The evolution of pore-fracture system in shale and its geological implications as investigated via in situ loading by synchrotron X-ray microtomography

The evolution of fracture and pore networks in organic-rich shale is crucial for predicting sweet spots and optimizing hydraulic-fracturing development of shale oil and gas, which is intimately linked to the in situ stress. Here, the quantitative characterization and evolution of microscale pore-fracture networks in shale plugs from the Shahejie Formation, Bohai Bay Basin, was monitored via in situ loading synchrotron X-ray microtomography. The shale exhibited obvious compaction, fracturing, and destruction processes, along with energies conversion. The initiation, closure, fracture, and growth of pore-fracture networks were captured and quantified using 4-D image processing and digital volume correlation. Obvious vertical compression and horizontal expansion were observed with increasing internal strain ranging from −14% to 9%. The initial microfractures extend along the region with low grayscale values, accompanied by new fracture formation. The pore spaces gradually increase with internal heterogeneity and initial isolated pore spaces gradually increase interconnectivity. The spatial clusters, referring to the integrations of pores and microfractures at different scales within rock, were further classified into nine types based on geometry. The induced pores and fractures have a significant impact on the porosity and permeability related to the rock lithology. The contribution of induced fractures to the porosity exceeds 80%, and ranges from 36.8% to 59.5% to the permeability. However, the deformation behavior is noticeably affected by lithology and sample size, strain rate, temperature, and fluid pressure. Our experimental results deepen the understanding of fracture quantitative evolution in organic-rich shale under rising stress, suggest a potential new mechanism for hydrocarbon enrichment in structural hinge zones and over-pressured regions, and reveal the dynamic behavior of fractures during in situ fracturing in organic-rich shale.