Supercritical CO2 Rock Fracturing Mechanism Analysis

Literature Overview

This paper provides a comprehensive analysis of the fundamental mechanisms governing rock fracturing by supercritical CO2. The research combines numerical simulation with laboratory experiments to elucidate the coupled thermo-mechanical-chemical processes that occur when supercritical CO2 is injected into a confined rock specimen. The study is particularly relevant to engineers designing high-pressure CO2 storage and delivery systems, as understanding the fracturing mechanism directly influences the required pressure ratings, material specifications, and safety margins of the associated equipment.

Core Mechanism Analysis

The supercritical CO2 rock fracturing mechanism operates through a sequence of interrelated physical and chemical processes:

The dominant mechanism shifts depending on the injection conditions. At higher injection pressures above 20 MPa, the mechanical pressure component dominates, producing a single dominant fracture. At lower pressures in the range of 8-15 MPa, the phase change energy and thermal shock effects become more significant, promoting the formation of complex fracture networks with higher surface roughness.

High-Pressure Equipment Design Implications

The understanding of fracturing mechanisms has direct implications for the design of the CO2 storage vessels and high-pressure delivery piping. The supercritical CO2 storage vessels typically operate at pressures between 15 and 25 MPa, requiring design in accordance with ASME Section VIII Division 1 or NB/T 47002. The design pressure must include a safety margin above the maximum operating pressure to account for thermal expansion of the CO2 in the event of ambient temperature increase.

For vessels designed to withstand the maximum fracturing pressure, the material selection follows a hierarchy:

1. Carbon steel (Q345R / 16MnR): Suitable for dry CO2 service at temperatures above 31.1 degrees Celsius, with minimum thickness determined by ASME Section VIII pressure vessel formulas.
2. Low-alloy steel (12Cr1MoV / 15CrMo): Required for high-temperature service above 200 degrees Celsius, where the CO2 may be at elevated temperature during the fracturing process.
3. Stainless steel clad or overlay: Applied to the internal surface of carbon steel vessels to provide corrosion resistance against wet CO2, using electroslag welding (ESW) or submerged arc welding (SAW) overlay techniques.

The weld overlay of stainless steel on carbon steel pressure vessels requires careful control of the dilution ratio to ensure the overlay layer maintains its corrosion resistance properties. The typical dilution ratio should not exceed 10% carbon steel in the overlay layer to maintain adequate resistance to CO2 corrosion. The weld procedure qualification must include intergranular corrosion testing of the weld overlay in accordance with ASTM A263 or ASTM A264 requirements.

Numerical Simulation and Experimental Validation

The paper employs finite element analysis to model the stress field distribution around the injection point during supercritical CO2 fracturing. The simulation results show that the maximum tangential stress at the borehole wall reaches a critical value at approximately 70% of the final breakdown pressure, indicating the onset of crack initiation. This finding has practical significance for the design of pressure relief systems on the CO2 delivery equipment, as the pressure relief valve setting should be calibrated to open at a pressure that prevents over-pressurization beyond the rock breakdown pressure.

The experimental validation uses a custom-built high-pressure triaxial testing apparatus capable of applying confining pressures up to 30 MPa and injection pressures up to 40 MPa. The apparatus is equipped with acoustic emission monitoring, pressure transducers with resolution of 0.01 MPa, and temperature sensors to capture the thermal effects of the phase change. The pressure vessels used in the testing apparatus are manufactured from 16MnR steel with a wall thickness of 25 mm, designed according to GB/T 150 pressure vessel codes, and inspected by radiographic testing (RT) and ultrasonic testing (UT) to ensure weld integrity.

Safety Considerations and FMEA Analysis

A failure mode and effects analysis (FMEA) of the supercritical CO2 fracturing system identifies the following critical failure modes:

• Vessel over-pressurization: Caused by failure of the pressure relief valve or control system malfunction, leading to catastrophic vessel failure. The risk is mitigated by installing redundant pressure relief devices and regular inspection of the relief valves.
• CO2 leakage: From weld joints or flange connections under high pressure, posing an asphyxiation hazard. The risk is reduced by using high-quality welding procedures, full-penetration welds, and hydrostatic testing at 1.5 times the design pressure.
• Material degradation: Due to CO2 corrosion or hydrogen embrittlement of the pressure vessel material. The risk is managed by material selection, overlay protection, and periodic thickness monitoring using ultrasonic testing.

Summary

The analysis of supercritical CO2 rock fracturing mechanisms reveals a complex interplay of phase change, thermal, chemical, and mechanical effects that govern the fracture process. These mechanisms have direct implications for the design of high-pressure CO2 storage vessels and delivery systems, including material selection, welding procedures, pressure relief system design, and safety analysis. The integration of numerical simulation with experimental validation provides a robust framework for predicting fracturing behavior and optimizing equipment design parameters. Engineers working on supercritical CO2 fracturing systems must ensure that all pressure-containing equipment is designed, fabricated, and inspected in accordance with applicable codes and standards to guarantee safe and reliable operation.