CO2 Pure Dry-Fracturing Technology and Equipment Research Progress

Overview of the Literature

This study note examines the research progress on CO2 pure dry-fracturing technology and equipment. The literature focuses on the development of equipment and systems that enable CO2 phase-change fracturing without the use of water-based fluids. From a pressure vessel and materials engineering perspective, the equipment development represents a significant advancement in high-pressure system design and fabrication.

Technology Principles

CO2 pure dry-fracturing technology differs from conventional hydraulic fracturing in several key aspects:

Feature	Hydraulic Fracturing	CO2 Pure Dry-Fracturing
Fluid Type	Water-based slurry	Liquid CO2 only
Energy Source	Pump pressure	Phase-change energy
Proppant	Added during fracturing	Not required or added separately
Environmental Impact	High water usage	Minimal water usage
Equipment Complexity	Moderate	High
Cost	Lower	Higher

Energy Release Mechanism

The CO2 pure dry-fracturing process relies on the energy released during the phase change of liquid CO2:

Compression phase — CO2 is compressed to 7–14 MPa and stored as a liquid
Trigger phase — The CO2 is rapidly depressurized by ignition or mechanical release
Expansion phase — The CO2 expands 500–700 times, generating high pressure
Fracturing phase — The high pressure creates fractures in the rock formation
Recovery phase — The CO2 is recovered and recycled for future use

Equipment Development

The literature describes several key equipment components for CO2 pure dry-fracturing:

CO2 Phase-Change Device

The core component of the system is the CO2 phase-change device, which must meet stringent design and fabrication requirements:

Pressure rating: 100–150 MPa design pressure
Temperature range: -56°C to 60°C operating temperature
Cycle life: 50–200 cycles minimum
Weight: Minimized for transport and handling
Safety features: Multiple redundant safety systems

The device fabrication requires:

Base metal selection — High-strength, low-alloy steel (e.g., 16MnR, Q345R)
Cladding material — Nickel-based alloy (Inconel 625, Hastelloy C-276) or austenitic stainless steel (316L, 347H)
Welding process — SAW for base metal, ESW or PTA for cladding
Heat treatment — PWHT to relieve residual stresses
Inspection — Full NDT including RT, UT, MT, and PT

CO2 Compression and Storage System

The CO2 compression and storage system includes:

Component	Function	Key Specifications
Compressor	Compresses CO2 to storage pressure	7–14 MPa output pressure
Storage tank	Stores compressed CO2	10–50 MPa design pressure
Piping	Transports CO2 to device	High-pressure, cryogenic service
Valves	Controls CO2 flow	High-pressure, quick-acting
Safety devices	Protects against overpressure	Relief valves, rupture discs

Control and Monitoring System

The control and monitoring system provides:

Real-time monitoring — Pressure, temperature, and vibration monitoring
Automated control — Automated operation of the fracturing process
Safety interlocks — Multiple redundant safety systems
Data acquisition — Comprehensive data logging for analysis
Remote operation — Remote control for safety

Recovery and Recycling System

The CO2 recovery and recycling system includes:

Recovery equipment — Captures CO2 after fracturing
Purification system — Removes contaminants from recovered CO2
Compression system — Re-compresses recovered CO2 for reuse
Storage system — Stores purified CO2 for future use
Monitoring system — Monitors CO2 quality and quantity

Materials and Fabrication Challenges

The CO2 pure dry-fracturing equipment presents several materials and fabrication challenges:

High-Pressure Vessel Design

The high-pressure vessels must be designed to withstand:

Extreme pressures — 100–150 MPa design pressure
Cyclic loading — 50–200 pressure cycles
Thermal cycling — Temperature variations from -56°C to 60°C
Corrosive environment — CO2-water interactions creating carbonic acid

The design must comply with applicable codes and standards:

ASME VIII Div.2 — For high-pressure vessel design
NB/T 47002 — Chinese standard for pressure vessel design
GB/T 150 — Chinese standard for pressure vessel fabrication
ASME IX — For welding procedure qualification
API 934 — For cladding requirements

Welding and Cladding Requirements

The welding and cladding requirements for CO2 pure dry-fracturing equipment are demanding:

Welding procedure qualification — Full procedure qualification per ASME IX
Welder qualification — Qualified welders for each welding process
Cladding process selection — Based on thickness, quality requirements, and cost
Weld inspection — Full NDT including RT, UT, MT, and PT
Post-weld heat treatment — To relieve residual stresses and improve toughness
Hydrogen control — Preventing hydrogen-induced cracking in high-strength steels

Inspection and Quality Control

The inspection and quality control requirements include:

Inspection Type	Method	Acceptance Criteria
Material verification	Chemical analysis, mechanical testing	Per applicable specifications
Weld inspection	RT, UT, MT, PT	Per applicable code
Cladding inspection	Thickness measurement, bond strength testing	Per API 934 or equivalent
Pressure testing	Hydrostatic testing	1.5 times design pressure
Cyclic testing	Fatigue testing	Per design life requirements
Corrosion testing	Immersion testing, salt spray testing	Per service requirements

Equipment Performance and Reliability

The literature presents data on the performance and reliability of CO2 pure dry-fracturing equipment:

Performance Metrics

The key performance metrics include:

Fracturing efficiency — Energy delivered to the rock formation per unit of CO2
Fracture network quality — Density and connectivity of the fracture network
CO2 recovery rate — Percentage of CO2 recovered after fracturing
Equipment availability — Percentage of time the equipment is available for use
Maintenance interval — Time between required maintenance activities

Reliability Considerations

The reliability of the equipment depends on:

Materials quality — Using high-quality materials with consistent properties
Fabrication quality — Ensuring high-quality welding and fabrication
Inspection and testing — Comprehensive inspection and testing before and during service
Maintenance and repair — Regular maintenance and timely repair of issues
Operational procedures — Proper operation and handling procedures

Study Insights and Implications

The CO2 pure dry-fracturing technology represents a significant advancement in subsurface stimulation methods with important implications for pressure vessel and materials engineering:

The technology creates demand for specialized high-pressure, low-temperature pressure vessels with advanced materials and cladding
The cyclic loading conditions require rigorous fatigue design and inspection
The corrosive environment necessitates careful materials selection and cladding technology
The safety-critical nature of the application demands the highest quality standards and inspection requirements

The development of CO2 pure dry-fracturing equipment requires close collaboration between materials scientists, welding engineers, pressure vessel designers, and equipment manufacturers. The pressure vessel fabrication community must develop specialized capabilities to meet the demanding requirements of this emerging technology, including advanced welding procedures, specialized inspection techniques, and comprehensive quality control systems.

The future of CO2 pure dry-fracturing technology depends on continued advancements in materials science, fabrication technology, and equipment design. The pressure vessel and materials engineering community plays a critical role in enabling these advancements, and must stay at the forefront of technological development to support the successful implementation of this important technology.