Experimental Study on the Distribution and Height of Spontaneous Imbibition Water of Chang 7 Continental Shale Oil

After multi-stage volume hydraulic fracturing in a shale oil reservoir, massive amounts of water can be imbibed into the matrix pores. One of the key imbibition characteristics of a shale reservoir is the imbibition water and its height distribution. Based on high pressure mercury injection (HPMI) experiments and nuclear magnetic resonance (NMR) analyses, this study quantitatively evaluated the pore-size distribution of Chang 7 continental shale oil reservoirs in Yanchang Formation, Ordos Basin. The pores could be divided into three types as micropores (& LE;0.1 & mu;m), mesopores (0.1-1.0 & mu;m), and macropores (>1.0 & mu;m), while the volume of micropores and mesopores accounted for more than 90%. This demonstrated that there were strong heterogeneity and micro-nano characteristics. According to the spontaneous imbibition (SI) experiments, the cumulative proportion of imbibition water content was the largest in micropores, exceeding 43%, followed by mesopores around 30%, and that of macropores was the lowest, and basically less than 20%. The negative values of stage water content in the macropore or mesopore indicated that these pores became a water supply channel for other dominant imbibition pores. Additionally, combining the fractal theory with the NMR T-2 spectrum, the relative imbibition water and actual height were calculated in different pores, while the height distribution varied with cores and shale oil. The shorter the core, the higher was the relative height, while the radius of macropores filled with imbibition water was reduced. This indicates that the height distribution was affected by the pore structure, oil viscosity, and core length.

Automated summary

Based on HPMI experiments and NMR analyses, this study quantitatively evaluated the pore-size distribution of a shale oil reservoir in Ordos Basin. The results showed strong heterogeneity and micro-nano characteristics, with micropores and mesopores accounting for over 90% of the volume. The imbibition water content was highest in micropores, followed by mesopores, while macropores had the lowest content.