Abstract

During the stimulating unconventional reservoirs, the vertical propagation of hydraulic fractures is crucial for enlarging the stimulated reservoir volume, especially in multi-layers of sandstone, mudstone and shale (sand-mud-shale). To investigate the effects of lithological interface and fracturing fluid viscosity on the fracture propagation vertically in the multi-layers, hydraulic fracturing experiments in laboratory were performed on the outcrop samples of 30 cm × 30 cm × 30 cm collected from Yanchang Formation in Ordos Basin. The results show that hydraulic fractures are multi-branched and zig-zagged when they initiate in shale, simple when they commence in sandstone or mudstone. Hydraulic fractures created with low-viscosity fracturing fluid can only cross sandstone from mudstone, but those induced by high-viscosity fracturing fluid can cross the sand-mud-shale layers. Furthermore, the high-viscosity fracturing fluid reduces the fractures complexity in shale, facilitating vertical fracture propagation. The injection pressure fluctuates slightly as the hydraulic fracture extends from shale to sandstone or mudstone, otherwise it fluctuates significantly. From the laboratory investigation, a hydraulic fracturing scheme for Chang 7 was proposed, with its feasibility proved in field tests.