CLADDING TECHNOLOGY SHANXI CO., LTDKey Parameters Research on Energy Release of CO2 Fracturer
Literature Overview
This study note examines a technical paper focused on the key parameters governing energy release in CO2 fracturing devices. The CO2 fracturer is a high-pressure vessel that stores liquid CO2 and releases it in a controlled manner to fracture coal seams or rock formations. The energy release process involves a rapid phase change from liquid to gas, generating high pressure and temperature that propagate fractures into the surrounding material. The paper investigates the key parameters that influence the energy release process, including injection pressure, injection rate, borehole geometry, and coal seam properties.
From a pressure vessel and materials engineering perspective, this research is directly relevant to the design and safety of CO2 fracturing devices. The energy release process imposes severe cyclic loading on the fracturing device, and the key parameters governing this process must be understood to ensure the long-term integrity and safety of the equipment.
Core Technical Concepts
The energy release in a CO2 fracturer is governed by the thermodynamic properties of CO2 and the geometry of the borehole and fracturing device. The key parameters include:
Thermodynamic Parameters
The thermodynamic properties of CO2 determine the pressure and temperature generated during the phase change process. The critical properties of CO2 are:
| Property | Value |
|---|---|
| Critical temperature | 31.1°C (304.25 K) |
| Critical pressure | 7.38 MPa |
| Triple point temperature | -56.6°C (216.55 K) |
| Triple point pressure | 0.518 MPa |
| Liquid density at 20°C | 770 kg/m³ |
| Gas density at 20°C, 1 atm | 1.98 kg/m³ |
| Expansion ratio (liquid to gas at 1 atm) | ~390:1 |
When liquid CO2 is heated above its critical temperature, it undergoes a rapid phase change to a supercritical gas, expanding in volume by a factor of 100–200 at constant pressure. This expansion generates high pressure in the borehole, which is the primary driving force for fracture propagation.
Key Energy Release Parameters
The paper identifies several key parameters that influence the energy release process:
| Parameter | Typical Range | Influence on Energy Release |
|---|---|---|
| Injection pressure | 20–50 MPa | Determines maximum fracture pressure |
| Injection rate | 1–10 L/s | Affects rate of pressure build-up |
| CO2 mass | 50–500 kg | Determines total energy available |
| Borehole diameter | 75–150 mm | Affects fracture geometry and propagation |
| Borehole depth | 50–300 m | Determines accessible coal seam volume |
| Heating power | 5–50 kW | Controls rate of phase change |
| Coal seam permeability | 1–100 mD | Affects fracture propagation resistance |
| Coal seam strength | 5–30 MPa | Determines fracture initiation pressure |
The injection pressure is the most critical parameter, as it determines the maximum pressure available to fracture the coal seam. The injection rate affects the rate of pressure build-up and the final fracture pressure. The CO2 mass determines the total energy available for fracturing and the extent of the fracture network.
Energy Release Mechanism
The energy release mechanism in a CO2 fracturer involves several stages:
Stage 1: CO2 Injection
Liquid CO2 is injected into the borehole under high pressure, typically 20–50 MPa. The injection process pressurizes the borehole and compresses any existing gas or fluid in the borehole. The pressure in the borehole rises as CO2 is injected, and the temperature may increase slightly due to the compression of the CO2.
Stage 2: Phase Change Initiation
When the temperature of the liquid CO2 reaches its critical temperature (31.1°C), the phase change process begins. The CO2 undergoes a rapid transition from liquid to supercritical gas, expanding in volume and generating high pressure in the borehole. The rate of phase change depends on the heating power applied to the CO2 and the thermal conductivity of the surrounding coal seam.
Stage 3: Fracture Propagation
As the pressure in the borehole increases, it eventually exceeds the fracture strength of the coal seam, causing fractures to propagate from the borehole into the surrounding coal. The fracture propagation is governed by the stress field around the borehole, the fracture toughness of the coal, and the pressure gradient between the borehole and the fracture tips.
Stage 4: Energy Dissipation
The energy released during the phase change process is dissipated through fracture propagation, friction, and heat transfer to the surrounding coal seam. The total energy released is determined by the CO2 mass, the initial and final temperatures and pressures, and the efficiency of the energy transfer process.
Design Implications for CO2 Fracturing Devices
The key parameters governing energy release have direct implications for the design of CO2 fracturing devices. The device must be designed to withstand the maximum injection pressure and temperature, and to safely contain and release the CO2 during the fracturing process.
Pressure Vessel Design
The CO2 fracturing device is a high-pressure vessel that must be designed in accordance with applicable pressure vessel codes such as ASME VIII Div. 1, GB/T 150, or EN 13445. The design must account for the maximum working pressure, the design temperature, and the cyclic loading imposed by the fracturing process.
| Design Requirement | Specification |
|---|---|
| Maximum working pressure | 40–50 MPa |
| Design pressure | 60–75 MPa |
| Design temperature | -20°C to 200°C |
| Material | 34CrMo4, 42CrMo4, or equivalent |
| Corrosion allowance | 3–6 mm |
| Fatigue life | ≥ 10,000 cycles |
| NDT requirements | 100% RT + 100% MT of welds |
| Hydrostatic test | 1.25 × design pressure |
The material selection is critical for ensuring the long-term integrity of the fracturing device. Carbon steel is susceptible to CO2 corrosion, particularly in the presence of moisture and at elevated temperatures. The corrosion rate can be as high as 1 mm/year in severe conditions, which would rapidly reduce the wall thickness of the vessel and compromise its structural integrity. To mitigate this risk, the vessel may be designed with a corrosion allowance of 3–6 mm, or the internal surface may be protected with a corrosion-resistant overlay or coating.
Welding and Fabrication
The fabrication of CO2 fracturing devices requires specialized welding procedures and strict quality control. The welds must be designed and fabricated to withstand the cyclic loading imposed by the fracturing process, and must be inspected to ensure they are free of defects that could lead to fatigue cracking or pressure boundary failure.
| Welding |
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