Combined micro-proppant and supercritical carbon dioxide (SC-CO₂) fracturing in shale gas reservoirs: a review

The poor proppant-carrying capacity of SC-CO₂ fracturing (SCF) negates its potential advantages for the exploitation of deep unconventional gas reservoirs over traditional water-based fracturing (WBF). The low proppant concentration in SCF-created fractures leads to accelerated proppant damage behaviours under high in-situ stress, such as proppant crushing and proppant embedment, resulting in a reduction in hydrocarbon production. Micro-proppants (>100 nm, <100 µm) with greater carrying capacity have great potential for SCF to improve proppant concentration in created tiny fractures and even massive natural fractures. The present study investigates the importance of fracturing fluid viscosity, flow states, proppant size and fracture morphology on micro-proppant carrying capacity in complex SCF fracture networks. The increased carrying capacity of micro-proppants in low viscosity SC-CO₂ narrows the difference between stimulated reservoir volume (SRV) and propped stimulated reservoir volume (PSRV), and an injection strategy with adjustable flow velocity promotes proppant sand embankment height and length, which are crucial to enhance the effective fracturing volume. In addition, this study reviews potential micro-proppant damage behaviours in SCF by analysing proppant concentration, proppant type, fluid saturation, proppant size and closure stress. A lower stress concentration between fracture surface and micro-proppant alleviates proppant embedment, and a thin proppant embedment layer maintains adequate flow channels for multilayers of micro-proppants, especially in ductile shale gas formations. Micro-proppants, with less elastic-plastic deformation and crushing, prevent the closure of created fractures under high closing pressure, which is critical to the fracture conductivity of micro-proppant SCF in water-sensitive shale gas reservoirs with abundant clay minerals.