Supercritical CO2 Gas Explosion Coal Body Fracturing Mechanism Experimental Study

Literature Overview and Scope

This paper investigates the fracturing mechanism of coal body induced by supercritical CO2 gas explosion through systematic experimental research. The study employs high-pressure vessel experiments to simulate downhole conditions and examines how supercritical CO2 transitions from liquid-like to gas-like behavior during expansion, generating sufficient energy to fracture coal matrices. The research is significant for engineers involved in pressure vessel design and operation because supercritical CO2 systems require specialized containment equipment, high-pressure piping, and vessels designed to withstand extreme pressure-temperature combinations.

Core Fracturing Mechanism

The fundamental principle relies on the unique phase behavior of CO2 above its critical point at 31.1°C and 7.38 MPa. When supercritical CO2 is injected into coal seams and undergoes rapid expansion, it generates a high-energy gas explosion that creates complex fracture networks. The mechanism involves three sequential stages: initial fluid injection and coal matrix swelling, pressure buildup to the critical threshold, and rapid phase transition generating shock waves that propagate through the coal body.

The energy density of supercritical CO2 gas explosion is substantially higher than conventional hydraulic fracturing fluids. This is because the phase change from supercritical state to gas releases latent energy directly within the formation, creating a more intense fracture initiation event. The resulting fracture network includes both primary radial fractures extending from the injection point and secondary branched fractures that enhance overall permeability.

Pressure Vessel and Equipment Implications

From a pressure equipment design perspective, the experimental apparatus and field equipment for supercritical CO2 fracturing present several critical engineering challenges. The containment vessels must be designed according to applicable codes such as ASME VIII Div.1 or GB/T 150, with design pressures typically exceeding 35 MPa to provide adequate safety margins above the maximum operating pressure.

The thermal cycling experienced during repeated injection and expansion cycles creates fatigue concerns. The temperature differential between the supercritical CO2 at injection (typically 50-80°C) and the ambient or downhole conditions can induce thermal stress concentrations at vessel welds and geometric discontinuities. Engineers must carefully evaluate thermal stress relief requirements and consider the accumulation of thermal fatigue damage over multiple operating cycles.

Material selection for these high-pressure systems requires consideration of CO2 compatibility. At supercritical conditions, CO2 exhibits enhanced solvency properties that can lead to corrosion of certain carbon steel grades. The study confirms that low-alloy steels such as Cr-Mo grades or austenitic stainless steels provide adequate resistance, while plain carbon steels may suffer from CO2 corrosion in the presence of trace moisture.

Key Technical Points and Defect Analysis

The experimental results reveal that fracture initiation is highly sensitive to the initial stress state of the coal body. When the horizontal stress difference exceeds a critical value, fractures preferentially propagate in the direction of maximum horizontal stress. This directional dependence has implications for wellbore trajectory planning and fracture network geometry.

Common failure modes observed in the experimental apparatus include:

• Overpressure rupture at vessel-to-piping weld joints due to inadequate heat-affected zone toughness
• Stress corrosion cracking in carbon steel components exposed to wet supercritical CO2
• Fatigue cracking at nozzle penetrations subjected to cyclic thermal loading
• Leakage at bolted flange connections due to gasket degradation under high-pressure CO2 service

Engineering Practice Integration

For engineers designing pressure equipment for supercritical CO2 fracturing operations, several practical recommendations emerge from this literature. First, the design pressure should incorporate a minimum 20% margin above the maximum expected operating pressure to account for pressure transients during the gas explosion phase. Second, impact testing of vessel materials at the minimum design temperature is essential, with Charpy V-notch energies exceeding 47 J for carbon and low-alloy steels.

The weld procedures for these high-pressure vessels should be qualified per ASME IX or NB/T 47014 with consideration of the cyclic loading conditions. Post-weld heat treatment is mandatory for vessel wall thicknesses exceeding 25 mm to relieve residual stresses and prevent delayed cracking. Non-destructive examination should include both volumetric methods (RT or UT) for weld integrity and surface methods (MT or PT) for fatigue crack detection during periodic inspections.

Study Insights and Implications

This research demonstrates that supercritical CO2 gas explosion fracturing represents a fundamentally different energy delivery mechanism compared to conventional hydraulic fracturing. The implications for pressure equipment design are substantial, requiring engineers to consider not only the steady-state pressure design but also the dynamic pressure transients and thermal cycling inherent in the fracturing process. Understanding the phase behavior of CO2 and its interaction with containment materials is essential for ensuring safe and reliable equipment operation throughout the service life of fracturing equipment.