Characteristics of oil distributions in forced and spontaneous imbibition of tight oil reservoir

Matrix imbibition, which includes spontaneous imbibition (SI) and forced imbibition (FI), is the main mechanism of water-based methods, and can play a significant role in unlocking tight oil potentials as a tremendous amount of oil remains in the matrix following primary production. Previously, SI and FI have been investigated separately in pore-scale studies for several years. However, it is difficult for the results to provide guidance for selecting water-based methods owing to the different core samples and pore classification criteria adopted. Therefore, an integrated study of SI and FI is conducted on tight cores in order to understand the characteristics of oil contributions from different pores. In this work, 68 tight cores from the Chang 8 formation, Ordos Basin (China) are investigated. Nine cores are used to test native wettabilities; then, rate-controlled porosimetry is conducted on a typical tight core. Finally, nuclear magnetic resonance is implemented to determine the oil distributions before and after SI and FI for six cores. Based on the petrophysical properties, the cores are classified into three permeability levels (0.06 mD, 0.1 mD, and 0.22 mD). The SI and FI results demonstrate that FI can always provide more than twice the oil recovery factor of SI in each permeability level. For FI, more than 40% of the produced oil is contributed by mesopores. With increasing permeability, macropores contribute more oil than micropores. For SI, the oil contribution from micropores can reach 53.34%. The permeability of 0.1 mD is a critical point at which the oil contribution of mesopores surpasses that of micropores.