CCUS-Based CO2 Water-Free Fracturing and Oil Displacement Technology System Study Notes

Literature Overview and Relevance to Pressure Vessel Engineering

This paper presents a comprehensive technology system design for carbon capture, utilization, and storage (CCUS) applications involving CO2 water-free fracturing and enhanced oil recovery (EOR). The study addresses the design, operation, and industrial implementation of a system that captures CO2, compresses it to high pressures, mixes it with fracturing fluids or hydrocarbons, and injects it into reservoirs for both fracturing and oil displacement purposes.

From a pressure vessel and piping engineering perspective, the CCUS technology system involves several critical high-pressure components that require careful design, fabrication, and inspection. The CO2 compression and storage system operates at pressures up to 15–25 MPa, and the injection system may operate at pressures exceeding 30 MPa depending on reservoir conditions. These components include high-pressure compressors, separators, storage vessels, injection wells, and associated piping systems.

Key Technical Parameters and Pressure Vessel Design Requirements

The CCUS technology system involves several critical process parameters that directly influence the design and fabrication requirements for pressure-containing equipment. The following table summarizes the key parameters and their implications:

The presence of CO2 in a wet environment creates a significant corrosion challenge. Carbonic acid corrosion (CO2 corrosion) occurs when CO2 dissolves in water to form carbonic acid, which attacks carbon steel surfaces. The corrosion rate depends on CO2 partial pressure, temperature, pH, and flow velocity. For the CCUS system, where CO2 is kept in a dry state during storage and transportation, the corrosion risk is reduced. However, at injection points and in the reservoir, water contact is inevitable, and corrosion control measures must be implemented.

Material Selection and Welding Requirements

The material selection for CCUS system components must account for the unique challenges posed by CO2 service. The following table presents typical material selections and their justification:

For components requiring weld overlay to achieve corrosion resistance, the overlay material selection and welding procedure must be carefully specified. Common overlay materials for CO2 service include:

• 309L / 316L stainless steel: Suitable for dry CO2 and mildly wet CO2 at moderate temperatures. Provides good resistance to carbonic acid corrosion.
• Duplex stainless steel (2205): Higher strength and improved corrosion resistance compared to austenitic stainless steels. Suitable for wet CO2 at elevated temperatures.
• Ni-Cr-Mo alloys (Inconel 625, Hastelloy C-276): Excellent resistance to CO2 corrosion in aggressive environments. Used for critical components where failure is not acceptable.
• Hardfacing alloys: Used for wear and corrosion resistance in high-flow areas such as valve seats and pump impellers.

The welding procedure for overlay applications in CCUS systems must be qualified per ASME IX or NB/T 47014, with additional requirements for corrosion testing of the overlay material. The overlay thickness must be sufficient to withstand the expected corrosion rate over the design life of the component, typically 20–30 years. A corrosion allowance of 1.5–3.0 mm is typical for wet CO2 service, in addition to any overlay thickness.

System Design and Engineering Integration

The CCUS technology system design involves the integration of multiple subsystems, each with specific fabrication and welding requirements:

1. CO2 capture and purification: The captured CO2 must be purified to remove impurities such as H2S, SOx, NOx, and water vapor. The purification equipment includes absorbers, strippers, and dryers, which may require corrosion-resistant linings or overlays.
2. CO2 compression and liquefaction: CO2 is compressed to high pressure and cooled to liquid state for storage and transportation. The compression system includes multi-stage compressors with interstage coolers, aftercoolers, and surge vessels. The high-pressure components require careful welding and inspection.
3. CO2 storage and transportation: Liquid CO2 is stored in pressurized vessels and transported via pipeline or truck. The storage vessels are designed per ASME VIII Div.1 with full volumetric examination of all critical welds. The transportation pipeline may require corrosion-resistant coatings or internal linings.
4. CO2 injection and fracturing: CO2 is injected into the reservoir at high pressure for fracturing and oil displacement. The injection system includes high-pressure pumps, injection wells, and surface equipment. The injection wells require corrosion-resistant tubing and casing, and the surface equipment must withstand high-pressure cyclic loading.

Quality Control and Inspection Requirements

The quality control program for CCUS system components must be comprehensive and rigorous, given the high-pressure and potentially corrosive service conditions. The following inspection and testing requirements are typical:

The hydrostatic test is particularly critical for CCUS system components, as it verifies the integrity of the pressure boundary under conditions that simulate operational loading. For high-pressure systems operating at 25–35 MPa, the hydrostatic test pressure may exceed 40–50 MPa, requiring specialized testing equipment and procedures.

Study Insights and Engineering Implications

The CCUS technology system design study highlights the importance of material selection and welding quality in ensuring the long-term reliability of high-pressure CO2 systems. Several key insights emerge from the study:

• The dry CO2 approach used in the fracturing and EOR application significantly reduces corrosion risk compared to conventional wet CO2 injection. However, the system must be designed to handle potential water ingress during maintenance or upset conditions.
• The high-pressure requirements of the injection system drive the need for careful weld design, thorough inspection, and regular in-service inspection programs. The fatigue life of welded joints in cyclic pressure service must be evaluated using appropriate methods.
• The integration of multiple materials in the CCUS system (carbon steel, stainless steel, nickel alloys, titanium) creates challenges for welding and inspection. Dissimilar metal welds require careful procedure qualification and enhanced inspection.
• The long design life of 20–30 years requires a corrosion allowance and inspection program that accounts for the expected degradation over the service life. Regular in-service inspection, including thickness monitoring and NDT, is essential.

Conclusion

The CCUS-based CO2 water-free fracturing and oil displacement technology system presents a significant opportunity for carbon mitigation and enhanced oil recovery, but it also poses substantial challenges for pressure vessel and piping engineering. The high-pressure, potentially corrosive, and cyclic loading conditions require rigorous design, fabrication, and inspection practices. Engineers involved in CCUS system fabrication must ensure that all pressure-containing components meet the applicable standards, that welding procedures are properly qualified, and that comprehensive quality control programs are implemented. The study provides a valuable framework for the design and implementation of CCUS systems, and the insights gained from this study can be applied to other high-pressure, corrosive service applications in the energy sector.