Multiscale Pore Structure Response of Shale to Forced Slickwater Imbibition

Abstract

Forced imbibition, where the wetting phase displaces the nonwetting phase under external forces and capillary pressure, is a critical phenomenon during fracturing and shut-in stages of shale gas development. This study systematically investigated the multiscale pore structure response of shale to slickwater forced imbibition using nuclear magnetic resonance (NMR) T2 spectra and a nonlinear power-law pore size distribution (PSD) conversion model. The results indicated the following: (1) The forced imbibition pressure governs the hierarchical response of multiscale pores. At 1–2 MPa, it preferentially drives fluid invasion into micropores and mesopores, whereas at 3–4 MPa, it establishes macropores as the primary flow pathways. (2) Micropores were pressure-sensitive, with high pressures inhibiting their structural adjustment. Conversely, increasing pressure advanced the inflection point of mesopore proportion changes, enhancing their alterability above 2 MPa. Macropore development strengthened, and its equilibrium time shortened, under higher pressures. (3) Under typical field-operational concentrations (0.1% friction reducer, 0.1% flowback aid, and 0.3% clay stabilizer), individual additives exhibited distinct, differentiated modification patterns: the friction reducer preferentially promoted micropore and macropore development in the early stage, with this effect weakening later; the flowback aid strongly facilitated micropore development; and the clay stabilizer promoted micropore and mesopore development but significantly suppressed macropores.