Experimental investigation on micro-fracture evolution and fracture permeability of oil shale heated by water vapor

The study investigated the relationship between the temperature field distribution and fracture connectivity of oil shale heated by water vapor using infrared thermal imaging experiment. High temperature triaxial permeability testing system was then used to study the permeability of oil shale containing single penetrating fracture under the condition of water vapor heating. Finally, micro CT imaging and hot stage microscope imaging experiments were conducted to study the microstructure characteristics of oil shale at different temperatures. It was observed that when the oil shale was heated by water vapor, fractures became the main flow channels, and the connectivity of fractures affected the heating area of oil shale. The magnitude of fracture permeability of oil shale decreased from 10−15 m2 at 25 °C to 10−17 m2 at 350 °C. When the temperature was higher than 350 °C, the permeability of oil shale increased rapidly and recovered to the order of 10−15 m2 after 500 °C. The micro-fractures had little contribution to permeability in the temperature range of 25 °C–350 °C. The decrease in the permeability in this temperature range was caused by the closure of the single penetrating fracture. When the temperature reached 400 °C, some new penetrating fractures appeared. This resulted in the increase in permeability. •Fractures constitute the main channels for fluid flow in oil shale.•The distribution of fractures determines the range of oil shale heated by superheated water vapor.•The threshold temperature of permeability of fractured oil shale was 350 °C.•The influence of fracture evolution on oil shale permeability was analyzed.