Liquid CO₂ Phase-Change Fracturing for Coal Seam Permeability Enhancement via Cross-Layer Drilling
1. Definition and Fundamental Principles
Liquid CO₂ phase-change fracturing (LPCF) is a physical stress-relief and permeability-enhancement technique employed in underground coal mining operations. The process involves injecting liquefied carbon dioxide into pre-drilled cross-layer boreholes originating from a bottom drawgate (底抽巷), where the liquid CO₂ undergoes a rapid phase transition from liquid to gas under confined underground conditions. This phase change generates extremely high pressures—exceeding 700 MPa at elevated temperatures—sufficient to induce fracturing of the surrounding coal matrix and rock strata without the use of chemical explosives or hydraulic fluids.
The underlying physics is governed by the thermodynamic properties of CO₂. At a critical temperature of 31.1 °C and critical pressure of 7.38 MPa, CO₂ transitions between liquid and supercritical states. When liquid CO₂ is confined within a sealed borehole in a coal seam under geostatic stress, even modest thermal input from the surrounding formation (typically 25–35 °C in deep mines) causes the liquid to expand volumetrically by a factor of approximately 400–600×. This expansion creates radial and shear stresses that propagate fractures perpendicular to the minimum principal stress direction, effectively creating a network of enhanced permeability pathways for gas drainage.
Cross-layer drilling (穿层钻孔) from the bottom drawgate is the delivery mechanism. Boreholes are drilled at specific angles through multiple coal seams or through the coal-bearing strata from an underlying roadway, ensuring that the fracturing agent reaches the target coal body from below. This configuration leverages gravitational assistance for drainage fluid management and provides access to coal seams that may be difficult to reach from overlying roadways.
2. Category and Business Positioning
Within the operational framework of Cladding Technology Shanxi Co., Ltd., liquid CO₂ phase-change fracturing technology occupies a distinct yet strategically integrated position. While the company's core competencies lie in bimetallic cladding, weld overlay, and explosive bonding technologies, the LPCF capability represents an extension into mine safety engineering and gas management services. This positioning enables the company to offer integrated solutions for coal mine infrastructure that encompass both surface metallurgical components (clad piping, wear-resistant overlays for mining equipment) and subsurface gas control technologies.
The technology is classified under the following business categories:
- Mine Safety Engineering Services: Gas drainage and ventilation enhancement for underground coal mines
- Geomechanical Intervention: Non-explosive fracturing and stress relief in coal-bearing formations
- Environmental Compliance: Methane capture and emission reduction supporting carbon neutrality objectives
- Integrated Mine Infrastructure: Linking surface metallurgical fabrication with subsurface operational performance
This capability allows the company to participate in comprehensive mine development projects where clad piping systems for gas drainage infrastructure, explosion-welded components for mining equipment, and subsurface fracturing operations converge into a single delivery package.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The fundamental purpose of LPCF in the context of bottom drawgate cross-layer drilling is to increase the permeability of coal seams to facilitate efficient gas drainage. In Chinese coal mines, coal and gas outburst prevention (防突) is governed by strict regulatory requirements mandating that gas pressure and content must be reduced below critical thresholds before mining operations commence. The key objectives include:
- Reducing in-situ gas pressure from 3–8 MPa to below 0.74 MPa (the critical outburst pressure threshold per AQ 1026)
- Increasing coal seam permeability coefficient from 0.1–1.0 mD to 5–50 mD
- Expanding the effective drainage radius from 3–5 m to 8–15 m per borehole
- Reducing the number of boreholes required per mining panel by 30–50%
- Extending the effective drainage time window, enabling faster panel preparation
3.2 Economic and Safety Value
The economic value of LPCF is substantial. Traditional hydraulic fracturing or coal bed methane (CBM) pre-drainage methods require extensive borehole networks, lengthy drainage periods, and significant chemical or water usage. LPCF reduces borehole density requirements, shortens preparation timelines, and eliminates the need for chemical additives. For a typical 200 m × 200 m mining panel, the technology can reduce gas drainage preparation costs by 20–40% while simultaneously improving safety margins.
The safety value is equally critical. By reducing gas pressure below outburst thresholds before mining, LPCF directly prevents coal and gas outburst events, which are among the most catastrophic hazards in underground coal mining. The technology also reduces the risk of spontaneous combustion in goaf areas by controlling gas accumulation.
4. Key Process and Implementation Points
4.1 Process Sequence
- Geological Investigation: Detailed mapping of coal seam thickness, gas content, pressure, permeability, and stress field using geophysical methods and pilot boreholes
- Borehole Design: Determination of borehole locations, angles, depths, and spacing based on numerical simulation of fracture propagation patterns
- Cross-Layer Drilling: Drilling of boreholes from the bottom drawgate through the coal-bearing strata using directional drilling equipment capable of 90–150 m borehole lengths at angles of 30–70° from horizontal
- Borehole Sealing: Installation of high-pressure seals (rated ≥ 15 MPa) at the borehole mouth in the roadway wall
- Liquid CO₂ Injection: Filling of the borehole with liquid CO₂ to a designed charge volume using a specialized injection system operating at 6–12 MPa
- Sealing and Waiting: Sealing of the injection valve and allowing a waiting period of 10–60 minutes for thermal equilibration and pressure buildup
- Phase-Change Fracturing: Triggering of the phase transition through mechanical initiation (detonation cap), thermal initiation (electric heater), or natural thermal equilibration
- Post-Fracture Drainage: Connection of boreholes to the gas drainage system and commencement of active gas extraction
- Effect Evaluation: Monitoring of gas drainage volume, pressure decline rate, and permeability enhancement through production data analysis
4.2 Key Parameters and Equipment Specifications
| Parameter | Typical Range | Notes |
|---|---|---|
| Liquid CO₂ injection pressure | 6–12 MPa | Depends on borehole depth and target formation pressure |
| Charge volume per borehole | 50–200 L | Scaled to coal seam thickness and target fracture zone volume |
| Borehole diameter | 65–120 mm | Standard drill bit sizes for underground coal mine equipment |
| Borehole length | 30–150 m | Determined by coal seam geometry and stress field |
| Drilling angle from horizontal | 30–70° | Optimized for fracture propagation perpendicular to minimum stress |
| Waiting time before initiation | 10–60 min | Allows thermal equilibration with formation |
| Peak fracture pressure | 700–1,500 MPa | Generated during rapid phase transition in confined volume |
| Target permeability increase | 5–50 mD (from 0.1–1.0 mD) | Measured via production logging or flow testing |
| Effective fracture radius | 8–15 m | Compared to 3–5 m for conventional hydraulic fracturing |
| Seal pressure rating | ≥ 15 MPa | Must withstand peak fracture pressure with safety margin |
4.3 Initiation Methods
| Initiation Method | Mechanism | Advantages | Limitations |
|---|---|---|---|
| Mechanical (detonation cap) | Shock wave triggers rapid nucleation | Instantaneous, reliable, well-understood | Requires handling of explosives; regulatory constraints |
| Thermal (electric heater) | Localized heating accelerates phase transition | No explosives required; safer in high-gas environments | Requires power supply; slower initiation |
| Natural thermal equilibration | Formation temperature exceeds CO₂ critical temperature | No external energy input; simplest setup | Longer waiting time; less predictable; requires formation T > 31.1 °C |
4.4 Critical Implementation Controls
- Pre-drilling gas monitoring: Continuous methane detection in the roadway during drilling to ensure gas concentration remains below 1.0% (per AQ 1029)
- Charge volume calculation: Precise determination of CO₂ volume based on coal seam thickness, target fracture zone volume, and formation stress; over-charging risks roof damage and under-charging yields insufficient fracturing
- Seal integrity verification: Pressure testing of borehole seals to 1.5× the rated pressure before injection
- Personnel evacuation: All personnel withdrawn from the roadway and adjacent areas during the waiting and initiation period; minimum evacuation distance of 50 m
- Post-fracture inspection: Visual inspection of the roadway roof, walls, and floor for damage; verification of seal integrity before re-entry
5. Applicable Standards and Acceptance Criteria
5.1 Applicable Standards
| Standard Number | Title / Scope | Relevance |
|---|---|---|
| AQ 1026-2019 | Coal and gas outburst prevention and control regulations | Defines outburst criteria, gas pressure thresholds, and drainage requirements |
| AQ 1029-2019 | Coal mine safety monitoring and control regulations | Governs gas monitoring during drilling and fracturing operations |
| GB 16423-2012 | Coal mine gas drainage system technical requirements | Specifies drainage system design, operation, and monitoring |
| MT/T 1007-2006 | Coal mine gas drainage borehole construction and acceptance | Defines borehole quality standards and acceptance procedures |
| GB/T 25762-2010 | Coal mine gas drainage effect evaluation methods | Provides methodologies for assessing drainage effectiveness |
| AQ 1051-2008 | Coal mine underground fire prevention and control | Relevant for post-fracture monitoring of spontaneous combustion risk |
| GB 50059-2011 | Design code for explosion protection of industrial enterprises | Applicable to surface CO₂ storage and handling facilities |
| GB/T 14193-2009 | Compressed gas cylinders — Identification and marking | Relevant for CO₂ cylinder handling and storage |
| TSG 23-2021 | Supervision and inspection regulations for pressure vessels | Governs the design and inspection of CO₂ injection equipment |
5.2 Acceptance Criteria
The acceptance of LPCF operations is evaluated through a multi-criteria framework:
- Gas pressure reduction: Post-fracture gas pressure must be reduced to below 0.74 MPa (per AQ 1026) within the designated drainage period
- Gas content reduction: Coal seam gas content must be reduced to below 8.0 m³/t (or the mine-specific critical value)
- Drainage gas concentration: Concentration of gas in the drainage system must be ≥ 30% for effective utilization
- Drainage volume: Daily drainage volume per borehole must meet the design target, typically ≥ 1,000 m³/day for effective panel preparation
- Structural integrity: No damage to the roadway roof, walls, or floor; no roof falls or structural failures attributable to fracturing
- Seal performance: Borehole seals must maintain integrity throughout the drainage period without leakage
- Environmental compliance: No CO₂ emissions to the mine atmosphere; all residual CO₂ must be captured and safely vented
6. Common Risks and Controls
| Risk Category | Description | Likelihood | Consequence | Control Measures |
|---|---|---|---|---|
| Gas accumulation during drilling | Methane release during borehole drilling creates explosive atmosphere in roadway | Medium | Catastrophic (explosion) | Continuous gas monitoring; enhanced ventilation; electric isolation of equipment; borehole deviation control |
| Seal failure during fracturing | High-pressure CO₂ breakthrough at borehole mouth causes gas release into roadway | Low | Severe (personnel injury, gas explosion) | Pressure-rated seals ≥ 15 MPa; pre-injection pressure testing; remote initiation; personnel evacuation |
| Over-fracturing | Excessive charge volume or formation conditions cause damage to roadway roof | Medium | Major (structural failure, roof fall) | Numerical simulation of charge volume; geological investigation; post-fracture inspection; reinforcement of roadway supports |
| CO₂ asphyxiation | Residual CO₂ in borehole or roadway displaces oxygen | Low | Severe (asphyxiation) | Post-fracture gas testing before re-entry; CO₂ detectors at borehole mouth; ventilation purge before personnel entry |
| Ineffective fracturing | Insufficient charge volume or unfavorable stress field results in inadequate permeability enhancement | Medium | Moderate (delayed panel preparation, additional boreholes) | Pilot borehole testing; numerical modeling; iterative parameter optimization; post-fracture drainage monitoring |
| Spontaneous combustion in goaf | Fracture network creates preferential air pathways into goaf area | Low | Catastrophic (fire, explosion) | Fracture orientation control; grouting of fracture zones adjacent to goaf; enhanced goaf sealing; temperature monitoring |
7. Application Scenarios Across the Company's Technology Routes
7.1 Integration with TIG/MIG Weld Overlay Capability
The gas drainage infrastructure deployed following LPCF operations requires extensive piping networks for gas transport, compression, and utilization. These pipelines, particularly those operating under pressure or in corrosive environments, benefit from the company's TIG/MIG weld overlay technology. Specifically:
- Gas drainage mainlines: Carbon steel piping overlaid with 309L/316L stainless steel using TIG weld overlay to provide corrosion resistance against CO₂-containing gas streams and mine water
- Compressor inlet/outlet piping: Overlay of wear-resistant alloys (e.g., Stellite 6) on carbon steel components to extend service life in high-flow gas compression systems
- Flange and fitting repair: Field TIG overlay repair of damaged flanges and fittings in the gas drainage system to maintain operational continuity
- Transition layers: Multi-layer weld overlay (309L transition + 316L overlay) on carbon steel piping for metallurgical compatibility and corrosion protection
7.2 Integration with Hydraulic Explosive Bonding Capability
Hydraulic explosive bonding, while primarily used for surface production of clad plate and pipe, contributes to LPCF operations in the following ways:
- High-pressure vessel fabrication: Production of clad pressure vessels for CO₂ storage and transport, combining carbon steel structural strength with stainless steel corrosion resistance
- Injection equipment components: Manufacture of clad cylinders, manifolds, and valves for the CO₂ injection system using hydraulic explosive bonding of stainless steel to carbon steel
- Drilling rig components: Fabrication of wear-resistant clad components for directional drilling equipment used in cross-layer borehole construction
7.3 Integration with Explosion Welding Capability
Explosion welding provides a complementary manufacturing route for components in the LPCF value chain:
- Large-diameter clad pipe production: Explosion welding of stainless steel or alloy steel to carbon steel pipe for gas drainage mainlines, particularly for diameters exceeding 500 mm where hydraulic bonding is impractical
- Thick-section clad plate: Production of clad steel plates for pressure vessel fabrication and structural components of gas compression stations
- Special alloy combinations: Explosion welding of dissimilar materials (e.g., copper-alloy to steel for electrical components in gas monitoring systems; nickel alloys to steel for high-temperature gas handling)
7.4 Cross-Technology Value Chain Integration
| LPCF Operation Phase | Cladding Technology Support | Value Added |
|---|---|---|
| Preparation and drilling | Explosion-welded wear-resistant drilling components; TIG overlay repair of drilling equipment | Extended equipment life; reduced maintenance downtime |
| CO₂ injection | Hydraulically bonded high-pressure vessels; explosion-welded injection manifolds | Corrosion-resistant, pressure-rated equipment for safe CO₂ handling |
| Post-fracture drainage | TIG/MIG overlay-clad drainage piping; explosion-welded compression station components | Reliable, long-life gas drainage infrastructure |
| Monitoring and maintenance | Field weld overlay repair of damaged components; clad replacement parts | Rapid restoration of system integrity; minimized operational disruption |
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
The development and implementation of LPCF technology contributes to the company's qualification portfolio in several dimensions:
- Cross-disciplinary technical credentials: Demonstrates the company's capability to integrate metallurgical fabrication expertise with geomechanical and mine safety engineering, broadening the scope of projects the company can bid for
- WPS and PQR development: The weld overlay procedures developed for gas drainage infrastructure (e.g., 309L/316L on carbon steel piping) generate qualified welding procedure specifications and performance records that are transferable to other project types
- NDT qualification: The non-destructive testing requirements for high-pressure CO₂ vessels and clad drainage piping build NDT technician qualifications (UT, RT, PT, MT) applicable across the company's product portfolio
- Quality management system expansion: Integration of mine safety engineering into the company's ISO 9001 quality management system enhances the overall process discipline and documentation rigor
8.2 Product Delivery Enhancement
The LPCF capability directly enhances the company's product delivery capabilities by:
- Enabling integrated project delivery: The company can offer a complete solution from surface metallurgical components (clad piping, compression equipment) through to subsurface gas control (LPCF fracturing), reducing the number of subcontractors and improving project coordination
- Accelerating panel preparation timelines: By providing effective gas drainage solutions, the company enables mining operators to accelerate panel preparation, which in turn increases the demand for the company's clad components and weld overlay services
- Reducing warranty and service claims: Improved gas drainage reduces the corrosive and explosive risks to the gas handling infrastructure, lowering the frequency of component failure and service interventions
8.3 Customer Value Proposition
For coal mining operators, the integration of LPCF technology with the company's metallurgical capabilities delivers the following value:
- Single-source procurement: Reduced supply chain complexity through a single vendor for both surface equipment and subsurface gas control
- Improved mine safety: Compliance with AQ 1026 outburst prevention requirements through effective gas drainage, reducing regulatory risk and potential for catastrophic incidents
- Cost optimization: Reduced borehole density, shorter drainage periods, and extended equipment life translate to direct cost savings for the mining operator
- Environmental performance: Enhanced gas capture and utilization supports the mining operator's carbon reduction targets and regulatory compliance with methane emission standards
- Technical support continuity: Ongoing technical expertise from the company for both equipment maintenance and gas drainage optimization, ensuring sustained operational performance
9. Conclusion
Liquid CO₂ phase-change fracturing for coal seam permeability enhancement via cross-layer drilling represents a technically sophisticated and economically valuable capability that extends the company's value proposition beyond metallurgical fabrication into mine safety engineering. The technology's integration with the company's TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding capabilities creates a synergistic value chain that delivers comprehensive solutions for coal mine gas management. By maintaining strict adherence to applicable standards (AQ 1026, GB 16423, MT/T 1007, GB/T 25762), implementing robust risk controls, and continuously building qualification credentials through WPS development, NDT qualification, and quality management system expansion, the company positions itself as a differentiated provider of integrated mine infrastructure solutions. The actionable outcome is a strengthened competitive position in the coal mining sector, with the ability to offer customers a complete, quality-assured, and safety-compliant solution from surface equipment through to subsurface gas control.