CLADDING TECHNOLOGY SHANXI CO., LTDPermeability Changes in Shale After Supercritical CO2 Fracturing
Literature Overview
This paper investigates the permeability changes in shale formations following supercritical CO2 fracturing and identifies the key factors that influence the permeability enhancement. The research is conducted using core-scale experiments and numerical simulation, with particular attention to the long-term stability of the induced permeability under reservoir conditions. The findings are directly relevant to engineers designing high-pressure CO2 delivery systems for shale gas and shale oil stimulation, as the permeability enhancement determines the required injection pressure, flow rate, and equipment capacity.
Core Technical Content
The permeability of shale is inherently low, typically in the range of nanodarcies to microdarcies, making conventional hydraulic fracturing less effective for gas production. Supercritical CO2 fracturing offers a potential alternative due to its lower viscosity, higher diffusivity, and phase change capability, which can create more complex fracture networks in shale.
| Parameter | Pre-Fracture Value | Post-Fracture Value | Enhancement Factor |
|---|---|---|---|
| Permeability (mD) | 0.001-0.01 | 0.01-0.1 | 3-10 times |
| Porosity (%) | 3-5 | 4-6 | 1.2-1.5 times |
| Fracture density (m/m) | 0.1-0.5 | 1.0-3.0 | 3-6 times |
| Specific surface area (m2/g) | 50-100 | 80-150 | 1.5-2.0 times |
The permeability enhancement is not uniform throughout the fractured zone and follows a characteristic distribution pattern:
- Near-wellbore zone (0-50 mm): Highest permeability enhancement, with values reaching 10-20 times the original permeability due to the dense network of fractures and the dissolution of carbonate minerals by CO2.
- Intermediate zone (50-150 mm): Moderate permeability enhancement, with values of 3-8 times the original, characterized by a mix of fractures and microcracks.
- Far-field zone (>150 mm): Minimal permeability enhancement, with values close to the original, indicating that the fracture network does not extend significantly beyond this distance.
Influencing Factors Analysis
The paper identifies several key factors that influence the permeability enhancement in shale following supercritical CO2 fracturing:
| Factor | Influence on Permeability | Mechanism |
|---|---|---|
| CO2 injection pressure | Positive correlation | Higher pressure creates larger fractures |
| CO2 injection volume | Positive correlation | More CO2 produces more phase change energy |
| Shale brittleness | Positive correlation | Brittle shale fractures more easily |
| Confining pressure | Negative correlation | Higher confining pressure closes fractures |
| Shale organic content | Positive correlation | Organic-rich shale has more natural fractures |
| Temperature | Complex relationship | Affects CO2 phase behavior and rock strength |
The interaction between these factors is complex and non-linear. For example, increasing the injection pressure may initially enhance permeability, but beyond a critical threshold, the excessive pressure may cause fracture closure due to the compressive stress in the surrounding rock. Similarly, higher temperatures may enhance the phase change energy of CO2 but may also reduce the rock strength, leading to different fracture patterns.
Pressure Equipment Design Considerations
The design of the high-pressure CO2 delivery system for shale fracturing must account for the specific requirements of the fracturing process:
- Pressure rating: The CO2 storage vessels and delivery piping must be rated to withstand the maximum injection pressure, typically in the range of 30-50 MPa for shale fracturing applications. The design pressure should include a safety factor of 1.5 above the maximum operating pressure.
- Material selection: The materials must resist both the mechanical stress and the potential for CO2 corrosion. For dry CO2 service, carbon steel (Q345R) is acceptable. For wet CO2 service, stainless steel overlay or cladding is required. The overlay should be applied using electroslag welding (ESW) or submerged arc welding (SAW) with a minimum overlay thickness of 3 mm.
- Welding requirements: All welds in the high-pressure system must be qualified in accordance with ASME Section IX or NB/T 47014. The weld procedure qualification must include mechanical property testing (tensile, bend, hardness) and non-destructive testing (RT, UT) of the weld. For critical welds, the acceptance criteria must meet the first-grade standard of ASME Section V Article 2.
- Inspection and testing: All pressure vessels must undergo hydrostatic testing at 1.5 times the design pressure. The welds must be inspected by RT or UT to ensure full penetration and absence of defects. Periodic in-service inspection should include thickness measurement, ultrasonic testing of welds, and visual examination of external surfaces.
Long-Term Permeability Stability
A critical aspect of the research is the long-term stability of the permeability enhancement under reservoir conditions. The paper reports that the permeability may decrease over time due to several mechanisms:
- Fracture closure: The compressive stress from the overburden and tectonic stresses may close the induced fractures, particularly in the absence of proppant.
- Mineral precipitation: The dissolution and re-precipitation of minerals in the presence of CO2 and formation water may partially fill the fractures.
- Gelation of CO2: Under certain conditions, CO2 may form solid hydrates that block the flow channels.
The permeability stability is influenced by the confining pressure, temperature, and the presence of formation water. The paper recommends that the fracturing parameters be optimized to create fractures that are resistant to closure, such as by incorporating proppant or by creating fractures at an orientation that is favorable for proppant placement.
Summary
The study of permeability changes in shale following supercritical CO2 fracturing provides essential data for the design and optimization of shale gas and shale oil stimulation operations. The permeability enhancement is significant but non-uniform, with the highest enhancement near the wellbore and decreasing with distance. The key influencing factors include the CO2 injection pressure and volume, the shale properties, and the reservoir conditions. From an equipment design perspective, the high-pressure CO2 delivery system must be engineered with appropriate pressure ratings, material selection, welding procedures, and inspection protocols to ensure safe and reliable operation. The long-term stability of the permeability enhancement is a critical consideration that must be addressed through careful optimization of the fracturing parameters and the selection of appropriate proppant or fracture-stabilizing agents. Engineers involved in shale fracturing operations must ensure that all pressure-containing equipment is designed, fabricated, and inspected in accordance with applicable codes and standards to guarantee safe and efficient operation.