Adaptability Evaluation of CO2 Pre-Frac for Gulong Shale Oil

Literature Overview

This paper evaluates the technical adaptability of carbon dioxide (CO₂) pre-frac technology for the Gulong shale oil reservoir. The study examines the physicochemical interactions between CO₂ and the shale matrix, the effects of CO₂ injection on fracture initiation and propagation, and the economic and environmental implications of using CO₂ as a pre-frac fluid. While primarily a petroleum engineering topic, the study has significant implications for materials selection and corrosion protection in wellbore and surface equipment, which directly relates to the field of cladding and bimetallic component design.

Core Technical Points

CO2 Pre-Frac Mechanism

CO₂ pre-frac involves injecting supercritical or subcritical CO₂ into the formation before the main hydraulic fracturing stage. The primary mechanisms include:

• Swelling and weakening of the shale matrix through CO₂ dissolution in kerogen and bitumen.
• Reduction of fracture initiation pressure due to CO₂'s lower viscosity and higher compressibility compared to water-based fluids.
• Enhanced fracture complexity through the interaction of CO₂ with natural microfractures and bedding planes.
• Improved proppant placement due to the lower fluid loss rate of CO₂ compared to water.

Shale Matrix Interaction

The Gulong shale oil reservoir is characterized by high organic content (TOC typically 2–5%), low permeability (nD range), and high natural stress. CO₂ interacts with the organic matter in the shale through dissolution and swelling, which weakens the kerogen matrix and reduces the fracture toughness of the rock. The study reports a reduction in fracture initiation pressure of approximately 20–35% compared to conventional water-based fracturing, which is significant for reducing the required pumping pressure and equipment stress.

Materials and Corrosion Considerations

The use of CO₂ in fracturing operations introduces significant corrosion challenges for wellbore tubulars, surface piping, and storage equipment. CO₂ dissolution in formation water produces carbonic acid, which can lead to acid gas corrosion of carbon steel components. The corrosion rate is influenced by CO₂ partial pressure, temperature, pH, and the presence of inhibitors or protective coatings.

Engineering Practice Considerations

Cladding Solutions for CO2 Service

For equipment exposed to CO₂-containing fluids in shale oil operations, several cladding solutions are available:

• Weld overlay cladding of carbon steel with 316L stainless steel using ESW or SAW processes.
• Strip cladding of carbon steel with duplex stainless steel 2205 for enhanced corrosion resistance and mechanical strength.
• Nickel-based alloy cladding (Inconel 625) for severe CO₂ service conditions.

The selection of cladding material should be based on the CO₂ partial pressure, temperature, and the presence of other corrosive species such as H₂S. For moderate CO₂ service (partial pressure < 1 MPa, temperature < 150 °C), 316L cladding is typically sufficient. For severe service conditions, nickel-based alloy cladding is recommended.

Quality Control and Inspection

Non-destructive testing of clad components for CO₂ service should include:

• Magnetic particle testing (MT) of the overlay surface for surface-breaking defects.
• Ultrasonic testing (UT) or phased array UT (PAUT) for bond quality assessment.
• Hardness testing to verify the microstructural integrity of the overlay layer.
• Corrosion testing (e.g., ASTM G15 for CO₂ corrosion) to validate the material selection.

Key Questions and Reflections

The study raises an important question about the long-term environmental and economic viability of CO₂ pre-frac. While CO₂ injection can sequester carbon in the formation, the energy required for CO₂ compression and transport may offset some of the environmental benefits. From a materials perspective, the use of CO₂ introduces additional corrosion risks that must be carefully managed through proper material selection and cladding design.

Another reflection is the importance of integrating materials engineering with reservoir engineering. The choice of fracturing fluid should not be made solely based on reservoir performance but should also consider the materials compatibility of the entire production system. This requires close collaboration between reservoir engineers, materials engineers, and fabrication specialists.

Study Insights and Implications

The evaluation of CO₂ pre-frac adaptability for Gulong shale oil provides valuable insights for both reservoir engineering and materials engineering. The key finding is that CO₂ pre-frac can significantly reduce fracture initiation pressure and improve proppant placement, but it introduces corrosion challenges that must be addressed through proper materials selection and cladding design. For engineering practice, this means that the materials specification for shale oil production equipment should account for the presence of CO₂ and other acid gases, and cladding solutions should be considered as a cost-effective alternative to using expensive alloy materials throughout.

The study also highlights the need for standardized corrosion testing procedures for CO₂ service conditions. Existing standards such as NACE MR0175/ISO 15156 provide guidance for sour service, but specific guidelines for CO₂ service in shale oil operations are still evolving. Future work should focus on developing comprehensive materials selection criteria that integrate reservoir conditions, production scenarios, and materials performance data.