Research on Relative Permeability of Shale Oil Reservoir in Jimsar Area Using Digital Cores

Abstract

Clarifying the laws of relative permeability of Jimsar shale reservoir holds the most promising potential in the optimization of fracturing design. There is, however, as an unconventional tight reservoir, the shale oil reservoir in Jimsar area has ultra-low porosity and permeability, which leads the difficulty to clarify its seepage characteristics with conventional physical model experiments. Therefore, in this work, constructing the digital cores of Jimsar shale characteristic reservoir, and forming three-dimensional pore network model of the upper and lower sweet spots. Afterward, under the formation condition, the digital cores displacement simulation was carried out, and the complete relative permeability characteristics were obtained. Furthermore, the wettability physical model test was carried out to investigate its effect for relative permeability. Combining with the analysis results of the microscopic pore throat structure of the digital cores, the law of Jimsar shale relative permeability is clarified. The results show that the digital cores can finely describe the difference between the upper and the lower sweet pots in pore throat structure. The upper sweet spot has a larger pore size (about 3 times, compared with the lower sweet pot), but its connectivity is less difference (the coordination number is about 13% lower than the lower sweet pot). The displacement simulation demonstrate that the upper sweet spot has a higher irreducible water saturation (about 36%) and a higher residual oil saturation (about 34%), indicating that the lower sweet spot fluid has stronger fluidity. After wettability analysis, the upper sweet spot is weakly water-wet (Wetting angle 40–80°), the lower sweet spot is weakly oil-wet (wetting angle 100–130°), which had a significant impact on the relative permeability. Combining with the pore structure characteristics of the upper and lower sweet spots, it shows that the water-wet shale reservoir with a relatively high pore size has an adversely affect in fluid flow. Furthermore, we can get a clear understanding of the relative permeability characteristics in the upper and lower sweet spots of the Jimsar characteristic shale reservoirs by combining the digital cores, and which will helpful for forming a targeted strategy in reservoir reconstruction designs. The results of this research have certain reference significance for the stimulation and reconstruction of tight reservoirs such as the shale oil.