CO₂ Phase-Transformation-Induced Rock Fracturing: Damage Range Calculation and Influencing Factors
1. Definition and Technical Principles
CO₂ phase-transformation-induced rock fracturing is an advanced subsurface stimulation technology that exploits the thermodynamic phase change of carbon dioxide—from gas to supercritical state, or from liquid to gas under confined pore conditions—to generate internal fracture networks within rock formations. Unlike conventional hydraulic fracturing that relies on high-viscosity slurry fluids, this technique leverages the unique compressibility, low viscosity, and phase-transition pressure behavior of CO₂ to propagate micro-cracks and damage zones within reservoir rocks.
The fundamental principle operates as follows: When CO₂ is injected into a confined rock matrix under specific pressure and temperature conditions, it undergoes a phase transformation. The transition from supercritical CO₂ (above 31.1 °C and 7.38 MPa) back to a gaseous or liquid phase within the pore space generates differential pressure differentials that exceed the tensile strength of the rock. This induces micro-fractures, shear failures, and progressive damage propagation. The damage range—the spatial extent of the fractured zone from the injection point—is governed by rock mechanical properties, injection parameters, and boundary conditions.
The damage range calculation typically employs fracture mechanics models, including:
- Linear Elastic Fracture Mechanics (LEFM): Using the stress intensity factor (KI) to determine crack propagation conditions under mode-I loading.
- Extended Finite Element Method (XFEM): Modeling crack initiation and propagation paths through heterogeneous rock media.
- Thermo-elastic-plastic coupled models: Incorporating the volumetric strain associated with CO₂ phase change as a body force driving crack growth.
- Discrete Element Method (DEM): Capturing grain-scale fracture behavior and damage evolution in brittle rock formations.
The governing equation for damage range (rd) can be expressed as:
rd = f(Pinj, σt, E, ν, ρrock, T, Sw, kperm)
where Pinj is injection pressure, σt is rock tensile strength, E is Young's modulus, ν is Poisson's ratio, ρrock is rock density, T is formation temperature, Sw is water saturation, and kperm is permeability.
2. Category and Business Positioning
Within the technical capability portfolio of Cladding Technology Shanxi Co., Ltd., this research entry occupies a critical position at the intersection of subsurface engineering science and surface/subsurface equipment protection technology. The company's primary business—bimetallic cladding and weld overlay manufacturing—serves the upstream and midstream oil and gas sector, where CO₂ fracturing operations are increasingly deployed for:
- Enhanced oil recovery (EOR) through CO₂ injection
- Unconventional reservoir stimulation (tight gas, shale, coalbed methane)
- Carbon capture, utilization, and storage (CCUS) reservoir conditioning
- Geothermal energy resource development
The study of CO₂-induced rock damage range directly informs the design requirements for the equipment that Cladding Technology Shanxi manufactures. Understanding the fracture propagation geometry, stress fields, and chemical environment at the wellbore face enables the company to:
- Specify appropriate overlay alloys for well casings and tubing exposed to aggressive CO₂ environments
- Determine the required thickness and grade of clad plates for pressure vessels and injection manifolds
- Qualify weld overlay procedures for downhole tools subjected to cyclic CO₂ phase-transformation pressures
- Provide technical consulting to operators on equipment selection and corrosion protection strategies
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The research on CO₂ phase-transformation-induced rock damage range serves several critical technical purposes:
- Predictive modeling: Quantifying the spatial extent of rock damage to optimize injection parameters for maximum reservoir contact while minimizing formation damage.
- Equipment integrity assessment: Determining the mechanical and chemical loading conditions that wellbore equipment experiences during and after CO₂ fracturing operations.
- Material selection guidance: Identifying the specific corrosion and mechanical degradation mechanisms that clad and overlay-protected equipment must withstand.
- Operational safety: Establishing safe operating envelopes that prevent casing failure, formation collapse, and CO₂ breakthrough to surface.
3.2 Value to Cladding Technology Shanxi's Operations
This research entry contributes directly to the company's value proposition in three dimensions:
Qualification Building: Demonstrating deep technical understanding of CO₂ fracturing mechanisms positions Cladding Technology Shanxi as a qualified supplier to operators deploying CO₂-based stimulation technologies. This supports WPS (Welding Procedure Specification) qualification for overlay procedures designed specifically for CO₂ service environments, aligned with API 16C and NACE MR0175/ISO 15156 requirements.
Product Delivery: Knowledge of damage range parameters enables the company to deliver optimized clad products—correct alloy selection, appropriate overlay thickness, and validated weld procedures—that match the specific operational conditions at the customer's site.
Customer Value: By integrating subsurface engineering knowledge with surface equipment manufacturing expertise, the company provides integrated solutions that reduce lifecycle costs, extend equipment service intervals, and minimize unplanned shutdowns.
4. Key Process and Implementation Points
4.1 Damage Range Calculation Methodology
The calculation of CO₂-induced rock damage range involves a systematic, multi-step analytical and numerical approach:
| Step | Description | Key Parameters | Output |
|---|---|---|---|
| 1 | Rock property characterization | Tensile strength (σt), compressive strength (σc), Young's modulus (E), Poisson's ratio (ν), permeability (k), porosity (φ) | Baseline mechanical model |
| 2 | CO₂ phase behavior determination | Injection temperature (T), pressure (P), phase state, density change (Δρ), volumetric expansion ratio | Phase transformation pressure threshold |
| 3 | Stress field calculation | In-situ stress (σH, σh, σv), pore pressure, effective stress | Stress distribution around injection point |
| 4 | Fracture propagation modeling | Fracture toughness (KIc), crack tip stress intensity, energy release rate | Fracture initiation and propagation paths |
| 5 | Damage range quantification | Fracture density, damage zone radius, connectivity index | Final damage range (rd) and geometry |
4.2 Influencing Factors and Their Impact
| Influencing Factor | Effect on Damage Range | Typical Range | Control Strategy |
|---|---|---|---|
| Injection pressure (Pinj) | Directly proportional; higher pressure extends damage range | 5–35 MPa | Optimize to match σt + fracture toughness margin |
| Rock tensile strength (σt) | Inversely proportional; stronger rock limits damage extent | 5–30 MPa | Characterize accurately via triaxial testing |
| Formation temperature (T) | Affects CO₂ phase state; above 31.1°C enables supercritical behavior | 25–150°C | Match CO₂ state to target phase transition |
| Injection rate (Q) | Higher rates increase pressure transient; can extend or limit range | 1–50 m³/min | Ramp injection to avoid sudden pressure spikes |
| In-situ stress anisotropy | Controls fracture orientation and asymmetry of damage zone | σH/σv ratio 0.5–2.0 | Account for in fracture geometry predictions |
| Rock permeability (k) | Higher permeability allows CO₂ to travel further before phase change | 0.01–1000 mD | Use multi-stage injection in low-permeability formations |
| Water saturation (Sw) | Affects CO₂ solubility and phase transition kinetics | 20–95% | Account for brine-CO₂ interaction in models |
| Rock type (lithology) | Sandstone vs. carbonate vs. shale—different fracture behavior | Various | Calibrate models to specific lithology |
4.3 Implementation in Equipment Design
The damage range calculation results feed directly into equipment design specifications:
- Casing and tubing selection: Damage range determines the extent of CO₂ exposure zone, which dictates the required corrosion resistance level of the casing material and the thickness of weld overlay protection.
- Overlay thickness design: Based on the predicted CO₂ partial pressure and temperature at the wellbore face, the minimum overlay thickness is calculated using diffusion models (Fick's second law) to prevent carbonation corrosion penetration through the overlay.
- Clad plate specification: For surface equipment (injection manifolds, separators, CO₂ compressors), the damage range data informs the design pressure and temperature ratings of clad vessels.
- WPS qualification scope: The operating envelope defined by damage range parameters determines the qualifying range of welding procedure specifications for overlay work.
5. Applicable Standards and Acceptance Criteria
5.1 Equipment and Material Standards
| Standard | Scope | Relevance to CO₂ Fracturing Equipment |
|---|---|---|
| NACE MR0175/ISO 15156 | Materials for H₂S-containing environments (also covers CO₂ service) | Material selection for wellbore equipment in CO₂ service |
| API 16C | Materials and welding requirements for sour service | Welding procedure requirements for clad/overlay equipment |
| ASME BPV Code Section VIII | Pressure vessel design and fabrication | Design of injection manifolds, separators, and pressure vessels |
| ASTM A240 | Stainless steel plate and sheet | Specification of clad plate materials (304L, 316L, 321, etc.) |
| ASTM A592 | Carbon steel plate with stainless steel cladding | Specification of bimetallic clad plates for CO₂ service |
| GB/T 13296 | Stainless steel seamless tubes | Specification of clad tubing for downhole applications |
| NB/T 47015 | Pressure vessel fabrication | Chinese national standard for pressure vessel manufacturing |
5.2 NDT and Acceptance Criteria
Equipment manufactured for CO₂ fracturing applications must meet rigorous non-destructive testing (NDT) requirements:
- Weld overlay qualification: RT (Radiographic Testing) per ASME Section V Article 2, Type 2 or Type 4; acceptance per ASME Section IX and API 16C
- Clad plate inspection: Ultrasonic testing (UT) for bond quality per ASTM E1280 or GB/T 23698; magnetic particle testing (MT) for surface defects per ASTM E1444
- Explosion-welded joints: Ultrasonic testing per ASTM E2491; peel testing per ASTM E1280; hardness traverse testing to verify no intermetallic formation
- Hydraulic explosive bonding: Bond quality verification via UT and destructive peel tests on coupon samples
- Overlay thickness verification: UT thickness measurement per ASTM A999; minimum thickness acceptance per design specification
5.3 Acceptance Criteria for CO₂ Service Equipment
- Corrosion resistance: Overlay alloy must demonstrate ≤0.025 mm/year corrosion rate in CO₂-containing brine at operating temperature per NACE TM0177 (Linear Polarization Resistance)
- Mechanical integrity: Overlay weld metal must achieve minimum hardness of 22 HRC and maximum of 35 HRC per NACE MR0175/ISO 15156
- Intergranular corrosion resistance: 100-hour ASTM A262 Practice E (65°C) test for sensitization resistance
- Carbonation corrosion protection: Overlay thickness must exceed calculated diffusion depth at end-of-life per design life assumption (typically 20–25 years)
6. Common Risks and Controls
6.1 Technical Risks in CO₂ Fracturing Operations
| Risk | Mechanism | Impact on Equipment | Control Measures |
|---|---|---|---|
| CO₂ corrosion (carbonation) | CO₂ dissolves in formation water forming carbonic acid (H₂CO₃), lowering pH to 3.0–5.5 | Pitting, general thinning, intergranular corrosion of carbon steel base | Apply 309L/316L weld overlay ≥3 mm; use duplex 2205 for high-chloride environments |
| Thermal cycling | CO₂ phase change causes rapid temperature fluctuations at wellbore face | Thermal fatigue cracking in overlay welds; base metal fatigue | Select overlay alloys with good thermal fatigue resistance (321, 347); control cooling rates during WPS qualification |
| Pressure cycling | Repeated injection/production cycles create cyclic stress | Fatigue crack initiation at overlay/base metal interface | Design overlay with fatigue-resistant microstructure; apply post-weld heat treatment per WPS |
| Galvanic corrosion | Electrochemical potential difference between overlay and base metal in electrolyte | Preferential corrosion at overlay edges and defects | Ensure full coverage of overlay; apply edge bead; use intermediate transition layers (309L between carbon steel and 316L) |
| Fracture propagation beyond design envelope | Damage range exceeds predicted model, exposing equipment to unexpected conditions | Unexpected corrosion exposure, mechanical overload | Apply safety factor of 1.5× on predicted damage range; design overlay for maximum credible conditions |
6.2 Manufacturing Risks and Controls
- Porosity in overlay welds: Controlled by using high-purity shielding gas (99.99% Ar for TIG, 98% Ar/2% CO₂ for MIG), maintaining proper gas flow rates, and ensuring adequate gas coverage for multi-pass builds.
- Cracking in overlay welds: Mitigated by preheating base metal to 100–150°C for carbon steel, controlling interpass temperature below 250°C, and using low-hydrogen consumables.
- Bond loss in explosion-welded joints: Prevented by precise control of flyer plate velocity (typically 300–1200 m/s), impact angle (15°–30°), and stand-off distance.
- Intermetallic formation: Avoided by limiting heat input in TIG overlay (≤0.8 kJ/mm) and using rapid multi-pass techniques that limit diffusion time.
7. Application Across Cladding Technology Shanxi's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
The TIG (Tungsten Inert Gas) and MIG (Metal Inert Gas) weld overlay route is the primary method for applying corrosion-resistant overlays to well casings, tubing, and surface equipment used in CO₂ fracturing operations.
Application to CO₂ fracturing equipment:
- Well casing overlay: 309L transition layer (2–3 mm) followed by 316L or duplex 2205 overlay (5–10 mm) on API 5CT P110 or Q125 casing. This protects against carbonation corrosion in the damage zone identified by the CO₂ fracturing damage range model.
- Injection manifold cladding: Multi-pass TIG overlay of 321 stainless steel on carbon steel manifolds that handle supercritical CO₂ at pressures up to 35 MPa. The overlay thickness is calculated based on the maximum CO₂ partial pressure predicted by the damage range model.
- Downhole tool protection: MIG overlay of 309L on packer mandrels and anchor assemblies that experience cyclic CO₂ exposure during multi-stage fracturing operations.
Key parameters for CO₂ service overlay:
| Parameter | TIG Overlay | MIG Overlay |
|---|---|---|
| Base material | API 5CT P110/Q125, A106 Gr.B, A516 Gr.70 | API 5CT P110/Q125, A106 Gr.B, A516 Gr.70 |
| Overlay alloy | 309L → 316L → 2205 (multi-layer) | 309L → 316L → 2205 (multi-layer) |
| Current | 80–180 A | 200–350 A |
| Voltage | 10–16 V | 22–28 V |
| Travel speed | 150–350 mm/min | 300–600 mm/min |
| Shielding gas | 99.99% Ar | 98% Ar / 2% CO₂ or 95% Ar / 5% CO₂ |
| Preheat | 100–150°C | 100–150°C |
| Interpass temperature | ≤250°C | ≤250°C |
| Typical overlay thickness | 3–10 mm | 5–15 mm |
| Deposition rate | 0.5–1.5 kg/h | 3–8 kg/h |
7.2 Hydraulic Explosive Bonding Route
Hydraulic explosive bonding (also known as hydraulic explosion welding or fluid-assisted explosive bonding) is a solid-state bonding technique that uses the energy of a controlled explosion to achieve metallurgical bonding between dissimilar metals. In the context of CO₂ fracturing equipment, this route is particularly valuable for manufacturing clad plates and pipes where the overlay material must be applied uniformly over large areas.
Application to CO₂ fracturing equipment:
- Large-diameter clad pipes: Production of CO₂ injection lines (DN300–DN1200) with 316L or 2205 cladding on carbon steel base pipes. The hydraulic explosive bonding process achieves uniform bond quality across the entire pipe circumference, critical for handling supercritical CO₂ at high pressures.
- Pressure vessel cladding: Manufacturing of clad separators and accumulators that receive and process CO₂-containing fluids. The hydraulic explosive bonding process provides superior bond quality compared to mechanical cladding for vessels operating at 15–35 MPa.
- Heat exchanger tubes: Production of clad tubes for CO₂-heat exchange systems in geothermal and CCS applications, where both sides of the tube wall require corrosion protection.
Key parameters for hydraulic explosive bonding:
| Parameter | Typical Value | Control Requirement |
|---|---|---|
| Flyer plate velocity | 300–1200 m/s | Must exceed critical jetting velocity for target material pair |
| Impact angle | 15°–30° | Controls jetting behavior and bond quality |
| Stand-off distance | 5–25 mm | Calibrated for specific flyer/target thickness combination |
| Explosive charge | High explosive (TNT equivalent) or hydraulic pressure (up to 600 MPa) | Precisely calculated for target material pair and geometry |
| Bond quality | 100% metallurgical bond with wavy interface | Verified by UT (ASTM E2491) and peel testing |
| Maximum clad thickness | Up to 25% of base thickness | Ensures uniform bonding without delamination |
7.3 Explosion Welding Route
Explosion welding (explosive welding) is a variant of the hydraulic explosive bonding technique that uses solid explosive charges to achieve the required flyer plate velocity. It is particularly suited for batch production of clad plates and for manufacturing components where the geometry and material combination require precise energy input control.
Application to CO₂ fracturing equipment:
- Clad plate production for surface facilities: Manufacturing of large-format clad plates (up to 6000 mm × 2500 mm) for injection skid fabrication, CO₂ storage tanks, and processing facility structures. Materials typically include 304L/316L on A36/A516 Gr.60 base.
- Explosion-welded clad pipe for wellhead equipment: Production of wellhead spools, Christmas tree components, and manifolds that require corrosion-resistant cladding on high-strength carbon steel or low-alloy base materials.
- Specialty component fabrication: Manufacturing of valve bodies, flanges, and fittings for CO₂ injection systems where the explosion welding process provides superior bond quality and uniformity compared to mechanical or weld overlay methods.
Key parameters for explosion welding:
| Parameter | Typical Value | Control Requirement |
|---|---|---|
| Explosive charge | HE (high explosive), typically TNT or equivalent | Precisely calculated based on flyer/target mass ratio and geometry |
| Flyer plate velocity | 300–1500 m/s | Must exceed critical velocity for jetting; typically 300–500 m/s for steel/stainless |
| Impact angle | 10°–25° | Controls jetting direction and bond zone geometry |
| Stand-off distance | 3–15 mm | Calibrated for specific flyer/target combination |
| Material pair | 316L/A516 Gr.70, 2205/A36, 304L/A516 Gr.60 | Must be within qualified material compatibility matrix |
| Bond quality verification | UT (ASTM E2491), peel test (ASTM E1280), hardness traverse | 100% UT inspection; representative peel tests per batch |
| Post-weld treatment | Stress relief at 550–650°C for 2 hours (if required) | Prevents residual stress-induced distortion and cracking |
8. Integration of Damage Range Research with Manufacturing Capabilities
8.1 Design-to-Manufacture Workflow
The CO₂ phase-transformation-induced rock damage range research integrates with Cladding Technology Shanxi's manufacturing capabilities through the following workflow:
- Customer provides reservoir data: Formation properties, in-situ stress, temperature profile, and target stimulation parameters.
- Damage range model execution: The company's technical team runs the damage range calculation using the customer's reservoir data to predict the extent of CO₂ exposure at the wellbore and surface facilities.
- Equipment specification derivation: Based on the predicted CO₂ partial pressure, temperature, and exposure duration, the team derives the required overlay material, thickness, and performance criteria.
- WPS development and qualification: Welding procedure specifications are developed and qualified per ASME Section IX and API 16C for the specific material combination and service conditions.
- Manufacturing and NDT: Products are manufactured using the appropriate technology route (TIG/MIG overlay, hydraulic explosive bonding, or explosion welding) and subjected to full NDT per the applicable standards.
- Performance verification: Laboratory testing (corrosion testing, mechanical testing, microstructural analysis) confirms that the manufactured product meets the design requirements derived from the damage range model.
8.2 Qualification Building Impact
This research entry directly supports Cladding Technology Shanxi's qualification building in several ways:
- Technical capability demonstration: Demonstrates the company's ability to integrate subsurface engineering knowledge with surface equipment manufacturing, a differentiator in the competitive market for CO₂ service equipment.
- WPS qualification support: The damage range model provides the engineering basis for qualifying overlay procedures for specific CO₂ service conditions, supporting API 16C and ASME Section IX compliance.
- Customer trust building: Demonstrates deep technical understanding of the customer's operational environment, building confidence in the company's ability to deliver reliable, long-lasting clad products.
- Regulatory compliance: Supports compliance with national and international standards for CO₂ service equipment, including NACE MR0175/ISO 15156, API 16C, and ASME BPV Code requirements.
9. Conclusion
The research on CO₂ phase-transformation-induced rock damage range calculation and influencing factors represents a critical technical capability that bridges subsurface engineering science with surface equipment manufacturing. For Cladding Technology Shanxi Co., Ltd., this knowledge enables the company to deliver optimized, standards-compliant clad and overlay-protected equipment for the rapidly growing CO₂ fracturing and CCS markets.
By integrating damage range modeling with the company's three technology routes—TIG/MIG weld overlay for precise, localized protection; hydraulic explosive bonding for large-format, uniform cladding; and explosion welding for batch production of specialty components—the company provides a comprehensive solution set that addresses the full spectrum of equipment needs in CO₂ fracturing operations. The resulting products meet or exceed the requirements of NACE MR0175/ISO 15156, API 16C, ASME BPV Code, and applicable GB/NB national standards, ensuring long-term reliability and customer value.