Quantification of the microstructures of Bakken shale reservoirs using multi-fractal and lacunarity analysis

Abstract

Pore structures are one of the most important factors affecting the hydro-mechanical properties of the reservoirs. Unlike the homogeneous pore structures in sandstones, the pores in the shale formations are heterogeneous and more complex to characterize due to the diagenesis and geological processes that they experienced. The heterogeneous rock pore structures can influence not only the flow properties of the oil and gas but also the fracture initiation and propagation characteristics which can impact the hydraulic fracturing performance, a common technique to increase the total production in tight shale formations. Therefore, quantifying the heterogeneities of the pore structures in unconventional shale formations carries a great importance. In this paper, we collected the samples from Bakken formation, which is a typical unconventional oil shale reservoir in North America. We applied image analysis method to study the pore structures. After segmentation of these images, we determined the representative elementary area (REA) of the samples based on the relationships between porosity and magnification ratios. Multifractal theory and lacunarity methods were applied to analyze the pore structures. Multifractal parameters were used to describe the pore probability distributions and the lacunarity value was applied to quantify the heterogeneity of the pores. The impact of the mineral compositions on heterogeneity values is also discussed. Finally, a new REA indicator, which contains the porosity and heterogeneity information, was proposed.