CLADDING TECHNOLOGY SHANXI CO., LTDCO2 Fracturing and Permeability Enhancement Technology for Low-Permeability Coal Seams
Literature Overview
This literature investigates the application of carbon dioxide (CO₂) fracturing technology for enhancing the permeability of low-permeability coal seams at the Hongfa Coal Mine. Low-permeability coal seams are characterized by gas permeability values below 1 × 10⁻³ μm², which severely limits the effectiveness of conventional gas drainage methods and poses significant safety risks due to the accumulation of coal and rock gas (methane). The study explores the use of supercritical CO₂ as a fracturing agent to create fractures in the coal seam, thereby improving gas drainage efficiency and reducing the risk of gas outbursts. The research also examines the environmental implications of CO₂ injection, including potential CO₂ sequestration in the coal seam.
Technical Principles and Process Design
The CO₂ fracturing technology is based on the principle that supercritical CO₂ (at temperatures above 31.1 °C and pressures above 7.38 MPa) exhibits unique physical properties, including low viscosity, high diffusivity, and high density, which make it an effective fracturing agent. When injected into the coal seam under high pressure, supercritical CO₂ creates fractures through a combination of hydraulic pressure and thermal effects, as the CO₂ undergoes a phase change from supercritical to gaseous state upon depressurization.
The following table summarizes the key process parameters for CO₂ fracturing:
| Parameter | Value | Notes |
|---|---|---|
| Injection pressure | 20–35 MPa | Depends on coal seam depth and strength |
| Injection temperature | 40–60 °C | Supercritical conditions |
| CO₂ injection rate | 5–15 m³/min | Controlled to prevent excessive fracturing |
| Total CO₂ injection volume | 500–2000 m³ | Per fracturing operation |
| Fracture propagation length | 10–30 m | Measured by microseismic monitoring |
| Post-fracturing permeability increase | 3–10 times | Measured by gas flow rate |
The process design involves the following steps:
- Drilling of injection boreholes into the coal seam at an appropriate angle (typically 90° or slightly deviated).
- Installation of packers to isolate the target coal seam section.
- Injection of supercritical CO₂ at controlled pressure and rate.
- Monitoring of fracture propagation using microseismic sensors and pressure gauges.
- Termination of injection when the target fracture length is achieved.
- Installation of drainage pipes and initiation of gas drainage operations.
Permeability Enhancement and Gas Drainage Performance
The study reports significant improvements in coal seam permeability following CO₂ fracturing. The following table presents the measured permeability values before and after fracturing for different coal seam sections:
| Coal Seam Section | Pre-Fracturing Permeability (× 10⁻³ μm²) | Post-Fracturing Permeability (× 10⁻³ μm²) | Enhancement Factor |
|---|---|---|---|
| Section A | 0.5 | 3.2 | 6.4 |
| Section B | 0.3 | 1.8 | 6.0 |
| Section C | 0.8 | 5.5 | 6.9 |
| Section D | 0.2 | 1.5 | 7.5 |
The gas drainage efficiency also improved significantly, with the gas concentration in the drained gas increasing from 20–30% to 60–80%, and the gas flow rate increasing by 3–5 times. The study attributes these improvements to the creation of a network of fractures that provide preferential pathways for gas flow, as well as the adsorption of CO₂ on the coal matrix, which reduces the effective stress and further enhances permeability.
The study also examines the long-term stability of the enhanced permeability. Over a monitoring period of 6 months, the permeability remained stable at approximately 80–90% of the initial post-fracturing value, indicating that the fractures created by CO₂ injection are stable and do not undergo significant closure due to coal matrix swelling or stress redistribution.
Environmental Considerations and CO2 Sequestration
An important aspect of the study is the evaluation of CO₂ sequestration potential in the coal seam. The study reports that approximately 60–70% of the injected CO₂ is retained in the coal seam through adsorption and dissolution in the coal pore structure. This sequestration not only reduces the net CO₂ emissions associated with the fracturing operation but also provides a potential pathway for CO₂ storage in deep coal seams. The study estimates that the Hongfa Coal Mine could sequester approximately 50,000–80,000 tonnes of CO₂ annually if the technology is applied to all low-permeability coal seams.
However, the study also acknowledges potential environmental risks, including CO₂ leakage through abandoned boreholes or fault zones, which could lead to gas accumulation in the mine and pose asphyxiation hazards to miners. The recommended mitigation measures include the installation of monitoring systems to detect CO₂ leakage, the use of sealed boreholes with multiple packer sets, and the implementation of a comprehensive gas monitoring and ventilation plan.
Key Questions and Reflections
The literature raises several important questions about the scalability and economic viability of CO₂ fracturing technology. First, the high injection pressures required (20–35 MPa) necessitate specialized equipment and infrastructure, which increases the capital and operational costs of the technology. The study suggests that the economic feasibility of CO₂ fracturing is most favorable for coal seams with high gas content and significant outburst risk, where the safety benefits justify the additional costs.
Second, the study acknowledges that the long-term environmental impact of CO₂ injection into coal seams is not fully understood. While the short-term sequestration potential is promising, the behavior of CO₂ over decades or centuries, including potential migration and leakage, requires further investigation. The study recommends the development of long-term monitoring programs to track CO₂ behavior and ensure environmental safety.
Third, the study highlights the need for regulatory frameworks to govern CO₂ fracturing operations, including guidelines for injection pressure limits, CO₂ purity requirements, and monitoring protocols. The development of such frameworks is essential for the widespread adoption of the technology.
Study Insights and Implications
The most significant contribution of this literature is the demonstration that CO₂ fracturing is a viable technology for enhancing the permeability of low-permeability coal seams, with significant benefits for gas drainage efficiency and mine safety. The study provides practical guidance for the design and implementation of CO₂ fracturing operations, including process parameters, monitoring protocols, and environmental safeguards. For engineers involved in coal mine gas management, this study reinforces the importance of innovative approaches to gas drainage and the potential for combining safety improvements with environmental benefits through CO₂ sequestration. The study also highlights the need for continued research and development to optimize the technology, reduce costs, and ensure long-term environmental safety.