CLADDING TECHNOLOGY SHANXI CO., LTDCO2 Miscible Fracturing Technology Application in G Block Study Note
Literature Overview
This study documents the field application of CO2 miscible fracturing technology in the G block, a reservoir characterized by low permeability and complex fluid phase behavior. The technology leverages the miscibility between CO2 and formation fluids to enhance fracture propagation, improve sweep efficiency, and reduce formation damage compared to conventional water-based fracturing systems. For pressure vessel and cladding engineers, this study provides critical insights into the operational parameters and material requirements for CO2 fracturing equipment deployed in field conditions.
Core Technical Content
Operational Parameters and Performance Results
| Parameter | Conventional Fracturing | CO2 Miscible Fracturing | Improvement |
|---|---|---|---|
| Fracturing fluid viscosity (mPa·s) | 3–5 | 0.1–0.5 | Reduced formation damage |
| Proppant placement efficiency (%) | 65–75 | 80–90 | 15–20% improvement |
| Initial production rate (m³/d) | 15–25 | 35–55 | 2–3x increase |
| 90-day cumulative production (m³) | 800–1200 | 2500–4000 | 3x increase |
| Formation damage index | 1.0 (reference) | 0.3–0.5 | 50–70% reduction |
| Water cut (%) | 60–80 | 10–25 | Significant reduction |
The study demonstrates that CO2 miscible fracturing achieves superior reservoir contact and reduced formation damage due to the low viscosity of CO2 at reservoir conditions and its miscibility with formation hydrocarbons. The technology requires surface equipment capable of handling high-pressure CO2 injection at rates of 5–20 m³/min and pressures up to 70 MPa.
Pressure Vessel and Cladding Engineering Implications
Equipment Requirements for CO2 Miscible Fracturing
| Equipment Component | Design Pressure (MPa) | Operating Temperature (°C) | Material Requirement | Cladding Specification |
|---|---|---|---|---|
| CO2 storage tank | 20 | 20–40 | SA-516 Gr.70 + 304L overlay | SAW, 2.5 mm overlay |
| High-pressure pump | 70 | 20–50 | 17-4PH or Alloy 625 | PTA, 3.0 mm overlay |
| Fracturing manifold | 50 | 20–60 | P91 + Inconel 625 | ESW, 3.2 mm overlay |
| Surface choke valve | 40 | 20–80 | 316L or Alloy 625 | GTAW + SAW overlay |
| Wellhead assembly | 70 | 20–100 | API 5CT P110 + 316L | SAW overlay |
Corrosion Mechanisms in CO2 Miscible Fracturing Service
The study identifies several corrosion mechanisms that must be addressed in equipment design:
- Carbonic acid corrosion: CO2 dissolved in residual moisture forms carbonic acid, attacking carbon steel surfaces. The corrosion rate increases with CO2 partial pressure and decreases with temperature above 60°C due to the formation of protective iron carbonate scales.
- Erosion-corrosion: High-velocity CO2 flow, particularly at fittings and flow restrictions, causes mechanical erosion that removes protective scales, exposing fresh metal to corrosion attack.
- Galvanic corrosion: At the interface between carbon steel base metal and stainless steel overlay layers, differential aeration can create galvanic cells that accelerate localized corrosion at the weld interface.
Engineering Practice Integration
Overlay Design for CO2 Miscible Fracturing Equipment
Based on the operational parameters identified in this study, the following overlay design recommendations are proposed:
| Service Condition | Base Metal | Overlay Material | Minimum Overlay Thickness | Inspection Method |
|---|---|---|---|---|
| Low-pressure CO2 storage (<20 MPa) | SA-516 Gr.70 | 304L | 2.5 mm | MT + PT |
| Medium-pressure injection (20–50 MPa) | P91 | Inconel 625 | 3.2 mm | RT + UT + MT |
| High-pressure surface equipment (>50 MPa) | Alloy 625 (solid) | N/A | N/A | RT + UT |
| Wellhead and christmas tree | API 5CT P110 | 316L | 3.0 mm | MT + PT + UT |
| Pump barrels and pistons | 17-4PH | Stellite 6 | 2.0 mm | UT + PT |
NDT Protocol for CO2 Service Equipment
The NDT protocol recommended for equipment in CO2 miscible fracturing service includes:
- Initial inspection: 100% MT and PT of all overlay welds, 100% RT of all pressure-containing welds, UT thickness measurement at 200 locations minimum
- In-service inspection: Annual UT thickness survey at critical locations, semi-annual MT/PT of overlay surfaces, hydrostatic test every 5 years
- Post-maintenance inspection: Full RT and UT of all repair welds, MT and PT of all overlay surfaces, dimensional verification of critical components
Study Insights and Reflections
The G block application study provides valuable field data that validates the economic and technical advantages of CO2 miscible fracturing. From a pressure vessel engineering perspective, the study confirms that the high-pressure, high-cycle operating conditions of CO2 fracturing equipment demand rigorous material selection, overlay specification, and inspection protocols.
The most significant engineering implication is the recognition that CO2 service equipment requires overlay layers designed for combined mechanical and corrosion resistance, with particular attention to erosion-corrosion at high-velocity flow interfaces. The specification of multi-layer overlay schemes, with a transition layer followed by a functional alloy layer, provides the best balance of mechanical compatibility and corrosion protection.
The study also highlights the importance of considering the full life-cycle of CO2 fracturing equipment, including the effects of thermal cycling, pressure cycling, and corrosion on the long-term integrity of overlay welds and pressure vessel components. Regular inspection and maintenance programs, based on risk assessment and FMEA analysis, are essential for ensuring safe and reliable operation.