Spontaneous imbibition (SI) is an important method to improve oil recovery in tight sandstone reservoirs. Commonly, the physical simulation of SI is performed at atmospheric pressure but the characteristics of spontaneous imbibition under confining pressure (SIUCP) is often neglected. In this study, oil distribution in tight cores was obtained in combination of high pressure mercury intrusion (HPMI) measurements and low-field nuclear magnetic (LF-NMR) measurements. After that, oil recovery for SI and SIUCP of tight core samples with all faces open (AFO) were obtained using LF-NMR measurements. Finally, a new scaling law for SIUCP was proposed to predict shut-in time in field scale. The results showed that 95.94–98.12 wt % of the oil was distributed in nanopores (0.1 ms < T 2 < 100 ms) of core samples, and the average amount of oil in nanomicro-pores, nanomeso-pores and nanomacro-pores were 34.04, 40.15, and 22.75 wt %, respectively. Ultimate oil recovery for core samples were 22.41, 44.41, 57.27, 61.84, and 62.82 wt %, respectively, as confining pressure increased from 0 to 2175 psi. The improved oil recovery for SIUCP was associated with the decline of effective pore radius as a function of confining pressure, which results in the effect of enhanced SI and compaction. Finally, a modified dimensionless time model was proposed in combination of Mason’s dimensionless time model and effective pore radius as a function confining pressure.

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Spontaneous imbibition (SI) generally occurs under forced pressure (the difference between hydraulic fluid pressure and original pore pressure) during shut-in time. However, the experimental study of SI is commonly performed at atmospheric pressure and the effect of the forced pressure is often neglected. How the forced pressure influences the SI behaviors under different factors is still not clear. In this paper, low-field nuclear magnetic resonance (LF-NMR) was adopted to study the mechanism of SI in the tight sandstone rock sample under forced pressure (forced imbibition, FI). The effects of boundary conditions, initial water saturation, bedding plane (BP) direction, and fluid salinity on oil recovery were also systematically investigated. Results showed that the ultimate oil recovery (UOR) varied from different boundary conditions. An inverse correlation also exists between the water uptake and salinity. As osmotic pressure exists, more water was imbibed into core samples with the decrease of KCl salinity. The rock sample with perpendicular BP direction has a higher UOR than that with parallel BP direction. Moreover, the initial water saturation has a great effect on UOR. Higher water saturation would result in a lower UOR. This study aims to provide some insights into understanding the mechanism of FI in UOR enhancement.

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