Imbibition Behavior of Oil-Saturated Rock: Implications for Enhanced Oil Recovery in Unconventional Reservoirs

The exploitation of oil from unconventional reservoirs remains a challenging engineering project, and enhanced oil recovery by imbibition is a crucial approach to tackle this problem. However, there has been a lack of detailed characterization of water and oil distribution during imbibition. In this study, the enhanced oil recovery by imbibition was monitored by the nuclear magnetic resonance (NMR) technique with dodecane as the saturation oil and distilled water as the imbibition liquid. The analyzed samples include tight gas sandstones, marine shale gas rocks, and lacustrine shale oil rocks. Our results show that gas and oil as testing fluids have similar porosities in tight sandstones, while the gas porosity in shale rocks is usually much higher than the oil porosity. Tight sandstones are extracted from the Shihezi and Taiyuan formations. Water increases gradually among different pores in Shihezi formation, which means water displaces oil and a lot of residual oil in a sample at last. Both the left and right wings move toward the right direction in Taiyuan formation, and there are two types of water from NMR T (1)-T (2) maps. Marine shales are collected from the Niutitang and Longmaxi formations. The initial oil content mainly in absorbed state is very low in the Niutitang Formation, water significantly increases among different pores during imbibition. Conversely, the initial oil content is very high in Longmaxi Formation, some water enters small pores and water is nearly no change in big pores during imbibition. Lacustrine shales are collected from the Qingshankou formation, among which two samples are broken up due to full of clay-swelling induced cracks, and the water increases in small pores while the oil decreases in large pores. Our study of imbibition-based enhanced oil recovery is beneficial to understanding the mechanism of water displacing oil in different unconventional reservoirs.