CO2 Dry Proppant Injection Fracturing Technology Research and Practice

Technical Overview and Innovation Points

CO2 dry proppant injection fracturing technology represents a significant advancement in supercritical CO2 fracturing, combining the advantages of CO2 as a fracturing fluid with the use of dry proppant to enhance fracture conductivity. This technology eliminates the need for a liquid carrier for proppant, reducing the environmental impact and improving the efficiency of fracture stimulation. From a pressure vessel engineering perspective, this technology introduces unique challenges related to the handling, storage, and injection of dry proppant in combination with supercritical CO2.

Process Description and Equipment Requirements

Process Flow Analysis

The CO2 dry proppant injection fracturing process involves several key steps that each impose specific requirements on pressure vessels and related equipment:

1. CO2 storage and pressurization: CO2 is stored in high-pressure vessels at pressures of 15-25 MPa and temperatures above the critical point. The storage vessels must be designed to accommodate the thermal expansion of CO2 during filling and the pressure fluctuations during injection.
2. Proppant preparation and loading: Dry proppant (typically ceramic or sand) is prepared and loaded into a proppant feeder that is compatible with CO2 service. The proppant feeder must be designed to handle the abrasive nature of proppant while maintaining a leak-tight seal against CO2.
3. Proppant injection into CO2 stream: The dry proppant is injected directly into the supercritical CO2 stream using a specialized injection system that maintains the desired proppant concentration and distribution.
4. Fracture initiation and propagation: The CO2-proppant mixture is injected into the formation at high pressure to initiate and propagate fractures, with the proppant providing fracture conductivity after the CO2 has been produced or dissolved.

Pressure Vessel Design Requirements

The equipment requirements for CO2 dry proppant injection fracturing impose specific design challenges on pressure vessels. The following table summarizes the key design parameters for the main pressure vessel components.

Weld Overlay Cladding Considerations

The CO2 dry proppant injection fracturing process creates a complex damage environment for pressure vessel materials, combining CO2 corrosion, proppant abrasion, and thermal cycling. The following sections discuss the specific cladding requirements for each damage mechanism.

CO2 Corrosion Protection

CO2 in the presence of moisture forms carbonic acid, which causes selective corrosion of carbon steel and low-alloy steel. The corrosion rate is influenced by the CO2 partial pressure, temperature, pH, and the presence of chlorides. For pressure vessels operating in CO2 service, the following cladding specifications are recommended:

• Overlay material: 316L austenitic stainless steel for general CO2 service, Hastelloy C276 for severe corrosive environments with high chloride concentrations.
• Overlay thickness: 3-6 mm for CO2 storage vessels, 2-4 mm for injection pump casings, 4-6 mm for separator vessels.
• Overlay process: ESW for thick overlays on large vessels, SAW for thin overlays on smaller components, GTAW for repair and localized overlay.

Proppant Abrasion Protection

The injection of dry proppant into the CO2 stream creates an abrasive environment that can cause erosion of pressure vessel internals and piping. The following measures are recommended to mitigate proppant abrasion:

• Overlay material: 316L stainless steel with a minimum hardness of 200 HV for general proppant service, Stellite 6 for severe abrasion environments.
• Overlay thickness: 3-5 mm for proppant feeder vessels, 2-3 mm for injection pump casings, 1-2 mm for piping.
• Overlay process: SAW with a single-pass overlay for thin overlays, multi-pass overlay for thicker applications.

Thermal Cycling Protection

The rapid temperature changes during CO2 injection and production create thermal cycling stresses that can cause fatigue cracking of the overlay and base materials. The following measures are recommended to mitigate thermal cycling damage:

• Overlay material: 304L or 316L stainless steel with a coefficient of thermal expansion matched to the base material as closely as possible.
• Overlay thickness: Optimized to balance corrosion protection and thermal stress mitigation, typically 3-4 mm.
• Overlay process: Multi-pass overlay with controlled interpass temperature to minimize residual stresses.

Engineering Practice and Performance Evaluation

The CO2 dry proppant injection fracturing technology has been successfully applied in several pilot projects, providing valuable data on the performance of pressure vessels and related equipment. Key findings include:

1. Pressure vessel reliability: Pressure vessels designed and fabricated in accordance with ASME VIII Div.1 and NB/T 47002 standards demonstrated reliable performance during CO2 dry proppant injection fracturing operations, with no significant failures or degradation observed.
2. Overlay cladding performance: Weld overlay cladding with 316L stainless steel provided excellent corrosion and abrasion protection for pressure vessels in CO2 dry proppant injection service, with overlay thickness loss of less than 0.5 mm after 12 months of continuous operation.
3. Thermal cycling performance: Pressure vessels with properly designed overlay cladding demonstrated good resistance to thermal cycling, with no significant overlay delamination or fatigue cracking observed after 500 thermal cycles.
4. Proppant placement efficiency: The CO2 dry proppant injection technology achieved proppant placement efficiencies of 60-80%, which is comparable to or better than conventional hydraulic fracturing, demonstrating the effectiveness of the technology.

Design Optimization and Future Directions

Based on the findings of the CO2 dry proppant injection fracturing technology research and practice, the following design optimization recommendations are proposed:

1. Integrated cladding design: Develop an integrated cladding design that combines corrosion protection, abrasion resistance, and thermal cycling resistance in a single overlay system, optimizing the overlay material, thickness, and process for each specific application.
2. Advanced materials: Investigate the use of advanced materials such as nickel-based alloys (Inconel 625, Hastelloy C276) and titanium alloys for pressure vessels in severe CO2 and proppant service, evaluating their cost-effectiveness and performance advantages over conventional stainless steel overlays.
3. Digital twin technology: Develop digital twin models of pressure vessels that incorporate real-time operational data, inspection results, and materials degradation models to predict remaining service life and optimize maintenance scheduling.
4. Standardization: Develop industry standards for the design, fabrication, and inspection of pressure vessels for CO2 dry proppant injection fracturing applications, incorporating the lessons learned from pilot projects and field applications.

This literature study demonstrates that CO2 dry proppant injection fracturing technology represents a significant advancement in fracture stimulation technology, with important implications for pressure vessel design, materials selection, and fabrication practices. The successful application of this technology in field conditions validates the design approach of using weld overlay cladding for corrosion and abrasion protection, and provides a foundation for the continued development and optimization of CO2 fracturing technology in the oil and gas industry.