Screen Hole Protection and CO₂ Phase-Change Fracturing Permeability Enhancement Technology for Low-Permeability Soft Coal Seams

1. Definition and Fundamental Principles

This combined technology addresses one of the most persistent challenges in coalbed methane (CBM) extraction: enhancing gas drainage efficiency in low-permeability soft coal seams. The approach integrates two complementary techniques—screen hole protection for borehole integrity and CO₂ phase-change fracturing for permeability enhancement—into a unified workflow designed to maximize gas recovery rates in geologically difficult formations.

1.1 Low-Permeability Soft Coal Seam Characteristics

Low-permeability soft coal seams are defined as coal formations with initial gas permeability typically below 0.5 × 10^-3 μm² (approximately 0.5 mD), where the coal body exhibits low mechanical strength, high plasticity, and a tendency toward compaction and collapse under stress. These formations present a dual challenge: the coal matrix itself resists gas flow due to its fine pore structure, and the soft coal body causes borehole wall instability during and after drilling operations, leading to reduced effective drainage length and premature borehole closure.

1.2 Screen Hole Protection Principle

Screen hole protection involves the installation of precision-manufactured perforated or slotted screen pipes within drainage boreholes to maintain borehole integrity in soft coal formations. The screen pipe allows gas ingress while preventing coal fines and debris from entering the borehole, thereby sustaining hydraulic conductivity along the drainage path. The technology relies on the following principles:

• Mechanical support: The screen pipe provides structural reinforcement to the borehole wall, resisting collapse caused by overburden stress and coal body compaction.
• Selective permeability: The slotted or perforated screen geometry permits gas and water flow while filtering out particulate matter, maintaining long-term borehole permeability.
• Stress redistribution: The screen pipe distributes radial stress from the surrounding coal body, reducing differential stress concentrations that would otherwise cause localized failure.

1.3 CO₂ Phase-Change Fracturing Principle

CO₂ phase-change fracturing exploits the thermodynamic properties of carbon dioxide to generate fractures in the coal matrix. The process involves injecting high-pressure CO₂ into the target formation, where it undergoes a phase transition from liquid to supercritical or gaseous state. The rapid expansion generates localized stress concentrations that exceed the coal body's tensile strength, creating a network of micro-fractures and secondary fracture channels. Key thermodynamic parameters governing this process include:

• Phase transition threshold: CO₂ transitions from liquid to gas at approximately 31.1°C (critical temperature) and 7.38 MPa (critical pressure). Below these conditions, the liquid-to-gas expansion ratio can reach 450:1, generating sufficient stress to fracture the coal matrix.
• Fracture network formation: The CO₂ phase-change process creates both primary fractures (radial, emanating from the borehole) and secondary fractures (perpendicular or oblique to the primary fractures), significantly expanding the effective drainage area.
• Fracture propping: Residual CO₂ gas and optionally introduced proppant materials maintain fracture aperture post-treatment, preventing closure under in-situ stress.

2. Category and Business Positioning

This technology falls within the category of coalbed methane drainage enhancement and borehole completion technology. Within the operational framework of Cladding Technology Shanxi Co., Ltd, this entry represents a strategic expansion into the coal mining services sector, leveraging the company's metallurgical expertise in clad materials, weld overlay fabrication, and specialized pipe manufacturing to deliver integrated solutions for CBM extraction challenges.

2.1 Strategic Positioning within the Company Portfolio

The combined application of screen hole protection and CO₂ phase-change fracturing positions the company at the intersection of its core metallurgical capabilities and the energy sector's growing demand for enhanced gas recovery. The technology serves as a bridge between the company's traditional cladding and weld overlay operations and high-value mining engineering services, creating opportunities for:

• Specialized screen pipe fabrication with corrosion-resistant cladding layers
• CO₂ injection system components requiring high-pressure weld overlay
• Integrated service delivery combining material manufacturing with field application

2.2 Value Chain Integration

The technology entry reflects the company's commitment to deepening its value chain participation. Rather than serving solely as a materials supplier, the company is developing the technical competency to understand and support the full application context of its products—particularly in demanding environments such as CO₂-exposed drilling and completion systems where material integrity directly impacts operational safety and economic outcomes.

3. Technical Purpose and Value

3.1 Primary Technical Objectives

The combined technology is designed to achieve the following quantifiable objectives in low-permeability soft coal seam CBM drainage operations:

• Permeability enhancement: Increase effective coal seam permeability by 3 to 8 times relative to the untreated baseline, expanding the effective drainage radius from a typical 5–10 m to 15–30 m.
• Borehole longevity: Extend effective drainage borehole service life by 40–60% through screen pipe protection, reducing the frequency of borehole re-drilling.
• Gas recovery rate improvement: Achieve single-borehole daily gas drainage volumes of 800–2,000 m³ in formations where untreated boreholes yield only 100–300 m³/day.
• Drainage efficiency: Reduce the time required to reach steady-state gas drainage conditions from 6–12 months to 2–4 months.

3.2 Economic Value

The economic value of this combined technology is realized through multiple channels:

• Increased gas production: Enhanced permeability directly translates to higher CBM volumes, with estimated incremental revenue of ¥300–800 per meter of treated borehole length.
• Reduced operational costs: Screen hole protection reduces borehole maintenance and re-drilling frequency, saving ¥15,000–40,000 per borehole over its service life.
• Safety improvement: Effective gas drainage reduces the risk of gas outbursts and coal and gas outburst incidents, providing immeasurable safety value.
• Environmental benefit: Captured CBM is a clean energy source that displaces natural gas and reduces methane emissions, contributing to carbon reduction targets.

3.3 Contribution to Qualification Building

This technology entry contributes to the company's qualification building in several dimensions:

• Technical competency demonstration: Mastery of this integrated technology demonstrates the company's capability to operate across material science and field engineering disciplines.
• Standard compliance track record: Successful implementation generates documented performance data that supports future qualification applications under industry standards.
• Customer trust development: Delivering measurable permeability enhancement results builds credibility with coal mining operators who require proven technology for safety-critical operations.

4. Key Process and Implementation Points

4.1 Process Flow Overview

The combined technology follows a structured implementation sequence:

1. Geological assessment: Characterize coal seam permeability, mechanical properties, stress field, and gas content through core analysis and field measurements.
2. Borehole drilling: Drill drainage boreholes to target depth using appropriate bit and mud systems for soft coal conditions.
3. Screen pipe installation: Lower pre-manufactured screen pipes into the borehole, ensuring proper centralization and cementation of non-productive intervals.
4. CO₂ injection preparation: Charge CO₂ into injection vessels at controlled pressure and temperature, verify equipment integrity through pressure testing.
5. CO₂ phase-change fracturing: Inject CO₂ into the formation through the screen pipe at controlled pressure rates, monitor injection parameters, and allow phase transition to occur in-situ.
6. Fracture stabilization: Introduce proppant materials (if required) to maintain fracture aperture, and allow the system to stabilize.
7. Drainage initiation: Connect the borehole to the gas collection system and initiate gas drainage, monitoring flow rates and composition.
8. Performance monitoring: Track drainage performance over time, compare against baseline expectations, and document results for technology optimization.

4.2 Screen Pipe Design and Fabrication Parameters

Parameter	Specification Range	Rationale
Screen pipe outer diameter	73–114 mm	Matched to borehole diameter (typically 1.5–2× screen OD) for soft coal conditions
Slit width	0.2–0.5 mm	Optimized for gas permeability while filtering coal fines in soft coal formations
Slit length	50–100 mm	Provides adequate flow area while maintaining structural integrity of the pipe wall
Slit pitch	20–50 mm (center-to-center)	Balances flow capacity with mechanical strength requirements
Screen opening area ratio	15–30%	Ensures sufficient gas ingress while maintaining pipe structural strength
Material grade	API 5CT J55/K55 or higher, with optional cladding layer	Provides corrosion resistance and mechanical strength for CO2-exposed environments
Connection type	API thread or threaded coupling with seal	Ensures pressure integrity and leak prevention at joints

4.3 CO₂ Injection Process Parameters

Parameter	Typical Range	Control Method
CO2 injection pressure	10–25 MPa	Controlled by high-pressure pump or accumulator system with pressure relief valve
CO2 injection volume	500–2,000 L per borehole (formation-dependent)	Measured by calibrated flow meters and accumulation tanks
Injection rate	50–200 L/min	Controlled by variable-speed pump or throttling valve
CO2 purity	≥99.5% CO2	Verified by gas chromatography analysis prior to charging
Injection temperature	Ambient to 40°C (controlled)	Monitored by thermocouples at injection point; heating may be applied if necessary
Phase transition monitoring	Pressure drop rate, acoustic emission	Real-time pressure transducers and acoustic sensors at surface
Post-injection stabilization time	24–72 hours	Controlled by timer; pressure monitoring confirms stabilization

4.4 Critical Implementation Considerations

Formation-specific parameter optimization: The CO₂ injection pressure and volume must be calibrated to the specific coal seam's mechanical properties. Over-pressurization risks creating excessive fractures that reduce coal body stability and compromise mining safety; under-pressurization fails to achieve meaningful permeability enhancement. A pre-treatment geomechanical assessment is essential to establish safe and effective injection parameters.

Screen pipe integrity verification: Prior to installation, screen pipes must undergo pressure testing to 1.5 times the design pressure to confirm weld integrity and slit geometry accuracy. Non-destructive testing (NDT) of weld joints, including ultrasonic testing (UT) and dye penetrant inspection (PT), is required per applicable standards.

CO₂ system safety: High-pressure CO₂ systems present significant hazards including asphyxiation risk, pressure vessel failure, and rapid depressurization effects. All system components must be certified for high-pressure gas service, and comprehensive safety protocols including pressure relief, gas detection, and emergency response procedures must be implemented.

5. Applicable Standards and Acceptance Criteria

5.1 Screen Pipe Standards

• GB/T 19299 — Steel pipes for oil and gas well casing (applicable to screen pipe base material)
• API 5CT — Specifications for casing and tubing (material grade and mechanical property requirements)
• SY/T 5437 — Technical requirements for screen pipes for oil and gas wells (slit geometry, opening area ratio, mechanical performance)
• ASTM A53 — Standard specification for welded and seamless wrought iron pipe (alternative material specification)

5.2 CO₂ Equipment and System Standards

• GB 150 — Pressure vessel design and fabrication (applicable to CO₂ storage and transfer vessels)
• GB/T 12337 — Refrigeration pressure vessels (applicable to cryogenic CO₂ storage)
• TSG 21 — Supervision regulation for stationary pressure vessels (mandatory safety supervision requirements)
• ASME BPV Section VIII — Boiler and Pressure Vessel Code (alternative for pressure vessel certification)
• GB 50157 — Design code for compressed gas station (applicable to CO₂ storage and handling facilities)

5.3 CBM Drainage and Permeability Enhancement Standards

• GB 50484 — Design code for coal mine gas drainage systems
• AQ 1026 — Safety specifications for coal mine gas drainage
• SY/T 6610 — Technical requirements for coalbed methane development wells
• NB/T 10075 — Technical specifications for coal seam gas drainage borehole completion
• ISO 13679 — Petroleum and natural gas industries — Well completion equipment (general reference for completion standards)

5.4 Acceptance Criteria

Acceptance Item	Criterion	Verification Method
Screen pipe pressure test	Withstand 1.5× design pressure for 10 minutes without leakage or deformation	Hydraulic pressure test with visual and gauge monitoring
Screen pipe NDT (weld joints)	No cracks, porosity, or incomplete fusion per acceptance level Ⅱ	Ultrasonic testing (UT) per GB/T 11345 or equivalent
CO2 system pressure test	Withstand 1.25× maximum operating pressure for 30 minutes	Hydrostatic or pneumatic test with certified pressure gauges
Post-fracturing permeability	Increase of ≥3× baseline permeability (measured or estimated)	Flow test or pressure transient analysis post-treatment
Drainage gas flow rate	≥800 m3/day within 7 days of treatment (formation-dependent)	Gas flow meter at borehole outlet
Drainage gas concentration	≥30% CH4 at borehole outlet	Gas chromatography or portable gas analyzer

6. Common Risks and Controls

6.1 Technical Risks

Risk Category	Description	Control Measures
Over-pressurization during CO2 injection	Excessive injection pressure may cause uncontrolled fracturing, coal body damage, or equipment failure	Install pressure relief valves set at 110% of maximum operating pressure; implement real-time pressure monitoring with automated shutoff at alarm thresholds; conduct pre-injection pressure calibration
Screen pipe collapse	Soft coal compaction may deform or collapse the screen pipe, reducing effective drainage area	Select screen pipe with adequate wall thickness and mechanical strength for formation stress; use centralizers to maintain screen pipe position; consider casing support in highly unstable intervals
CO2 leakage	Leakage from high-pressure CO2 systems poses asphyxiation and environmental hazards	Implement comprehensive leak detection systems; ensure all connections are properly sealed and pressure-tested; establish confined space entry procedures; provide personal protective equipment (PPE)
Insufficient permeability enhancement	CO2 phase-change fracturing may not achieve target permeability increase due to formation heterogeneity or incorrect parameter selection	Conduct thorough pre-treatment geomechanical assessment; use pilot injections to calibrate parameters; implement multi-stage injection if initial results are suboptimal; consider alternative or supplementary stimulation methods
Fracture closure	Fractures created by CO2 phase-change may close under in-situ stress, reducing long-term permeability enhancement	Introduce proppant materials (sand, ceramic, or resin-coated particles) during or after CO2 injection; design fracture geometry to maximize stress-shadow effects; monitor long-term drainage performance

6.2 Safety Risks

• Asphyxiation hazard: CO₂ is heavier than air and can accumulate in low-lying areas. Implement fixed CO₂ gas detectors at ground level and at working elevations; establish exclusion zones during injection operations; require continuous air monitoring for personnel in the vicinity.
• Pressure vessel failure: CO₂ storage and transfer vessels operating at high pressure present rupture and projectile hazards. Ensure all vessels are certified per GB 150 or ASME BPV Section VIII; implement regular inspection and hydrostatic testing programs; establish safe exclusion distances during operations.
• Rapid depressurization effects: Sudden release of high-pressure CO₂ can cause rapid temperature drop (Joule-Thomson effect) leading to frostbite or brittle fracture of equipment. Use insulated and cryogenically rated piping; implement controlled depressurization procedures; train personnel on emergency response.
• Coal and gas outburst: In high-gas-pressure formations, drilling and fracturing operations may trigger coal and gas outburst. Implement outburst prediction and prevention measures per AQ 1026; use outburst-resistant drilling techniques; establish emergency evacuation procedures.

6.3 Environmental Risks

• Surface subsidence: Extensive fracturing may cause localized surface subsidence. Limit fracturing intensity in areas with surface infrastructure; monitor surface deformation with survey markers or InSAR; design fracture geometry to avoid critical surface features.
• Groundwater contamination: CO₂ injection or leakage may affect groundwater quality. Conduct pre-treatment groundwater baseline sampling; implement monitoring wells around treatment areas; use casing and cementing to isolate non-productive intervals and protect aquifers.
• CO₂ emissions: Uncontrolled CO₂ release contributes to greenhouse gas emissions. Implement closed-loop CO₂ recovery systems where feasible; monitor and report CO₂ emissions; consider using captured CO₂ from industrial sources to achieve carbon-neutral operations.

7. Application Scenarios Across the Company's Technology Routes

While this technology entry is primarily a coal mining engineering application, it creates significant synergies with the company's three core technology routes. The following analysis demonstrates how each route contributes to the delivery of this combined technology.

7.1 TIG/MIG Weld Overlay Applications

TIG (Tungsten Inert Gas) and MIG (Metal Inert Gas) weld overlay technology is directly applicable to the manufacturing of specialized components required for this combined technology:

• CO₂ injection system components: High-pressure CO₂ is corrosive, particularly in the presence of moisture. TIG weld overlay of corrosion-resistant alloys (e.g., 309L, 316L, or duplex stainless steel) on carbon steel pipe fittings, valves, and flanges extends service life and ensures leak-tight integrity. The overlay layer provides a diffusion barrier against CO₂-induced corrosion while maintaining the mechanical strength of the base material.
• Screen pipe repair and refurbishment: Damaged or worn screen pipes can be refurbished through TIG weld overlay of the slit areas or damaged sections, restoring structural integrity and extending service life without full replacement. This is particularly valuable for proprietary screen pipe designs where replacement parts are unavailable.
• High-pressure pump components: CO₂ injection pumps require wear-resistant and corrosion-resistant surfaces on plungers, barrels, and seals. MIG weld overlay with hardfacing alloys (e.g., Ni-based or Cr-based) enhances surface durability under high-pressure cyclic loading.
• Transition layer fabrication: When connecting dissimilar materials in CO₂ systems (e.g., carbon steel to stainless steel), TIG weld overlay creates a transition layer that prevents intergranular corrosion and hydrogen embrittlement at the weld interface.

7.2 Hydraulic Explosive Bonding Applications

Hydraulic explosive bonding technology contributes to the manufacturing of clad components for this application:

• Clad pipe for CO₂ transport: Hydraulic explosive bonding produces high-integrity clad pipes with a corrosion-resistant outer or inner layer bonded to a structural base material. These clad pipes are ideal for CO₂ transport lines where the combination of high pressure and corrosive environment requires both mechanical strength and corrosion resistance without the cost of fully alloy piping.
• Clad plate for pressure vessel linings: CO₂ storage and transfer vessels can be fabricated using clad plate construction, where a corrosion-resistant inner cladding layer is bonded to a structural steel outer layer. This approach reduces material costs while ensuring the internal surface resists CO₂ corrosion.
• Multi-layer clad components: For applications requiring both corrosion resistance and wear resistance (e.g., CO₂ injection nozzles), multi-layer clad construction can be achieved through sequential hydraulic explosive bonding or combination with weld overlay, providing tailored surface properties.

7.3 Explosion Welding Applications

Explosion welding technology provides additional capabilities for this technology entry:

• Large-diameter clad pipe fabrication: Explosion welding is particularly effective for producing large-diameter clad pipes (up to 2,000 mm or more) that may be required for CO₂ collection and transport mainlines in large-scale CBM operations. The explosive bonding process produces metallurgical bonds with superior interface quality compared to mechanical cladding methods.
• Clad plate for surface preparation: Large-format clad plates produced by explosion welding can serve as raw material for fabrication of CO₂ system components, including pressure vessel shells, pipe spools, and structural supports.
• Specialty alloy combinations: Explosion welding enables bonding of material combinations that are not achievable through fusion welding, such as titanium to carbon steel or aluminum to steel. These specialty clad materials may be required for specific CO₂ system components where unique corrosion resistance or weight considerations apply.

7.4 Integrated Value Delivery

The convergence of this coal mining technology with the company's metallurgical capabilities creates a differentiated value proposition:

• End-to-end solution delivery: The company can manufacture clad pipes and weld-overlay components for CO₂ systems, supply screen pipes with corrosion-resistant cladding, and provide technical support for permeability enhancement operations—delivering a complete solution from material to field performance.
• Technical qualification leverage: Experience in this application domain generates documented performance data that supports qualification for additional CBM and coal mining projects, creating a positive feedback loop for business development.
• Customer value amplification: By understanding the full application context, the company can optimize material specifications, provide application-specific recommendations, and deliver higher-value solutions that address the customer's total cost of ownership rather than just material cost.

8. Conclusion and Forward Outlook

The combined application of screen hole protection and CO₂ phase-change fracturing permeability enhancement technology represents a significant technical capability for Cladding Technology Shanxi Co., Ltd. This technology addresses a critical industry challenge—enhancing gas drainage in difficult geological formations—while creating meaningful opportunities to leverage the company's core metallurgical competencies in weld overlay, hydraulic explosive bonding, and explosion welding.

The strategic value of this technology entry extends beyond immediate project delivery. It establishes the company as a technically sophisticated partner capable of integrating material science with field engineering, a positioning that is increasingly valuable in the energy sector where customers demand integrated solutions rather than discrete material supplies. The documented learning and technical mastery reflected in this entry contribute to the company's overall qualification portfolio, supporting future business development in the coalbed methane and coal mining services markets.

Future development priorities should include: standardizing the technical protocol for CO₂ phase-change fracturing across different geological conditions; developing proprietary screen pipe designs with enhanced cladding protection; establishing a database of treatment performance data to support predictive modeling; and pursuing relevant industry certifications and standard participation to formalize the company's technical authority in this domain.