Spontaneous Water Imbibition Behavior of Chang 7 Continental Shale in the Longdong Area, Ordos Basin: An Experimental Study

For further understanding of water spontaneous imbibition in continental shales, the pore structure and pore-size distribution of Chang 7 shale samples were investigated by scanning electron microscope, high-pressure mercury injection, and nuclear magnetic resonance. The results showed that micropores and small pores were the main types. Next, the vertical spontaneous imbibition experiments were conducted to reveal the imbibition process and investigate the temporal variation law on core-scale and pore-scale levels. The results indicated that the process could be divided into three stages. The first stage was the main period for the imbibition amount rising and accounted for more than 86%. The imbibition water saturation with a shorter core was higher, and was the largest in micropores. Additionally, the imbibition ripple rate at the core-scale and pore-scale levels implied that the imbibition front broke through the obstruction point to flow into the smaller pores or other paths conducive to imbibition. It fluctuated greatly in the first stage to show that there were massively stronger imbibition capacity flow paths, and the shale samples were strongly heterogeneous to a certain extent.