CO₂ Dry-Method Proppant-Filled Fracturing Technology: Principles, Implementation, and Integration with Clad Pipe Systems
1. Definition and Fundamental Principles
CO₂ dry-method proppant-filled fracturing (CO₂干法加砂压裂) is an advanced reservoir stimulation technique that employs supercritical or subcritical carbon dioxide as the primary carrier fluid to transport proppant (sand) into naturally or artificially created fractures in tight, low-permeability, or unconventional reservoirs. Unlike conventional water-based (slickwater) hydraulic fracturing, the dry CO₂ method eliminates the need for large volumes of water, chemical additives, and associated flowback water management.
The fundamental operating principle relies on the unique thermophysical properties of carbon dioxide at reservoir conditions. When CO₂ is injected at pressures exceeding its critical pressure (7.38 MPa) and temperatures above its critical temperature (31.1 °C), it enters a supercritical state where it exhibits gas-like viscosity and diffusivity combined with liquid-like density and solvency. This supercritical state enables:
- Enhanced fracture propagation: The low viscosity of supercritical CO₂ (0.05–0.1 mPa·s) compared to water (0.3–0.6 mPa·s) allows for complex, branching fracture networks to develop, increasing the effective stimulated reservoir volume (SRV).
- Reduced formation damage: The absence of water eliminates clay swelling, water-blocking, and fines migration in water-sensitive reservoirs such as tight gas sands, coalbed methane, and shale gas formations.
- Efficient proppant placement: The rapid phase transition of CO₂ upon entering the reservoir (from supercritical to gas phase as pressure drops) generates a powerful expansion force that pushes proppant deeper into fracture networks.
- Zero water footprint: Eliminates flowback water, produced water handling, and associated environmental compliance burdens.
2. Category and Business Positioning
Within the operational framework of Cladding Technology Shanxi Co., Ltd., CO₂ dry-method fracturing technology occupies a strategic position at the intersection of materials engineering and oilfield services. The company's core competencies in bimetallic cladding, weld overlay, and explosion welding directly serve the demanding material requirements of CO₂ fracturing operations:
- Upstream materials supply: CO₂ environments are highly corrosive to conventional carbon steel and even some stainless grades. Clad pipes, overlay-lined tubing, and corrosion-resistant surface treatments are essential for wellbore integrity during and after fracturing operations.
- Technology knowledge integration: Understanding CO₂ fracturing process parameters, downhole conditions, and material degradation mechanisms enables the company to design and qualify cladding solutions that withstand specific service environments.
- Value chain extension: The company positions itself as a materials and surface engineering partner to oilfield service companies performing CO₂ fracturing, providing the critical infrastructure materials that enable safe and efficient operations.
3. Technical Purpose and Value
3.1 Reservoir Stimulation Objectives
CO₂ dry-method proppant-filled fracturing addresses several critical challenges in modern hydrocarbon production:
- Tight reservoir productivity enhancement: In formations with permeabilities below 1 mD, conventional stimulation methods are insufficient. The complex fracture geometry created by CO₂ significantly increases drainage area and well productivity.
- Low-permeability coalbed methane (CBM) stimulation: Water-sensitive CBM reservoirs respond poorly to water-based fracturing. CO₂'s dry nature preserves coal matrix integrity while creating effective gas pathways.
- Environmental compliance: Eliminates the 90–95% water content of conventional fracturing fluids, reducing surface water consumption from 10,000–50,000 m³ per stage to zero, and eliminating flowback water treatment requirements.
- Carbon utilization: When CO₂ is sourced from industrial capture, this technology contributes to carbon utilization pathways while simultaneously enhancing hydrocarbon recovery.
3.2 Value to Cladding Technology Shanxi Co., Ltd.
The study and practical implementation of CO₂ fracturing technology provides direct value to the company's core business through:
- Market qualification: Demonstrates technical competence in understanding downstream application environments, strengthening bids for clad pipe and overlay-lined equipment supply contracts.
- Product specification development: Enables the development of material specifications tailored to CO₂ fracturing service conditions (temperature, pressure, corrosion potential, proppant abrasion).
- Customer relationship deepening: Provides technical dialogue capability with oilfield service companies and operators, positioning the company as a technically informed partner rather than a pure materials supplier.
4. Key Process and Implementation Points
4.1 CO₂ Dry Fracturing Process Flow
- CO₂ sourcing and preparation: Industrial-grade CO₂ (purity ≥99.5%) is sourced from natural gas processing, flue gas capture, or dedicated CO₂ production facilities. The CO₂ is compressed to liquid state at surface conditions (typically 5–15 MPa at 15–25 °C).
- Proppant preparation: Proppant (typically 20/40 mesh or 40/70 mesh ceramic or quartz sand) is loaded into a dry mixing system. Unlike water-based fracturing, no slurry is prepared—the proppant is injected separately or mixed with CO₂ in a dry blending device.
- Injection system deployment: A specialized high-pressure CO₂ injection system (capable of 35–70 MPa) with dry proppant injection capability is rigged at the wellhead. The system includes CO₂ storage tanks, high-pressure pumps, proppant dry blender, and downhole pressure control equipment.
- Fracture initiation and propagation: CO₂ is injected at controlled rates (typically 5–30 m³/min for gas wells) to initiate and propagate fractures. The low viscosity of CO₂ creates tortuous, branched fracture networks.
- Proppant placement: Dry proppant is injected either simultaneously with CO₂ or in a separate stage. The CO₂'s phase transition energy at reservoir conditions provides the driving force for proppant placement into fracture networks.
- Well shut-in and flowback: After injection, the well is shut in to allow proppant settling and fracture closure. Flowback is primarily CO₂ gas with minimal liquid return.
4.2 Critical Process Parameters
| Parameter | Typical Range | Function/Significance |
|---|---|---|
| CO₂ injection pressure | 35–70 MPa | Exceeds fracture initiation pressure; maintains supercritical state in wellbore |
| CO₂ injection rate | 5–30 m³/min | Controls fracture geometry; higher rates create wider, more complex fractures |
| Proppant concentration | 0.5–2.0 kg/L (CO₂ equivalent) | Controls fracture conductivity; must balance placement efficiency with fracture height |
| Proppant type | Ceramic (3.5 SG) or quartz (2.65 SG) | Higher density proppant resists closure better in high-stress environments |
| Proppant size | 20/40 mesh (primary), 40/70 mesh (lead) | Graded sizes optimize near-wellbore and far-field fracture conductivity |
| Reservoir temperature | 35–90 °C | Must exceed CO₂ critical temperature (31.1 °C) for supercritical behavior |
| Reservoir pressure | 8–30 MPa | Determines phase transition energy available for proppant placement |
| Total CO₂ volume per stage | 50–500 m³ | Determines fracture volume and stimulated reservoir volume |
| Shut-in time | 24–72 hours | Allows proppant settling, fracture closure, and CO₂ diffusion into formation |
4.3 Dry Mixing Technology
The critical innovation in "dry-method" (干法) CO₂ fracturing is the elimination of water-based slurry preparation. Proppant is introduced into the CO₂ stream using one of the following dry mixing approaches:
- Side-injection dry blender: Proppant is pneumatically conveyed into the high-pressure CO₂ line using nitrogen or compressed air as the carrier gas. Mixing occurs in a cyclonic or venturi-type blender before the combined stream enters the wellbore.
- Sequential injection: Proppant is injected as a dense gas-solid mixture in discrete slugs, followed by CO₂ "pad" to push proppant into the formation.
- Pre-mixed CO₂-proppant slurry (semi-dry): A minimal amount of CO₂ is used to create a dense supercritical fluid that can suspend proppant without water, leveraging the high density of liquid CO₂ at surface conditions.
5. Applicable Standards and Acceptance Criteria
5.1 Equipment and Materials Standards
| Standard | Title/Scope | Relevance to CO₂ Fracturing |
|---|---|---|
| API 5CT | Specification for casing and tubing | Wellbore integrity during high-pressure CO₂ injection; corrosion resistance requirements |
| API 5L | Pipe line pipe specifications | Surface CO₂ transport piping; pressure rating and material selection |
| API 6A | Specification for wellhead and Christmas tree equipment | Wellhead equipment rated for CO₂ service conditions |
| ASME B31.3 | Process piping | Surface CO₂ piping system design, pressure testing, and inspection |
| ASME BPVC Section VIII | Pressure vessels | CO₂ storage tanks and high-pressure equipment design and fabrication |
| NACE MR0175 / ISO 15156 | Materials for use in H₂S-containing environments | Material selection for sour gas wells where CO₂ fracturing is applied |
| ASTM A398 / A399 | Friction stir welding consumable materials | Related to clad pipe joint integrity in CO₂ service |
| GB/T 21832 | Oil and natural gas industry — Welded steel pipe for casing and tubing | Domestic standard for wellbore pipe used in CO₂ fracturing operations |
| SY/T 5448 | Hydraulic fracturing proppant — Ceramic | Proppant quality specifications for Chinese oilfield applications |
| SY/T 5329 | Hydraulic fracturing — Proppant requirements | Proppant strength, roundness, sphericity, and crush strength criteria |
5.2 Operational Acceptance Criteria
- Fracture initiation confirmation: Injection pressure must exceed calculated fracture initiation pressure (typically determined from mini-fracture tests or geomechanical models) by a minimum of 5–10 MPa.
- Proppant placement verification: Post-fracturing microseismic monitoring or pressure transient analysis must confirm proppant placement within target fracture geometry.
- Wellbore integrity: No evidence of casing deformation, connection leakage, or corrosion initiation during or after CO₂ injection operations.
- Productivity improvement: Post-fracturing production testing (PLT or extended production) must demonstrate measurable rate enhancement compared to pre-fracturing baseline (typically 3–10× improvement expected for tight gas wells).
- Environmental compliance: Zero water discharge; CO₂ emissions from flowback must be captured or vented in compliance with local environmental regulations.
6. Common Risks and Controls
6.1 Technical Risks
| Risk Category | Description | Mitigation Measures |
|---|---|---|
| CO₂ phase transition damage | Rapid expansion from supercritical to gas phase can cause sanding, formation damage, or wellbore instability | Controlled injection rates; staged proppant placement; post-fracturing wellbore stabilization |
| Low fracture conductivity | CO₂'s low viscosity may result in narrow fractures with insufficient proppant placement for long-term conductivity | Higher proppant concentrations; ceramic proppant with higher crush strength; multi-stage treatment design |
| Equipment corrosion | Supercritical CO₂ is highly corrosive to carbon steel, especially in the presence of trace H₂S, H₂O, or chlorides | Use of clad pipe (304L/316L overlay on carbon steel); hardfacing on high-wear components; regular NDT inspection |
| Proppant bridging | Dry proppant may bridge in narrow wellbore sections or at perforation tunnels | Proper perforation density; lead-in with fine mesh proppant; adequate CO₂ volume for proppant transport |
| Wellbore temperature drop | Joule-Thomson cooling effect during CO₂ injection can drop wellbore temperature significantly, risking hydrate formation or cement sheath damage | Controlled injection rate; wellbore insulation; pre-warming procedures where applicable |
| Asphyxiation hazard | CO₂ is heavier than air and can accumulate in low-lying areas, creating asphyxiation risk for personnel | Continuous gas monitoring; emergency response procedures; adequate ventilation; personal protective equipment |
6.2 Material Integrity Risks (Direct Relevance to Cladding Technology)
- Carbon steel degradation: Supercritical CO₂ attacks carbon steel surfaces, causing rapid corrosion rates of 0.5–2.0 mm/year in aggressive environments. Control: Use of 304L/316L stainless steel weld overlay (TIG) on carbon steel pipes and equipment, providing 3–5 mm corrosion-resistant barrier.
- Hydrogen embrittlement: CO₂ corrosion can generate atomic hydrogen that diffuses into steel, causing hydrogen embrittlement cracking. Control: Selection of overlay alloys with high resistance to hydrogen uptake; avoidance of high-strength steels in CO₂ contact zones.
- Proppant abrasion: High-velocity proppant-laden CO₂ flow causes erosion of wellbore components. Control: Hardfacing with Stellite or tungsten carbide overlay at high-flow zones; use of abrasion-resistant clad tubing.
- Thermal cycling fatigue: Temperature fluctuations during injection and shut-in cycles cause thermal stress in clad interfaces. Control: Proper WPS qualification for thermal expansion mismatch; NDT verification of clad bond integrity after thermal cycling testing.
7. Application Scenarios Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
The TIG (Tungsten Inert Gas) and MIG (Metal Inert Gas) weld overlay technology route directly serves CO₂ fracturing operations through the following applications:
- Wellbore casing and tubing overlay: Internal weld overlay of 304L/316L stainless steel (3–6 mm thickness) on API 5CT carbon steel casing and tubing to provide corrosion resistance in CO₂ environments. The TIG process provides excellent control over dilution (typically <10%), ensuring the overlay retains full austenitic corrosion resistance. Multi-pass TIG overlay with 309L transition layer followed by 316L cover layers provides optimal performance.
- Surface equipment protection: Overlay of CO₂ injection pumps, valves, and high-pressure fittings with corrosion-resistant alloys to extend equipment life in supercritical CO₂ service.
- Repair and maintenance: Field-applied weld overlay repair of corroded wellhead components, Christmas tree equipment, and flowlines exposed to CO₂ during fracturing operations.
- Proppant handling equipment: Hardfacing of proppant dry blender components, pneumatic conveying pipes, and hopper internals with abrasion-resistant alloys (e.g., Stellite 6, tungsten carbide) to withstand proppant erosion.
Relevant WPS/PQR requirements: Welding procedures for CO₂ service overlay must be qualified per ASME Section IX or AWS D10.9, with additional requirements for:
- Dilution control verification (metallographic examination showing <10% base metal dilution in overlay)
- Corrosion testing per ASTM G101 or NACE TM0177 (CO₂ corrosion rate <0.025 mm/year)
- Hydrogen embrittlement resistance testing per ASTM G178 (slow strain rate testing)
7.2 Hydraulic Explosive Bonding Route
The hydraulic explosive bonding (water jet + explosive) technology route contributes to CO₂ fracturing applications through:
- Clad pipe for CO₂ transport: Production of explosion-bonded clad pipes (e.g., 316L stainless steel on Q345B carbon steel) for surface CO₂ transport pipelines connecting CO₂ storage facilities to fracturing sites. The metallurgical bond achieved through explosive welding provides superior corrosion resistance compared to mechanical bonding methods, with bond strength typically exceeding 100 MPa.
- Large-diameter vessel cladding: Internal cladding of CO₂ storage tanks (ASME Section VIII) with stainless steel using hydraulic explosive bonding, providing uniform corrosion protection over large surface areas without the labor-intensive overlay welding required for tank internals.
- Wellhead manifold protection: Clad construction of wellhead manifolds and high-pressure headers that must withstand both high injection pressures and CO₂ corrosion, combining the structural strength of carbon steel with the corrosion resistance of stainless steel cladding.
Quality assurance for hydraulic explosive bonded products in CO₂ service:
- Bond verification per ASTM A781 (tensile, shear, and peel testing)
- Ultrasonic thickness and bond quality inspection per ASTM E2775
- Corrosion testing of bonded interface per ASTM G5 (immersion) and ASTM G101 (cyclic corrosion)
- NDT per GB/T 11345 (UT) and ASTM E1444 (MT) for defect detection
7.3 Explosion Welding Route
The explosion welding technology route provides the most robust cladding solutions for the most demanding CO₂ fracturing service conditions:
- High-pressure CO₂ injection equipment: Explosion-welded clad construction of high-pressure cylinders, accumulators, and surge tanks that must contain supercritical CO₂ at pressures up to 70 MPa. The explosion welding process creates a metallurgical bond with no intermetallic degradation, maintaining full mechanical and corrosion properties of both base and cladding materials.
- Downhole tools and equipment: Clad construction of downhole valves, packers, and completion tools that must withstand combined high-pressure CO₂, proppant abrasion, and potential H₂S coexistence. Explosion welding allows thick cladding layers (5–15 mm) that provide extended service life.
- Proppant storage and handling: Explosion-welded clad hoppers, silos, and conveying equipment for proppant storage, where the combination of abrasion resistance (hardfacing overlay) and corrosion resistance (stainless cladding) is required.
- CO₂ capture and compression equipment: Clad heat exchangers, compressors, and processing equipment used in CO₂ capture facilities that supply CO₂ for fracturing operations, where sour gas corrosion (H₂S + CO₂) creates extreme material demands.
Explosion welding specifications for CO₂ service:
- Explosion parameters qualified per ASTM A781 for the specific material combination (e.g., 316L/Q345B, Inconel 625/Q345B)
- Wave pattern verification through macrographic examination (consistent, periodic bonding waves)
- Full radiographic or ultrasonic inspection of clad interface per GB/T 3323 or ASTM E164
- Pressure testing per ASME BPVC Section VIII (hydrostatic test at 1.5× MAWP)
- Corrosion resistance verification per NACE MR0175/ISO 15156 for sour service qualification
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The study and practical implementation of CO₂ dry-method fracturing technology contributes to the company's qualification portfolio in the following ways:
- Technical competence demonstration: Demonstrates understanding of the end-use environment for cladding products, enabling the company to qualify for bids requiring knowledge of CO₂ service conditions and material selection criteria.
- WPS/PQR expansion: Develops and qualifies welding procedures specifically for CO₂ service overlay, including procedures for 316L/304L overlay on carbon steel with verified low dilution, corrosion resistance, and hydrogen embrittlement resistance. These WPS/PQRs can be submitted to operators and oilfield service companies as part of qualification packages.
- Material specification development: Creates proprietary material specifications for CO₂ fracturing service that exceed minimum API/NACE requirements, providing a competitive differentiation for product qualification.
- Cross-industry certification: Positions the company for participation in oilfield service company qualification programs (e.g., Schlumberger, Halliburton, CNPC supplier qualification), where understanding of fracturing technology is a prerequisite.
8.2 Product Delivery Enhancement
- Application-specific product design: Enables the development of product specifications tailored to specific CO₂ fracturing service conditions (temperature, pressure, corrosion potential, proppant abrasion level), resulting in products that better match customer needs and reduce field failures.
- Reduced warranty claims: Understanding of CO₂ corrosion mechanisms and proppant erosion patterns enables better overlay thickness specification, alloy selection, and WPS design, reducing premature failure and associated warranty costs.
- Accelerated project timelines: Pre-qualified WPS/PQR packages for CO₂ service overlay enable faster project execution, as customers do not need to undergo lengthy WPS qualification cycles.
- Value-added testing services: The company can offer corrosion testing, overlay dilution analysis, and service life prediction as value-added services to customers, enhancing product competitiveness.
8.3 Customer Value Creation
- Extended equipment life: Properly specified and applied cladding solutions extend the service life of CO₂ fracturing equipment from 1–2 years (unprotected carbon steel) to 8–15 years, providing significant ROI to operators.
- Reduced non-productive time (NPT): Reliable clad equipment reduces unplanned downtime due to corrosion failures, saving operators significant costs in well intervention and equipment replacement.
- Environmental compliance support: By enabling reliable CO₂ fracturing operations through superior materials solutions, the company supports operators in meeting environmental regulations and achieving sustainable production targets.
- Technical partnership: The company's understanding of CO₂ fracturing technology enables it to serve as a true technical partner, providing engineering support, material selection guidance, and field service support beyond simple product supply.
9. Integration Framework: Connecting CO₂ Fracturing Knowledge to Core Capabilities
| CO₂ Fracturing Requirement | Technical Challenge | Company Solution (Technology Route) | Key Standard/Specification |
|---|---|---|---|
| Wellbore corrosion resistance | Supercritical CO₂ attacks carbon steel at 0.5–2.0 mm/year | 304L/316L TIG weld overlay (3–6 mm) on casing/tubing | API 5CT; NACE MR0175; ASTM G101 |
| Surface CO₂ transport piping | High-pressure CO₂ transport requires corrosion-resistant, pressure-rated pipe | Explosion-welded 316L/Q345B clad pipe | ASME B31.3; ASTM A781; GB/T 11345 |
| Proppant handling equipment | Abrasion from high-velocity proppant-laden CO₂ flow | Stellite 6 hardfacing overlay on blender components | AWS A5.15; ASTM A276 (substrate) |
| High-pressure CO₂ storage | Large-volume CO₂ storage at 5–15 MPa with corrosion resistance | Hydraulic explosive bonded clad pressure vessels | ASME BPVC VIII Div.1; ASTM E2775 |
| Downhole completion tools | Combined high-pressure, corrosion, and abrasion in downhole environment | Explosion-welded thick clad (5–15 mm) tool bodies | API 17D; NACE MR0175; ASTM A781 |
| Wellhead equipment | High-pressure CO₂ with potential H₂S coexistence | Multi-layer overlay (309L transition + 316L cover) on wellhead components | API 6A; NACE MR0175; ASME IX |
10. Conclusion and Forward Outlook
The study and practical implementation of CO₂ dry-method proppant-filled fracturing technology represents a strategic knowledge investment for Cladding Technology Shanxi Co., Ltd. that directly enhances the company's ability to serve the oil and gas industry's growing demand for corrosion-resistant and abrasion-resistant materials solutions.
As global energy transition accelerates and unconventional resource development intensifies, CO₂ fracturing is expected to see significant growth, particularly in:
- Carbon utilization projects: CO₂ captured from industrial sources being used for both EOR (enhanced oil recovery) and reservoir stimulation, creating dual value from captured carbon.
- Unconventional gas development: Tight gas, shale gas, and coalbed methane formations where water-based fracturing is technically or environmentally impractical.
- Environmental compliance drivers: Regulatory pressure to reduce water usage and flowback water management in fracturing operations.
By maintaining deep technical understanding of CO₂ fracturing processes, the company ensures that its TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding capabilities remain aligned with evolving market demands. This knowledge integration enables the company to:
- Proactively develop product offerings for emerging CO₂ fracturing markets
- Provide technically informed material selection and engineering support to customers
- Maintain competitive differentiation through application-specific qualification and testing capabilities
- Position itself as a strategic materials partner in the oil and gas value chain
The practical experience gained from CO₂ fracturing technology research directly translates into improved product specifications, expanded WPS/PQR portfolios, enhanced NDT protocols, and deeper customer relationships—ultimately driving revenue growth and market share expansion in the high-value oilfield materials segment.