Microfluidic Experimental Investigation on Seepage Mechanism During Shut-In and Flowback Stages in Tight Oil Reservoirs of the Sichuan Basin

The Shaximiao Formation in the Sichuan Basin hosts abundant tight oil resources; however, its reservoirs are typified by low porosity, low permeability, pronounced pore–throat structural heterogeneity, and highly complex microscopic crude oil seepage behavior. This study systematically investigates the microscopic flow mechanisms of crude oil in the Shaximiao Formation using a microfluidic experimental platform coupled with an integrated physical simulation system that enables real-time monitoring of dynamic imbibition throughout the fracturing–shut-in–flowback cycle. Experiments were conducted across three reservoir quality classes (Class I, II, and III), seven discrete shut-in durations (4, 8, 12, 24, 36, 42, and 54 h), and two representative fracturing fluid injection rates (12 and 20 m3/min). The results show that (1) residual-oil exhibits a distinct spatial distribution pattern: enrichment in large pores and large throats, with minimal retention in small pores and small throats; (2) moderate extension of shut-in duration significantly enhances movable oil saturation, whereas excessive shut-in time drives partial movable oil to transform into film flow or become trapped in dead-end pores, thereby exacerbating residual-oil retention; (3) the proportion of movable oil decreases gradiently with declining reservoir quality, following the order: Class I > Class II > Class III reservoirs; and (4) for the same reservoir type, a lower injection rate (12 m3/min) improves sweep efficiency in small pore–small throat regions and reduces residual oil retention, while a higher rate (20 m3/min) tends to induce an unbalanced seepage phenomenon, “preferential breakthrough in large pores and persistent retention in small pores”, which impairs the overall reservoir stimulation effect.