Experimental investigation of the leak-off effect on proppant transportation and distribution in a vertical fracture

Proppant placement inside hydraulic fractures is a key factor during gas production from unconventional gas reservoirs. Fracturing fluid somewhat leaks off through natural fractures when fracturing naturally fractured reservoirs, which affects proppant transportation and distribution. However, this phenomenon has often been neglected in existing experimental fracture models for simplicity. To date, the influence of leak-off on proppant placement remains unclear. This study aimed to build a leak-off vertical fracture model. Laboratory tests on proppant transportation and distribution were conducted using this model at different locations and controlled rates of fluid leak-off through valves simulating natural fractures. The experimental results showed that when the leak-off at the front was more severe than other leak-off locations, the deep fracture suffered the problem of large empty areas and thus ineffective proppant support. The interaction between leak-off location and alternating proppant injection mode was also studied to mitigate the adverse effect of leak-off. Accordingly, the pumping of small-sized proppant particles and then large-sized proppant particles was proposed for the case of severe leak-off in the front fracture. Such a mode led to a uniform proppant distribution throughout the fracture.