Experimental Investigation on the Characteristics of Microscopic Pore Structure and Spontaneous Imbibition of Chang 7 Continental Shale Oil Reservoir

Spontaneous imbibition is an important mechanism governing the shale oil reservoir production performance, especially in the early stage of production after multi-stage volume hydraulic fracturing. The microscopic pore structure of reservoirs is an important factor affecting imbibition. In this study, for the samples drilled from Chang 7 continental shale reservoirs of the Yanchang Formation in Ordos Basin, China, scanning electron microscope (SEM), high-pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR) T2 with water saturated were preformed to study pore structure characteristics and characterize quantitatively pore-size distribution. The results showed that the rock is tight shale and contains various clays and minerals, and micropores (≤0.1 μm) and small pores (0.1 μm–1.0 μm) are the main pore types and accounts for almost more than 90% of total pore volume. Next, conducted spontaneous imbibition experiments to reveal the SI law of Chang 7 continental shale, using NMR technology. Results show that the imbibition recovery is inverse proportion to the length of cores. And the micro-characteristics of spontaneous imbibition recovery is different with the pores, the imbibition amount of micropores is most, next is small pores, that of macropores is least, and the recovery degree is macropores, micropores and small pores from large to small. Furthermore, the initial imbibition rate is the largest in macropores, followed by micropores. Meanwhile, the spontaneous imbibition of macropores reach the equilibrium stage firstly, followed by small pores and finally micropores. The results show that the imbibition process of Chang 7 continental shale is not only affected by capillary force, but also affected by obvious oil-water viscous resistance.