Study on the multifractal characterization and seepage of the shale matrix: A case study of the Longmaxi Formation in southwestern Sichuan Basin, China

Abstract

The pore structure and heterogeneity of shale significantly impact the occurrence and permeability of shale gas. Studying these aspects is beneficial for exploring and developing shale gas. In this study, organic-rich shale from the Longmaxi Formation in the Luzhou-Changning area was selected as the research subject. Experiments, including spontaneous imbibition, pressure saturation, and nuclear magnetic resonance, were conducted to characterize the pore structure and heterogeneity of the shale in this area. Using multifractal theory, we analyzed and discussed the multifractal characteristics of pores with different connectivities in relation to permeability. The results indicate that the T2 cutoff value (T2f) of the shale is greater than the T2c value, suggesting that the pore size of connected pores exceeds that of pores retaining bound water. The sizes of connected pores primarily range from 1 to 30 nm, predominantly between 1 and 10 nm. Significant differences were observed in the multifractal spectrum functions under dry, spontaneous imbibition, and water-saturated conditions, reflecting varying degrees of heterogeneity. The multifractal parameters differ, with higher ΔD values under water-saturated conditions, indicating that pores with weaker connectivity enhance the overall heterogeneity of the shale when saturated. Conversely, lower ΔD values under dry conditions suggest less heterogeneity in pores containing bound water. Total organic carbon (TOC) and clay content were identified as the main factors influencing the development of connected pores. Based on these findings, a discriminant 3D diagram correlating permeability with porosity, tortuosity, and multifractal parameters (D0-Dmax) of strongly connected pores was established for the Luzhou-Changning area. This diagram serves as an effective tool for evaluating the flow capacity of shale reservoirs and offers guidance for identifying sweet spots within the reservoir.