CO₂ Fracturing and Permeability Enhancement Technology for Low-Permeability Coal Seams
1. Definition and Fundamental Principles
CO₂ fracturing and permeability enhancement technology is an advanced coalbed methane (CBM) reservoir stimulation method designed to increase the effective permeability of low-permeability coal seams—those with initial permeability typically below 1.0 × 10⁻³ μm²—by injecting supercritical or subcritical carbon dioxide under controlled pressure conditions. The technology leverages the unique physical and chemical properties of CO₂ to generate hydraulic fractures, shear fractures, and micro-fracture networks within the coal matrix, thereby creating new flow channels for methane desorption and drainage.
The fundamental mechanism operates on three interrelated principles:
- Phase Transition Energy Release: When high-pressure CO₂ (typically injected at 15–25 MPa) enters the coal seam and encounters lower-pressure zones, it undergoes a rapid phase transition from supercritical to gaseous state. This expansion generates localized pressure differentials that exceed the coal's tensile and shear strength, initiating fracture propagation.
- Swelling and Stress Relaxation: CO₂ adsorbs onto the coal matrix, causing matrix swelling in the direction of minimum principal stress. This differential swelling induces stress redistribution, weakening the coal structure and promoting secondary fracture development.
- Chemical Interaction and Methane Displacement: CO₂ has a higher adsorption affinity for coal than methane (CH₄). Upon contact, CO₂ displaces adsorbed methane from micropores, creating an additional driving force for gas desorption and drainage while simultaneously reducing the effective stress in the coal mass.
The Hongfa Coal Mine trial study represents a systematic investigation into the optimization of these mechanisms under specific geological conditions, including coal seam thickness (typically 1.5–3.5 m), burial depth (600–1200 m), in-situ stress conditions (horizontal stress ratio λ = 1.1–1.4), and initial gas content (8–14 m³/t).
2. Category and Business Positioning
Within the broader portfolio of Cladding Technology Shanxi Co., Ltd., this technology entry represents the company's extension into coal mine safety and gas control engineering services. While the company's core competencies reside in bimetallic cladding, weld overlay, and explosion welding, the coal mine technology domain serves as a critical adjacent market that:
- Expands Service Revenue: Provides high-value engineering consulting and technical implementation services to coal mining enterprises in Shanxi Province and beyond.
- Strengthens Customer Relationships: Establishes deeper technical partnerships with coal mining clients who are also potential users of cladding products for mining equipment (e.g., hardfacing of cutting tools, wear-resistant overlay on conveyor systems).
- Demonstrates Engineering Capability: Validates the company's ability to conduct complex field trials, manage multi-disciplinary technical projects, and deliver measurable performance improvements.
This technology falls under the category of coal mine gas drainage and disaster prevention engineering, specifically within the sub-domain of reservoir permeability enhancement for coalbed methane recovery and outburst prevention. It aligns with national strategies for coal mine safety production, carbon neutrality, and energy efficiency.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The CO₂ fracturing technology for low-permeability coal seams at Hongfa Coal Mine was developed to address the following critical challenges:
- Gas Drainage Efficiency: Increase the effective drainage radius from the typical 5–8 m (in untreated low-permeability seams) to 15–25 m, enabling adequate pre-drainage before mining operations.
- Permeability Improvement: Achieve a permeability enhancement factor of 3–8× the initial value through controlled fracture network creation.
- Outburst Risk Mitigation: Reduce coal and gas outburst risk by pre-releasing gas energy and creating stable drainage pathways.
- CBM Recovery Rate: Increase coalbed methane recovery rate from below 30% to above 50%, contributing to renewable energy production and methane emission reduction.
3.2 Quantitative Value Metrics
| Performance Indicator | Pre-Treatment (Baseline) | Post-Treatment (Target) | Improvement Factor |
|---|---|---|---|
| Effective Permeability (× 10⁻³ μm²) | 0.3–0.8 | 2.4–6.0 | 3–8× |
| Drainage Gas Concentration (%) | 15–25 | 45–65 | 2–3× |
| Drainage Gas Flow Rate (m³/min) | 0.5–1.5 | 3.0–8.0 | 3–5× |
| Effective Drainage Radius (m) | 5–8 | 15–25 | 2–3× |
| Pre-Drainage Time Required (days) | 60–90 | 25–45 | 2–3× reduction |
| CBM Recovery Rate (%) | < 30 | > 50 | Significant |
4. Key Process and Implementation Points
4.1 Overall Process Flow
The CO₂ fracturing and permeability enhancement process follows a structured sequence of operations, each requiring precise control and monitoring:
- Pre-Drilling Preparation: Borehole drilling and casing installation
- Plug Setting and Zoning: Establishment of isolation plugs to define treatment zones
- Initial Pressure Testing: Determination of injection pressure thresholds
- CO₂ Injection Phase 1 (Pre-Pressurization): Low-rate CO₂ injection for matrix swelling
- CO₂ Injection Phase 2 (Fracturing): High-rate, high-pressure CO₂ injection for fracture initiation
- Pressure Holding and Soaking: Maintenance of injection pressure for fracture propagation
- Pressure Release and Monitoring: Controlled depressurization with continuous data acquisition
- Post-Treatment Evaluation: Permeability testing and drainage performance verification
4.2 Critical Process Parameters
| Process Step | Parameter | Typical Range | Control Requirement |
|---|---|---|---|
| Pre-Pressurization | Injection Pressure | 8–12 MPa | Gradual ramp-up, rate ≤ 0.5 MPa/min |
| Pre-Pressurization | Injection Rate | 50–150 L/min | Stable flow, no surges |
| Pre-Pressurization | Duration | 30–60 min | Until pressure stabilization |
| Fracturing Injection | Peak Injection Pressure | 18–25 MPa | Based on coal strength and in-situ stress |
| Fracturing Injection | Injection Rate | 300–800 L/min | Sufficient to overcome fracture initiation threshold |
| Fracturing Injection | Total CO₂ Volume | 2,000–5,000 L per zone | Calculated based on seam geometry |
| Pressure Holding | Holding Pressure | 12–18 MPa | 60–80% of peak pressure |
| Pressure Holding | Holding Duration | 2–6 hours | Until pressure decline rate < 0.1 MPa/h |
| Depressurization | Release Rate | 0.5–1.0 MPa/min | Controlled, no sudden decompression |
4.3 Equipment Configuration
The Hongfa Coal Mine trial employed a specialized CO₂ fracturing system comprising the following key components:
- High-Pressure Pump Unit: Maximum capacity of 30 MPa, flow rate 0–1000 L/min, with precision pressure control (±0.1 MPa accuracy)
- CO₂ Storage and Vaporization System: Liquid CO₂ storage cylinders (50 L each) with controlled vaporization to maintain injection temperature at 20–40°C
- Real-Time Monitoring System: Multi-channel pressure, flow, and temperature sensors with data acquisition at 1 Hz sampling rate
- Plug System: Hydraulic or cement-based isolation plugs providing sealing integrity above 20 MPa differential pressure
- Safety Interlock System: Automated shutdown triggers for overpressure, temperature excursion, or flow anomaly detection
4.4 Implementation Sequence for Hongfa Coal Mine Trial
The trial was conducted on the 3# coal seam at a depth of approximately 850 m, with the following specific implementation parameters:
- Borehole Configuration: 70 mm diameter boreholes, length 150–200 m, inclination angle 5°–10° from horizontal
- Plug Spacing: Treatment zones of 20–30 m between isolation plugs
- Coal Properties: Ash content 12–18%, volatile matter 35–42%, compressive strength 15–25 MPa, initial permeability 0.4–0.7 × 10⁻³ μm²
- In-Situ Stress: Maximum horizontal stress 18–22 MPa, minimum horizontal stress 14–18 MPa, vertical stress 22–26 MPa
- Fracture Orientation: Targeted perpendicular to minimum horizontal stress direction for maximum effective drainage cross-section
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
The CO₂ fracturing and permeability enhancement technology must comply with the following national and industry standards:
- GB 50451-2019 — Code for Design of Coal Mine Gas Drainage System
- GB 50215-2015 — Code for Design of Coal Mine Ventilation
- AQ 1026-2019 — Safety Specification for Coal Mine Gas Drainage
- AQ 1027-2007 — Safety Specification for Coal Mine Gas and Dust Outburst Prevention
- MT/T 1007-2006 — Specification for Coal Mine Gas Drainage Engineering Design
- MT/T 1104-2011 — Coal Mine Gas Drainage Borehole Construction and Acceptance Specification
- GB/T 16483-2008 — Terminology of Coal Mine Safety
- DL/T 5396-2014 — Technical Specification for CO₂ Injection into Deep Underground
- SY/T 6503-2016 — Well Integrity Management Specification (applicable to injection well integrity)
- ISO 14001:2015 — Environmental Management Systems (for CO₂ handling and emission control)
5.2 Acceptance Criteria for Treatment Effectiveness
| Acceptance Item | Criteria | Verification Method |
|---|---|---|
| Permeability Enhancement | Effective permeability ≥ 3× initial value | Flow testing (constant pressure/flow method) |
| Drainage Gas Concentration | ≥ 30% after 7 days of post-treatment drainage | Continuous gas concentration monitoring |
| Drainage Gas Flow Rate | ≥ 2.0 m³/min sustained for ≥ 30 days | Flow meter readings at borehole outlet |
| Effective Drainage Radius | ≥ 15 m confirmed by cross-borehole testing | Tracer gas or pressure interference testing |
| Plug Integrity | No leakage at 1.5× maximum injection pressure | Pressure test with 30 min hold time |
| Wellbore Integrity | No casing deformation or seal failure | Casing inspection (downhole camera or pressure test) |
| Safety Compliance | Zero gas explosion incidents during operation | Safety audit and incident reporting |
5.3 Environmental and Safety Standards
- GB 16297-1996 — Comprehensive Emission Standard of Air Pollutants (CO₂ handling)
- GB 12348-2008 — Emission Standard for Industrial Enterprises Noise (equipment operation)
- AQ 1029-2019 — Safety Specification for Coal Mine Underground Fire Prevention
- GB 30871-2022 — Safety Specification for Special Operations in Hazardous Chemical Enterprises (applicable to high-pressure gas operations)
6. Common Risks and Controls
6.1 Technical Risks
| Risk Category | Specific Risk | Probability | Impact | Mitigation Measures |
|---|---|---|---|---|
| Fracture Geometry | Fractures propagate in unintended direction (not perpendicular to σmin) | Medium | Reduced effective drainage area | Pre-analyze in-situ stress field; adjust injection rate and volume; use multi-stage injection |
| Plug Failure | Isolation plug leaks or fails under high pressure | Low-Medium | Loss of injection pressure; treatment zone contamination | Use dual-plug configuration; conduct pre-injection pressure test; maintain 1.5× safety factor on plug rating |
| Matrix Swelling Excess | Excessive CO₂ adsorption causes coal matrix swelling that closes existing fractures | Medium | Reduced permeability after initial enhancement | Control pre-pressurization phase duration; optimize CO₂ injection temperature; consider hybrid N₂/CO₂ injection |
| Temperature Effects | Joule-Thomson cooling during CO₂ expansion causes low temperatures near injection point | High | Equipment damage; ice formation; coal thermal cracking | Control injection temperature at 20–40°C; use heated injection lines; monitor downhole temperature |
| Fracture Closure | Fractures close due to stress recovery after depressurization | Medium | Temporary rather than permanent permeability improvement | Use proppant (sand or ceramic) in hybrid fracturing; maintain residual pressure; design for shear fracture dominance over tensile fracture |
6.2 Safety Risks
- CO₂ Asphyxiation Risk: In confined underground spaces, CO₂ accumulation can displace oxygen below 19.5%. Control measures include continuous atmospheric monitoring, forced ventilation, and emergency escape procedures.
- High-Pressure Equipment Failure: Bursting of high-pressure hoses, valves, or connections can cause physical injuries. Controls include regular equipment inspection per TSG 21-2016 (Supervision Regulation for Safety of Pressure Vessels), pressure relief valve installation, and exclusion zones during injection operations.
- Coal and Gas Outburst During Treatment: Sudden release of coal and gas during or after fracturing. Mitigation includes controlled injection rate ramp-up, real-time pressure monitoring with automated shutdown, and ensuring adequate drainage infrastructure is in place before treatment.
- Wellbore Collapse: Loss of wellbore integrity due to stress redistribution. Prevention through proper casing design, cement sheath quality assurance, and post-treatment wellbore stability monitoring.
6.3 Risk Control Framework
A comprehensive risk management framework aligned with ISO 31000:2018 (Risk Management — Guidelines) should be implemented, encompassing:
- Risk Identification: Systematic hazard analysis for each process step using Job Hazard Analysis (JHA)
- Risk Assessment: Quantitative evaluation using probability × consequence matrix
- Risk Mitigation: Implementation of engineering controls, administrative controls, and PPE
- Risk Monitoring: Continuous real-time monitoring during operations with defined alarm thresholds
- Emergency Response: Pre-planned emergency procedures with regular drill exercises
7. Application Scenarios Across Company Technology Routes
7.1 Integration with TIG/MIG Weld Overlay Route
The CO₂ fracturing technology creates synergistic opportunities with the company's weld overlay capabilities:
- Hardfacing of Injection Equipment: CO₂ injection pumps, valves, and connectors operating under high-pressure, corrosive conditions benefit from TIG weld overlay with ASTM A388 or GB/T 12470 compliant hardfacing alloys to extend service life.
- Wear-Resistant Overlay on Borehole Tools: Drill bits, reamers, and plug-setting tools used in borehole preparation can receive MIG weld overlay of GB/T 11352 compliant alloy layers for enhanced durability in abrasive coal formations.
- Transition Layer Welding for Hybrid Systems: When integrating CO₂ injection equipment with existing gas drainage infrastructure (steel piping, valves), TIG transition layer welding ensures metallurgical compatibility between dissimilar materials per ASME B31.3 piping code requirements.
7.2 Integration with Hydraulic Explosive Bonding Route
- Clad Pipe for CO₂ Injection Lines: Hydraulic explosive bonding can produce lined pipes (e.g., stainless steel-lined carbon steel) for CO₂ injection circuits, combining structural strength with corrosion resistance at reduced cost compared to full stainless construction.
- Pressure Vessel Linings: CO₂ storage and transfer vessels can utilize bonded cladding to provide internal corrosion protection while maintaining structural integrity under cyclic pressure loading per GB/T 17748 (Explosion-Welded Clad Plates).
- Valve Body Cladding: High-pressure valves in the injection system can be clad with wear and corrosion-resistant alloys using hydraulic bonding techniques for extended maintenance intervals.
7.3 Integration with Explosion Welding Route
- Explosion-Welded Flanges and Fittings: Critical high-pressure connections in the CO₂ injection system can utilize explosion-welded flanges providing metallurgically sound joints between dissimilar materials (e.g., 316L SS facing on carbon steel body) per ASTM A491 specifications.
- Clad Plate for Pressure Test Chambers: Test chambers used for plug integrity verification and equipment qualification can be constructed using explosion-welded clad plates, providing internal corrosion resistance with external structural strength.
- Instrumentation Mounting Plates: Sensor housings and mounting brackets for downhole monitoring equipment can be clad using explosion welding to ensure signal integrity and corrosion resistance in the harsh underground environment.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The Hongfa Coal Mine CO₂ fracturing trial study contributes to the company's qualification portfolio in several dimensions:
- Engineering Service Qualification: Demonstrates capability to design, implement, and evaluate complex underground engineering projects, supporting applications for coal mine safety engineering service qualifications per AQ 2013-2008 (Safety Production License Management Regulations for Coal Mines).
- Technical Expertise Documentation: The systematic study and documented results establish the company as a technical authority in coal mine gas control, supporting R&D tax credits and high-tech enterprise certification under GB/T 24001 and relevant national science and technology commission guidelines.
- Multi-Disciplinary Integration: Combining metallurgical engineering (cladding) with coal mine engineering demonstrates the company's ability to deliver integrated solutions, a key criterion for government-backed technology demonstration projects.
8.2 Product Delivery Enhancement
- Validated Field Conditions: Understanding the actual operating conditions (pressure, temperature, corrosive environment) in coal mine gas systems enables more accurate specification of cladding products for mining applications.
- Customer-Specific Solutions: Direct engagement with coal mine operators provides first-hand knowledge of equipment failure modes, enabling proactive product improvement and custom cladding solutions.
- Accelerated Product Development: Field trial data on material performance under CO₂ exposure and cyclic pressure loading informs alloy selection and cladding thickness optimization for mining equipment.
8.3 Customer Value Proposition
| Value Dimension | Specific Contribution | Quantifiable Benefit |
|---|---|---|
| Safety | Reduced outburst risk through enhanced gas drainage | Outburst probability reduction of 60–80% |
| Productivity | Shorter pre-drainage time enabling faster mining advance | 25–40% reduction in pre-drainage duration |
| Revenue | Increased CBM recovery for sale as renewable energy | Additional 500,000–2,000,000 m³ CBM per treatment zone |
| Environmental | Reduced methane emissions (25× more potent GHG than CO₂) | 1,000–5,000 tCO₂e avoided per treatment zone |
| Equipment Longevity | Clad injection equipment with extended service life | 3–5× longer replacement intervals vs. unclad equipment |
9. Conclusions and Forward Outlook
The CO₂ fracturing and permeability enhancement technology for low-permeability coal seams, as demonstrated through the Hongfa Coal Mine trial study, represents a technically sophisticated and commercially valuable capability that extends the company's service portfolio beyond traditional cladding manufacturing. The technology addresses critical safety, environmental, and economic needs of the coal mining industry while creating natural integration points with the company's core cladding and weld overlay products.
Future development directions include:
- Hybrid Fracturing Systems: Combining CO₂ fracturing with water-based hydraulic fracturing and proppant placement for enhanced fracture conductivity and permanence.
- Multi-Gas Injection Strategies: Evaluating N₂/CO₂/CH₄ blended injection for optimized matrix interaction and reduced asphyxiation risk.
- Real-Time Fracture Monitoring: Integration of distributed acoustic sensing (DAS) and microseismic monitoring for real-time fracture geometry characterization.
- AI-Driven Process Optimization: Machine learning models for injection parameter optimization based on real-time pressure and flow data.
- Carbon Credit Integration: Structuring CBM recovery and CO₂ sequestration projects to generate tradable carbon credits under China's national carbon market framework.
By maintaining technical rigor in process development, strict adherence to applicable standards (GB 50451-2019, AQ 1026-2019, MT/T 1007-2006), and continuous integration of cladding technology solutions into coal mine equipment, the company positions itself as a comprehensive technology provider serving the coal mining sector's evolving safety and efficiency requirements.