Mitigating proppant flowback: multi-factor mechanisms and optimization Strategies from laboratory studies

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Proppant flowback after hydraulic fracturing significantly impairs fracture conductivity, yet the quantitative effects of multiple factors remain poorly understood. In this study, based on similarity criteria, an improved API conductivity cell was used to systematically evaluate the influence of nine factors on the critical flowback flow rate: closure pressure, lithology, fiber concentration, proppant concentration, proppant size and its combinations, placement pattern, proppant type, and fluid viscosity. The results show that proppant size, fluid viscosity, placement pattern, and closure pressure are the most influential factors (variation > 5-fold), with closure pressure following an exponential relationship. The segmented placement (large proppant near the wellbore) gives a critical flow rate of 165 mL/min, which is 5.3 times that of uniform mixing (31 mL/min). The optimal fiber concentration is 0.4%–0.5%, and low viscosity (1–2 mPa·s) can increase the critical flow rate by more than 5 times. A combined strategy is proposed: large-size proppant + segmented placement + 0.5% fibers + broken-gel viscosity < 3 mPa·s, which elevates the critical flow rate from about 30 mL/min to over 150 mL/min. This study provides experimental support for flowback design in unconventional reservoirs.