Simulation and Suppression of Pipeline Blockage in Liquid CO2 Pre-Fracturing Ground Systems

Literature Overview and Problem Statement

Liquid carbon dioxide (CO2) fracturing has emerged as an environmentally attractive alternative to conventional hydraulic fracturing, offering advantages in reducing water usage, enhancing fracture complexity, and improving production rates in tight gas reservoirs. However, the ground pipeline system used to transport liquid CO2 from storage to the wellhead is susceptible to blockage caused by phase change, pressure drop, and temperature reduction during transport. This study investigates the mechanisms of pipeline blockage through numerical simulation and proposes engineering countermeasures to suppress or prevent blockage occurrence. The research is directly relevant to engineers designing high-pressure pipelines and pressure vessels for supercritical or subcritical CO2 service.

Blockage Mechanism and Simulation Approach

The blockage phenomenon in liquid CO2 pipelines is attributed to the following mechanisms:

The simulation employs a one-dimensional transient flow model coupled with thermodynamic property calculations for CO2. The model accounts for heat transfer between the fluid and the pipe wall, heat exchange with the ambient environment, and the effects of insulation on the thermal boundary conditions. The governing equations include mass conservation, momentum conservation (with friction factor), and energy conservation with enthalpy-temperature relations for CO2.

Key simulation results indicate that blockage risk increases significantly when:

• Pipeline length exceeds 500 meters without intermediate heating
• Ambient temperature falls below -10 °C
• Flow velocity drops below 2 m/s (increasing residence time and heat loss)
• Pipeline insulation thermal conductivity exceeds 0.05 W/(m·K)

Suppression Methods and Engineering Countermeasures

The study proposes a multi-layered approach to blockage suppression:

1. Thermal insulation optimization: Using vacuum-insulated piping (VIP) with thermal conductivity below 0.01 W/(m·K) reduces heat loss by 80% compared to conventional insulation, maintaining fluid temperature above the critical point throughout the pipeline length.
2. Active heating systems: Electric trace heating or hot water circulation along the pipeline maintains minimum fluid temperature. The heating power requirement is estimated at 50–150 W/m depending on ambient conditions and insulation quality.
3. Pressure maintenance: Maintaining pipeline pressure above 10 MPa ensures the CO2 remains in the supercritical or liquid phase, avoiding the phase transition region where blockage risk is highest. Pressure regulation valves at strategic intervals along the pipeline provide this function.
4. Flow velocity management: Minimum flow velocity of 3 m/s is recommended to reduce heat loss per unit mass and prevent stratification of gas and liquid phases.
5. Material selection: Pipeline materials must withstand the low temperatures that may occur at startup or shutdown transients. Carbon steel is acceptable for temperatures above -20 °C, while low-temperature carbon steel (such as 16MnDR or A333 Grade 6) is required for temperatures below -30 °C. Austenitic stainless steel (304 or 316) provides excellent low-temperature toughness and corrosion resistance.

Engineering Practice Integration and Quality Considerations

From a pressure vessel and pipeline engineering perspective, several additional considerations arise from this study. The thermal cycling experienced by liquid CO2 pipelines during startup, normal operation, and shutdown creates fatigue loading that must be accounted for in the design life assessment. The temperature range may span from ambient (up to +50 °C in summer) to -40 °C during cold winter conditions, resulting in thermal stress ranges that require careful material selection and stress analysis.

The welding quality of pipeline joints is critical, as any imperfection at the weld interface may initiate fatigue crack propagation under thermal cycling. Welding procedures should be qualified in accordance with ASME IX or equivalent standards, with specific attention to post-weld heat treatment for materials requiring stress relief. Non-destructive testing should include both volumetric methods (radiographic testing or ultrasonic testing) and surface methods (dye penetrant or magnetic particle testing) to detect both internal and surface defects.

The study also highlights the importance of pressure relief devices in liquid CO2 pipeline systems. Rapid temperature reduction during a blockage event may cause pressure buildup upstream, requiring properly sized relief valves to protect the pipeline integrity. The relief valve set pressure should be coordinated with the design pressure of the pipeline and any connected pressure vessels, following the principles outlined in ASME VIII Div.1 or GB/T 150.

Summary

This literature provides valuable insights into the blockage mechanisms affecting liquid CO2 ground pipelines and presents a comprehensive set of engineering countermeasures for blockage suppression. The multi-layered approach combining insulation optimization, active heating, pressure maintenance, and flow velocity management represents a practical strategy for ensuring reliable pipeline operation. Engineers designing liquid CO2 transport systems should adopt a holistic design philosophy that addresses thermal management, material selection, welding quality, and pressure protection as an integrated system rather than isolated design elements. The findings have broader implications for any cryogenic or supercritical fluid transport application where phase change and thermal management are critical design considerations.