Liquid CO₂ Pre-Fracturing Ground Pipeline Blockage Simulation and Inhibition Methods

1. Definition and Fundamental Principles

Liquid CO₂ (LCO₂) pre-fracturing is an advanced well-stimulation technique in which supercritical or liquid carbon dioxide is injected into subsurface formations to generate fractures with enhanced conductivity and reduced formation damage. The "ground pipeline blockage simulation and inhibition" discipline encompasses the systematic study of flow-assurance phenomena that occur within surface piping, flowlines, and manifolds during LCO₂ injection, storage, and transport operations. The core technical challenge arises from the unique thermodynamic behavior of CO₂ under pressure-temperature conditions typical of surface facilities.

The fundamental physical principles governing blockage mechanisms include:

Phase Transition and Joule-Thomson Cooling: When liquid CO₂ depressurizes through valves, restrictors, or long pipeline sections, the Joule-Thomson effect causes rapid temperature reduction. CO₂ can transition from liquid to supercritical state or, at low pressures, undergo solidification into dry ice (solid CO₂) at the triple point (5.18 bar, −56.6 °C). This phase change is the primary driver of blockage in surface piping.
Hydrate Formation: In the presence of water contamination, CO₂ hydrates (carbonate hydrates, e.g., CO₂·5.75H₂O) can form at temperatures above the freezing point of water, creating crystalline plugs that obstruct flow paths. Hydrate formation is pressure-dependent and typically occurs between 2–20 °C at injection pressures of 5–15 MPa.
Two-Phase Flow Instability: Simultaneous flow of liquid CO₂ and gas-phase CO₂ within surface pipelines creates slug flow, churn flow, or stratified flow regimes. These unstable flow patterns cause pressure surges, vibration, and localized accumulation of CO₂ in low points, promoting blockage.
Thermal Gradient Effects: Ambient temperature fluctuations along exposed ground pipelines (particularly in cold-climate regions such as northern China and the Sichuan Basin) create differential cooling rates, accelerating solidification at cold spots.

Blockage simulation employs computational fluid dynamics (CFD) coupled with thermodynamic equations of state (EOS) — typically the Peng-Robinson or Span-Wagner EOS for CO₂ — to model pressure, temperature, and phase behavior along pipeline routes under operational and transient conditions. Inhibition methods encompass both chemical (hydrate inhibitors: methanol, ethylene glycol, MEG; kinetic inhibitors: THF, THN) and mechanical/thermal approaches (trace heating, insulation, flow assurance management systems).

2. Category and Business Positioning

Within the operational framework of Cladding Technology Shanxi Co., Ltd., this technical capability falls under the category of Process Engineering Support and Flow Assurance Technology. While the company's primary revenue-generating technology routes are TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding, the LCO₂ pre-fracturing pipeline blockage discipline serves a critical enabling function:

Upstream Customer Enablement: The company's clad piping products (typically carbon steel/304L, carbon steel/316L, or carbon steel/9Cr-1Mo cladding) are deployed in oil and gas production systems, including LCO₂ injection facilities. Understanding and addressing blockage risks ensures that clad pipe products maintain integrity and deliverability in demanding service conditions.
Technical Advisory Services: The company provides engineering consultation to EPC contractors and oilfield service providers on flow assurance challenges specific to LCO₂ systems, positioning itself as a value-added technical partner rather than a pure material supplier.
Cross-Disciplinary Integration: This capability bridges metallurgy, thermodynamics, fluid mechanics, and corrosion engineering — disciplines that are all represented in the company's technical workforce, creating synergy across product development and customer support.

Business positioning: This is classified as a Level 2 Technical Competency — not a standalone product but a critical support capability that enhances product qualification, reduces warranty exposure, and strengthens customer relationships in the unconventional oil and gas and CCUS (Carbon Capture, Utilization, and Storage) markets.

3. Technical Purpose and Value

3.1 Primary Objectives

Develop validated simulation models that predict blockage onset conditions (temperature, pressure, flow rate, water content) for specific pipeline configurations used in LCO₂ pre-fracturing surface facilities.
Establish quantitative inhibition criteria — minimum inhibitor concentration, minimum trace-heating power, maximum allowable pipeline length without intermediate reheat — to ensure continuous, unobstructed LCO₂ flow from storage to injection point.
Define inspection intervals and monitoring parameters for early detection of developing blockages before operational shutdown occurs.
Provide engineering justification for clad pipe material selection and specification in LCO₂ service environments where low-temperature embrittlement and CO₂ corrosion are concurrent concerns.

3.2 Quantifiable Value

Operational Uptime: Each hour of pipeline blockage in a fracturing operation costs an average of USD 15,000–30,000 in lost productivity, crew standby, and equipment demobilization. Proactive inhibition reduces unplanned shutdowns by 60–80%.
Asset Protection: Pressure surges caused by blockage-induced slug flow can exceed design pressure by 20–50%, risking fatigue damage to clad pipe welds, flanges, and supports. Flow assurance management prevents such overpressure events.
Regulatory Compliance: Properly managed LCO₂ systems meet environmental and safety regulations (particularly NACE MR0175/ISO 15156 for sour service and relevant environmental discharge controls for CO₂ emissions).
Product Differentiation: Companies that can demonstrate comprehensive flow-assurance understanding alongside cladding manufacturing capability command 15–25% price premiums in competitive bids.

4. Key Process and Implementation Points

4.1 Simulation Methodology

The blockage simulation workflow follows a structured approach:

Process Characterization: Document the LCO₂ injection process parameters including storage pressure (typically 15–20 MPa), injection pressure (5–15 MPa), mass flow rate (10–50 t/h), ambient temperature range (−30 °C to +45 °C), and water content in the CO₂ stream (target <10 ppmv for anhydrous systems).
Thermodynamic Modeling: Apply the Span-Wagner equation of state to calculate CO₂ phase envelope, density, enthalpy, and Joule-Thomson coefficient across the operating pressure-temperature domain.
Hydraulic Simulation: Use steady-state and transient hydraulic models (e.g., OLGA, HYSYS, or ANSYS Fluent) to determine pressure drops, temperature profiles, and flow regime transitions along the pipeline route.
Blockage Criterion Definition: Establish the minimum fluid velocity below which hydrate or dry ice accumulation becomes self-sustaining. For LCO₂ systems, the critical velocity is typically 1.5–3.0 m/s for liquid-phase flow and 10–15 m/s for two-phase flow.
Inhibition Strategy Development: Determine the minimum inhibitor injection rate, trace-heating power density, and insulation thermal resistance required to maintain fluid temperature above the hydrate/dry ice formation threshold with a safety margin of ≥10 °C.

4.2 Key Process Parameters

Parameter	Typical Range	Critical Threshold	Monitoring Method
Storage Pressure	15–20 MPa	—	Pressure gauge/transmitter
Injection Pressure	5–15 MPa	Minimum 5.18 MPa (triple point)	Pressure transmitter at wellhead
Pipeline Temperature	Ambient to −60 °C (JT cooled)	Must remain above −56.6 °C (dry ice)	Thermocouple array (PT100)
Water Content in CO₂	Target <10 ppmv	Below 50 ppmv (hydrate risk)	Capillary tube hygrometer
Flow Velocity	2–5 m/s (liquid)	Minimum 1.5 m/s	Coriolis mass flowmeter
Hydrate Inhibitor Concentration	1.5–3.0 wt% (MEG)	Minimum 1.0 wt% (sub-inhibitor)	Refractometer / online analyzer
Trace Heating Power	15–30 W/m	Minimum 10 W/m (cold climate)	Power supply monitoring
Ambient Temperature (Design)	−30 °C to +45 °C	Design for −30 °C (northern China)	Weather station / DCS

4.3 Inhibition Method Comparison

Inhibition Method	Effectiveness	Cost (Relative)	Environmental Impact	Best Application
Methanol (MeOH)	High (thermodynamic)	Medium	High (toxic, VOC)	Short-term, high-pressure systems
Ethylene Glycol (MEG)	High (thermodynamic)	Medium-High	Low (recoverable)	Long-term, continuous operation
THF (Kinetic)	Medium (delays formation)	Low	Medium	Sub-inhibitor dosing, short pipelines
Trace Heating + Insulation	High (temperature control)	Medium (capital + operating)	Low	Cold-climate, long pipelines
Flow Assurance Management	High (system-level)	Low (software/monitoring)	None	All scenarios (complementary)

5. Applicable Standards and Acceptance Criteria

5.1 Design and Material Standards

ASME B31.3 — Process Piping: Governs design, materials, fabrication, and inspection of LCO₂ surface piping. Minimum design temperature must account for Joule-Thomson cooling to at least −60 °C.
ASME B31.4 / B31.8 — Pipeline Transportation Systems for Liquids and Gas: Applicable to long-distance CO₂ transport segments.
API 5L — Specification for Line Pipe: Base pipe material specification for clad pipe manufacture.
ASTM A388 — Specification for Clad Plate for Pressure Vessels: Material specification for clad plate used in LCO₂ storage vessels and heat exchangers.
GB/T 16545 — Carbon Steel and Low Alloy Steel Clad Plate: Chinese national standard for clad plate manufacturing and acceptance.
GB 50540 — Code for Design of Gas Station Engineering: Relevant for CO₂ storage and distribution facility design in China.
ISO 15156 / NACE MR0175 — Petroleum and Natural Gas Industries — Materials for H₂S-Containing Environments: Applicable where CO₂ systems also contain hydrogen sulfide (wet CO₂ service).
GB/T 19624 — Carbon Dioxide: Specification for industrial-grade CO₂ used in fracturing operations.

5.2 Inspection and Testing Standards

ASME Section V — Non-Destructive Examination: UT, RT, MT, PT methods for clad pipe weld inspection.
ASME Section IX — Welding Qualifications: WPS/PQR requirements for clad-to-base metal welds in LCO₂ service.
GB/T 3323 — Radiographic Testing of Welds: Acceptance criteria for RT inspection of pipe welds.
GB/T 11345 — Ultrasonic Testing of Welds: UT acceptance criteria for full-penetration welds in clad pipe.
NACE SP0472 — Recommended Practice for Cathodic Protection of Underground or Submerged Metallic Piping Systems: Applicable to buried LCO₂ pipelines.

5.3 Acceptance Criteria for Flow Assurance

Pipeline minimum temperature at all points must remain ≥10 °C above the predicted hydrate/dry ice formation temperature under worst-case conditions (maximum JT cooling, minimum flow rate, coldest ambient).
Pressure drop along the pipeline must not exceed 15% of injection pressure, ensuring adequate wellhead pressure delivery.
Flow velocity must remain above the critical velocity for hydrate transport at all times, including during start-up and shutdown transients.
Inhibitor concentration must be maintained at or above the sub-inhibitor level (typically 1.0 wt% MEG or 0.5 wt% MeOH) with a continuous monitoring system and automated alarm at 80% of minimum threshold.
All pipeline segments must be designed for a minimum impact energy of 34 J at the design minimum temperature (per ASME B31.3, Table 323.2.2).

6. Common Risks and Controls

6.1 Risk Identification and Mitigation Matrix

Risk	Likelihood	Consequence	Mitigation Measures
Dry ice formation in JT-cooled sections	Medium	Critical (complete blockage)	Trace heating, insulation, minimum flow velocity maintenance, pressure control above 5.18 MPa
Hydrate plug formation	Medium-High	High (partial/complete blockage)	Water removal (drying to <10 ppmv), thermodynamic inhibitor injection, temperature management
Slug flow-induced overpressure	Medium	High (weld fatigue, flange leak)	Flow regime optimization, surge volume provision, pressure relief valves, pipeline slope design
Low-temperature embrittlement of clad welds	Low-Medium	Critical (catastrophic failure)	WPS qualification at design minimum temperature, Charpy V-notch testing (≥20 J at −60 °C), appropriate filler metal selection
CO₂ corrosion of base metal	Medium	Medium (gradual wall thinning)	Clad layer thickness ≥3 mm (carbon steel/304L or 316L), corrosion monitoring (ER probes), inhibitor injection
Inhibitor dosing failure	Low	High (rapid hydrate formation)	Redundant dosing systems, online concentration monitoring, automated shutdown at zero-flow condition
Insulation failure in cold weather	Medium (cold climate)	Medium-High (localized cooling)	Insulation integrity inspection, redundant trace heating circuits, thermal imaging surveys

6.2 Control Philosophy

The risk management approach follows the hierarchy of controls: (1) Elimination — design pipelines to avoid conditions that promote blockage (adequate slope, minimum bends, appropriate diameter); (2) Substitution — use anhydrous CO₂ to eliminate hydrate risk; (3) Engineering Controls — trace heating, insulation, inhibitor injection systems; (4) Administrative Controls — operating procedures, monitoring protocols, training; (5) Personal Protective Equipment — for emergency response to CO₂ release.

7. Application Scenarios Across Company Technology Routes

7.1 TIG/MIG Weld Overlay Application

In LCO₂ pre-fracturing surface facilities, TIG and MIG weld overlay technology is applied to:

Corrosion-resistant overlay on carbon steel piping: Applying 304L, 316L, or duplex stainless steel overlay to carbon steel pipe surfaces in LCO₂ injection manifolds and skid-mounted equipment where wet CO₂ corrosion is a concern. The overlay thickness is typically 3–6 mm, applied in 2–3 passes using ER308L or ER316L filler wire per ASME Section IX WPS.
Repair of corroded pipeline segments: Field repair of existing production pipelines that have experienced wall thinning from CO₂ corrosion, using TIG weld overlay to restore wall thickness and corrosion resistance without complete pipe replacement.
Wear-resistant overlay on valve bodies and fittings: Applying hardfacing alloys (e.g., Stellite 6, 21-6-6) to valve seats, gate valve bodies, and ball valve internals that experience erosion from high-velocity LCO₂ flow and solid particle abrasion.

Key qualification requirement: Weld overlay WPS must be qualified per ASME Section IX for service at the design minimum temperature (typically −40 °C to −60 °C for LCO₂ surface piping), with Charpy impact testing demonstrating ≥20 J absorption at the test temperature per ASME B31.3 requirements.

7.2 Hydraulic Explosive Bonding (HEB) Application

Hydraulic explosive bonding (water-jet explosive bonding) is applicable to LCO₂ systems in the following scenarios:

Production of clad plate for LCO₂ storage vessels: Manufacturing carbon steel/304L or carbon steel/316L clad plate for fabrication of LCO₂ storage tanks (typically 50–200 m³ capacity) operating at 15–20 MPa. HEB provides metallurgical bonding without dilution, preserving the corrosion resistance of the stainless cladding layer.
Large-diameter clad pipe for main injection lines: Producing OD 219–406 mm clad pipe for main LCO₂ injection lines from storage to wellhead, where the combination of high pressure and potential CO₂ corrosion requires reliable cladding. HEB is particularly advantageous for producing consistent bond quality over large plate areas required for large-diameter pipe rolling.
Heat exchanger cladding: Producing clad plate for CO₂ cooling/condensation heat exchangers that must handle liquid CO₂ on the process side while maintaining structural integrity of the carbon steel shell. The HEB process ensures bond strength ≥120 MPa (per ASTM E2348 test method) without heat-affected zone degradation.

Quality assurance for HEB products in LCO₂ service requires: (1) Full-length ultrasonic bond inspection per ASTM E2348; (2) Peel testing per ASTM E2348 demonstrating ≥120 MPa bond strength; (3) Chemical analysis of cladding layer confirming composition within specification limits (ASTM A240 for 304L/316L); (4) Hydrostatic pressure testing at 1.5× design pressure per ASME Section VIII Div. 1.

7.3 Explosion Welding Application

Explosion welding (explosive cladding) serves LCO₂ systems in high-demand applications:

High-pressure LCO₂ compressor casing cladding: Manufacturing clad components for reciprocating or centrifugal compressors that handle liquid CO₂ at pressures up to 25 MPa. Explosion welding provides superior bond quality and eliminates the risk of intermetallic compound formation that can occur with diffusion bonding at elevated temperatures.
Large-format clad plate for vessel fabrication: Producing clad plate up to 12 m × 4 m for fabrication of large LCO₂ storage spheres (1000+ m³) and buffer tanks. Explosion welding handles large format production efficiently with consistent bond quality across the entire surface.
Special alloy cladding for extreme conditions: Applying nickel-base alloy cladding (Inconel 625, Hastelloy C-276) to carbon steel substrates for LCO₂ systems with high H₂S content (acid gas service), where the combination of CO₂ and H₂S creates severe corrosion conditions requiring premium alloy cladding. Explosion welding is the preferred method for achieving metallurgical bonds with these difficult-to-weld alloy combinations.

Explosion welding qualification for LCO₂ service requires compliance with: (1) ASTM A388 for clad plate specifications; (2) ASTM E2348 for bond testing; (3) NACE MR0175/ISO 15156 for materials in sour service; (4) ASME Section VIII Div. 1 for pressure vessel fabrication using clad materials. The explosion welding process parameters (velocity ratio, collision angle, standoff distance) must be qualified per ASTM A770 and validated through full-scale coupon testing at the design minimum temperature.

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

Technical competence in LCO₂ pipeline blockage simulation and inhibition directly supports the company's qualification portfolio:

WPS/PQR Development: Understanding of LCO₂ service conditions (temperature, pressure, corrosion environment) enables development of welding procedure specifications that are specifically qualified for cryogenic and sour service, enhancing the company's WPS library and reducing the need for additional qualifications on a project-by-project basis.
QMS Enhancement: Integration of flow-assurance knowledge into the company's Quality Management System (per ISO 9001:2015 and NACE SP0184) demonstrates a holistic approach to product quality that extends beyond manufacturing to include service-life performance considerations.
Certification Support: Documentation of technical competence in LCO₂ service conditions supports applications for API Q1 (Quality Management Systems for Petroleum, Petrochemical, and Natural Gas Industry) and ASME "U" Stamp authorization for pressure vessel fabrication using clad materials.
Technical Personnel Training: The study and application of blockage simulation principles provides a structured training framework for engineers and welders, building institutional knowledge that reduces reliance on external consultants for LCO₂-related projects.

8.2 Product Delivery Enhancement

Specification Optimization: Knowledge of LCO₂ blockage mechanisms enables the company to recommend optimal clad pipe specifications — wall thickness, cladding material, minimum bend radius, and support spacing — that address flow assurance concerns proactively, reducing field modifications and change orders.
Delivery Confidence: With validated simulation models, the company can provide customers with quantitative assurance that delivered products will perform reliably under specified operating conditions, reducing warranty claims and enhancing brand reputation.
Value-Added Documentation: Each product delivery can include a technical data package that documents the flow-assurance analysis performed, demonstrating engineering rigor and supporting the customer's own regulatory compliance submissions.
Lead Time Reduction: Pre-qualified WPS, established process parameters, and validated material specifications for LCO₂ service reduce the qualification lead time for new projects by 4–8 weeks compared to first-time qualification.

8.3 Customer Value Creation

Risk Reduction: Customers gain confidence that their LCO₂ surface facilities will operate continuously without unplanned blockage events, protecting their production schedules and revenue streams. The quantifiable value of avoided downtime alone (USD 15,000–30,000/hour) justifies the premium for technically qualified suppliers.
Regulatory Compliance: In China's rapidly evolving regulatory environment for CCUS and unconventional oil/gas development, customers require suppliers who can demonstrate technical competence in compliance with emerging standards (GB/T 40577, SY/T 6843, and forthcoming national standards for CO₂ utilization). The company's technical knowledge positions it as a preferred supplier for regulated projects.
Total Cost of Ownership Reduction: By providing integrated solutions that address both material integrity (clad pipe quality) and flow assurance (blockage prevention), the company reduces the customer's total cost of ownership by minimizing lifecycle maintenance, unplanned repairs, and production losses.
Strategic Partnership: Technical expertise in LCO₂ systems enables the company to transition from a component supplier to a strategic technical partner, participating in customer process design, materials selection, and flow assurance strategy development — creating long-term, high-value commercial relationships.
Market Access: Demonstrated competence in LCO₂ pipeline technology opens access to China's growing CCUS market (targeting 100+ demonstration projects by 2025 per the "14th Five-Year Plan") and the domestic shale oil/gas fracturing market, both of which are rapidly expanding and require specialized clad materials and technical expertise.

9. Implementation Roadmap and Recommendations

9.1 Short-Term Actions (0–6 Months)

Complete a comprehensive LCO₂ process flow diagram and P&ID review for all existing and planned customer projects to identify specific blockage risk scenarios.
Develop a standardized flow assurance data collection template for use during customer technical consultations and product specification development.
Conduct a gap analysis of existing WPS/PQR qualifications against LCO₂ service requirements (minimum temperature, impact energy, corrosion resistance) and initiate necessary requalification activities.
Establish a technical library of LCO₂-related standards, codes, and reference documents (ASME B31.3, API 5L, GB 50540, ISO 15156, NACE SP0184) for internal reference and customer support.

9.2 Medium-Term Actions (6–18 Months)

Develop and validate a CFD-based simulation model for a representative LCO₂ surface pipeline configuration, calibrated against published experimental data and customer field data.
Establish partnerships with academic institutions (e.g., China University of Petroleum, Southwest Petroleum University) for joint research on CO₂ hydrate formation and inhibition under Chinese field conditions.
Develop a standardized technical data package for LCO₂ service that includes flow assurance analysis, material selection rationale, weld qualification data, and inspection/monitoring recommendations.
Train technical sales and engineering teams on LCO₂ service fundamentals to enable effective customer communication and technical proposal development.

9.3 Long-Term Actions (18–36 Months)

Pursue formal certification or membership in relevant industry bodies (API, NACE International, China Petroleum and Chemical Industry Federation) to validate technical competence and enhance market credibility.
Develop proprietary flow assurance software tools or simulation capabilities that can be offered as a value-added service to customers, creating an additional revenue stream.
Participate in the development of Chinese national and industry standards for LCO₂ fracturing equipment and materials, establishing the company as a technical leader in the emerging market.
Expand the clad material portfolio to include specialized alloys (duplex stainless, nickel-base alloys) qualified for the most demanding LCO₂ service conditions, supporting the full spectrum of customer requirements.

10. Conclusion

The technical capability in liquid CO₂ pre-fracturing ground pipeline blockage simulation and inhibition represents a strategically valuable addition to Cladding Technology Shanxi Co., Ltd.'s technical portfolio. While not a standalone product, this capability creates significant value through enhanced product qualification, improved customer service, reduced operational risk, and expanded market access in China's rapidly growing unconventional oil/gas and CCUS sectors. The integration of flow assurance knowledge with the company's core cladding manufacturing technologies — TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding — creates a differentiated value proposition that positions the company as a comprehensive technical partner rather than a commodity material supplier. Continued investment in this technical discipline will yield compounding returns as the LCO₂ fracturing market matures and regulatory requirements for flow assurance and materials qualification become more stringent.