Effects of fracture interference on pore-scale CO₂ flow and mass transfer during multi-cycle CO₂ huff-n-puff in shale

Fracture interference strongly controls cross-scale CO₂ mass transfer and flow behavior between matrix and fractures during huff-n-puff, while residual water further complicates these processes. However, the governing mechanisms remain unclear. Here we construct artificial fractures with finite lengths in core samples to simulate distinct flow regions near fractures. Then, we investigate how fracture interference and residual water control the pore-scale mass transfer through online nuclear magnetic resonance (NMR). A recognition method is established to identify the distinct flow behaviors and quantify the effects of fracture length, fracture number, and their coupling. The results indicate that fracture interference with short length intensifies distal liquid redistribution and micropore backflow, but this response cannot be efficiently converted into recovery because of the rapid decay of pressure gradient and the long return distance. Recovery therefore mainly depends on the expansion and carrying capacity associated with CO₂ phase transition, with a final recovery of 46.72%. Long fractures advance fluid redistribution and improve its conversion during single-phase flow due to the shorter return distance, giving a final recovery of 65.16%. In later cycles, however, bubble-assisted carrying remains confined near the fractures because gas propagation lags behind pressure propagation in the distal matrix. Residual water weakens fracture effectiveness and redirects CO₂ through locally less water-blocked pathways. As cumulative water film migration strengthens the barrier near fractures in later cycles, the additional recovery is only 6.49%, 16.63% lower than water-free case. These findings provide theoretical support for fracturing design and CO₂ injection optimization.