Experimental Study on Internal Pressure Changes in CO2 Phase Transformation Explosion Fracture Pipes

Overview and Technical Background

This literature investigates the pressure dynamics within pipes subjected to CO2 phase transformation explosion fracture (PTFE), a technique used for rock fracture in mining, quarrying, and geothermal applications. The CO2 phase transformation process involves the rapid expansion of liquid CO2 upon depressurization, generating pressures of 300-800 MPa within confined pipe sections. Understanding the internal pressure evolution is critical for pipe design, safety assessment, and process optimization. This study has direct relevance to pressure vessel engineering as it addresses extreme transient pressure loading conditions.

Experimental Setup and Methodology

The study employs instrumented test pipes with embedded pressure transducers to capture the rapid pressure changes during CO2 phase transformation. The experimental configuration includes:

Component	Specification	Purpose
Test pipe	Q345B carbon steel, Φ108×8 mm	Simulate downhole conditions
CO2 charge	50-200 g liquid CO2	Energy source for fracture
Pressure transducers	0-1000 MPa range, 10 kHz response	Record pressure-time history
Ignition system	Electric detonator or laser	Initiate phase transformation
Data acquisition	100 kHz sampling rate	Capture rapid pressure changes
Confinement	Sand-filled or water-filled	Simulate rock formation

Pressure Evolution Characteristics

The internal pressure dynamics during CO2 phase transformation follow a characteristic multi-stage pattern:

Stage 1 - Initial pressurization (0-5 ms): Upon ignition, the liquid CO2 rapidly expands as the confining charge breaks down. Pressure rises from ambient to 50-150 MPa as the CO2 transitions from liquid to supercritical state. This stage is governed by the rate of heat transfer from the ignition source to the CO2 mass.

Stage 2 - Peak pressure development (5-20 ms): The pressure continues to rise rapidly, reaching peak values of 400-800 MPa depending on the charge mass and confinement conditions. The expansion ratio of CO2 from liquid (density ~770 kg/m³) to gas (density ~1.2 kg/m³ at atmospheric conditions) provides the energy for fracture.

Stage 3 - Pressure decay (20-100 ms): After reaching peak pressure, the pressure decays as gas flows through the fracture channels created in the surrounding medium. The decay rate depends on fracture propagation speed and gas flow resistance.

Stage 4 - Residual pressure (100-1000 ms): A residual pressure of 10-50 MPa persists as the system equilibrates, with gradual dissipation through continued gas flow.

Pressure-Velocity Relationship Analysis

The study establishes correlations between key parameters:

CO2 Charge (g)	Peak Pressure (MPa)	Pressure Rise Rate (MPa/ms)	Optimal for
50	350-450	30-50	Soft rock fracture
100	500-650	40-70	Medium-hard rock
150	600-750	50-90	Hard rock
200	700-850	60-100	Very hard rock

The pressure rise rate is equally important as the peak pressure value for effective fracture initiation. A rapid pressure rise (steep dP/dt) creates stress waves that initiate crack propagation more effectively than a slower pressure increase of similar magnitude.

Pipe Design and Safety Considerations

From a pressure vessel design perspective, the study highlights several critical considerations:

Burst pressure margin: The design burst pressure of the test pipe should exceed the peak internal pressure by a factor of at least 1.5. For expected peak pressures of 800 MPa, the pipe must withstand at least 1200 MPa without failure.
Dynamic loading effects: The rapid pressure rise creates inertial effects that increase the effective stress beyond what static analysis predicts. The dynamic amplification factor typically ranges from 1.2 to 2.0 depending on the rate of pressure increase relative to the pipe's natural frequency.
Material selection: The study demonstrates that conventional carbon steel (Q345B) can withstand multiple CO2 phase transformation events if the wall thickness is adequate (minimum 8 mm for Φ108 mm pipe with 800 MPa peak pressure). However, repeated cycling leads to fatigue damage accumulation at the ignition point.
Thermal effects: The rapid expansion creates localized temperature drops (Joule-Thomson effect) that can reach -78.5°C at the CO2 charge location, potentially causing low-temperature brittle fracture in susceptible materials.

Engineering Practice Applications

The findings have practical implications for:

Downhole tool design: Understanding pressure evolution enables optimization of charge mass and pipe geometry for specific rock formations.
Safety assessment: The pressure-time history provides data for evaluating personnel safety distances and equipment protection requirements.
Process optimization: The correlation between charge parameters and pressure output enables prediction of fracture effectiveness before field deployment.

Study Insights and Reflections

The experimental data presented provides valuable quantitative information for engineers designing systems that experience extreme transient pressure loading. The multi-stage pressure evolution pattern—initial rapid rise, peak plateau, and gradual decay—is characteristic of confined gas expansion events and has parallels in other high-energy processes such as controlled demolition and hydraulic fracturing. In my experience with pressure vessel design, the distinction between static and dynamic pressure loading is fundamental, and this study reinforces the importance of considering rate effects when evaluating structural integrity under transient conditions. The practical value of establishing empirical correlations between charge parameters and pressure output cannot be overstated, as it enables rational design rather than trial-and-error approaches to process development.