Thickeners for Anhydrous CO2 Fracturing Fluids: From Molecular Design to Field Applications

Conventional hydraulic fracturing consumes massive amounts of water and causes severe formation damage. Anhydrous CO2 fracturing fluids offer a promising low-carbon alternative. Nevertheless, the low intrinsic viscosity of CO2 severely restricts proppant transport and complex fracture propagation, hindering large-scale field application. Over the past three decades, research on CO2 thickeners has evolved from empirical screening to mechanism-guided molecular design, with the thickening paradigm shifting from simple polymer chain entanglement to dynamic supramolecular self-assembly networks. However, systematic and critical reviews focusing on their development, key challenges, and paradigm limitations are rare. This paper presents a comprehensive review of CO2 thickeners, covering their molecular design, synthesis approaches, performance control, thickening mechanisms, and field applications. We summarize their evolution from fluorinated and siloxane-based polymers to hydrocarbon modifiers, supramolecular small molecules, and surfactant systems, and clarify the intrinsic thickening mechanisms of each category. Field case studies show that although siloxane-based and fluorinated systems exhibit application potential, they are still constrained by strong cosolvent dependence or environmental persistence risks. To address these bottlenecks, this review proposes prospective development directions, including smart-responsive materials, bioinspired green chemistries, and AI-assisted inverse molecular design. This review is expected to provide a theoretical reference and practical guidance for the development of high-efficiency, eco-friendly, and field-scalable CO2 fracturing technologies.