Fracturing controlled primary migration of hydrocarbon fluids during heating of organic-rich shales
Résumé
Time-resolved three-dimensional in situ high resolution synchrotron x-ray tomographic imaging was used to investigate the effects of slowly heating organic-rich Green River Shale from 60° to 400°C, in air without confinement, to better understand primary migration of hydrocarbon fluids in very low permeability source rock. Cracks nucleate in the interior of the sample at a temperature around 350°C. As the temperature increases, they grow and coalesce along lamination planes to form bigger cracks. This process is accompanied by a release of light hydrocarbons generated by decomposition of the initially immature organic matter, as determined by thermogravimetry and gas chromatography. These results provide the first 4D monitoring of an invasion percolation-like fracturing process in organic-rich shales. This process increases the permeability of the sample and provides pathways for fluid expulsion - an effect that might also be relevant for primary migration under natural conditions. We propose a 2D fracture model that reproduces both the observed non-linear crack growth in a lamination plane and the irregular geometry of the crack fronts.
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