Time-lapse X-ray imaging of deformation modes in organic-rich Green River Shale heated under confinement

Shales are layered sedimentary rocks, which can be almost impermeable for fluids and act as seals and cap-rock, or, if a shale layer hosts a fracture network, it can act as a fluid reservoir and/or a conduit.  Organic-rich shales contain organic matter - kerogen, which can transform from solid-state to oil and gas during shale burial and exposure to heat. When the organic matter is decomposing into lighter molecular weight hydrocarbons, the pore-pressure inside the shale rock increases and can drive propagation of hydraulic fractures and strongly modify the permeability of these tight rocks. Density, geometry, extension, and connectivity of the final fracture network depend on the combination of the heating conditions and history of external loading experienced by the shale reservoir. Here, we have performed a series of rock physics experiments where organic-.rich shale samples were heated, under in situ conditions, and the development of microfractures was imaged through time. We used the high-energy X-ray beam produced at the European Synchrotron Radiation Facility to acquire dynamic microtomography images and monitor different modes of the shale deformation in-situ in 3D. We reproduce natural conditions of the shale deformation processes using a combination of vertical load, confining and heating of the shale samples. Shales feature natural mineral and silt lamination and hydraulic fractures easily propagate parallel to these laminae if no overburden stress is applied. However, if the principal external load becomes vertical, perpendicular to the shale lamination, the fracture propagation direction can deviate from the horizontal one. Together horizontal and vertical fractures form a three-dimensional connected fracture network, which provides escaping pathways for generated hydrocarbons. Our experiments demonstrate that tight shale rocks, which are often considered as impermeable, could have hosted transient episodes of micro-fracturing and high permeability during burial history.