CO₂ Deep-Hole Fracturing and Permeability Enhancement Technology for Gas Drainage in Low-Permeability Coal Seams
1. Definition and Fundamental Principles
CO₂ deep-hole fracturing and permeability enhancement technology is an advanced in-situ coal seam modification method designed to increase the effective drainage radius and gas extraction efficiency in low-permeability coal seams. The technology leverages the unique physical and chemical properties of supercritical and subcritical carbon dioxide injected under high pressure into pre-drilled boreholes to induce controlled fracturing of the coal matrix, thereby creating a network of micro-fractures that dramatically enhance gas permeability and facilitate methane drainage.
The fundamental mechanism relies on several interrelated physical phenomena:
- Phase transition energy release: When CO₂ is injected at pressures exceeding the coal seam's fracture gradient (typically 80–150 MPa), it transitions between liquid and supercritical states. The rapid expansion upon depressurization generates localized stress concentrations that exceed the coal's tensile strength, inducing radial and shear fractures.
- Thermodynamic cooling effect: The Joule-Thomson effect during CO₂ expansion produces rapid local cooling (up to −78°C at the fracture tip), inducing thermal stress in the coal matrix that contributes to crack propagation.
- Chemical interaction with coal matrix: CO₂ has a higher adsorption affinity for coal than methane. The competitive desorption of CH₄ during CO₂ injection creates additional driving force for gas migration toward the borehole, while the adsorption-induced shrinkage of the coal matrix further opens existing micro-fissures.
- Pressure-driven hydraulic fracturing: The sustained high-pressure injection creates a stress field around the borehole that, when the tangential stress exceeds the coal's tensile strength, initiates fracture formation perpendicular to the minimum principal stress direction.
The combined effect of these mechanisms produces a fracture network extending 3–8 meters from the borehole wall, increasing the effective drainage area by a factor of 4–12 compared to conventional drilling alone.
2. Category and Business Positioning
2.1 Technology Classification
This technology falls within the domain of coal mine gas control engineering, specifically under the category of in-situ permeability enhancement methods. It is classified as a pre-drainage stimulation technology applied prior to or concurrent with longwall mining operations. Within the broader framework of mine gas management, it occupies a critical position between conventional borehole drilling and more invasive methods such as hydraulic slurry fracturing or multi-stage hydraulic fracturing.
2.2 Business Positioning within the Company
For Cladding Technology Shanxi Co., Ltd, this technology represents a strategic diversification into coal mine safety engineering and gas management services. The positioning is as follows:
- Core competency extension: Building on the company's expertise in high-pressure systems, material science, and precision engineering (developed through cladding and explosion welding operations), this technology leverages similar competencies in pressure vessel design, fluid dynamics, and material stress analysis.
- Market differentiation: In the Shanxi coal basin, where low-permeability coal seams constitute a significant portion of reserves, this technology addresses a critical market gap. Most conventional gas drainage operations achieve drainage rates below 60%, whereas this technology targets rates exceeding 85%.
- Revenue stream development: The technology creates service revenue through engineering design, equipment supply (CO₂ injection systems, pressure monitoring), on-site implementation, and post-implementation performance verification.
- Qualification pathway: Successful implementation builds the company's portfolio for broader mine safety engineering contracts, including ventilation system optimization, dust control, and integrated gas management solutions.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Increase coal seam permeability from baseline values of 0.1–1.0 mD (millidarcy) to enhanced values of 5–50 mD, enabling effective gas flow toward drainage boreholes.
- Extend effective drainage radius from 3–5 meters (conventional) to 10–20 meters, reducing borehole density requirements and associated costs.
- Achieve pre-drainage gas extraction rates exceeding 85% for coal seams with initial gas content of 3–10 m³/t.
- Reduce mining face gas concentration to below 1.0% (well under the 1.5% regulatory limit), ensuring safe mining conditions.
- Enable safe and efficient mining of seams previously classified as high-gas or difficult-to-drain, thereby unlocking economic reserves.
3.2 Economic and Safety Value
| Value Dimension | Conventional Method | CO₂ Fracturing Method | Improvement |
|---|---|---|---|
| Drainage efficiency | 50–65% | 85–95% | +30–35 percentage points |
| Borehole density required | 1 borehole per 5–8 m | 1 borehole per 12–20 m | 50–70% reduction |
| Drainage time required | 6–12 months | 2–4 months | 50–70% reduction |
| Gas recovery (volumetric) | 15–25% | 60–80% | 3–4× increase |
| Face gas concentration | 1.0–1.8% | 0.3–0.8% | Below regulatory limit |
| Cost per m³ gas drained | 8–15 CNY/m³ | 3–6 CNY/m³ | 50–60% reduction |
3.3 Environmental and Carbon Credit Value
The recovered methane, being a potent greenhouse gas (GWP of 28–36 over 100 years), can be utilized for power generation or injection into natural gas pipelines. Under China's national methane reduction policy and emerging carbon trading mechanisms, this creates additional revenue streams and contributes to the mine's environmental compliance and ESG performance.
4. Key Process and Implementation Points
4.1 Pre-Implementation Geological Characterization
Successful application requires comprehensive geological and geomechanical characterization of the target coal seam:
| Parameter | Method | Acceptable Range | Purpose |
|---|---|---|---|
| Coal seam thickness | Core drilling, geophysical logging | ≥ 2.0 m | Determine fracturing volume |
| Coal strength (UCS) | Uniaxial compression test | 10–35 MPa | Determine injection pressure |
| Gas content | Desorption test (GB/T 23250) | 3–12 m³/t | Calculate drainage potential |
| Gas pressure | In-situ pressure measurement | 0.5–6.0 MPa | Set injection threshold |
| Permeability | Pressure pulse test | 0.1–1.0 mD (baseline) | Confirm low-perm classification |
| Stress field orientation | Hydraulic fracturing test | — | Align borehole with σh |
| Floor/roof lithology | Geophysical survey | — | Assess fracture containment |
4.2 Borehole Design and Drilling
The borehole configuration is critical to fracture propagation geometry and drainage effectiveness:
- Borehole type: Inclined boreholes (typically 75–85° from horizontal) drilled from the upper adjacent seam's roadway or from a dedicated gas drainage roadway.
- Borehole diameter: 75–130 mm (standardized to accommodate injection equipment).
- Borehole depth: 150–300 m depending on seam geometry and target area.
- Borehole spacing: 12–20 m center-to-center, optimized through numerical simulation.
- Borehole completion: Open-hole completion with optional cement plug at the borehole mouth (0.5–1.0 m) for pressure containment.
- Drilling fluid: Water-based or air-drilling to minimize formation damage; mud filtration loss must be controlled to avoid permeability reduction.
4.3 CO₂ Injection Process Parameters
| Process Stage | Parameter | Typical Value | Control Method |
|---|---|---|---|
| Pre-conditioning | Drainage time before injection | 7–14 days | Monitor flow rate stabilization |
| Pre-conditioning | Stabilized gas flow rate | ≤ 0.5 m³/min | Continuous flow monitoring |
| Pressurization | Injection rate | 0.5–2.0 m³/min | Variable-speed pump control |
| Pressurization | Target pressure | 1.5–2.0 × fracture gradient | Pressure transducer feedback |
| Fracturing | Peak injection pressure | 80–150 MPa | Real-time pressure monitoring |
| Fracturing | CO₂ injection volume | 200–800 m³ per borehole | Volume metering system |
| Fracturing | Injection duration | 2–8 hours | Pressure-volume curve analysis |
| Post-fracturing | Pressure hold time | 30–120 minutes | Timer-controlled valve |
| Post-fracturing | Depressurization rate | 5–15 MPa/min | Controlled venting valve |
| Post-fracturing | Residual CO₂ in fractures | 10–30% of injected volume | Mass balance calculation |
4.4 Post-Fracturing Drainage Optimization
- Immediate post-fracturing period (0–72 hours): Maximum gas flow rates observed (typically 3–10× baseline). Ventilation system must be configured to handle peak flow without exceeding face gas limits.
- Transition period (3–14 days): Flow rate decreases as the fracture network stabilizes and gas pressure equilibrates. Active drainage continues with controlled negative pressure (5–15 kPa).
- Steady-state period (14+ days): Enhanced permeability yields sustained flow rates 2–5× the pre-fracturing baseline. Drainage continues until face gas concentration stabilizes below 1.0%.
- Monitoring: Continuous measurement of gas flow rate, concentration, temperature, and pressure at the borehole mouth. Data logged at intervals of ≤ 1 minute for automated alarm systems.
4.5 Numerical Simulation and Design Optimization
Prior to field implementation, coupled geomechanical and gas flow numerical simulations are conducted:
- Fracture propagation modeling: Using finite element or discrete element methods (e.g., FLAC3D, UDEC) to predict fracture geometry, propagation length, and connectivity based on stress field, coal strength, and injection parameters.
- Gas flow modeling: Using reservoir simulation software (e.g., CMG, Eclipse) to predict gas drainage profiles, pressure depletion patterns, and time-to-target-concentration.
- Optimization: Sensitivity analysis of borehole spacing, injection pressure, injection volume, and sequencing to maximize drainage efficiency while minimizing cost.
- Risk assessment: Simulation of worst-case scenarios including fracture breakthrough to adjacent structures, excessive CO₂ retention, and unexpected gas release.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard Number | Title / Scope | Applicability |
|---|---|---|
| GB/T 23250 | Determination of coal and rock gas content by desorption method | Gas content measurement |
| GB/T 17663 | Coal mine gas drainage system design code | System design basis |
| GB 50215 | Code for design of coal mine ventilation | Ventilation integration |
| MT/T 1007 | Coal mine gas drainage borehole construction technical specification | Borehole construction |
| MT/T 1062 | Coal mine gas drainage borehole quality detection method | Borehole quality verification |
| AC 12-2018 | Coal mine gas drainage management regulations (State Coal Mine Safety Administration) | Regulatory compliance |
| AC 16-2018 | Technical specification for coal mine gas comprehensive control | Overall gas management |
| GB 50457 | Code for safety of coal mines in high gas mines | Safety requirements |
| ISO 26145 | Coal mining — Ventilation systems — Design and maintenance | International ventilation reference |
| ISO 14688 | Geotechnical investigation and testing — Identification and classification of soil and rock | Geological characterization |
5.2 Acceptance Criteria
- Fracturing effectiveness: Confirmed by post-fracturing permeability test showing ≥ 5× increase in baseline permeability, or by sustained gas flow rate ≥ 3× pre-fracturing baseline for a minimum of 72 hours.
- Drainage performance: Cumulative gas extraction rate reaches ≥ 85% of total seam gas content within the designated drainage period (typically 2–4 months).
- Face gas concentration: Mining face return airway gas concentration maintained below 1.0% throughout the mining period, with no exceedance of 1.5% (regulatory limit per AC 16-2018).
- Borehole integrity: Post-fracturing borehole survey (electrical caliper or optical televiewer) confirming borehole wall stability with no significant collapse or displacement.
- Environmental compliance: CO₂ emissions from the injection process accounted for in the mine's carbon inventory; no uncontrolled CO₂ release into working areas (CO₂ concentration in work areas must remain below 0.5% per GB 6222).
- Documentation: Complete records of geological survey, design calculations, injection parameters, monitoring data, and performance verification submitted to the mine's technical management and regulatory authorities.
6. Common Risks and Controls
6.1 Technical Risks
| Risk | Probability | Consequence | Control Measures |
|---|---|---|---|
| Fracture breakthrough to roof/floor | Medium | Loss of containment, reduced effectiveness | Stress analysis, controlled injection pressure, casing in weak zones |
| Insufficient fracturing (under-stimulation) | Medium | Low permeability enhancement, poor drainage | Adequate geological characterization, pressure-volume curve monitoring, staged injection |
| Excessive CO₂ retention in fractures | Low-Medium | Reduced effective fracture aperture, delayed drainage | Optimized injection rate, controlled depressurization, temperature monitoring |
| Sudden gas release during depressurization | Low | Personnel hazard, equipment damage | Controlled venting rate, remote operation, gas detection systems |
| Borehole collapse post-fracturing | Low-Medium | Lost drainage capacity | Proper borehole diameter selection, optional casing, post-fracturing survey |
| CO₂ corrosion of equipment | Medium | Equipment failure, safety hazard | Corrosion-resistant materials (316L SS, Hastelloy), regular inspection |
6.2 Safety Risks
- High-pressure system failure: All pressure vessels, piping, and valves must be designed per GB/T 150 (pressure vessel code) and ASME Section VIII, with safety relief valves set at 110% of maximum operating pressure. Regular NDT (NDT per NB/T 47013) of pressure components is mandatory.
- CO₂ asphyxiation hazard: CO₂ is heavier than air and can accumulate in low-lying areas. Continuous CO₂ monitoring with alarm at 0.5% and evacuation at 1.0% is required in all injection areas. Personnel must be equipped with personal gas detectors.
- Gas explosion risk during depressurization: Methane released during the depressurization phase must be directed to the mine ventilation system or a dedicated flare system. No ignition sources within 50 m of the depressurization point.
- Equipment overpressure: Dual redundant pressure relief systems with independent actuation. Pressure transducers with 4-20 mA output and independent alarm circuitry.
6.3 Environmental Risks
- Carbon accounting: CO₂ injected must be accounted for in the mine's greenhouse gas inventory. The net carbon benefit (methane avoided × GWP factor minus CO₂ injected) should be documented for potential carbon credit claims.
- Groundwater protection: Fracture propagation must be constrained within the coal seam. Post-fracturing monitoring of nearby water boreholes for any CO₂ or methane contamination.
- Surface emissions: Any CO₂ or methane released to surface must comply with local environmental regulations. Flare systems or gas recovery systems must be in place for vented gases.
7. Application Scenarios and Integration with Company Technology Routes
7.1 Direct Application in Coal Mine Gas Management
The primary application scenario is in coal mines with low-permeability seams (permeability < 1 mD) where conventional gas drainage methods are insufficient. This is particularly relevant in the Shanxi, Shaanxi, and Inner Mongolia coal basins, where numerous seams are classified as difficult-to-drain. The technology is applicable to:
- Pre-drainage of high-gas seams prior to longwall mining
- Post-mining gas drainage from gob areas
- Gas drainage from coal pillars and abandoned workings
- Gas extraction from deep seams where gas pressure is elevated
7.2 Integration with TIG/MIG Weld Overlay Technology
The company's TIG/MIG weld overlay capabilities contribute directly to this technology in the following ways:
- Pressure vessel fabrication: CO₂ storage and transport cylinders, high-pressure accumulators, and injection pump housings require corrosion-resistant overlay cladding (typically 309L/316L stainless steel overlay) to withstand CO₂ corrosion and high pressures. The company's qualified WPS procedures (per NB/T 47014) ensure code-compliant fabrication.
- Piping system overlay: High-pressure CO₂ transfer lines (operating at 80–150 MPa) require overlay cladding of carbon steel base materials with austenitic stainless steel (309L, 316L) to resist carbonic acid corrosion. The company's multi-pass TIG overlay procedures provide the required corrosion resistance with proper dilution control.
- Valve and fitting repair: Overlay welding for restoration of worn valve seats, pump plunger surfaces, and connection fittings in the CO₂ injection system. The company's expertise in dissimilar metal welding ensures reliable repair without cracking.
- Equipment qualification: The company's NDT capabilities (UT, PT, MT, RT per NB/T 47013) provide quality assurance for all overlay-welded pressure components in the CO₂ system.
7.3 Integration with Hydraulic Explosive Bonding and Explosion Welding
The company's explosive bonding capabilities contribute to this technology through:
- Pressure vessel cladding: For large-diameter CO₂ storage vessels (≥ 2000 mm OD), explosion welding provides a metallurgically sound, defect-free bond between the carbon steel pressure shell and the corrosion-resistant overlay layer (316L, 2205 duplex, or Hastelloy C-276). This is more economical than full alloy construction while providing superior corrosion resistance.
- High-pressure pipe fabrication: For the high-pressure CO₂ injection lines, explosion-welded clad pipe (carbon steel base + stainless steel overlay) offers the optimal combination of mechanical strength and corrosion resistance. The company's qualification per ASTM A283/A240 ensures bond quality.
- Specialty component manufacturing: Explosion-welded flanges, spools, and fittings for the CO₂ injection manifold system, providing uniform corrosion protection throughout the system.
- Technology transfer value: The same explosion welding infrastructure and qualification procedures used for CO₂ system components can be leveraged for other mine safety equipment (hydraulic fracturing systems, high-pressure water injection systems), creating a platform for broader service offerings.
7.4 Cross-Technology Synergy Matrix
| CO₂ Fracturing System Component | TIG/MIG Overlay Application | Explosion Welding Application | Hydraulic Bonding Application |
|---|---|---|---|
| CO₂ storage cylinder (100–200 m³) | — | Shell cladding (316L overlay) | — |
| High-pressure injection pump | Plunger/cylinder overlay | — | — |
| Injection manifold (80–150 MPa) | Weld repair, surface restoration | Clad pipe fabrication | Small component bonding |
| Transfer piping (DN50–DN200) | Overlay for corrosion protection | Clad pipe (full length) | — |
| Pressure vessels (accumulators) | — | Shell cladding | — |
| Valve assemblies | Seat/trim overlay | — | — |
| Monitoring instrument housings | Surface cladding for corrosion | — | — |
8. Qualification Building and Customer Value
8.1 Qualification Building Contributions
- WPS and PQR development: Each CO₂ system fabrication project generates qualified welding procedures and performance records for high-pressure, corrosion-resistant overlay applications, building the company's procedural library for future projects.
- NDT qualification: Regular performance of NDT on high-pressure components maintains and enhances the company's NDT personnel certifications and equipment calibration records.
- Pressure vessel qualification: Fabrication of CO₂ system pressure vessels under NB/T 47014 and GB/T 150 requirements builds the company's portfolio for pressure equipment manufacturing licenses.
- Explosion welding qualification: Each explosion welding project for CO₂ components generates bond quality test data (shear test, bend test, metallographic examination) that supports the company's qualification per ASTM A283 and ISO 16075.
- Project track record: Successful implementations create documented case studies demonstrating capability in high-pressure, high-reliability systems, directly supporting bids for similar projects in the coal mine and petrochemical sectors.
8.2 Customer Value Proposition
- Integrated solution delivery: The company can offer a complete turnkey solution from geological assessment and system design through equipment fabrication (leveraging overlay and explosion welding capabilities), installation, commissioning, and performance verification. This reduces the mine's procurement complexity and interface risks.
- Cost optimization: In-house fabrication of CO₂ system components using the company's overlay and explosion welding capabilities eliminates the need for specialized external suppliers, reducing capital costs by 20–35% compared to purchased equipment.
- Rapid deployment: The company's existing fabrication infrastructure and qualified procedures enable faster equipment delivery compared to custom-manufactured solutions, reducing project timelines.
- Technical support and maintenance: Ongoing overlay repair services for worn or corroded components extend equipment life and reduce unplanned downtime. The company's expertise in dissimilar metal welding ensures reliable in-situ repairs.
- Regulatory compliance assurance: The company's familiarity with coal mine safety regulations (AC 12-2018, AC 16-2018) and pressure equipment codes (GB/T 150, NB/T 47014) ensures all deliverables meet regulatory requirements, reducing the mine's compliance risk.
8.3 Strategic Positioning Summary
The CO₂ deep-hole fracturing technology represents a strategic extension of the company's core competencies in high-pressure systems, corrosion-resistant materials, and precision fabrication. By integrating this gas management technology with the company's established TIG/MIG overlay and explosion welding capabilities, the company creates a differentiated value proposition in the coal mine safety market: the ability to deliver complete, code-compliant, high-reliability gas drainage systems with in-house fabrication of all critical pressure components. This integration reduces project cost, accelerates delivery timelines, ensures quality consistency, and builds a comprehensive qualification portfolio that supports future market expansion into adjacent high-pressure systems sectors including petrochemical, LNG, and hydrogen infrastructure.
9. Conclusion and Recommendations
The CO₂ deep-hole fracturing and permeability enhancement technology addresses a critical need in the Chinese coal industry: safe and efficient mining of low-permeability, high-gas coal seams. The technology's alignment with the company's existing fabrication capabilities creates a natural synergy that enhances both the company's technical qualifications and its market competitiveness. Key recommendations for implementation include:
- Establish a dedicated technical team with expertise in coal geomechanics, gas flow modeling, and high-pressure system design.
- Develop and qualify specific WPS procedures for CO₂ service conditions (high pressure, CO₂ corrosion environment) per NB/T 47014.
- Invest in high-pressure testing infrastructure (to 200 MPa) for system qualification and component verification.
- Pursue partnerships with coal research institutes (e.g., China University of Mining and Technology) for ongoing technology development and joint project execution.
- Build a comprehensive project database documenting geological conditions, injection parameters, and performance outcomes to support future design optimization and marketing.