Application Study of CO2 Fracturing Technology in Xingnan Pilot Area

Project Overview and Technical Framework

The Xingnan pilot area represents a significant field application of CO2 fracturing technology in China's oil and gas industry. This pilot project provides valuable practical data on the performance of CO2 fracturing equipment, including pressure vessels, injection systems, and completion hardware, under actual reservoir conditions. From a pressure vessel engineering perspective, this application study offers critical insights into the design, fabrication, and operational challenges of CO2 fracturing equipment in a real-world setting.

Technical Challenges and Solutions

Reservoir Conditions and Equipment Requirements

The Xingnan pilot area presents specific reservoir conditions that impose unique requirements on the CO2 fracturing equipment. The following table summarizes the key reservoir parameters and their implications for pressure vessel design.

Pressure Vessel Design Challenges

The application study identifies several specific challenges in the design of pressure vessels for CO2 fracturing in the Xingnan pilot area:

1. High-temperature design: The reservoir temperature of 55-75 degrees C requires the use of vessel materials with adequate high-temperature strength. The allowable stress values for Q345R decrease by approximately 5-8% at 75 degrees C compared to room temperature, necessitating thicker vessel walls or the use of higher-grade materials such as 16MnR.
2. Corrosion resistance: The presence of chlorides and formation water creates a corrosive environment that challenges the integrity of carbon steel pressure vessels. The application study recommends the use of weld overlay cladding with 316L stainless steel on the interior surfaces of pressure vessels in contact with CO2 and formation water.
3. Thermal cycling: The temperature variations during fracturing operations, including the Joule-Thomson cooling effect during CO2 expansion, create thermal cycling stresses that must be evaluated for fatigue life. The application study recommends the use of low-temperature impact-tested materials and appropriate PWHT to mitigate fatigue cracking.

Weld Overlay Cladding Applications

The application study highlights the importance of weld overlay cladding in protecting pressure vessels from CO2 corrosion. The following table summarizes the recommended cladding specifications for different equipment components.

Operational Performance and Lessons Learned

The Xingnan pilot area application provides valuable operational data that informs future pressure vessel design and fabrication practices. Key lessons learned include:

1. Reliability of CO2 charge cylinders: The pilot study confirmed that CO2 charge cylinders fabricated to NB/T 47002 standards with proper weld inspection and hydrostatic testing demonstrate reliable performance in field conditions. The minimum detonation length of 0.8 meters was confirmed under actual reservoir conditions.
2. Corrosion performance of overlay cladding: Pressure vessels with 316L weld overlay cladding demonstrated excellent corrosion resistance during the pilot operations, with no significant overlay degradation observed after 12 months of continuous operation. This validates the design approach of using austenitic stainless steel overlay for CO2 service.
3. Thermal management: The pilot study identified thermal management as a critical issue during fracturing operations. The rapid cooling of CO2 during expansion can cause localized thermal stresses in pressure vessels, particularly at weld seams and material transitions. The use of proper insulation and thermal barrier coatings on pressure vessels was recommended to mitigate this issue.
4. Inspection and maintenance: The pilot study established a comprehensive inspection and maintenance program for CO2 fracturing pressure vessels, including regular UT inspection of weld overlay bonds, RT inspection of critical welds, and periodic hydrostatic testing. This program has proven effective in maintaining the integrity of pressure vessels throughout the pilot operations.

Engineering Recommendations

Based on the findings of the Xingnan pilot area application study, the following engineering recommendations are proposed for future CO2 fracturing projects:

1. Adopt a risk-based inspection (RBI) approach for pressure vessel inspection, with inspection intervals and methods tailored to the specific operating conditions and materials used.
2. Use weld overlay cladding with 316L stainless steel as the standard corrosion protection measure for pressure vessels in CO2 service, with ESW as the preferred overlay process for thick overlays and SAW for thin overlays.
3. Implement a comprehensive materials management program that includes material certification, traceability, and periodic re-certification to ensure the continued compliance of pressure vessel materials with applicable standards.
4. Develop and maintain a digital twin of the pressure vessel fleet, incorporating real-time operational data and inspection results to optimize maintenance scheduling and predict remaining service life.

This application study demonstrates that CO2 fracturing technology can be successfully implemented in field conditions with appropriate pressure vessel design, fabrication, and inspection practices. The Xingnan pilot area experience provides a valuable foundation for the continued development and deployment of CO2 fracturing technology in China's oil and gas industry.