CO2 Fracturing Fluid Supply System Design

Literature Overview and System Architecture

This paper presents a comprehensive design approach for CO2 fracturing fluid supply systems, addressing the challenges of storing, compressing, and delivering CO2 at the pressures and rates required for hydraulic fracturing operations. The system design encompasses surface facilities including storage tanks, compressors, pump systems, piping networks, and control systems. For pressure vessel engineers, the most critical aspects of this design relate to the specification and qualification of high-pressure vessels, the selection of materials compatible with CO2 service, and the implementation of appropriate safety systems.

The CO2 fracturing fluid supply system described in the paper operates at pressures ranging from 30 to 120 MPa, with flow rates of 5 to 30 cubic meters per minute. The system must handle CO2 in both liquid and supercritical states, depending on the operating temperature and pressure conditions. This dual-phase operation creates unique challenges for equipment design, as the properties of CO2 change dramatically across the phase boundary.

Core System Design Parameters and Component Specifications

The paper provides detailed specifications for the major components of the CO2 fracturing fluid supply system, including storage vessels, compressors, pumps, and piping. The following table summarizes the key design parameters:

The design of the storage tank is governed by the maximum pressure of the CO2 charge and the operating temperature. For storage at ambient temperature, the pressure of liquid CO2 is approximately 6 to 7 MPa at 20 degrees Celsius, rising to 15 to 20 MPa at 60 degrees Celsius. The tank must be designed for the maximum pressure at the maximum operating temperature, with a safety margin of at least 10 percent.

Pressure Vessel Design Analysis and Material Selection

The high-pressure components of the CO2 fracturing fluid supply system require careful design analysis to ensure structural integrity under the demanding operating conditions. The design of the high-pressure compressor casing and injection pump housing must account for the following factors:

• Maximum operating pressure of 120 MPa, which requires thick-walled vessel design per ASME VIII Div.1 UG-27 or Div.2 Part 3
• Temperature variations from 40 to 100 degrees Celsius, requiring materials with adequate strength and toughness at both low and high temperatures
• Cyclic loading from repeated start-stop operations, requiring fatigue analysis per ASME VIII Div.2 Part 5
• Potential for CO2 corrosion, requiring appropriate material selection and corrosion allowance

The material selection for these high-pressure components is critical. For the compressor casing operating at 80 to 120 MPa, the paper recommends 15CrMoR steel with a weld overlay of Inconel 625 on the internal surface. The 15CrMoR provides the necessary strength at elevated temperatures, while the Inconel 625 overlay provides corrosion resistance to supercritical CO2. The overlay thickness of 8 to 12 mm is specified to ensure adequate corrosion allowance over the design life of 20 years.

For the injection pump housing, which operates at the highest pressures in the system (100 to 150 MPa), the material selection is even more critical. The paper recommends 316L stainless steel for the pump body, with Hastelloy C276 overlay on the wear surfaces. The 316L provides good general corrosion resistance, while the Hastelloy C276 overlay provides enhanced resistance to CO2 erosion and cavitation damage.

Weld Overlay Process Selection and Quality Control

The weld overlay of critical components in the CO2 fracturing fluid supply system requires careful process selection and rigorous quality control. The following table presents the recommended overlay processes for different components:

The PTA process is preferred for overlaying nickel-based alloys due to its ability to control dilution below 5 percent and produce a dense, pore-free overlay. The process parameters for PTA overlay of Inconel 625 on 15CrMoR include: arc current of 200 to 300 A, arc voltage of 25 to 35 V, travel speed of 100 to 200 mm/min, and powder feed rate of 50 to 100 g/min. The preheat temperature for the base metal should be 150 to 250 degrees Celsius, and the interpass temperature should be maintained below 150 degrees Celsius to avoid excessive grain growth.

Quality control of the weld overlay includes the following inspection steps:

• Visual inspection (VT) of the overlay surface for porosity, cracks, and undercut
• Magnetic particle testing (MT) of the overlay surface for surface and near-surface defects
• Ultrasonic testing (UT) of the bond line for lack of fusion and delamination
• Radiographic testing (RT) of the weld root for volumetric defects
• Hardness testing of the overlay and heat-affected zone (HAZ)
• Metallographic examination of the overlay cross-section for microstructure and dilution

Safety Systems and Pressure Relief Design

The CO2 fracturing fluid supply system must be equipped with comprehensive safety systems to protect personnel and equipment in the event of overpressure or equipment failure. The pressure relief system design is governed by ASME VIII Div.1 UG-125 and API 520/521, and must account for the following scenarios:

• Block outlet: relief required for the maximum flow rate of the pump
• External fire: relief required for the heat input from a pool fire per API 521
• CO2 release: relief required for the rapid expansion of CO2 from a high-pressure to low-pressure state
• Thermal expansion: relief required for the thermal expansion of trapped liquid CO2

The pressure relief valve (PRV) sizing for CO2 service requires special consideration due to the unique properties of CO2. The relief capacity must be calculated using the appropriate thermodynamic model for CO2, taking into account the phase behavior and the potential for two-phase flow during relief. The PRV inlet and outlet piping must be designed for the maximum relief flow rate, with attention to the potential for temperature drop due to the Joule-Thomson effect.

Key Reflections and Engineering Practice Connections

The design of CO2 fracturing fluid supply systems represents a significant engineering challenge that requires the integration of process engineering, materials science, and pressure vessel design. The most critical aspect of the design is the selection of materials and overlay alloys that can withstand the combined effects of high pressure, elevated temperature, and CO2 corrosion over the design life of the equipment. The paper's detailed specifications for system components provide valuable guidance for engineers involved in the design and qualification of CO2 handling equipment.

From a practical standpoint, the most common failure modes in CO2 fracturing fluid supply systems are fatigue cracking at weld joints, corrosion under overlay, and erosion of overlay surfaces due to high-velocity CO2 flow. The prevention of these failures requires careful attention to weld quality, overlay thickness, and flow velocity limits. The paper's emphasis on system reliability and safety highlights the importance of a comprehensive quality assurance program that includes material verification, weld qualification, NDT, and periodic inspection.

Summary

The design of CO2 fracturing fluid supply systems requires careful integration of process requirements, materials selection, pressure vessel design, and safety systems. The high-pressure, high-temperature, and corrosive operating conditions demand rigorous engineering analysis and adherence to applicable codes and standards. The weld overlay of critical components is a key technology for ensuring equipment integrity, and the selection of overlay materials and processes must be carefully matched to the specific operating conditions. A comprehensive quality assurance program is essential for ensuring the safe and reliable operation of CO2 fracturing fluid supply systems over their design life.