Multi-scale fractal analysis of pores in shale rocks

Pore structures is a very critical parameter that affects the physical, mechanical and chemical properties of the reservoir rock. Pore shapes and pore size distributions can impact the transport and storage capacity of the reservoir rocks. This necessitates the adequate knowledge of the pore structures of the rocks. In this paper, we characterized and quantified the pore structures of rock samples from the Bakken Formation which is a typical unconventional shale oil reservoir. Samples of Upper and Middle Bakken were collected and studied based on the Scanning Electron Microscope (SEM) images. First, the threshold of each image was determined from overflow criteria and then the related pores were extracted from the corresponding image. In the next step, the pore microstructures such as pore size, pore shape distributions of different samples were calculated and compared. Finally, we used fractal theory to describe the pore structures of the shale formation and investigated the relationship between fractal dimension and pore structures. The results showed that pores with various sizes and shapes were widely distributed in the shale samples. Compared with samples from Middle Bakken, samples from Upper Bakken Formation with higher clay content showed higher fractal dimension and more complex pore structures. Finally, the fractal dimension was used to quantify the impact of the magnification on the pore structures.