Comparative investigation of in situ hydraulic fracturing and high-temperature steam fracturing tests for meter-scale oil shale

The in situ heating technology for mining oil shale through steam injection involves creating interconnected horizontal fractures in the orebody. The authors use meter-scale oil shale as the experimental object to create a unified, integral sample containing oil shale, a filling body, and a rigid pressure chamber containing drilled holes as well as temperature and pressure sensors. A 1000 ton large press was used to apply load to the sample, and water pumps and steam boilers were used to comparatively analyze the processes of in situ hydraulic fracturing and high-temperature steam fracturing. The results showed that the starting pressure for hydraulic fracturing was only 0.34 times lower than the in situ stress, it featured rapid fracture propagation, with a synchronous pressure response between the boreholes as well as a short connection time for the group wells. Furthermore, the horizontal cracks were more easily formed than vertical cracks in layered rock masses. Moreover, high-temperature steam fracturing was required to overcome the constraints imposed by the superposition of the formation and thermal stresses. The fracturing pressure was 1.2 times higher than the in situ stress and fracture propagation was slow, with a negligibly small pressure response between the boreholes and a long connection time for the group wells. For this heterogeneous shale with obvious bedding plane structure, the influence of the thermal stress on the fracture-inducing stress of the orebody was much stronger than that of its own tensile strength. Finally, an appropriate fracturing technology for group wells should be selected according to the rated pressure of the steam boiler.