Experimental Study on Spontaneous Imbibition under Confining Pressure in Tight Sandstone Cores Based on Low-Field Nuclear Magnetic Resonance Measurements

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.