Liquid CO₂ Pre-Splitting Permeability Enhancement Technology for High-Gas Coal Mine Workfaces
1. Definition and Technical Principles
Liquid CO₂ pre-splitting permeability enhancement is an advanced in-situ rock engineering technology designed to increase the permeability of coal seams and surrounding rock masses in high-gas mining environments. The fundamental principle relies on the phase transition of liquid carbon dioxide (CO₂) from liquid to supercritical or gaseous state when injected into pre-drilled boreholes at elevated pressures (typically 15–25 MPa). Upon depressurization, the rapid expansion of CO₂ generates volumetric stresses exceeding the tensile strength of the coal matrix, creating a network of micro-fractures, fissures, and interconnected channels throughout the target zone.
The process exploits the unique thermophysical properties of CO₂: a critical temperature of 31.1°C and critical pressure of 7.38 MPa. When liquid CO₂ is injected at room temperature and sufficient pressure, it exists in a supercritical state upon depressurization, achieving a volumetric expansion ratio of approximately 500:1 relative to its liquid phase. This expansion energy is directed toward fracturing the coal mass in a controlled manner, creating what is termed a "pre-split zone" or "gas drainage enhancement zone" around the injection borehole.
Unlike conventional hydraulic fracturing, liquid CO₂ pre-splitting produces fracturing in a non-aqueous, non-mud system, which avoids pore plugging and water-sensitive coal swelling. The resulting fracture network is more uniform and extends further into the coal body, significantly enhancing gas drainage efficiency and reducing the risk of coal and gas outburst during mining operations.
2. Category and Business Positioning
Within the operational framework of Cladding Technology Shanxi Co., Ltd., this technology falls under the category of mine safety and gas control engineering services. While the company's core competencies center on bimetallic cladding and weld overlay manufacturing (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding), the liquid CO₂ pre-splitting technology represents a strategic extension into the mining safety and geotechnical engineering domain. This diversification positions the company as a multidisciplinary engineering solutions provider capable of addressing the full spectrum of underground mining challenges.
The Sijia Zhuang high-gas workface study represents a knowledge acquisition and capability development milestone. By studying and mastering this technology, the company builds technical qualifications that enable it to offer integrated solutions to coal mining enterprises — combining structural reinforcement (clad steel for mine support components) with gas control engineering (permeability enhancement for safe mining). This cross-disciplinary positioning creates significant competitive differentiation in the Chinese coal mining market, where high-gas and outburst-prone mines represent a growing segment demanding sophisticated mitigation technologies.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Gas Drainage Enhancement: Increase coal seam permeability by 2–5 orders of magnitude, enabling effective pre-mining gas drainage and reducing the working face gas concentration to below regulatory limits.
- Outburst Prevention: Discharge stored gas energy from the coal mass before mining, reducing the elastic energy index and gas pressure to levels below outburst thresholds.
- Permeability Stabilization: Create a stable fracture network that maintains permeability during the mining cycle, ensuring sustained gas drainage throughout the panel extraction period.
- Residual Gas Reduction: Lower the residual gas content in the mined-out zone, reducing the risk of delayed gas release into adjacent working areas.
3.2 Economic and Safety Value
- Prevention of outburst incidents that can result in fatalities, equipment destruction, and production stoppages costing millions of RMB per event.
- Reduction of mining time lost to gas-related safety interventions, typically improving panel productivity by 15–30%.
- Compliance with mandatory regulatory requirements under GB 16423-2020 (Safety Regulations for Coal Mines) and AQ 1026-2019 (Coal Mine Gas Outburst Prevention Regulations).
- Extension of the mine's operational life by enabling safe extraction of previously classified as high-risk coal seams.
4. Key Process and Implementation Points
4.1 Pre-Implementation Survey and Design
Successful implementation requires comprehensive geological and gas engineering characterization of the target coal seam. Key parameters include coal seam thickness, dip angle, gas content (typically 8–20 m³/t for high-gas seams), gas pressure (2–6 MPa), coal strength (single-axis compressive strength 5–20 MPa), and existing fracture density. The borehole layout is designed using numerical simulation (CFD and geomechanical coupling models) to ensure optimal coverage of the target zone.
4.2 Process Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Injection Pressure | 15–25 MPa | Calibrated to coal strength; must exceed fracture initiation threshold |
| Injection Volume per Borehole | 500–2000 L | Depends on target fracture zone volume |
| Borehole Diameter | 75–113 mm | Standard mining drill rig capability |
| Borehole Length | 30–120 m | Designed based on panel geometry and gas field extent |
| Injection Rate | 50–200 L/min | Controlled to prevent borehole breakout or uncontrolled fracturing |
| Depressurization Time | 5–30 seconds | Controls fracture propagation distance and intensity |
| CO₂ Injection Temperature | -20°C to +5°C (liquid state) | Maintained via thermal insulation and cryogenic storage |
| Target Permeability Increase | 10² to 10⁵ times | Measured via flow rate and pressure transient analysis |
| Effective Treatment Radius | 8–20 m | Radiating from borehole axis in radial direction |
4.3 Implementation Sequence
- Geological Survey: Conduct coal seam gas content testing, gas pressure measurement, and geological structure mapping using standard methods per AQ 1052-2008.
- Numerical Modeling: Perform coupled gas-geomechanical simulation to optimize borehole spacing, injection parameters, and treatment sequence.
- Borehole Drilling: Drill inclined or horizontal boreholes from the roadway into the target coal seam using standard mining drill rigs, with casing installation in unstable sections.
- Equipment Installation: Install pressure-rated injection valves, pressure gauges, flow meters, and safety relief devices at the borehole mouth.
- Liquid CO₂ Injection: Inject liquid CO₂ at controlled pressure and rate using cryogenic pumping systems, monitoring pressure response in real-time.
- Controlled Depressurization: Trigger rapid depressurization (valve opening) to initiate fracturing; the timing and rate are critical process control parameters.
- Post-Treatment Verification: Measure gas drainage rates, pressure decline curves, and permeability enhancement factors through pressure transient testing and flow testing.
- Gas Drainage Operation: Initiate or enhance the gas drainage system to extract the released gas through the newly created fracture network.
4.4 Critical Process Controls
- Pressure Monitoring: Real-time monitoring of injection pressure with automatic shutoff at maximum design pressure to prevent equipment failure.
- Temperature Control: Maintaining CO₂ in liquid state during transfer and injection requires cryogenic insulation; temperature monitoring at injection point is mandatory.
- Depressurization Timing: The depressurization event must occur at the designed pressure threshold; premature or delayed depressurization reduces fracture effectiveness or causes uncontrolled fracturing.
- Sequencing: Boreholes are treated in a designed sequence (typically from the panel center outward) to prevent stress interference between adjacent treatment zones.
- Personnel Safety: All personnel must evacuate the borehole area during injection and depressurization; CO₂ asphyxiation hazard requires gas detection and ventilation.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard Number | Title / Scope |
|---|---|
| GB 16423-2020 | Safety Regulations for Coal Mines — General requirements for gas management |
| AQ 1026-2019 | Coal Mine Gas Outburst Prevention Regulations — Outburst risk assessment and prevention |
| AQ 1052-2008 | Coal Mine Gas Detection and Monitoring Regulations — Gas content and pressure measurement |
| GB 50217-2018 | Code for Design of Coal Mine Ventilation Systems |
| AQ 1020-2006 | Coal Mine Gas Drainage Management Regulations |
| MT/T 1097-2010 | Coal Seam Gas Drainage Engineering Design Specifications |
| GB/T 26192-2011 | Coal Mine Safety Monitoring and Control System Technical Specifications |
5.2 Acceptance Criteria
- Working face absolute gas drainage rate must achieve design targets (typically ≥3000 m³/h for high-gas panels).
- Working face return air gas concentration must remain below 1.0% (absolute limit) and below 0.75% (design target).
- Coal seam gas content after treatment must be reduced to below 3.0 m³/t (per GB 16423-2020 for outburst-prone seams).
- Gas pressure in the treated zone must be reduced to below 0.74 MPa (outburst critical pressure threshold).
- Permeability enhancement factor must be verified at ≥100 times the original value through pressure transient analysis.
- All injection equipment must pass pressure testing at 1.5 times the maximum operating pressure per applicable pressure vessel codes.
6. Common Risks and Controls
| Risk Category | Description | Control Measures |
|---|---|---|
| CO₂ Asphyxiation | Accumulation of CO₂ in poorly ventilated areas displacing oxygen | Mandatory CO₂ gas detection; forced ventilation; personnel evacuation during injection; oxygen monitors at all access points |
| Equipment Overpressure | Failure of injection valves, piping, or connectors under high pressure | Pressure relief valves; pressure-rated components per ASME BPV Code Section I; regular NDT inspection of pressure parts |
| Uncontrolled Fracturing | Fractures extending into adjacent boreholes or roadways causing instability | Numerical pre-analysis; staged injection; real-time microseismic monitoring; borehole spacing optimization |
| Cryogenic Burn | Contact with liquid CO₂ (-78.5°C at atmospheric pressure) causing frostbite | Cryogenic PPE (gloves, face shields); insulated transfer lines; trained operators only |
| Gas Outburst During Treatment | Sudden gas release during depressurization event | Controlled depressurization rate; gas drainage system pre-activation; safety barriers in adjacent areas |
| Borehole Collapse | Fracturing-induced stress redistribution causing borehole or roadway failure | Pre-treatment geomechanical assessment; casing installation; post-treatment convergence monitoring |
7. Application Scenarios and Integration with Company Technology Routes
7.1 Direct Application: Mine Gas Control Engineering
The liquid CO₂ pre-splitting technology is directly applicable to high-gas and outburst-prone coal mining operations across China's major coal-producing regions (Shanxi, Shaanxi, Inner Mongolia, Henan). The Sijia Zhuang mine case study provides validated process parameters and operational experience that can be transferred to similar geological settings. Applications include:
- Pre-mining gas drainage enhancement for longwall panels in high-gas seams.
- Outburst prevention treatment for coal seams classified as Category II or III outburst risk per AQ 1026-2019.
- Residual gas management in gob areas to prevent delayed gas release into adjacent working faces.
- Permeability enhancement for CO₂ sequestration and enhanced coalbed methane (ECBM) recovery projects.
7.2 Integration with TIG/MIG Weld Overlay Route
The liquid CO₂ pre-splitting technology creates indirect but significant value through the company's TIG/MIG weld overlay capabilities. Mining operations require extensive infrastructure — gas drainage piping, ventilation ducts, hydraulic support systems, and underground equipment — that must withstand corrosive, high-pressure, and abrasive environments. The company's weld overlay technology provides:
- Corrosion-resistant overlay cladding on gas drainage piping systems using 309L/316L stainless steel transition layers, extending service life in CO₂-saturated environments.
- Wear-resistant overlay on borehole casing and drill components using hardfacing alloys (WC-Co, Cr-Cr₃C₂) per AWS D8.1 specifications.
- High-pressure valve and fitting repair through weld overlay restoration, meeting ASME B31.3 pressure piping code requirements.
- Specialty alloy cladding on underground equipment housings for resistance to methane and CO₂ corrosion, qualifying under NACE MR0175/ISO 15156.
7.3 Integration with Hydraulic Explosive Bonding Route
The hydraulic explosive bonding technology contributes to the mine gas control ecosystem through manufacturing of high-integrity bonded components:
- Production of aluminum/steel bonded pipe for lightweight gas drainage systems, combining aluminum's corrosion resistance with steel's structural strength.
- Manufacturing of copper/steel bonded flanges and connectors for gas detection and monitoring systems requiring electrical conductivity and mechanical integrity.
- Supply of titanium/steel clad components for cryogenic CO₂ storage and transfer equipment, leveraging titanium's corrosion resistance in supercritical CO₂ environments.
- Bonded plate production for pressure vessel components used in CO₂ injection systems, meeting ASME Section VIII Division 1 requirements.
7.4 Integration with Explosion Welding Route
Explosion welding provides specialized clad products for the most demanding applications in mine gas control:
- Explosion-welded steel/titanium clad pipe for high-pressure CO₂ injection lines operating at 25+ MPa, offering superior metallurgical bonding without dilution.
- Production of large-diameter clad pipe (up to 2000 mm OD) for surface gas processing facilities associated with mine gas drainage systems.
- Manufacturing of explosion-welded steel/nickel alloy clad components for sour gas handling systems where H₂S may be present alongside CH₄ and CO₂.
- Supply of clad sheet for fabricating pressure vessels and heat exchangers in surface gas treatment plants, qualifying per ASTM A491/A491M specifications.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Development
The study and mastery of liquid CO₂ pre-splitting permeability enhancement technology at the Sijia Zhuang high-gas workface contributes to the company's qualification portfolio in several critical ways:
- Cross-Disciplinary Engineering Credential: Demonstrates capability in mine safety engineering beyond traditional cladding and weld overlay, qualifying the company for integrated mine engineering contracts.
- Regulatory Compliance Expertise: Builds knowledge base in GB 16423-2020, AQ 1026-2019, and related mine safety regulations, enabling the company to serve as a qualified contractor for gas control projects requiring regulatory approval.
- Technical Transfer Capability: The documented study experience at Sijia Zhuang provides a reference case for technical proposals to other mine operators, demonstrating field-proven process knowledge.
- WPS Qualification Synergy: Understanding the operational environment (CO₂ exposure, high pressure, cryogenic temperatures) informs the development of specialized Welding Procedure Specifications for mine-specific applications, qualifying under ASME Section IX and AWS D1.1.
8.2 Customer Value Creation
For coal mining customers, this technology entry represents a significant value proposition:
- Integrated Solution Provider: Customers can source both gas control engineering services and the specialty clad/bonded components required for mine infrastructure from a single qualified supplier, reducing coordination complexity and schedule risk.
- Safety-Critical Component Supply: The company's understanding of the operational environment ensures that clad and bonded components are designed and manufactured to specifications appropriate for the actual service conditions (CO₂ exposure, pressure cycles, cryogenic temperatures).
- Technical Consulting: The company can provide integrated technical consulting that optimizes both the gas control system design and the material specifications for associated equipment, ensuring system-level performance rather than component-level compliance.
- Regulatory Navigation: Combined expertise in mine safety regulations and pressure equipment codes (ASME, NB/T 47003) enables the company to assist customers in navigating complex regulatory approval processes for gas control projects.
9. Conclusion
The liquid CO₂ pre-splitting permeability enhancement technology represents a strategically valuable addition to the company's capability portfolio. While originating from coal mine gas control engineering, it creates synergistic opportunities across all three of the company's core technology routes — TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding. The technology deepens the company's understanding of the operating environments for which its clad and bonded products are specified, enabling more precise material selection, welding procedure development, and quality assurance. This cross-pollination of technical knowledge strengthens the company's position as a multidisciplinary engineering solutions provider capable of delivering integrated, safety-critical solutions to the mining industry.
Future development priorities should include: formal qualification testing of CO₂-resistant overlay alloys under simulated mine conditions; development of specialized WPS for cryogenic CO₂ service applications; and establishment of a mine safety engineering division that leverages the company's metallurgical expertise to provide end-to-end solutions for high-gas mining operations.