Due to the high shale content and low permeability in shaly unconsolidated sandstone (SUS) reservoirs, the crude recovery rate can be very low. Fracturing is the key technology for developing such reservoirs. Thus, whether smooth fractures can be formed or not is important to improve production and to maximize economic benefit. In the study, a SUS model based on PFC discrete element method (DEM) is established firstly. Through the stress-strain curve fitting of the uniaxial pressure experiment, the microcosmic parameters are obtained. Meanwhile, the hydraulic fracturing PFC model of SUS is built by considering the fluid-solid coupling theory. The simulation shows that natural fractures can be connected by fracturing fluid to form branching fractures in hydraulic fracturing, but the main fracture still extends along the direction vertical to the minor principal stress. The crustal stress field can obviously affect the shape of hydraulic fractures in SUS. The smaller the stress difference is, the more fractures have the tendency to form disorderly fracture zones; with the increase of stress difference, fractures become more and more straight. It is also found that the stress chain direction is obvious after loading stress field to SUS, but the uneven distribution of stress is serious. The phenomenon leads to the shear stress emergence in the process of fractures expansion, making fractures direction more easily turn to the branches and produce fracture zones. Combining PFC microcosmic parameters and fracturing curves, it is believed that the normal and shear stiffness of sandstone particles are larger than those of shaly ones. To achieve the same stress, shaly ones need larger displacement, requiring more hold pressure time. Besides, the Poisson's ratio and strong plasticity both are related to SUS shear failure. The study results can provide a basis for optimizing the design of hydraulic fracturing in SUS reservoirs.
The newest round of oil & gas resources evaluation in China indicates that shaly unconsolidated sandstone (SUS) reservoirs are widely distributed and equipped with large reserves, occupying a foremost position in the whole oil & gas exploitation and production. How to develop efficiently and rationally this type of oil & gas reservoir has become one of the essential growth points is China's relevant industry, which can be of great significance to improve the output and reduce production cost.
