Study on Pore Structure of Shale and Fluid Distribution Patterns of Surfactant-Enhanced Spontaneous Imbibition

Spontaneous imbibition modified by surfactants is a key technology for enhancing shale oil recovery. Currently, relevant studies mainly concentrate on marine shale worldwide, while the pore–fluid coupling characteristics of widely distributed medium-TOC terrestrial shale remain poorly understood. Against this background, this paper takes typical Paleogene terrestrial shale as the research object and integrates N2/CO2 adsorption and NMR T2 spectroscopy to jointly characterize multiscale pore structures and dynamic fluid evolution during imbibition. The results show that the shale is dominated by mesopores in terms of pore volume, while micropores provide most of the specific surface area. The zwitterionic surfactant HPSB can greatly reduce oil–water interfacial tension and alter rock wettability, thereby breaking the high capillary resistance of micropores. During imbibition, water invades macropores first, followed by mesopores and micropores, and the entire process exhibits remarkable nonlinear dynamics controlled by multiscale pores. The 0.15% HPSB solution shows the best effect on activating micropores. This study innovatively quantifies the influence of surfactant concentration on fluid migration across different pore scales and reveals the internal mechanism of staged imbibition and micropore lag activation in terrestrial shale. It not only complements the global research system of shale imbibition theory but also offers practical guidance for the optimization of fracturing fluid systems in mesopore-dominated shale oil reservoirs.