Impact of initial water saturation on pore structure and fluid infiltration during hydraulic fracturing mapped by real-time nuclear magnetic resonance

Although hydraulic fracturing of water-bearing tight sandstone gas reservoirs has been extensively investigated, little attention has been paid to the influence of water saturation ( w s ) on pore structure and fluid infiltration behavior during the fracturing process. To address this gap, hydraulic fracturing experiments were conducted on sandstone specimens with different water saturations ( w s = 0, 25%, 50%, 75%, and 100%) using a real-time nuclear magnetic resonance (NMR) system. Results show that increasing w s reduces both breakdown pressure and breakdown time. Fluid injection promotes progressive micropores dilation and their transformation into mesopores and macropores at w s = 0–50%, while higher saturation enhances macropores modification by coalescence of pre-existing smaller pores. Capillary tension at dry-wet interfaces and clay mineral dissolution are suggested as dominant mechanisms governing pore-scale damage at low and high w s , respectively. Fluid infiltration is enhanced at low w s due to improved pore connectivity and strong water absorption effect, but is suppressed at high w s owing to water-locking effects. An apparent transition in mesopore evolution and preferential infiltration direction is observed between w s = 50% and 75%. These findings provide mechanistic insights into the role of w s in hydraulic fracturing of water-bearing tight sandstone gas reservoirs.