Investigation of the key parameters controlling microfracture propagation in the Upper Bakken shale oil

Abstract

Abstract Microfractures generated by kerogen thermal maturation are among the most effective mechanisms for establishing hydraulic connectivity and hydrocarbon migration in organic-rich shales; however, the quantified roles of differential stress, kerogen geometry, and total organic carbon (TOC) in controlling the architecture of this network have not yet been systematically clarified. In this study, focusing on the upper member of the Bakken Formation, a fully coupled hydro–mechanical model based on the Discrete Element Method (DEM) and the Finite Volume Method (FVM) was employed to evaluate the mechanical response at the microscale and the evolution of the microfracture network under fluid pressure generated by kerogen maturation and in-situ stress. The low-porosity rock specimen was modeled as a dense packing of particles in the YADE software, fluid pressure in kerogen patches was applied under different differential stress states and various kerogen configurations, and fracture intensity was quantified by the P 32 parameter while the network geometry was characterized by the distribution of the dip angle of microfractures. In the first scenario, increasing differential stress led to a threshold-type and nonlinear behavior of P 32 ; such that within an intermediate range of Δ σ , a significant increase in fracture surface area occurred, whereas at higher stresses, additional loading mainly resulted in reorganization and verticalization of the existing network rather than the creation of new microfractures. In the second scenario, it was shown that alignment and small horizontal spacing between kerogen patches markedly enhance stress-field interaction and vertical connectivity of microfractures, whereas increasing the spacing confines the damage zone to local and relatively isolated regions. In the third scenario, increasing organic content at low levels causes the damage zones to remain disconnected; however, upon exceeding a critical threshold, the microfracture network abruptly transforms into a continuous skeleton at the specimen scale, and thereafter further increases in TOC mainly strengthen the existing pathways, accompanied by only a minor increase in the generation of new microfractures.These results indicate that in kerogen-rich shales, the architecture and connectivity of the microfracture network arise from the simultaneous interaction of differential stress, kerogen geometry and spatial arrangement, and mechanical anisotropy of the layers. The proposed numerical framework enables the explicit incorporation of kerogen-derived microfractures into reservoir modeling and stimulation design in a quantitative manner, and can provide a basis for optimizing well placement and hydraulic fracturing strategies in unconventional reservoirs.