ISO/TR 27925:2023 — Carbon Dioxide Capture, Transportation and Geological Storage — Flow Assurance for CO2 Transportation and Injection

1. Introduction to Flow Assurance for CO2 Systems

Flow assurance is the discipline of ensuring that fluids are transported reliably from one point to another under all expected operating conditions. For CO2 transportation and injection systems in CCS applications, flow assurance presents unique challenges that distinguish it from conventional oil and gas flow assurance. The thermodynamic properties of CO2 — particularly its critical point at 31°C and 7.38 MPa and its complex phase behavior — introduce flow assurance risks that are not encountered in hydrocarbon systems. ISO/TR 27925:2023 addresses these challenges comprehensively.

Developed by ISO/TC 265, this Technical Report covers the entire CO2 flow chain from the capture facility outlet through transportation pipelines to the injection wellhead. It addresses phase behavior prediction, hydrate formation and inhibition, solids transport and deposition, transient flow management, and operational risk assessment. The standard provides guidance applicable to both onshore and offshore CO2 transportation systems in gaseous, liquid, and dense-phase conditions.

2. CO2 Phase Behavior and Fluid Properties

2.1 Phase Envelope Characterization

The standard emphasizes that accurate characterization of the CO2 stream’s phase envelope is the foundation of all flow assurance analysis. Impurities present in the CO2 stream (N2, O2, H2, CH4, H2S, Ar, H2O) can significantly alter the phase envelope compared to pure CO2. Even small concentrations of certain impurities can cause the phase envelope to widen dramatically, creating conditions where two-phase flow can occur in regions where pure CO2 would remain single-phase. The standard requires that phase envelope calculations use equations of state validated for CO2-rich mixtures with impurities, such as the GERG-2008 equation of state or the Span-Wagner model for pure CO2.

2.2 Dense-Phase Transportation

Most CO2 pipelines operate in dense-phase conditions (pressure above 7.38 MPa and temperature below 31°C but above the saturation temperature) to avoid two-phase flow and maximize transportation efficiency. The standard provides detailed guidance on designing pipelines for dense-phase operation, including the need to maintain pressure above the saturation pressure at all points in the pipeline, accounting for elevation changes, frictional pressure drop, and heat transfer with the environment. A key design consideration is that the high density of dense-phase CO2 (600-900 kg/m³) creates significant hydrostatic pressure gradients in hilly terrain, which must be carefully modeled.

3. Hydrate Formation and Prevention

3.1 CO2 Hydrate Characteristics

CO2 hydrates (clathrate compounds where CO2 molecules are trapped in a water crystal lattice) can form at temperatures well above the freezing point of water, depending on pressure conditions. At 5 MPa, CO2 hydrates can form at temperatures up to 10°C — conditions commonly encountered in pipeline operations. Hydrate formation can cause partial or complete pipeline blockages, valve failures, and instrumentation malfunction. The standard provides comprehensive guidance on hydrate prediction methods, including thermodynamic models and industry correlations.

3.2 Hydrate Prevention Strategies

ISO/TR 27925 reviews multiple hydrate prevention strategies applicable to CO2 systems. Dehydration of the CO2 stream is the most effective method — if the water dew point is maintained below the minimum operating temperature, hydrates cannot form. The standard specifies typical water content targets (< 50 ppmv for dense-phase systems). Chemical inhibition using thermodynamic inhibitors (methanol, MEG) or low-dosage hydrate inhibitors (LDHIs) provides additional protection, particularly during transient operations such as start-up or shutdown. The standard emphasizes that inhibitor selection must consider the unique solubility behavior of inhibitors in dense-phase CO2.

4. Transient Flow and Operational Risk Management

4.1 Transient Flow Scenarios

The standard addresses transient flow conditions that pose particular risks for CO2 systems. Pipeline depressurization is the most critical transient — the rapid pressure drop causes a temperature decrease through the Joule-Thomson effect, potentially reaching temperatures low enough to embrittle pipeline steel or cause dry ice (solid CO2) formation. The standard provides guidance on controlled depressurization rates, heating requirements for vent systems, and transient modeling methodologies. Other transient scenarios include pipeline start-up (warming and pressurization), shutdown (cooling and depressurization), and batch injection of different CO2 quality streams.

4.2 Risk Assessment Methodology

ISO/TR 27925 recommends a systematic risk assessment methodology that considers the unique hazards of CO2 transportation. Key risk scenarios include pipeline rupture with rapid CO2 release (dense-phase CO2 releases expand violently as they transition to gas), accumulation in low-lying areas (CO2 gas is denser than air), and brittle fracture propagation. The standard references methodologies such as HAZOP, bow-tie analysis, and quantitative risk assessment (QRA) adapted for CO2-specific hazards.

5. Solids Transport and Deposition

The standard addresses the transport and deposition of solid particles in CO2 streams, including corrosion products, scale particles, and desiccant fines from dehydration units. Solid particles can accumulate in low points, erode pipeline components at bends and valves, and interfere with instrumentation and control valves. The standard provides guidance on erosion velocity limits, particle size tolerances, and filtration requirements. For systems with significant solids content, the standard recommends periodic pigging or cleaning operations to maintain pipeline integrity and flow efficiency.

6. Frequently Asked Questions