CO₂ Fracturing-Induced Secondary Gas Permeability Enhancement for High-Efficiency Mining of Outburst Coal Seams
1. Definition and Fundamental Principles
CO₂ fracturing-induced secondary gas permeability enhancement is an advanced in-situ coalbed methane (CBM) pre-conditioning technology designed to mitigate coal and gas outburst hazards prior to longwall or room-and-pillar extraction. The core principle involves injecting supercritical or subcritical CO₂ into a coal seam through a borehole network, exploiting the thermodynamic and geochemical interactions between CO₂ and coal matrix to create secondary permeability pathways that facilitate subsequent gas drainage.
The technology operates on three interrelated physical mechanisms:
- Thermodynamic Swelling and Desorption: Upon CO₂ contact with coal, preferential adsorption of CO₂ over CH₄ occurs due to CO₂'s higher adsorption affinity (approximately 3–4× that of methane on dry coal). This competitive desorption releases adsorbed CH₄ from micropore surfaces, creating a driving pressure differential that enhances gas flow.
- Thermal Stress-Induced Fracturing: The Joule-Thomson cooling effect and subsequent exothermic adsorption of CO₂ generate cyclic thermal stress within the coal matrix. Repeated injection cycles produce micro-cracks and propagate existing fracture networks, increasing permeability by 30–150% relative to baseline.
- Chemical Stress and Matrix Shrinkage: CO₂-induced coal matrix swelling followed by desorption-driven shrinkage creates volumetric strain cycles. These cycles widen existing cleats and generate new micro-fractures, establishing enhanced permeability pathways for gas and water drainage.
The "secondary permeability enhancement" designation distinguishes this process from primary fracturing methods (e.g., hydraulic fracturing, hydraulic explosive fracturing). Secondary enhancement specifically refers to the post-fracture permeability improvement achieved through CO₂ injection after initial borehole preparation and primary fracture creation, targeting the recovery of permeability that typically degrades within 48–72 hours following primary fracturing due to coal dust plugging, water film formation, and matrix swelling.
2. Technical Purpose and Operational Value
The primary technical objectives of this methodology are:
- Outburst Hazard Elimination: Reduce gas content in the mining face to below the regulatory threshold (typically ≤3.0 m³/t for strong outburst seams, per NB/T 10300-2019) before extraction commences.
- Permeability Restoration: Counteract the rapid permeability decline that follows primary hydraulic fracturing, maintaining enhanced flow capacity over extended drainage periods (60–180 days).
- Drainage Efficiency Improvement: Achieve gas drainage efficiency (E₁) of ≥85% and gas drainage rate (K) ≥1.0 m³/t·d, enabling safe mining at production rates of 4–8 m/d in previously hazardous conditions.
- Economic CBM Recovery: Pre-flush CBM for potential commercial recovery, with well flow rates of 5,000–20,000 m³/d achievable in seams with initial permeability of 1–5 mD.
From a business positioning perspective, this technology represents a critical enabling capability for the company's operations in high-risk coal mining environments. It bridges the gap between coal seam preparation (fracturing) and safe extraction, providing a complete gas control solution that supports the company's broader mission of delivering integrated coal mine safety and efficiency technologies.
3. Key Process Implementation Points
3.1 Pre-Conditioning Assessment and Design Parameters
Effective implementation requires rigorous pre-injection characterization of the target coal seam. The following parameters must be established through core sampling, well logging, and pilot testing:
| Parameter | Typical Range (Outburst Seam) | Measurement Method | Design Implication |
|---|---|---|---|
| Initial Gas Content (W) | 6.0–12.0 m³/t | Desorption method (GB/T 23255-2009) | Determines total CO₂ injection volume |
| Initial Permeability (K₀) | 0.5–5.0 mD | Pressure transient analysis | Controls injection pressure and rate |
| Coal Strength (f) | 0.3–0.8 MPa | Compressive strength test (MT/T 793-1999) | Assesses fracture propagation potential |
| Outburst Index (S) | 0.6–1.5 | Coal strength and gas pressure composite | Defines hazard classification (NB/T 10300-2019) |
| Swelling Coefficient | 0.8–2.5% | Triaxial compression with CO₂ | Predicts permeability change trajectory |
3.2 CO₂ Injection Process Parameters
The injection protocol follows a multi-stage approach optimized for maximum permeability enhancement while minimizing CO₂ loss through unintended migration:
| Process Stage | CO₂ Injection Pressure (MPa) | Injection Rate (m³/min) | Duration (h) | Objective |
|---|---|---|---|---|
| Stage 1: Low-pressure adsorption | 2.0–3.5 | 0.5–1.0 | 6–12 | Initial CO₂ adsorption and CH₄ desorption initiation |
| Stage 2: Medium-pressure cycling | 4.0–6.0 | 1.0–2.0 | 12–24 | Thermal stress cycling and micro-fracture generation |
| Stage 3: High-pressure consolidation | 6.5–8.0 | 0.5–1.5 | 4–8 | Fracture network densification and permeability locking |
| Stage 4: Pressure maintenance | Hold at 5.0–6.0 | 0 (closed system) | 24–48 | Equilibration and permeability stabilization |
3.3 Borehole Network Configuration
The effectiveness of CO₂ fracturing is directly dependent on borehole network design. Key design parameters include:
- Borehole spacing: 8–15 m for inclined boreholes; 10–20 m for horizontal boreholes, determined by fracture radius prediction (typically 3–8 m for outburst seams).
- Borehole angle: 45°–75° inclination from horizontal for inclined boreholes; horizontal boreholes drilled from adjacent roadways.
- Borehole length: 60–150 m for inclined boreholes; 100–300 m for horizontal boreholes.
- Sealing quality: Cement or chemical grout seal length ≥10 m; seal pressure resistance ≥1.5× maximum injection pressure.
- Drainage borehole integration: CO₂ injection boreholes must be compatible with subsequent drainage operations, with inner diameter ≥φ76 mm to accommodate drainage equipment.
3.4 Post-Injection Drainage Protocol
Following CO₂ injection and pressure maintenance, the borehole network is transitioned to active gas drainage. The drainage protocol follows these key principles:
- Immediate depressurization: Controlled pressure reduction at 0.2–0.5 MPa/h to prevent sudden gas release and coal ejection.
- Vacuum drainage initiation: Apply vacuum of −0.05 to −0.08 MPa within 2 hours of pressure release.
- Drainage period: Minimum 60 days for moderate outburst seams; 90–180 days for strong outburst seams (S ≥ 1.0).
- Drainage efficiency monitoring: Continuous measurement of gas flow rate, gas concentration, and gas content reduction; target ≥85% drainage efficiency.
- Gas content verification: Final gas content must be measured and confirmed below threshold before mining is authorized (per MT/T 977-2006).
4. Applicable Standards and Acceptance Criteria
4.1 Governing Standards
| Standard Number | Title / Scope | Relevance to CO₂ Fracturing |
|---|---|---|
| NB/T 10300-2019 | Classification and Control of Coal and Gas Outburst | Defines outburst hazard classification and pre-emptive measures including gas content thresholds |
| GB/T 23255-2009 | Coal and Gas Outburst Prediction — Desorption Method | Specifies gas content measurement methodology for pre- and post-drainage verification |
| MT/T 977-2006 | Coal Mine Gas Drainage System Design Specification | Governs drainage system design parameters including vacuum levels and borehole spacing |
| MT/T 793-1999 | Compressive Strength Determination of Coal | Coal strength characterization for outburst index calculation |
| GB 50809-2012 | Coal Mine Gas Drainage Design Code | Design requirements for gas drainage infrastructure and safety systems |
| MT/T 1101-2011 | Technical Specification for Coal Bed Methane Drilling and Completion | Borehole construction quality standards applicable to CO₂ injection boreholes |
| GB 50417-2007 | Design Code for Coal Mine Safety Monitoring and Control Systems | Monitoring system requirements for gas content and concentration tracking |
| ASTM D4747 | Standard Test Methods for Permeability of Core Samples | Reference methodology for permeability measurement of coal core samples (international) |
4.2 Acceptance Criteria
The following acceptance criteria must be met before mining operations are authorized in a face treated with CO₂ fracturing secondary permeability enhancement:
- Gas content: Post-drainage gas content ≤ 3.0 m³/t (strong outburst seam) or ≤ 8.0 m³/t (moderate outburst seam), verified by at least 3 independent measurements per face.
- Gas drainage efficiency (E₁): ≥ 85% for strong outburst seams; ≥ 75% for moderate outburst seams.
- Gas drainage rate (K): ≥ 1.0 m³/t·d for strong outburst seams.
- Permeability enhancement: Post-fracturing permeability ≥ 3× initial permeability (K₀), verified by pressure transient analysis.
- Borehole integrity: ≥ 95% borehole completion rate with effective seal integrity confirmed by pressure test.
- Gas monitoring: Continuous gas concentration monitoring at the mining face showing no exceedance of 1.0% CH₄ during mining operations.
5. Common Risks and Control Measures
| Risk Category | Description | Likelihood | Consequence | Control Measures |
|---|---|---|---|---|
| CO₂ migration to adjacent strata | CO₂ bypasses target seam through natural fractures or borehole seal failures | Medium | Reduced treatment effectiveness; potential roof instability in overlying strata | Multi-stage cement sealing; borehole pressure monitoring; pre-injection fracture mapping via microseismic |
| Sudden gas release during depressurization | Rapid pressure reduction triggers uncontrolled gas flow and potential coal ejection | Low-Medium | Equipment damage; personnel injury; borehole blockage | Controlled depressurization rate (≤0.5 MPa/h); remote operation; safety barriers at borehole mouth |
| Permeability re-decline | Post-fracturing permeability returns to baseline due to coal dust plugging, water film, or matrix swelling | High | Insufficient drainage efficiency; failure to meet gas content thresholds | Timely drainage initiation (within 24h); nitrogen backfill of treated boreholes; periodic CO₂ re-injection |
| Insufficient fracture propagation | Fractures fail to extend beyond borehole vicinity, limiting treatment volume | Medium | Treatment only effective in near-borehole zone; large untreated area remains hazardous | Optimized injection pressure based on coal strength and in-situ stress; multi-stage fracturing with intermediate drainage |
| Environmental and safety concerns | CO₂ accumulation in confined spaces; asphyxiation risk during borehole operations | Low | Personnel injury or fatality | Continuous O₂ and CO₂ concentration monitoring; ventilation protocols; personal protective equipment per GB 39800.1-2020 |
| Equipment failure under high pressure | CO₂ injection equipment failure at 6–8 MPa operating pressure | Low | Process interruption; potential high-pressure gas release | Equipment rated to ≥1.5× maximum operating pressure; pressure relief valves; regular inspection per TSG 21-2016 |
6. Application Scenarios and Integration with Company Technology Portfolio
6.1 Direct Application in Coal Mine Operations
CO₂ fracturing secondary permeability enhancement is primarily deployed in the following operational scenarios:
- Strong outburst seams (S ≥ 1.0): As a mandatory pre-emptive measure before longwall or room-and-pillar extraction, in compliance with NB/T 10300-2019 requirements for outburst prevention.
- Thick seams with high gas content (W > 8.0 m³/t): Where conventional drainage alone cannot achieve required gas content reduction within the planned mining schedule.
- Low-permeability seams (K₀ < 2.0 mD): Where hydraulic fracturing alone yields insufficient permeability improvement, CO₂ fracturing provides the additional permeability enhancement needed for effective drainage.
- CBM commercial recovery operations: Where gas drainage serves dual purposes of hazard elimination and commercial CBM production, with post-fracturing flow rates supporting economic viability.
6.2 Integration with Company's Technology Routes
While CO₂ fracturing is fundamentally a coal seam gas control technology, its integration with the company's broader technology portfolio creates a comprehensive solution framework:
6.2.1 Synergy with Hydraulic Explosive Bonding Technology
The hydraulic explosive bonding technology, originally developed for bimetallic cladding applications, can be adapted for coal seam fracturing applications. The controlled hydraulic explosive energy delivery creates complex fracture networks within the coal seam that provide enhanced pathways for subsequent CO₂ injection. Key integration points include:
- Primary fracture creation: Hydraulic explosive charges placed in boreholes generate complex fracture networks with higher connectivity than conventional hydraulic fracturing.
- Fracture network optimization: The energy density and charge configuration can be tuned to create fractures of specific orientation and extent, optimizing the fracture network for CO₂ distribution.
- Equipment commonality: The high-pressure hydraulic systems used in bonding operations are directly applicable to CO₂ injection operations, reducing capital expenditure.
6.2.2 Support for TIG/MIG Weld Overlay Operations in Mining Environments
In mining environments where CO₂ fracturing is deployed, the company's TIG/MIG weld overlay capabilities support the fabrication and repair of critical underground equipment:
- Pressure vessel fabrication: CO₂ storage and injection vessels require high-integrity welds meeting TSG 21-2016 requirements, fabricated using qualified WPS procedures.
- Drainage pipe overlay: Underground drainage piping subject to corrosive mine water and abrasive coal dust can be extended in service life through weld overlay of corrosion-resistant cladding layers.
- Equipment repair: Field repair of hydraulic fracturing equipment, pressure gauges, and injection manifolds using qualified weld overlay procedures ensures operational continuity.
6.2.3 Equipment Protection Through Cladding Solutions
The equipment deployed in CO₂ fracturing operations is exposed to aggressive conditions that benefit from the company's cladding and overlay expertise:
| Equipment Component | Service Condition | Recommended Cladding/Overlay | Standard Reference |
|---|---|---|---|
| CO₂ injection manifold | High-pressure CO₂ at 6–8 MPa, subcritical conditions | 309L/316L TIG weld overlay on carbon steel base | ASTM A240; AWS D10.6 |
| Drainage borehole casing | Abrasive coal dust, acidic mine water (pH 3–6) | Hardfacing overlay (Co-Cr or WC-Co) per AWS A5.15 | ASTM A540; NB/T 47013 |
| Pressure vessels (CO₂ storage) | Cyclic pressure loading, potential CO₂ corrosion | Full-wall 316L clad plate fabrication | ASME BPV Section VIII; GB 150-2011 |
| Gas drainage piping (underground) | Continuous gas flow, mine water ingress, mechanical impact | 13Cr or duplex 2205 clad pipe | ASTM A830; NACE MR0175/ISO 15156 |
7. Qualification Building and Customer Value
7.1 Qualification and Certification Pathway
The implementation of CO₂ fracturing technology supports the company's qualification building in the following areas:
- Coal Mine Safety Equipment Manufacturing License: Demonstrated capability in gas control technology supports qualification for manufacturing and supplying gas drainage and control equipment under the requirements of AQ 1029-2019.
- Coal Mine Construction Qualification: Experience with CO₂ fracturing operations supports qualification for undertaking coal mine gas control engineering projects at appropriate classification levels.
- WPS Qualification for Mining Applications: Development of Welding Procedure Specifications specifically qualified for mining equipment repair and fabrication in CO₂ fracturing applications, per NB/T 47014-2011.
- Pressure Vessel Manufacturing License: Fabrication of CO₂ storage and injection vessels supports qualification under TSG 21-2016 for pressure vessel manufacturing.
7.2 Customer Value Proposition
The CO₂ fracturing secondary permeability enhancement technology delivers measurable value to coal mining customers:
- Safety Enhancement: Reduction of outburst risk from Class III/IV to Class I/II, enabling safe mining operations and regulatory compliance with zero outburst incidents.
- Production Rate Improvement: Mining face advancement rate increase from 1–2 m/d (hazard-restricted) to 4–8 m/d (gas-controlled), representing a 200–400% production increase.
- Drainage Cost Reduction: Shortened drainage period from 180–240 days to 60–90 days, reducing capital tied up in borehole infrastructure and ventilation systems.
- CBM Revenue Generation: Pre-flushed CBM with flow rates of 5,000–20,000 m³/d can be commercially recovered, generating additional revenue streams of ¥5–15 million per face per year (at ¥1.5–3.0/m³ CBM price).
- Environmental Benefit: CO₂ utilization in fracturing partially offsets carbon emissions; recovered CBM displaces coal combustion, reducing overall mine carbon footprint by 15–25%.
7.3 Technology Maturity and Deployment Status
The CO₂ fracturing technology has progressed through the following maturity stages:
| TRL Level | Description | Status | Evidence |
|---|---|---|---|
| TRL 3 | Proof of concept in laboratory | Completed | Coal core CO₂ adsorption and permeability enhancement tests |
| TRL 5 | System prototype in simulated environment | Completed | Pilot borehole testing in laboratory-scale coal seam simulator |
| TRL 6 | Technology demonstration in operational environment | In Progress | Single-face pilot operations at partner coal mines |
| TRL 7 | System prototype demonstration in operational environment | Targeted | Multi-face deployment with full monitoring and data collection |
| TRL 8 | System complete and qualified | Future | Standardized procedures, training programs, and commercial deployment |
8. Conclusion
CO₂ fracturing-induced secondary gas permeability enhancement represents a critical technology capability for addressing coal and gas outburst hazards in high-risk mining operations. By combining thermodynamic, thermal, and chemical mechanisms to enhance coal seam permeability, this technology enables safe and efficient mining of seams that would otherwise be uneconomical or unsafe to extract using conventional drainage methods alone.
For Cladding Technology Shanxi Co., Ltd., this capability extends the company's value proposition beyond traditional bimetallic cladding and weld overlay into the high-demand coal mine safety and gas control market. The technology creates opportunities for equipment fabrication, overlay repair services, and integrated engineering solutions that leverage the company's existing manufacturing infrastructure, welding qualifications, and quality management systems. As China's coal mining industry continues to modernize and invest in safety technologies, the demand for proven, efficient gas control solutions is expected to grow significantly, positioning this technology as a strategic growth driver for the company.