CO₂-Induced Fracturing Permeability Enhancement Technology for Low-Permeability Coal Seams
1. Definition and Technical Principles
CO₂-induced fracturing permeability enhancement technology is an advanced reservoir stimulation method designed to increase gas flow capacity in low-permeability coal seams where conventional hydraulic fracturing is either ineffective or economically unviable. The fundamental principle relies on the phase-change behavior of carbon dioxide (CO₂) under reservoir conditions. When supercritical CO₂ is injected into a low-permeability coal seam at pressures exceeding the fracture initiation threshold (typically 6–12 MPa depending on in-situ stress), it creates a network of fractures and micro-fractures that significantly increase the effective permeability of the coal matrix.
The technology exploits three coupled mechanisms:
- Mechanical Fracturing: Supercritical CO₂, injected at high pressure, exceeds the minimum horizontal principal stress of the coal body, generating tensile fractures that propagate through the coal mass.
- Adsorption-Induced Swelling and Desorption: CO₂ has a higher adsorption capacity on coal surfaces compared to methane (CH₄). Upon adsorption, CO₂ displaces previously adsorbed CH₄, generating desorption gas pressure that further propagates existing fractures and creates new micro-fracture pathways.
- Phase-Change Volume Expansion: As supercritical CO₂ migrates through fractures and transitions to gaseous phase due to pressure reduction, the volume expansion ratio (approximately 1:1000 from liquid to gas phase) generates secondary fracturing energy that extends the fracture network.
For the Hongfa Coal Mine case study, the specific objective was to address coal seams with initial permeability values typically in the range of 0.001–0.01 mD (millidarcy), which are classified as ultra-low permeability and are unresponsive to conventional water-based hydraulic fracturing due to high capillary entry pressure and coal matrix swelling under aqueous fluids.
2. Category and Business Positioning
2.1 Technology Classification
This technology falls under the category of gas reservoir stimulation and coalbed methane (CBM) enhancement, specifically within the sub-domain of non-aqueous fracturing fluids. It represents an integrated approach combining:
- Reservoir engineering and geomechanics
- Chemical engineering (phase behavior and adsorption thermodynamics)
- Drilling and completion engineering
- Carbon capture, utilization, and storage (CCUS) technology
2.2 Positioning Within Cladding Technology Shanxi Co., Ltd.
While the primary technology focuses on reservoir stimulation, its implementation within the company's portfolio serves a critical role in equipment qualification and material engineering. The CO₂ injection infrastructure—including high-pressure pipelines, injection wells, surface equipment, and underground fittings—requires specialized metallurgical solutions that directly leverage the company's core competencies in:
- TIG/MIG Weld Overlay: Overlay cladding of injection well casings and surface piping with corrosion-resistant alloys to withstand CO₂ corrosion (carbonic acid formation in presence of trace moisture)
- Explosion Welding: Fabrication of clad pipes for CO₂ transport and injection systems where a carbon steel base with a stainless steel or nickel alloy cladding layer provides economic and functional advantages
- Hydraulic Explosive Bonding: Production of pressure vessel components and heat exchangers for CO₂ compression and liquefaction systems
3. Technical Purpose and Value
3.1 Primary Technical Objectives
| Objective | Target Metric | Baseline (Pre-Stimulation) | Post-Stimulation Target |
|---|---|---|---|
| Permeability Enhancement | Effective permeability (mD) | 0.001–0.01 mD | 0.1–1.0 mD |
| Gas Production Rate | CH₄ flow rate (m³/day) | 10–50 m³/day | 500–2000 m³/day |
| Fracture Network Density | Fracture length (m) | N/A (unstimulated) | 30–80 m from wellbore |
| CO₂ Sequestration | CO₂ retention (t/well) | N/A | 500–2000 t |
| Gas Drainage Efficiency | Drainage ratio (%) | <5% | 30–60% |
3.2 Value Proposition
The technology delivers multi-dimensional value:
- Mine Safety: Enhanced gas drainage reduces the risk of coal and gas outbursts, directly contributing to mine safety compliance under GB 16423-2020 (Coal Mine Safety Regulations).
- Resource Recovery: Converts previously uneconomic low-permeability coal seams into productive gas reservoirs, increasing overall resource recovery rates.
- Carbon Sequestration: CO₂ injected into coal seams is permanently sequestered through adsorption, contributing to national carbon neutrality goals.
- Equipment Demand: Generates substantial demand for high-performance clad pipes, corrosion-resistant fittings, and pressure vessels—all core products of the company's manufacturing capabilities.
4. Key Process and Implementation Points
4.1 CO₂ Injection System Configuration
The injection system consists of surface compression/liquefaction equipment, high-pressure injection pipelines, injection well completions, and monitoring instrumentation. The design parameters must account for:
- Maximum injection pressure: 12–20 MPa (depending on formation fracture pressure)
- CO₂ injection temperature: 20–40°C (supercritical state maintained above 31.1°C and 7.38 MPa)
- Injection rate: 0.5–3.0 m³/min (adjusted based on real-time pressure monitoring)
- Injection duration: 2–8 hours per stage
4.2 Material Requirements for Injection Infrastructure
CO₂ in the presence of trace moisture forms carbonic acid (H₂CO₃), creating a corrosive environment that demands specialized material selection. This is where the company's cladding technology expertise becomes critical:
| Component | Service Conditions | Recommended Cladding Solution | Applicable Standard |
|---|---|---|---|
| Injection Pipeline (Surface) | 15–20 MPa, 20–60°C, CO₂/H₂O | 304L/316L TIG weld overlay on Q345B base pipe | ASME B31.3, GB/T 150 |
| Injection Well Casing | 12–20 MPa, 40–80°C, CO₂/brine | Explosion-welded 304/20 steel clad pipe | ASTM A240, GB/T 18446 |
| Compression System Piping | 25–35 MPa, 40–80°C | 316L hydraulic explosive bonded cladding | ASME BPVC VIII Div.1 |
| Valves and Fittings | 15–20 MPa, cyclic loading | 309L/316L MIG weld overlay | ASME B16.34, NACE MR0175 |
| Storage Vessel | 2.5–4.0 MPa, ambient | Explosion-welded 304/16Mn clad plate | GB/T 150, TSG 21-2016 |
4.3 Fracturing Process Parameters
The fracturing process is executed in multiple stages to optimize fracture network geometry and coverage:
- Pre-fracturing Assessment: Measurement of in-situ stress, formation pressure, and permeability through well logging and mini-fracture tests.
- Stage Design: Determination of injection stages based on vertical wellbore geometry, target fracture spacing (typically 50–150 m between stages), and expected fracture height.
- Plug Setting: Placement of temporary plugs (cement or mechanical) to isolate injection intervals.
- CO₂ Injection: Controlled injection of supercritical CO₂ at designed rate and pressure, with real-time monitoring of wellhead pressure, injection rate, and ground vibration.
- Fracture Closure Monitoring: Tracking of pressure decline after injection cessation to evaluate fracture closure behavior and gas production onset.
- Production Testing: Flow testing at multiple choke sizes to determine deliverability and evaluate stimulation effectiveness.
4.4 Weld Overlay Specifications for CO₂ Service
For the injection infrastructure components requiring weld overlay cladding, the following WPS parameters apply:
| Parameter | Specification | Rationale |
|---|---|---|
| Base Material | Q345B / 20# / 16Mn | Structural strength at moderate cost |
| Overlay Alloy | ER309L (transition) + ER316L (corrosion layer) | Crack-free dilution control + Cl/CO₂ resistance |
| Number of Passes | 3–4 (1 transition + 2–3 corrosion) | Minimum 3 mm overlay thickness per NACE MR0175 |
| Interpass Temperature | ≤150°C | Prevent grain growth and sensitization |
| Preheat Temperature | 80–120°C | Reduce hydrogen cracking susceptibility |
| Post-Weld Heat Treatment | 620°C × 2h (solution treatment) | Precipitate dissolution, improve ductility |
| NDT Requirements | RT 100% + PT 100% + UT 100% (bond strength) | Ensure overlay integrity and full fusion |
| Minimum Bond Strength | ≥55 MPa (shear) | Per GB/T 25670-2010 |
5. Applicable Standards and Acceptance Criteria
5.1 Reservoir Stimulation Standards
- GB/T 23566-2009: Coalbed methane reservoir evaluation methods
- SY/T 6610-2016: Technical specifications for coalbed methane fracturing
- GB 16423-2020: Safety regulations for coal mines (gas drainage requirements)
- DZ/T 0227-2014: Technical specifications for coalbed methane exploration and development
- API RP 90: Recommended practice for design and installation of CBM wells
5.2 Materials and Equipment Standards
- ASME B31.3: Process piping design and fabrication
- ASME BPVC Section VIII Div.1: Pressure vessel design and construction
- GB/T 150-2011: Pressure vessel design, fabrication, inspection, and acceptance
- TSG 21-2016: Supervision regulations for stationary pressure vessels
- GB/T 18446-2001: Explosion-welded steel composite plates
- ASTM A240: Chromium and chromium-nickel stainless steel plate for pressure vessels
- NACE MR0175/ISO 15156: Materials for H₂S-containing environments (applicable to CO₂/H₂S co-existing conditions)
- GB/T 25670-2010: Testing methods for weld overlay cladding
- GB/T 3323-2005: Radiographic testing of welds
- GB/T 11345-2013: Ultrasonic testing of welds
5.3 Acceptance Criteria
| Test Category | Acceptance Standard | Reference |
|---|---|---|
| RT of Weld Overlay | No cracks, lack of fusion; porosity ≤ Grade II | GB/T 3323, AWS D10.6 |
| UT Bond Strength | ≥55 MPa shear strength | GB/T 25670 |
| PT of Overlay Surface | No linear indications; round indications ≤3 mm | GB/T 18851 |
| Hardness (Overlay) | ≤250 HBW (for NACE service) | NACE MR0175 |
| Fracture Network (Post-Fracturing) | Fracture length ≥30 m; conductivity ≥10 D·m | SY/T 6610 |
| Gas Production (Post-Stimulation) | ≥500 m³/day sustained for 30 days | GB/T 23566 |
6. Common Risks and Controls
6.1 Technical Risks
| Risk Category | Description | Mitigation Strategy |
|---|---|---|
| Insufficient Fracturing | CO₂ injection fails to initiate fractures due to underestimation of in-situ stress | Conduct comprehensive pre-fracturing stress analysis; implement stepwise pressure increase protocol with real-time monitoring |
| Fracture Height Control | Fractures propagate beyond target zone into overlying strata | Design injection rate and total volume based on fracture height prediction models; use fiber optic distributed acoustic sensing (DAS) for monitoring |
| CO₂ Leakage | CO₂ migrates to shallow strata or surface through natural fractures | Implement multi-barrier well completion design; conduct periodic well integrity testing per API RP 1191 |
| Equipment Corrosion | CO₂ corrosion of injection equipment reduces service life | Apply TIG/MIG weld overlay cladding per WPS qualification; implement corrosion monitoring per NACE SP0775 |
| Coal Matrix Swelling | Adsorption-induced swelling partially closes fractures post-injection | Design fracture network with sufficient aperture; implement staged production to manage swelling dynamics |
| Weld Overlay Cracking | Cracking in weld overlay due to thermal mismatch or hydrogen embrittlement | Strict control of preheat and interpass temperatures; use low-hydrogen filler metals; implement post-weld bake-out per AWS D10.6 |
6.2 Quality Control Measures
- WPS/PQR Qualification: All weld overlay procedures for CO₂ service must be qualified per GB/T 985 and AWS D10.6, with specific testing for corrosion resistance in CO₂ environments.
- In-Process Monitoring: Real-time monitoring of welding parameters (current, voltage, travel speed, gas flow) with automated recording and deviation alarms.
- Material Traceability: Full chemical and mechanical certification for base and overlay materials per ASTM A240 and GB/T 4237.
- Corrosion Testing: Overlay materials must pass 1000-hour immersion testing in simulated CO₂/H₂O environment at service temperature before deployment.
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Application
The CO₂ fracturing project generates significant demand for weld overlay cladding on injection infrastructure. Key applications include:
- Surface Injection Manifolds: Multi-pass TIG weld overlay of ER309L/ER316L on carbon steel manifold assemblies operating at 15–20 MPa. The overlay provides corrosion protection while maintaining the structural strength of the base material.
- Wellhead Equipment: MIG weld overlay on wellhead valves, chokes, and pressure gauges exposed to wet CO₂. Three-pass overlay (1× ER309L transition + 2× ER316L corrosion layer) achieves minimum 3 mm functional thickness.
- Compressor Discharge Lines: High-temperature weld overlay on discharge piping of CO₂ compressors, where temperatures reach 80–120°C and CO₂ partial pressure is elevated.
- Repair and Maintenance: Field weld overlay repair of damaged cladding on existing injection infrastructure, extending equipment service life and reducing replacement costs.
Qualification Building: Successful delivery of weld overlay components for CO₂ injection systems establishes the company's capability in high-pressure, corrosive service applications. This qualification supports future bids for similar projects in oil and gas, chemical processing, and power generation sectors.
7.2 Hydraulic Explosive Bonding Application
Hydraulic explosive bonding (water-jet explosive welding) is particularly suited for producing large-diameter clad pipes and plate products for the CO₂ fracturing infrastructure:
- Large-Diameter Clad Pipe: Production of Φ325–Φ610 mm explosion-welded clad pipes (304/20# or 316L/Q345B) for surface CO₂ transport and injection pipelines. The hydraulic explosive bonding process ensures uniform bond quality across large diameters without the distortion associated with thermal welding.
- Clad Plate for Vessels: Fabrication of explosion-welded clad plate (304/16Mn, minimum 3 mm clad) for CO₂ storage vessels and flash tanks. The process meets GB/T 18446 requirements for bond strength and microstructure quality.
- Heat Exchanger Components: Production of clad plate for CO₂ compression interstage coolers and gas-gas heat exchangers where temperature control is critical for maintaining supercritical conditions.
Product Delivery Advantage: Hydraulic explosive bonding provides a cold-process alternative to thermal cladding, eliminating concerns about heat-affected zone (HAZ) degradation, residual stress, and distortion. This is particularly valuable for large components where post-weld heat treatment is impractical.
7.3 Explosion Welding Application
Traditional air-gap explosion welding serves specific high-performance applications in the CO₂ fracturing project:
- High-Pressure Injection Valves: Explosion-welded valve bodies (316L/ASTM A105) for high-pressure injection control valves operating at 20–35 MPa with cyclic pressure loading. The explosion welding process provides superior fatigue resistance compared to weld overlay due to the metallurgically bonded interface with no heat-affected zone.
- Subsea/Underground Equipment: Explosion-welded components for underground CO₂ injection equipment where space constraints and maintenance accessibility are limited, requiring maximum reliability and minimum maintenance intervals.
- Specialty Alloys: Explosion welding of dissimilar metal combinations (e.g., Hastelloy C-276/steel) for components exposed to the most aggressive CO₂/H₂S/brine environments in the injection system.
Customer Value: Explosion-welded components offer a 2–3× service life extension compared to bare carbon steel and a 40–60% cost reduction compared to solid alloy construction. For a CO₂ fracturing project requiring thousands of linear meters of injection piping and numerous high-pressure fittings, this translates to significant lifecycle cost savings.
8. Integration and Synergy
The CO₂ fracturing permeability enhancement technology serves as a cross-disciplinary integration platform that connects the company's metallurgical capabilities with downstream energy applications. The technical learning and qualification gained from this project creates a multiplier effect:
- WPS Development: CO₂-specific weld procedures developed for this project are directly transferable to natural gas processing, LNG facilities, and chemical plant applications.
- NDT Protocol Development: Specialized inspection protocols developed for weld overlay in high-pressure CO₂ service enhance the company's NDT capability for all pressure-containing equipment.
- Customer Relationship: Successful delivery of qualified equipment for the Hongfa Coal Mine project establishes the company as a trusted supplier in the coalbed methane sector, opening opportunities for additional projects across Shanxi Province and nationally.
- Carbon Neutrality Alignment: Participation in CCUS-related projects aligns the company with national carbon neutrality goals, enhancing corporate social responsibility credentials and potentially qualifying for government subsidies and preferential financing.
9. Conclusion
The CO₂-induced fracturing permeability enhancement technology for low-permeability coal seams represents a technically sophisticated application that demands high-performance materials, rigorous quality control, and deep integration between reservoir engineering and materials science. For Cladding Technology Shanxi Co., Ltd., this project serves as a strategic platform for demonstrating and expanding capabilities in weld overlay, explosion welding, and hydraulic explosive bonding for demanding service conditions. The qualification, technical learning, and customer relationships developed through this project create lasting value that extends well beyond the immediate project scope, positioning the company for growth in the broader energy and industrial equipment markets.