Stress Sensitivity Analysis of Shale Oil Reservoir in Jimsar Based on Digital Cores

Due to the shale oil in Jimsar is an unconventional tight reservoir with ultra-low porosity and low permeability, which requires fracturing and reconstruction. Clarifying its stress-sensitivity characteristics can provide a theoretical basis for subsequent development. Prior to this study, cores from actual characteristic reservoirs in the upper sweet spot and lower spot of the Jimsar were obtained by in-situ coring. The multi-scale digital scanning images of characteristic shale reservoirs’ cores in Jimsar were obtained by micro-nano CT scanning technology, and the three-dimensional (3D) images were visualized to construct multi-scale digital cores. Based on the 3D digital cores, the physical parameters and structural characteristics of Jimsar shale under different stress states were obtained by compacting method. The research shows that the porosity and permeability of the samples both decrease with the increase of stress. The decrease trend of porosity in the study range is basically consistent (1%/10 MPa), while the decrease of permeability is rapid in the initial stage (0.06 mD/10 MPa), and relatively gentle in the later stage (0.02 mD/10 MPa). With the increasing of stress, the coordination number decreases, and the pore-throat connectivity deteriorates. Comparing the pore-throat structure between the upper and the lower sweet spot, the upper spot mainly developed large pores, while the lower sweet spot mainly developed long and narrow pore throats. Therefore, the upper sweet spot mainly reduced large pores, while the proportion of small pores in the lower sweet spot decreased significantly as the increasing of stress. Combining digital cores, we can get a clear understanding of the stress-sensitivity characteristics in the upper and lower sweet spots of the Jimsar characteristic shale reservoirs, and which will helpful for forming a targeted strategy in reservoir reconstruction designs. The research results have certain reference significance for the stimulation and reconstruction of tight reservoirs such as the shale oil.