Experimental Study of Hydraulic Fracturing in Unconsolidated Sandstone

Hydraulic fracturing is widely applied to enhance production in unconsolidated sandstone reservoirs. However, field fracturing does not always provide ideal stimulation for the lack of the understanding of fracturing mechanism in unconsolidated sandstone. We performed fracturing experiments on various artificial sandstone samples to explore the influencing factors. In this work, a large triaxial system was developed for the preparation of rock samples and injection experiments. The unconsolidated sandstone reservoirs were modeled by filling the cavity inside the system with diverse formulations. We considered homogeneous reservoirs with different permeability, reservoirs damaged near wells, and vertically heterogeneous reservoirs containing mud layers. Injection rates and fracturing fluid were also varied in the injection experiments. The experiments verify fracture fluid leak-off control is the key to successful fracturing stimulation in unconsolidated sandstone reservoirs. The results show the fracturing fluid may invade the sand resulting in an internal filter cake only in ultra-high permeability samples. On the other hand, suitable fracturing designs can ensure fractures penetrate through the damage zones, as the presence of damage zones is beneficial for fluid leak-off control. With no stress contrast, fractures preferentially propagate into lower-permeability formations, such as mud layers, which enable fracture penetration to other layers. The finding in this work provides valuable guidelines for the design of hydraulic fracturing in unconsolidated sandstone reservoirs.