Pore structure is one of the most important parameters affecting the properties of reservoir formations such as mechanical properties. The existence of the pores in the rocks will cause stress concentrations where isolated pores or cracks may nucleate favorably and crack growth will be strongly affected during hydraulic fracturing, as an enhanced production technique in unconventional reservoirs. Unlike the more homogeneous pores in sandstones, the pore structures in the shale formations are heterogeneous and complex to quantify due to the diagenesis and geological processes that the shale has undergone. Formations with heterogeneous microstructures may introduce a heterogeneous stress field during the fracturing stimulation, which can influence the crack growth and continuity. Therefore, quantifying the microstructural heterogeneity of the shale formations is essential in stimulation performance and production analysis from unconventional reservoirs. In this paper, samples from the Middle Bakken Formation, which is a typical unconventional reservoir in North America, were collected. Field-emission Scanning Electron Microscope (FESEM) was used to produce the images from the samples. After the segmentation of these images, the lacunarity method was applied to analyze the heterogeneous of the pore structures. Finally, based on the choosing of the REA (representative elementary area) using the porosity indicator, we calculated the heterogeneity of pore structures of different samples and compared the values.

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