Particle transport in fractured geo-energy reservoirs considering the effect of fluid inertia and turbulent flow: A review

Particle-fluid two-phase flows in rock fractures and fracture networks play a pivotal role in determining the efficiency and effectiveness of hydraulic fracturing operations, a vital component in unconventional oil and gas extraction. Central to this phenomenon is the transport of proppants, tiny solid particles injected into the fractures to prevent them from closing once the injection is stopped. However, effective transport and deposition of proppant is critical in keeping fracture pathways open, especially in low-permeability reservoirs. This review explores, then quantifies, the important role of fluid inertia and turbulent flows in governing proppant transport. While traditional models predominantly assume and then characterise flow as laminar, this may not accurately capture the complexities inherent in real-world hydraulic fracturing and proppant emplacement. Recent investigations highlight the paramount importance of fluid inertia, especially at the high Reynolds numbers typically associated with fracturing operations. Fluid inertia, often overlooked, introduces crucial forces that influence particle settling velocities, particle-particle interactions, and the eventual deposition of proppants within fractures. With their inherent eddies and transient and chaotic nature, turbulent flows introduce additional complexities to proppant transport, crucially altering proppant settling velocities and dispersion patterns. The following comprehensive survey of experimental, numerical, and analytical studies elucidates controls on the intricate dynamics of proppant transport under fluid inertia and turbulence - towards providing a holistic understanding of the current state-of-the-art, guiding future research directions, and optimising hydraulic fracturing practices.