Numerical simulation of in situ exploitation of oil shale by injecting high-temperature steam

Oil shale has a very important strategic value for energy safety, but in situ pyrolysis technology still faces both scientific and technical problems that urgently need to be overcome. Among the current world's oil shale in situ conversion technologies, the Chinese in situ pyrolysis technique, which consists in injecting high-temperature water vapor, is considered to be one of the most efficient methods in this field. The thermal-hydraulic-mechanical-chemical (THMC) coupling model for this method is presented. Based on the model, the temperature distribution and permeability evolution were simulated. It was concluded that, ,first, due to the existence of prefracturing cracks, the pyrolysis front tended to be focused on both sides of the prefracturing cracks and the region had a relatively high temperature and permeability. Also, the pyrolysis front formed a sharp protrusion at the leading edge on both sides of the crack. Second, in the process of pyrolysis, pore-crack and tensile stress in the high-temperature region increased rapidly, and the permeability coefficient increased by 3-4 orders of magnitude and formed an in situ pyrolysis zone of oil shale. Third, the thermal protective, seepage insulation and stress trap zones were formed around the pyrolysis zone, and the internal boundaries o f the three zones coincided, thus forming a good environment for underground pyrolysis similar to that of ground retorting.