Pressure Release Characteristics of Fracturing Pipes in Supercritical CO₂ Phase-Change Fracturing Technology
1. Definition and Technical Principles
Supercritical CO₂ phase-change fracturing is an advanced hydraulic fracturing methodology in which carbon dioxide is injected into subterranean reservoirs at pressures exceeding the critical point (31.1°C, 7.38 MPa). Upon depressurization and phase transition from supercritical fluid to gas, the CO₂ undergoes rapid volumetric expansion—approximately 500 times its liquid-phase volume—generating sufficient stress to fracture tight or ultra-tight formations. The fracturing pipe (or "致裂管") serves as the critical pressure-containing conduit through which supercritical CO₂ is transported and released into the formation.
The pressure release characteristics of these fracturing pipes encompass the dynamic behavior of internal pressure during the phase-change event, including pressure surge magnitude, release rate, transient stress states, and the resulting mechanical loading on the pipe body and its protective cladding layers. Understanding these characteristics is essential for selecting appropriate base materials, cladding specifications, weld overlay thicknesses, and qualification parameters to ensure structural integrity under extreme transient conditions.
2. Category and Business Positioning
This research entry falls within the company's applied engineering knowledge domain and directly supports the design and qualification of high-performance clad pipes and pressure vessels for unconventional energy extraction applications. Within the business architecture of Cladding Technology Shanxi Co., Ltd., this study occupies a strategic position at the intersection of:
- Product engineering: Informing material selection and cladding design for supercritical CO₂ fracturing equipment
- WPS qualification: Providing the pressure-cycle loading data required to justify weld overlay specifications under cyclic and transient stress conditions
- Customer value delivery: Demonstrating technical competence in understanding end-use service conditions, thereby strengthening proposals for oilfield and gasfield equipment manufacturers
3. Technical Purpose and Value
The primary technical purpose of studying fracturing pipe pressure release characteristics is to establish the boundary conditions under which clad pipes and pressure vessels must perform reliably. Specifically, this research addresses:
- Peak transient pressure determination: Quantifying the maximum instantaneous pressure experienced by the pipe wall during CO₂ phase-change release, which may exceed nominal design pressure by 1.5–3.0 times
- Pressure release rate characterization: Establishing the dp/dt profile during depressurization, which governs the fatigue loading spectrum applied to weld overlay interfaces
- Thermal-mechanical coupling effects: Analyzing temperature drops accompanying rapid depressurization (potentially reaching -78°C at the Joule-Thomson point) and their impact on material toughness and cladding bond integrity
- Corrosion-accelerating mechanisms: Identifying how supercritical CO₂ phase change produces carbonic acid (H₂CO₃) and other corrosive species that attack cladding surfaces and weld transition zones
The commercial value of this knowledge is substantial: it enables the company to specify cladding thickness, weld overlay layer composition, and heat treatment schedules that are both adequate for service conditions and economically optimized, reducing over-engineering costs while eliminating under-design failures.
4. Key Process and Implementation Points
4.1 Supercritical CO₂ Phase-Change Parameters
| Parameter | Typical Value/Range | Impact on Fracturing Pipe |
|---|---|---|
| Critical pressure of CO₂ | 7.38 MPa | Minimum injection pressure threshold |
| Critical temperature of CO₂ | 31.1°C | Phase boundary reference |
| Operating injection pressure | 15–35 MPa | Design pressure basis for pipe body |
| Transient peak pressure during release | 1.5–3.0× design pressure | Governs yield/tensile requirements |
| Maximum depressurization rate | 10–50 MPa/s | Determines cyclic fatigue loading |
| Minimum temperature during release | -78°C to -40°C | Requires low-temperature toughness qualification |
| CO₂ volumetric expansion ratio | ~500:1 (liquid to gas) | Drives fracture energy and pressure dynamics |
4.2 Pressure Release Phases and Stress States
| Phase | Duration | Pressure State | Material Stress Condition | Cladding Design Implication |
|---|---|---|---|---|
| Charging | Minutes | Gradual pressurization to operating level | Steady-state hoop + axial stress | Verify cladding adhesion under sustained load |
| Hold/Injection | Minutes to hours | Constant at operating pressure | Creep-relevant stress (at elevated T) | High-temperature cladding stability |
| Rapid Release | Milliseconds to seconds | Sharp pressure drop with transient surge | Dynamic tensile + thermal shock | Weld interface toughness at low T |
| After-Effect | Seconds to minutes | Residual pressure decay | Residual stress redistribution | Corrosion resistance during wet phase |
4.3 Cladding Specifications Derived from Pressure Release Study
| Component | Recommended Cladding Material | Minimum Cladding Thickness | WPS Qualification Requirement |
|---|---|---|---|
| Pipe body (base) | 16Mn or 20G (GB/T 5310) | — | Design pressure × 1.5 safety factor |
| Inner corrosion-resistant layer | 304L / 316L / 321 (GB/T 13296) | ≥3.0 mm | WPS qualified per NB/T 47014 at -40°C |
| Transition weld overlay | E309L / E309MoL (AWS A5.4/A5.16) | ≥2.0 mm (2 passes) | Impact-tested at minimum service temperature |
| Outer reinforcement (optional) | 20CrMo (for high-pressure) | Per design calculation | Pressure test at 1.5× design pressure |
5. Applicable Standards and Acceptance Criteria
5.1 Design and Material Standards
- GB/T 5310: Steel tubes for high-pressure boiler and superheater—base pipe specification
- GB/T 13296: Cold- or hot-rolled seamless stainless steel tubes—cladding tube specification
- ASTM A335 P91/P11: Ferritic alloy steel pipe for high-temperature service
- ASME BPV Section I: Power Piping—design pressure calculations for pressure-containing components
- API 5CT: Specification for casing and tubing—oilfield pipe mechanical requirements
- ISO 1143: Thermoplastics—reference for CO₂ phase behavior data
5.2 Welding and Qualification Standards
- NB/T 47014: Qualification test procedure for fusion welds—mandatory for pressure vessel weld WPS qualification
- GB/T 985: TIG welding of steel—process parameters for cladding weld overlay
- NB/T 47013.2: Ultrasonic testing of welded joints—acceptance for cladding interface inspection
- ASME Section IX: Welding and Brazing Qualifications—WPS/PQR qualification framework
- ISO 15614-1: Qualification testing of welding procedures for metallic materials
5.3 Non-Destructive Testing and Acceptance
- RT (Radiographic Testing): Per NB/T 47013.2, acceptance level II minimum for full-penetration welds; level I for overlay welds where feasible
- UT (Ultrasonic Testing): 100% coverage of cladding weld interface; acceptance per NB/T 47013.3
- PT (Penetrant Testing): 100% surface inspection of overlay welds; acceptance per NB/T 47013.4, level I
- MT (Magnetic Particle Testing): Applicable to ferromagnetic base materials; acceptance level I per NB/T 47013.5
- Hardness testing: Overlay weld hardness within ±50 HV of base metal specification; gradient verification across transition zone
5.4 Pressure Testing Criteria
- Hydrostatic pressure test: 1.5× design pressure, hold time ≥30 minutes, no visible deformation or leakage
- Pneumatic pressure test (alternative): 1.1× design pressure for components where water damage is unacceptable
- Cyclic pressure test: For fracturing pipe applications, minimum 100 cycles at 0–1.5× design pressure to validate cladding integrity under dynamic loading
6. Common Risks and Controls
| Risk Category | Description | Mechanism | Control Measure |
|---|---|---|---|
| Cladding delamination | Separation of overlay from base metal | Thermal shock during rapid depressurization; poor WPS qualification | WPS qualification at minimum service temperature; transition layer with compatible thermal expansion (E309L); 100% UT inspection |
| Hydrogen-induced cracking | Cracks in weld overlay or HAZ | H₂ generation from CO₂ + H₂O → H₂CO₃ dissociation; low-temperature embrittlement | Low-hydrogen welding consumables; post-weld heat treatment per NB/T 47014; impact testing at -40°C minimum |
| Carbonic acid corrosion | Pitting and general corrosion of cladding | Supercritical CO₂ + residual moisture → H₂CO₃; pH reduction | Select 316L or higher alloy cladding; ensure minimum 3.0 mm cladding thickness; corrosion allowance in design |
| Low-temperature brittle fracture | Catastrophic failure at Joule-Thomson temperatures | DBTT exceeded; loss of ductility in weld metal or HAZ | Specify base metal with DBTT ≤ -60°C; qualify weld procedure with Charpy V-notch at -40°C; minimum 27 J absorption energy |
| Fatigue failure at weld interface | Crack initiation at overlay/base interface | Cyclic pressure loading (dp/dt up to 50 MPa/s); residual stress concentration | Stress-relief welding; weld toe grinding; cyclic fatigue testing per ASTM E466 |
| Over-thickening of overlay | Excessive weld buildup reducing pipe ID | Operator skill variation; inadequate process control | Welding procedure with specified deposition rate; dimensional inspection at every 500 mm; automated TIG where feasible |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
The TIG/MIG weld overlay route is the primary manufacturing method for producing clad fracturing pipes where inner corrosion resistance is required. Based on the pressure release characteristics identified in this study, the following implementation parameters apply:
- Weld overlay sequence: Base pipe preparation (machining or GMAW bevel) → Transition layer (E309L, 2 passes, 1.5–2.0 mm each) → Functional layer (316L, 2–3 passes, 2.0–3.0 mm total) → Surface finishing
- Interpass temperature control: ≤150°C for austenitic overlay layers to prevent sensitization; ≤250°C for ferritic transition layers
- Travel speed: 150–250 mm/min (TIG); 300–500 mm/min (MIG) depending on pipe diameter and cladding thickness
- Shielding gas: Argon (99.99%) for TIG; Argon + 5% CO₂ or pure Argon for MIG to ensure full penetration and clean weld surface
- Post-weld treatment: Solution annealing at 1050–1100°C for 304L/316L overlays where ductility is critical; stress relief at 620–650°C for 2 hours for ferritic components
- Pressure test qualification: Each WPS must be qualified with a pressure test coupon subjected to cyclic loading equivalent to 1.5× design pressure for 100 cycles before production release
7.2 Hydraulic Explosive Bonding Route
Hydraulic explosive bonding (also known as hydraulic explosion cladding or water-assisted explosive bonding) is applicable for producing large-diameter clad pipe blanks or plate sections for fracturing manifold and wellhead components. The pressure release characteristics inform the following design considerations:
- Cladding configuration: Stainless steel (304L/316L) cladding on carbon steel (20G/16Mn) base plate, bonded under hydraulic explosive conditions
- Bond quality verification: Each bond must be verified by macrograph examination per ASTM E1022, with 100% intermetallic compound-free bonding across the interface
- Post-bond machining: After hydraulic explosive bonding, the clad plate is machined to final dimensions; minimum remaining cladding thickness must be ≥3.0 mm to maintain corrosion protection
- Pressure cycling validation: Bonded assemblies must survive 500 pressure cycles (0–35 MPa) without delamination, validated by UT monitoring of bond interface
- Temperature qualification: Bond interface must maintain mechanical integrity at -40°C; validated by Charpy impact testing on bond-line coupons per ASTM E23
7.3 Explosion Welding Route
Traditional explosion welding (air-gap explosive cladding) produces clad pipe and plate components with exceptional bond strength, suitable for the most demanding fracturing pipe applications where pressure release events are most severe:
- Applicability: Large-diameter pipe (DN ≥ 100) and thick-walled components where TIG/MIG overlay would require excessive weld passes
- Cladding thickness range: 2.0–10.0 mm depending on pipe diameter and corrosion severity
- Bond quality criteria: Per ASTM E1022, the bond interface must show full metallurgical bonding with no voids, cracks, or unmelted particles; intermetallic compound thickness ≤5 μm
- Dynamic loading qualification: Explosion-welded joints inherently possess superior fatigue resistance due to work-hardening at the bond interface; however, qualification testing per ASTM E466 is still required to confirm ≥10⁶ cycle endurance at 1.5× design pressure
- Corrosion resistance validation: Post-explosion-welding corrosion testing per NACE TM0177 (for carbonic acid environments) or ASTM G102 (immersion testing) to confirm ≥1000 hours without significant thickness loss
8. Contribution to Qualification Building and Customer Value
8.1 WPS Qualification Enhancement
The pressure release characteristic data obtained from this study directly enhances the company's WPS qualification database by:
- Providing quantified transient pressure and temperature boundary conditions for qualification coupon testing
- Justifying the selection of specific welding consumables (E309L transition + E316L functional) based on demonstrated service requirements
- Establishing the minimum impact energy requirement (≥27 J at -40°C) that must be met by qualified procedures
- Defining the cyclic fatigue test protocol that must accompany each PQR for fracturing pipe applications
8.2 Product Delivery Capability
This research enables the company to deliver fracturing pipe products with the following verified capabilities:
- Design pressure range: 15–50 MPa with validated cladding integrity
- Temperature range: -40°C to +150°C with confirmed toughness and corrosion resistance
- Cyclic life: ≥1000 pressure cycles without delamination or cracking
- Corrosion resistance: ≥1000 hours in carbonic acid environment without significant degradation
- Compliance: Full traceability from raw material through WPS qualification to final NDT acceptance
8.3 Customer Value Proposition
"The integration of supercritical CO₂ phase-change pressure release data into our cladding design methodology allows us to deliver fracturing pipes that are engineered for the actual service conditions—not merely nominal design pressure. This reduces the probability of in-service failure by an estimated 80–90% compared to conventionally specified clad pipes, while providing our customers with documented qualification evidence that meets the most stringent operator requirements."
9. Conclusion
The study of fracturing pipe pressure release characteristics in supercritical CO₂ phase-change fracturing technology represents a critical knowledge asset for Cladding Technology Shanxi Co., Ltd. By translating fundamental research into actionable engineering specifications—cladding material selection, minimum thickness requirements, WPS qualification parameters, NDT acceptance criteria, and cyclic fatigue validation protocols—the company positions itself as a technically differentiated supplier in the unconventional energy equipment market. The three technology routes (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding) each address distinct segments of the fracturing pipe market, and the pressure release data provides the unified technical basis for qualification across all routes. This integrated approach strengthens bid competitiveness, reduces warranty liability, and builds long-term customer trust through demonstrable technical authority.