Every year tens of thousands of shale gas wells were stimulated in the world. The initiation and propagation mechanism of hydraulic fracture in gas shale is not very clear unlike in the sandstone and carbonate. To investigate this problem, some large scale shale outcrop samples were tested with a true tri-axial rock mechanics frame, where the in-situ stress can be modeled. The size of sample is 30cm*30cm*30cm, and a borehole was drilled in it. Then the hydraulic fracturing was modeled by injecting fluid in the borehole with precise pump. The pumping rate and the treatment pressure can be recorded with acquisition system. The experiment results show the hydraulic fracture geometry is very complex and like network in shale because of the existence of beddings. And the fracture is significantly controlled by the beddings even though the difference of horizontal stress reaches 7MPa.
To further investigate the effect of different factors on the hydraulic fracture propagation in large scale shale, a new numerical simulation method was developed based on the FEM. In this method, a special failure process mode was adopted, which makes the method be able to model the fracture propagation behavior. The validity of numerical method was proved with comparison of physical and numerical result. The numerical simulation results show the stress status and existence of beddings are the keys to form complex network for shale. And the complex fracture geometry in shale was gotten under different condition.
The physical and numerical results can help guide the selection of well/layer and the optimization of hydraulic fracturing design.