Research on Triangular Area Hanging Roof CO2 Fracture Control Technology in Fully Mechanized Mining Face

Literature Overview

This paper presents research on the application of carbon dioxide (CO2) fracturing technology for controlling hanging roof instability in the triangular zone of fully mechanized mining faces. The triangular zone, formed at the intersection of adjacent working faces, is characterized by complex stress redistribution and roof instability due to the loss of lateral support. The study proposes the use of supercritical CO2 injection as a controlled fracturing method to relieve stress concentration and stabilize the hanging roof. While this topic belongs to the mining engineering domain, it involves important considerations of pressure vessel design, high-pressure gas handling, and material performance under extreme conditions that are directly relevant to pressure vessel and cladding engineering.

Core Technical Points

The triangular zone in fully mechanized mining faces is a critical area for roof control because it experiences combined effects of abutment pressure from adjacent faces and the loss of lateral support at the face intersection. The hanging roof in this zone is prone to sudden collapse, posing significant safety hazards to mining operations. The proposed CO2 fracturing technology involves injecting supercritical CO2 into the hanging roof strata through specially designed boreholes to create controlled fractures that relieve stress concentration and improve roof stability.

CO2 Fracture Control Parameters

The study demonstrates that supercritical CO2, when injected at pressures exceeding 15 MPa, enters the supercritical state where it exhibits both liquid-like density and gas-like diffusivity. This unique combination enables the CO2 to penetrate micro-fractures in the rock mass and create controlled hydraulic fractures that relieve stress concentration without causing uncontrolled roof collapse.

Material and Equipment Considerations

The CO2 fracturing technology requires specialized equipment including high-pressure pumps, injection valves, pressure monitoring systems, and safety relief devices. These components must be designed and manufactured to withstand repeated cycles of high pressure and temperature, with material selection and welding qualification being critical considerations.

Equipment Material Requirements

The buffer pressure vessel used in the CO2 injection system is a typical example of a clad plate pressure vessel, where the carbon steel shell provides structural strength and the stainless steel overlay provides corrosion resistance against CO2 and moisture. The design and fabrication of such vessels must comply with GB/T 150 or ASME VIII Div.1 requirements, with particular attention to the welding procedures and non-destructive testing of clad plate welds.

Engineering Practice Integration

The application of CO2 fracturing technology in mining operations presents several challenges that are directly relevant to pressure vessel engineering. First, the high-pressure CO2 handling system requires careful design to prevent brittle fracture under low-temperature conditions, particularly during depressurization events. The selection of materials with adequate low-temperature toughness, such as austenitic stainless steels or nickel-based alloys, is essential for ensuring safe operation.

Second, the repeated pressure cycling experienced by the CO2 injection equipment necessitates fatigue analysis in accordance with ASME VIII Div.2 or API 934. The cyclic loading can lead to fatigue cracking at weld joints and stress concentration areas, and the selection of weld overlay materials with high fatigue resistance is critical. The study does not explicitly address fatigue considerations, which represents a significant gap in the engineering design of the CO2 fracturing system.

Risk Assessment Summary

Key Questions and Reflections

A critical question that arises from this study is the long-term reliability of the CO2 injection equipment under harsh mining conditions. The combination of high pressure, temperature cycling, and potential exposure to corrosive gases in the mining environment creates a challenging service condition that demands rigorous material selection and quality assurance. The study focuses primarily on the fracturing technology itself and does not adequately address the mechanical design and fabrication requirements for the pressure equipment involved.

Another important consideration is the environmental impact of CO2 injection into underground strata. While the study emphasizes the safety benefits of the technology, the potential for CO2 migration to the surface and its contribution to greenhouse gas emissions must be evaluated. This consideration is particularly relevant in the context of global efforts to reduce carbon emissions and achieve sustainable development goals.

Study Insights and Implications

The most significant insight from this literature is the demonstration that supercritical CO2 can be effectively used as a controlled fracturing agent for roof stability in mining operations. For pressure vessel engineers, the study highlights the importance of material selection and design qualification for high-pressure gas handling systems, particularly in applications involving CO2 where the risk of corrosion and low-temperature brittleness must be carefully managed. The study also underscores the value of interdisciplinary collaboration, as the successful implementation of CO2 fracturing technology requires the integration of mining engineering, materials science, and pressure vessel design expertise. For engineers working in the field of bimetal products and pressure vessels, the study serves as a reminder that the principles of material compatibility, process control, and quality assurance are universal across diverse engineering applications.