CO2 Gas-Phase Fracturing Permeability Enhancement Technology Application at Xingyu Coal

Overview and Context

This literature describes the application of carbon dioxide gas-phase fracturing technology for permeability enhancement in coal reservoirs at Xingyu Coal Mine. The technology leverages the phase transition of liquid CO2 to gas under high temperature and pressure conditions to create complex fracture networks within low-permeability coal seams, thereby enhancing gas drainage efficiency and reducing outburst risk. From a pressure vessel and containment engineering perspective, this technology demands rigorous design and fabrication of high-pressure CO2 storage vessels, injection systems, and associated piping that can withstand repeated thermal cycling and cyclic pressure loading.

Core Technical Principles

CO2 gas-phase fracturing relies on the unique thermodynamic properties of carbon dioxide. When liquid CO2 is injected into a high-temperature formation, it undergoes a phase change from liquid to supercritical or gaseous state, generating expansion pressures exceeding 60 MPa at temperatures above the critical point of 31.1°C and 7.38 MPa. The rapid expansion creates tensile fractures that propagate through the coal matrix, while the dissolution of CO2 in formation water and coal micropores further reduces capillary resistance and improves gas flow pathways.

The key advantage over conventional water-based fracturing lies in the elimination of large volumes of fracturing fluid that can damage coal permeability through clay swelling and pore plugging. This is particularly significant for gas-bearing coal seams where maintaining matrix permeability is critical for effective gas drainage.

Engineering Practice Implications

From a fabrication standpoint, the CO2 injection system requires careful consideration of material selection and cladding strategies. The high-pressure storage vessels and injection cylinders must withstand pressures typically ranging from 15 to 25 MPa at ambient storage conditions, with potential transient pressures exceeding 35 MPa during injection into hot formations.

The CO2 environment at elevated temperatures presents a unique corrosion challenge. Above 40°C, CO2 becomes increasingly corrosive to carbon steel, forming iron carbonate scale and causing internal wall thinning. This necessitates the use of overlay cladding on wetted surfaces, where austenitic stainless steel (304L or 316L) or nickel-based alloys provide adequate corrosion resistance. The weld overlay process must ensure a metallurgically sound bond between the base carbon steel and the overlay layer, with bond strength exceeding 200 MPa as verified by tensile or shear testing per NB/T 47014.

Key Technical Parameters and Process Control

The fracturing effectiveness depends on several controllable parameters:

• Injection pressure: typically 15–30 MPa depending on formation depth and stress state
• CO2 injection rate: controlled to maintain fracture propagation without breakthrough
• Formation temperature: determines the phase state and expansion ratio of CO2
• Coal seam permeability: baseline permeability of 0.1–1.0 mD in low-permeability seams
• Fracture network complexity: measured by post-fracturing permeability enhancement factor

From a welding and fabrication quality perspective, the cyclic thermal loading experienced by the high-pressure components during repeated injection cycles can lead to fatigue cracking at the cladding interface. Post-weld heat treatment (PWHT) is essential to relieve residual stresses in the overlay welds, typically performed at 620–650°C for carbon steel base materials. The overlay layer composition must be carefully controlled to avoid intermetallic compound formation at the interface, which would compromise bond integrity under thermal cycling.

Study Insights and Reflections

This technology represents a significant advancement in coalbed methane extraction, but its engineering implementation demands the same rigor in pressure equipment fabrication as any high-pressure hydrogenation or petrochemical application. The cyclic nature of CO2 injection — repeated pressurization, thermal shock, and depressurization — creates a fatigue environment that requires careful attention to weld quality, residual stress levels, and material compatibility. Engineers working on such systems should pay particular attention to the hydrogen embrittlement potential of the base steel under repeated CO2 loading cycles, as dissolved CO2 can generate hydrogen at the metal surface.

The application of weld overlay cladding in CO2 systems also highlights the importance of post-weld inspection protocols. Ultrasonic testing of the cladding bond line should be performed both before and after PWHT, and periodic in-service inspections should include eddy current testing of the overlay layer thickness to detect any progressive thinning or delamination. This case study reinforces the principle that advanced energy extraction technologies cannot succeed without robust pressure equipment engineering as their foundation.