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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