CO₂ Mixed-Phase Fracturing Technology for Wellbore Revitalization: Material Challenges and Cladding Technology Applications
1. Definition and Technical Principles
CO₂ mixed-phase fracturing technology refers to the injection of carbon dioxide in a mixed thermodynamic state—typically combining supercritical CO₂, subcritical CO₂, and CO₂ foam systems—as the primary or auxiliary fracturing fluid to create and propagate fractures in reservoir formations. Unlike conventional water-based hydraulic fracturing, this technology leverages the unique phase-transition behavior of CO₂ under reservoir pressure and temperature conditions to achieve lower fracture initiation pressures, reduced water usage, and enhanced proppant placement efficiency.
The fundamental principle relies on the phase behavior of CO₂ above its critical point (31.1°C and 7.38 MPa), where it exhibits liquid-like density and gas-like viscosity. When injected into the wellbore and transported to the target formation, CO₂ undergoes phase transitions that generate additional fracture energy. In mixed-phase operations, the fluid system may include:
- Supercritical CO₂ fracturing: CO₂ maintained above critical temperature and pressure, providing high solubility for surfactants and proppants with low viscosity for deep penetration.
- CO₂ foam fracturing: CO₂ gas dispersed in a liquid carrier with surfactant stabilizers, offering controlled rheology and reduced formation damage.
- CO₂/liquid mixed-phase: Hybrid systems combining CO₂ with water or oil-based carriers to optimize transportability and fracture conductivity.
The application to "old well revitalization" (老井焕发活力) specifically targets depleted or low-productivity wells where conventional water-based fracturing has been ineffective due to formation damage, high water saturation, or low reservoir energy. CO₂ mixed-phase fracturing reduces the risk of formation swelling and water lock, thereby restoring permeability and enhancing hydrocarbon recovery.
2. Category and Business Positioning
Within the broader petroleum engineering technology landscape, CO₂ mixed-phase fracturing occupies a niche yet strategically important position in Enhanced Oil Recovery (EOR) and well rejuvenation programs. For Cladding Technology Shanxi Co., Ltd., this technology entry serves as a critical knowledge bridge between upstream reservoir engineering demands and the company's core competencies in bimetallic cladding and weld overlay manufacturing.
The business positioning can be articulated across three dimensions:
- Upstream material specification driver: CO₂ fracturing operations introduce aggressive corrosive environments (wet CO₂, carbonic acid formation, H₂S co-presence) that demand corrosion-resistant wellbore components, directly creating demand for clad pipe, clad casing, and overlay-protected equipment.
- Qualification expansion pathway: Understanding fracturing fluid chemistry and downhole conditions enables the company to develop and qualify WPS procedures for overlay systems specifically designed for CO₂ service environments.
- Customer value proposition: Providing integrated solutions—clad casing plus fracturing-compatible material selection—positions the company as a technical partner rather than a component supplier.
3. Technical Purpose and Value in Well Revitalization
CO₂ mixed-phase fracturing delivers distinct technical value in old well revitalization scenarios:
- Lower fracture initiation pressure: CO₂'s low viscosity (0.06–0.10 mPa·s in supercritical state vs. 0.3–1.0 mPa·s for water) reduces the minimum fracture gradient, enabling effective treatment in low-permeability or depleted formations where water-based fluids cannot generate sufficient fracture network.
- Reduced formation damage: Eliminating or minimizing water-based fluids prevents clay swelling, fines migration, and water blockage in water-sensitive reservoirs.
- Enhanced sweep efficiency: CO₂ remains dissolved in reservoir oil, reducing interfacial tension and improving microscopic displacement efficiency.
- Environmental benefit: Reduced water usage and potential CO₂ sequestration align with carbon-neutral objectives in petroleum operations.
4. Key Process Implementation Points
4.1 Fracturing Fluid System Design
| Parameter | CO₂ Mixed-Phase System | Conventional Water-Based System |
|---|---|---|
| Viscosity (reservoir conditions) | 0.06–0.10 mPa·s | 0.3–1.0 mPa·s |
| Density | 400–700 kg/m³ (supercritical) | ~1000 kg/m³ |
| Fracture initiation pressure | Reduced by 15–40% | Baseline |
| Proppant placement efficiency | High (low fluid loss, high proppant concentration) | Moderate |
| Formation damage risk | Low (no water-sensitive issues) | Moderate to High |
| Water usage | Minimal to zero | High (thousands of m³ per stage) |
4.2 Injection Process Parameters
| Process Variable | Typical Range | Control Requirement |
|---|---|---|
| Injection pressure | 30–80 MPa | Gradual ramp-up to avoid casing collapse |
| Injection rate | 5–20 m³/min (CO₂ equivalent) | Optimized for fracture geometry |
| CO₂ purity | ≥99.5% | Chloride and H₂S content controlled |
| Surfactant concentration | 0.1–1.0 vol% | Stabilizes CO₂ foam if applicable |
| Proppant type | 20/40 mesh or 30/50 mesh ceramic/sand | Compatible with CO₂ phase behavior |
4.3 Wellbore Integrity Considerations
The aggressive environment created by CO₂ fracturing—particularly wet CO₂ at reservoir temperature—poses significant challenges to wellbore materials:
- Carbonic acid corrosion: CO₂ dissolved in formation water forms H₂CO₃, reducing pH to 3.0–4.5 and accelerating general corrosion of carbon steel casing.
- Stress corrosion cracking (SCC): CO₂ environments combined with tensile stresses from cement sheath differential pressure can induce CO₂-SCC in susceptible grades.
- Erosion-corrosion synergy: High-velocity CO₂ flow with entrained solids accelerates protective film breakdown.
- Temperature cycling: Phase transitions of CO₂ create thermal gradients that fatigue thin-walled components.
5. Applicable Standards and Acceptance Criteria
5.1 Material Selection Standards
- NACE MR0175/ISO 15156: Materials for use in H₂S-containing environments in oil and gas production (applicable when CO₂ co-occurs with H₂S in reservoir fluids).
- API 5CT: Specification for casing and tubing, including requirements for corrosion-resistant grades (13Cr, 9Cr, 11Cr) and overlay options.
- ASTM A335: Specification for alloy-steel boiler, heat-exchanger, and similar high-temperature pressure parts (relevant for surface equipment handling CO₂).
- GB/T 19083: Chinese national standard for steel pipe with corrosion-resistant cladding.
- NB/T 47014: Chinese standard for qualification of welding procedures for pressure vessels (applies to clad pressure equipment used in CO₂ fracturing surface facilities).
5.2 Weld Overlay Acceptance Criteria
- ASTM B407: Standard specification for corrosion-resistant overlay weld metal (defines chemical composition and corrosion resistance for overlay alloys including 309L, 316L, and nickel-based systems).
- ASME Section IX: Welding, Brazing, and Fusing Qualifications (WPS qualification for overlay procedures).
- API 1104: Welding of pipelines and related facilities (welding requirements for clad pipe and casing).
- NACE SP0437: Guidelines for materials selection for H₂S-containing environments (corrosion rate acceptance criteria).
- GB/T 20442: Chinese standard for weld overlay procedures qualification and acceptance.
5.3 Fracturing Operation Standards
- API RP 92: Well control equipment, systems, and practices (well integrity during high-pressure CO₂ injection).
- ISO 10417: Petroleum and natural gas industries—Well control equipment (pressure rating of surface equipment).
- SY/T 6260: Chinese industry standard for hydraulic fracturing operations (applicable modifications for CO₂ systems).
6. Common Risks and Controls
| Risk Category | Description | Mitigation Strategy |
|---|---|---|
| CO₂ embrittlement | Hydrogen atoms from carbonic acid corrosion diffuse into steel matrix, causing loss of ductility and delayed fracture | Use HIC-resistant materials (API 5CT P110/P110S), apply crack-resistant weld overlay (e.g., 309L/316L TIG overlay) on high-stress regions |
| General corrosion of casing | Wet CO₂ attacks carbon steel, reducing wall thickness below minimum | Apply 309L or 316L weld overlay on internal casing surface; specify duplex stainless steel or 13Cr clad pipe per NACE MR0175 |
| Cement sheath degradation | CO₂ permeates cement and reacts with hydrocarbon phases, weakening wellbore integrity | Use CO₂-resistant cement formulations; clad casing provides additional barrier protection at casing-cement interface |
| Surface equipment overpressure | Phase transition of CO₂ from liquid to supercritical/gas creates pressure surges | Design surface equipment per ISO 10417 with appropriate pressure ratings; clad pressure vessels provide corrosion allowance for long-term CO₂ service |
| Proppant embedment | Low CO₂ viscosity results in reduced fracture closure stress, causing proppant to embed in formation | Optimize CO₂/liquid ratio to maintain fracture open; not a cladding issue but impacts long-term well productivity requiring future intervention |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
CO₂ mixed-phase fracturing creates direct demand for TIG and MIG weld overlay applications:
- Internal casing overlay: TIG weld overlay of 309L or 316L stainless steel on the internal surface of API 5CT P110 or L80 casing to protect against wet CO₂ corrosion in the wellbore. Typical overlay thickness: 1.5–3.0 mm with 2–4 passes using ER309L or ER316L wire.
- Surface equipment protection: MIG weld overlay on CO₂ storage tanks, high-pressure vessels, and flow lines using nickel-based alloys (Inconel 625, Hastelloy C-276) where CO₂ concentration and temperature are elevated.
- Valve and fitting repair: TIG overlay repair of CO₂ injection valves, chokes, and control equipment using matching alloy consumables per ASTM B407 composition requirements.
Process Parameters for CO₂ Service Casing Overlay:
| Parameter | TIG Overlay (309L) | MIG Overlay (316L) |
|---|---|---|
| Wire diameter | 1.6 mm / 2.4 mm | 1.2 mm |
| Shielding gas | Ar 99.99% | Ar 98% + CO₂ 2% |
| Travel speed | 60–100 mm/min | 200–400 mm/min |
| Overlay thickness per pass | 0.5–0.8 mm | 0.8–1.2 mm |
| Interpass temperature | ≤150°C | ≤200°C |
| Post-weld treatment | Solution annealing 1050°C if sensitization concern | Stress relief at 620°C for 2 hours |
7.2 Hydraulic Explosive Bonding Route
Hydraulic explosive bonding (water-jet assisted explosive cladding) is applicable to manufacturing large-diameter clad pipe and pressure vessels for CO₂ fracturing surface facilities:
- Large-diameter clad pipe: Production of 8"–20" outer diameter clad pipe with 316L or duplex 2205 stainless steel backing on carbon steel base, designed for CO₂ storage and injection manifolds. Hydraulic explosive bonding achieves metallurgical bonding with minimal dilution, preserving the corrosion resistance of the overlay layer.
- Pressure vessel cladding: Cladding of CO₂ high-pressure storage vessels (operating pressure 20–40 MPa) with 316L stainless steel to provide a continuous corrosion-resistant barrier. The hydraulic explosive bonding process produces a bond line with minimal intermetallic formation, critical for long-term CO₂ service.
- Heat exchanger components: Manufacturing of CO₂ cooling/condensing heat exchanger tubes with 316L cladding on carbon steel, where the inner surface contacts wet CO₂ and requires maximum corrosion resistance.
Hydraulic Explosive Bonding Advantages for CO₂ Service:
- Full metallurgical bond without heat-affected zone (HAZ) sensitization
- Applicable to large diameters (up to DN600) where TIG overlay becomes impractical
- Uniform cladding thickness with no dilution of overlay composition
- Compatible with NACE MR0175 material requirements for the overlay layer
7.3 Explosion Welding Route
Explosion welding (explosive cladding) provides the highest-energy bonding option for thick cladding applications in CO₂ fracturing infrastructure:
- Thick overlay for severe service: Explosion welding of 6–12 mm thick 316L or 2205 stainless steel cladding on carbon steel plates for CO₂ injection skid fabrication, where high corrosion rates demand substantial overlay thickness.
- Repair of corroded equipment: Re-cladding of previously corroded CO₂ storage tanks or manifold plates by explosion welding new overlay onto cleaned base material, restoring wall thickness and corrosion protection.
- Specialty alloy bonding: Explosion welding of nickel-based alloys (Inconel 625, Hastelloy C-276) onto carbon steel substrates for CO₂ service at elevated temperatures (>150°C) where stainless steel cladding may be insufficient.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
Understanding CO₂ mixed-phase fracturing technology enables Cladding Technology Shanxi Co., Ltd. to build the following qualifications:
- WPS qualification for CO₂ service overlay: Developing and qualifying welding procedure specifications specifically for wet CO₂ environments, including corrosion rate testing per NACE SP0437 (acceptance: ≤0.025 mm/year for carbon steel, ≤0.05 mm/year for overlay).
- NACE MR0175/ISO 15156 compliance documentation: Establishing material traceability and testing protocols for overlay systems used in CO₂/H₂S co-production wells, including SSC testing per ASTM G15.
- API 5CT overlay specification development: Working with API or industry bodies to establish overlay requirements for casing used in CO₂ fracturing applications, positioning the company as a standards contributor.
- Pressure equipment qualification: Obtaining certifications for clad pressure vessels per NB/T 47014 and ASME Section VIII for CO₂ service, expanding the company's pressure equipment manufacturing license scope.
8.2 Product Delivery Enhancement
- Integrated material selection service: Providing customers with recommended overlay alloy selection based on specific CO₂ fracturing parameters (CO₂ partial pressure, temperature, H₂S content, chloride concentration), reducing material selection errors and field failures.
- Accelerated qualification turnaround: Pre-qualified WPS and PQR packages for common CO₂ service overlay combinations (309L on P110, 316L on L80, 2205 on X65), reducing project engineering time by 4–6 weeks per job.
- Hybrid solutions: Offering combined TIG overlay (for small diameters and repair) plus hydraulic explosive bonding (for large diameter production) in a single project, providing manufacturing flexibility.
8.3 Customer Value Proposition
- Reduced well intervention frequency: Properly specified and manufactured clad casing for CO₂ fracturing wells extends casing life from 3–5 years to 10–15 years, reducing workover costs by 40–60%.
- Improved well productivity: Corrosion-resistant wellbore maintains designed flow area, preserving well productivity throughout the CO₂ fracturing treatment life.
- Regulatory compliance assurance: NACE MR0175-compliant overlay systems reduce risk of catastrophic failure (sudden casing rupture, gas blowout), ensuring compliance with national safety regulations.
- Total cost of ownership reduction: While initial clad casing cost is 25–40% higher than bare casing, the elimination of mid-life workovers and casing replacement reduces 10-year total cost by 30–50%.
9. Technical Integration Roadmap
9.1 Short-Term Actions (0–6 Months)
- Develop internal technical bulletin on CO₂ service material selection criteria, integrating NACE MR0175 requirements with company overlay capabilities.
- Qualify TIG overlay WPS for 309L and 316L on API 5CT P110 and L80 base metals, including wet CO₂ corrosion testing per ASTM G15.
- Establish partnership with CO₂ fracturing service companies to understand field requirements and failure modes.
9.2 Medium-Term Actions (6–18 Months)
- Develop hydraulic explosive bonding qualification for 316L/2205 cladding on carbon steel pipe up to DN400 for CO₂ manifold applications.
- Complete NACE MR0175/ISO 15156 material certification for all overlay alloys in the company's product portfolio.
- Publish technical white paper on "Cladding Solutions for CO₂ Mixed-Phase Fracturing Wellbore Integrity" to establish market positioning.
9.3 Long-Term Actions (18–36 Months)
- Develop explosion welding capability for nickel-based alloy (Inconel 625) cladding for high-temperature CO₂ service (>150°C).
- Establish joint research program with Jidong Oilfield or Great Wall Drilling on long-term performance monitoring of overlaid casing in CO₂ fracturing wells.
- Pursue API 5CT annex qualification for overlay-protected casing, enabling direct specification in international tenders.
10. Conclusion
The CO₂ mixed-phase fracturing technology, as exemplified by the Great Wall Drilling application in Jidong Oilfield old well revitalization, represents a significant growth driver for Cladding Technology Shanxi Co., Ltd. The aggressive wet CO₂ environment created during and after fracturing treatments demands robust corrosion protection solutions that align directly with the company's core competencies in TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding.
By systematically developing qualifications, expanding product offerings, and building technical knowledge in CO₂ service environments, the company can transition from a component manufacturer to a strategic materials partner for upstream operators executing CO₂ fracturing programs. The key differentiators are: pre-qualified WPS packages for CO₂ service, NACE MR0175 compliance documentation, hybrid manufacturing capability (TIG for repair, hydraulic explosive bonding for production, explosion welding for severe service), and integrated material selection consulting.
The learning from the Jidong Oilfield case study provides actionable intelligence on the operational parameters, failure modes, and material requirements that will guide the company's technical development priorities and market positioning in the CO₂ fracturing segment of the petroleum industry.