Liquid CO2 Phase-Change Fracturing Permeability Enhancement Technology for Bottom Drawgate Interlayer Drilling

Technical Background

The bottom drawgate interlayer drilling technique is a coal mining ventilation method used to create gas drainage pathways through rock strata from the bottom of a mine. The liquid CO2 phase-change fracturing method represents an innovative approach to enhance the permeability of coal seams by utilizing the rapid expansion of liquid CO2 upon depressurization to generate hydraulic fractures. This technology addresses the challenge of low permeability in coal seams, which limits the effectiveness of conventional gas drainage methods.

Principle of Liquid CO2 Phase-Change Fracturing

Liquid CO2 is injected into pre-drilled holes under high pressure (typically 15–25 MPa). When the injection valve is suddenly opened or the pressure is released, the liquid CO2 undergoes a rapid phase change from liquid to gas, expanding by a factor of approximately 450 times. This expansion generates a shock wave and high-pressure gas flow that propagates through the coal mass, creating a network of fractures that enhance gas drainage pathways.

Parameter	Typical Value	Range	Notes
Injection pressure	20 MPa	15–25 MPa	Dependent on coal strength
CO2 injection volume	5–15 L	3–20 L	Per fracture stage
Phase change expansion ratio	450:1	400–500:1	At atmospheric pressure
Fracture propagation distance	2–8 m	1–10 m	Depends on coal properties
Effective permeability increase	3–10 times	2–15 times	Post-fracturing
Fracturing temperature	-78.5 °C	-90 to -60 °C	CO2 sublimation point

Comparison with Conventional Permeability Enhancement Methods

The liquid CO2 phase-change fracturing method offers several advantages over conventional hydraulic fracturing and explosive fracturing. Unlike hydraulic fracturing, which requires large volumes of water and proppants, CO2 fracturing uses a minimal volume of working fluid and leaves no residual proppant in the fractures. Compared to explosive fracturing, CO2 fracturing is safer, more controllable, and does not generate toxic gases.

Method	Permeability Enhancement	Environmental Impact	Safety	Cost
Hydraulic fracturing	Moderate	High (water usage, proppant)	Moderate	High
Explosive fracturing	High	Moderate (vibration, gas)	Low	Moderate
Liquid CO2 fracturing	High	Low (CO2 is natural)	High	Moderate
Mechanical cutting	Low	Low	High	Low

Application in Bimetal Pressure Vessel Context

While this technology is primarily applied in coal mining, the principles of controlled phase-change energy release are relevant to the design of pressure vessels that handle cryogenic fluids. The understanding of CO2 phase-change mechanics informs the design of safety relief devices and the selection of materials that can withstand rapid temperature and pressure transients. For pressure vessels handling liquefied gases, the study of CO2 expansion behavior provides valuable data for predicting the consequences of overpressure events and for designing appropriate emergency depressurization systems.

Key Reflections

The liquid CO2 phase-change fracturing technology demonstrates the power of leveraging natural phase-change phenomena for engineering applications. The key insight is that the energy density of the phase change (approximately 130 kJ/kg for CO2) is sufficient to fracture coal seams without the need for chemical explosives. This principle could be adapted for other applications, such as controlled demolition of concrete structures or creation of permeability in tight gas reservoirs. The technology also highlights the importance of understanding the thermodynamic behavior of cryogenic fluids, which is directly relevant to pressure vessel design for liquefied gas storage.

Summary

Liquid CO2 phase-change fracturing is an effective and environmentally friendly method for enhancing coal seam permeability. Its application in bottom drawgate interlayer drilling provides a practical solution to the challenge of gas drainage in low-permeability coal seams. The underlying thermodynamic principles have broader implications for pressure vessel design and cryogenic fluid handling, making this technology relevant to the broader field of pressure equipment engineering.