Pressure Release Characteristics of Fracturing Tubes in Supercritical CO2 Phase Change Fracturing

Literature Overview

This paper focuses on the pressure release characteristics of the fracturing tube, a critical component in supercritical CO2 phase change fracturing technology. The fracturing tube is a high-pressure vessel that stores supercritical CO2 and releases it into the formation under controlled conditions. Understanding the pressure release behavior is essential for ensuring the safety and effectiveness of the fracturing operation, as well as for the proper design and fabrication of the fracturing tube itself. The research is of direct relevance to engineers specializing in pressure vessel design, welding, and non-destructive testing, as the fracturing tube is a pressure-containing component that must meet stringent design and fabrication requirements.

Core Technical Content

The fracturing tube in supercritical CO2 phase change fracturing operates by storing CO2 in a supercritical state at high pressure and then rapidly releasing it into the formation, causing a phase change that generates a shock wave. The pressure release characteristics determine the effectiveness of the fracturing process and the safety of the operation.

The pressure release process can be divided into three phases:

1. Initial pressure buildup: The CO2 is injected into the fracturing tube at a controlled rate, and the pressure increases linearly with the injected volume. The tube wall stress increases proportionally according to the thin-wall pressure vessel formula.
2. Pressure release initiation: When the pressure reaches the designed release threshold, the release mechanism (such as a fuse plug or pressure-sensitive seal) activates, and the CO2 begins to flow out of the tube.
3. Rapid depressurization: The CO2 undergoes a rapid phase change from supercritical to gas state, causing a sharp drop in pressure. The pressure release rate is influenced by the release hole size, the CO2 fill ratio, and the tube geometry.

Pressure Vessel Design and Fabrication

The fracturing tube is a pressure-containing component that must be designed and fabricated in accordance with applicable codes and standards. The design considerations include:

The material selection for the fracturing tube depends on the operating conditions. For dry CO2 service at temperatures above 31.1 degrees Celsius, carbon steel grades such as Q345R or 16MnR are suitable. For high-temperature service above 200 degrees Celsius, low-alloy steel grades such as 12Cr1MoV or 15CrMo are required. The material must also have adequate toughness at the minimum service temperature, which is particularly important for the pressure release phase where the temperature may drop significantly due to the Joule-Thomson effect.

Welding and Non-Destructive Testing

The welding of the fracturing tube is a critical process that directly affects the safety and reliability of the component. The following welding and inspection requirements must be met:

1. Weld procedure qualification: The welding procedure specification must be qualified in accordance with ASME Section IX or NB/T 47014, with the qualification test including tensile testing, bend testing, and hardness testing of the weld metal and heat-affected zone.
2. Welder qualification: The welders must be qualified in accordance with ASME Section IX or NB/T 47014, with the qualification test performed on a coupon of the same material and thickness as the fracturing tube.
3. Weld inspection: All welds must be inspected by radiographic testing (RT) or ultrasonic testing (UT) to 100% coverage. The acceptance criteria must meet at least the second-grade standard of ASME Section V Article 2 or NB/T 47013-2. For critical welds, the first-grade standard should be applied.
4. Hydrostatic testing: The completed fracturing tube must undergo hydrostatic testing at 1.5 times the design pressure. The test pressure must be held for a minimum of 10 minutes, with no visible deformation or leakage permitted.
5. Magnetic particle testing: After hydrostatic testing, the welds should be inspected by magnetic particle testing (MT) to detect any surface-breaking defects that may have been introduced during the testing process.

Pressure Release Mechanism and Safety Analysis

The pressure release mechanism of the fracturing tube is a critical safety feature that must be designed and tested to ensure reliable operation. The paper analyzes several release mechanisms, including:

A failure mode and effects analysis (FMEA) of the fracturing tube identifies the following critical failure modes:

• Premature release: The release mechanism activates before the designed pressure is reached, resulting in incomplete fracturing. This risk is mitigated by careful calibration of the release mechanism and regular testing of the release pressure.
• Failure to release: The release mechanism does not activate at the designed pressure, leading to over-pressurization of the tube. This risk is mitigated by installing a secondary pressure relief device and by regular inspection of the release mechanism.
• Catastrophic tube failure: The tube ruptures due to material defect, weld failure, or corrosion. This risk is reduced by strict material control, welding qualification, non-destructive testing, and periodic in-service inspection.

Engineering Practice and Quality Control

In practical applications, the fracturing tube must be manufactured and inspected in accordance with strict quality control procedures. The following quality control measures are recommended:

1. Material certification: All raw materials must be accompanied by mill certificates that verify the chemical composition and mechanical properties. The materials must meet the requirements of the applicable standards (e.g., GB/T 713, ASME II).
2. Welding procedure control: The welding procedure must be documented and followed strictly. Any deviation from the qualified procedure must be documented and approved by a qualified welding engineer.
3. Non-destructive testing: All welds must be inspected by RT or UT before hydrostatic testing. The inspection records must be documented and retained for the lifetime of the component.
4. Hydrostatic testing: The hydrostatic test must be performed in accordance with the applicable code requirements. The test results must be documented, including the test pressure, test duration, and any observations.
5. Final inspection and certification: Before delivery, the fracturing tube must undergo a final inspection that includes visual examination, dimensional verification, and review of all quality records. A final inspection certificate must be issued, confirming compliance with the design and fabrication requirements.

Summary

The study of pressure release characteristics of fracturing tubes in supercritical CO2 phase change fracturing technology provides essential information for the safe and effective design of this critical component. The fracturing tube is a pressure-containing vessel that must be designed, fabricated, and inspected in accordance with stringent codes and standards to ensure reliable operation under extreme pressure conditions. The pressure release mechanism is a key safety feature that must be carefully designed, calibrated, and tested to prevent premature release or failure to release. From a quality control perspective, strict adherence to material certification, welding qualification, non-destructive testing, and hydrostatic testing requirements is essential to ensure the integrity and safety of the fracturing tube. Engineers involved in the design and fabrication of supercritical CO2 fracturing equipment must prioritize safety and reliability in all aspects of the design, fabrication, and inspection process to ensure the successful and safe operation of the fracturing system.