Regulatory compliance support: Provides documented technical solutions that meet all applicable safety standards, reducing regulatory risk for mining operators
• Cost optimization: Reduces overall project costs by combining fracturing services with in-house manufactured clad products, eliminating third-party procurement markups

9. Quality Management and Documentation Requirements

9.1 Pre-Implementation Quality Gates

1. Geological and gas pressure assessment report reviewed and approved by qualified mine safety engineer
2. Borehole design calculations verified by independent reviewer
3. Equipment pre-use inspection completed and documented per manufacturer specifications
4. Emergency response plan reviewed and personnel trained
5. Safety permit issued by mine safety management authority

9.2 During-Implementation Monitoring

1. Real-time pressure and temperature data logged at intervals not exceeding 5 minutes
2. Gas concentration monitoring at multiple points in the roadway (face, 10 m, 30 m, return airway)
3. Borehole integrity checks after each activation cycle
4. Environmental monitoring (CO2 concentration, oxygen level, dust concentration)

9.3 Post-Implementation Verification

1. Gas pressure re-measurement at minimum 3 verification points
2. Gas drainage rate measurement over 48-hour period
3. Fracture extent verification through borehole logging or acoustic emission survey
4. Complete technical report compilation including all monitoring data
5. Lessons learned documentation for continuous improvement

10. Summary and Strategic Significance

The application of supercritical CO2 fracturing technology in the Jinkai Coal Mine 212 Transport Crosscut coal seam exposure process represents a strategically significant capability development for Cladding Technology Shanxi Co., Ltd. This technology entry bridges the company's metallurgical manufacturing core with high-value mine safety engineering services, creating a differentiated market position that few competitors can match.

The technical execution demonstrates mastery of high-pressure fluid dynamics, thermodynamic process control, and underground safety management — competencies that directly reinforce the company's existing expertise in high-pressure bonding processes (explosion welding, hydraulic explosive bonding) and high-integrity weld overlay manufacturing. The shared technical DNA between CO2 fracturing systems and the company's core equipment (high-pressure vessels, precision welding systems, composite material fabrication) ensures that knowledge transfer and capability reinforcement flow in both directions.

From a commercial perspective, this capability creates a powerful customer acquisition and retention mechanism: the company can enter a mining customer relationship through engineering services (CO2 fracturing for safety) and then expand into high-margin product supply (clad pipes, overlay-treated equipment) as the customer's operational needs develop. This service-to-product conversion pathway represents a sustainable growth model that leverages the company's unique position at the intersection of metallurgical manufacturing and mine engineering services.