CLADDING TECHNOLOGY SHANXI CO., LTDSupercritical CO2 Gas Explosion Coal Body Fracturing Mechanism
Literature Overview
This paper investigates the fracturing mechanism of coal bodies induced by supercritical CO2 gas explosion, a technique developed primarily for enhanced coalbed methane (CBM) recovery and coal seam gas drainage improvement. The research employs both laboratory-scale experiments and numerical modeling to characterize the damage patterns, gas release behavior, and permeability enhancement in coal following supercritical CO2 explosion fracturing. The study is of particular relevance to engineers involved in the design and fabrication of high-pressure CO2 containers and delivery systems used in coal mining applications, where the equipment must withstand extreme pressure differentials and cyclic loading conditions.
Core Technical Content
The supercritical CO2 gas explosion fracturing of coal bodies involves the rapid phase transition of supercritical CO2 from a high-pressure, high-density state to a low-pressure, low-density gas state within a confined space. This rapid expansion generates a shock wave that propagates through the coal body, creating a complex network of fractures and pores.
| Parameter | Typical Value | Range |
|---|---|---|
| Supercritical CO2 injection pressure | 20-30 MPa | 15-35 MPa |
| Supercritical CO2 injection temperature | 35-50 C | 32-60 C |
| CO2 injection volume per shot | 2-5 L | 1-10 L |
| Coal confining pressure | 5-15 MPa | 3-20 MPa |
| Fracture zone radius | 50-150 mm | 30-200 mm |
| Post-fracture permeability increase | 3-8 times | 2-12 times |
The fracturing process can be divided into three phases: the initial pressure buildup phase where the CO2 is injected and the coal matrix deforms elastically; the phase change and shock wave generation phase where the CO2 undergoes rapid depressurization and expansion; and the post-explosion damage consolidation phase where the fracture network stabilizes and gas flow channels are established.
Material and Welding Requirements for CO2 Delivery Equipment
The equipment used for supercritical CO2 gas explosion fracturing in coal bodies includes high-pressure CO2 cylinders, pressure regulators, and delivery hoses, all of which must be designed and fabricated to withstand the extreme operating conditions. The CO2 storage cylinders are typically designed in accordance with GB/T 34525 or ISO 9809, with a test pressure of 1.5 times the maximum working pressure.
The welding of these high-pressure components requires special attention to the following aspects:
- Weld procedure qualification: The welding procedure specification (WPS) must be qualified in accordance with NB/T 47014 or ASME Section IX, with additional requirements for the specific service conditions including CO2 exposure and cyclic pressure loading.
- Weld metal selection: The filler metal must be compatible with the base material and resistant to CO2 corrosion. For carbon steel cylinders, low-hydrogen electrodes such as E7018 or E8018 are commonly used, with the deposit chemistry ensuring adequate toughness at the minimum service temperature.
- Post-weld inspection: Full-penetration welds must be inspected by radiographic testing (RT) to ASME Section V Article 2 or NB/T 47013-2 requirements, with acceptance criteria meeting at least the second-grade standard for critical joints.
- Hydrostatic testing: All completed pressure vessels and cylinders must undergo hydrostatic testing at 1.5 times the maximum working pressure, with no visible deformation or leakage permitted.
Numerical Modeling and Damage Analysis
The paper employs a coupled thermo-mechanical numerical model to simulate the supercritical CO2 explosion fracturing process in coal. The model incorporates the equation of state for CO2, the thermodynamic properties of the phase change, and the constitutive behavior of coal under dynamic loading. The simulation results reveal that the shock wave pressure at the explosion center can reach values exceeding 100 MPa, decaying rapidly with distance from the source.
The damage pattern in the coal body following supercritical CO2 explosion is characterized by three distinct zones:
| Zone | Distance from Source | Damage Type | Description |
|---|---|---|---|
| Zone I | 0-30 mm | Crush zone | Complete pulverization of coal matrix |
| Zone II | 30-100 mm | Fracture zone | Dense network of radial and circumferential fractures |
| Zone III | 100-200 mm | Microcrack zone | Sparse microcracks and pores |
The fracture zone (Zone II) is the most important for gas drainage improvement, as it provides the primary flow channels for coalbed methane extraction. The density and orientation of fractures in this zone are influenced by the coal strength, confining pressure, and the CO2 injection parameters.
Engineering Practice and Safety Analysis
In practical coal mining applications, the supercritical CO2 gas explosion fracturing equipment must be designed and operated in accordance with strict safety regulations. The FMEA analysis identifies the following critical safety concerns:
- Cylinder over-pressurization due to thermal expansion: If the CO2 cylinder is exposed to ambient temperatures exceeding the design temperature, the internal pressure may increase beyond the design limit. This risk is mitigated by installing pressure relief devices on all cylinders and storing them in temperature-controlled environments.
- Weld failure under cyclic loading: The repeated pressurization and depressurization cycles can lead to fatigue cracking at weld joints. This risk is reduced by using high-quality welding procedures, performing periodic ultrasonic testing of critical weld joints, and applying appropriate fatigue design factors.
- CO2 leakage in underground environments: CO2 leakage in underground mines poses an asphyxiation hazard to personnel. The risk is managed by using high-integrity connections, regular leak detection testing, and ensuring adequate ventilation in the working areas.
Summary
The study of supercritical CO2 gas explosion fracturing of coal bodies provides valuable insights into the damage mechanisms and gas release behavior that are critical for enhanced CBM recovery. The research demonstrates that the fracturing process creates a complex fracture network that significantly enhances coal permeability, with the most effective gas drainage achieved in the fracture zone surrounding the explosion center. From an equipment design perspective, the high-pressure CO2 delivery system must be engineered with appropriate material selection, welding qualification, and non-destructive testing to ensure safe and reliable operation under the demanding conditions of underground coal mining. The integration of numerical modeling with experimental validation provides a robust framework for optimizing the fracturing parameters and predicting the resulting damage patterns in different coal seam conditions.