Evaluating the potential for oil recovery by imbibition and time-delay effect in tight reservoirs during shut-in

When stimulated only by natural energy, tight oil reservoirs have low production efficiency. Improving the potential for oil recovery by imbibition is essential to enhancing oil production. Shut-in is an effective way to make good use of the potential for oil recovery by imbibition. In this study, imbibition experiments and nuclear magnetic resonance (NMR) testing were combined to reveal the characteristics of spontaneous imbibition and the T-2 value of the oil recovery potential by imbibition in tight oil reservoirs. The main factors affecting the potential for oil recovery by imbibition were studied. We also discussed the relationship between clay minerals and the potential for oil recovery by imbibition, especially the time-delay effect of imbibition displacement. A modified model based on the static balance time for spontaneous imbibition was proposed to calculate the field shut-in time. The results show that the static spontaneous imbibition balance time of tight oil reservoirs are significantly different in various zones. The oil recovery by imbibition has an obvious time-delay effect and it could put the oil in non-movable pores under natural conditions to use. Internal conditions like microscopic pore structure and the type of clay minerals affect the oil recovery by imbibition on small pores (T-2 < 100 ms). External conditions like reservoir temperature, pressure, and bedding affect oil recovery by imbibition in small and large pores. The clay minerals expand after the interaction between fracturing fluids and rock. Meanwhile, oil is discharged with compression of those pore space. To optimize the shut-in time of a tight reservoir, these effects of clay behavior can suggest good practices in hydraulic fracturing.