Effect of fracture roughness and shear displacement on permeability and proppant transportation in a single fracture

Proppants are one of the essential parameters in fracking design. They not only provide fracture permeability but do prevent "healing" of fractures. Hence, the quantification of proppant transport characteristics is highly critical in a sustainable production from hydraulically fractured wells. Previous attempts were limited to smooth (parallel) fracture surfaces, to a great extent. The consensus reached in the literature, however, is that the roughness of fractures may play a crucial role on proppant transport affecting the permeability of hydraulic fractures. In this paper, an experimental scheme to visually and quantitatively investigate the hydraulic charac-teristics of rough fractures in the presence of proppants is presented. Seven rock samples of different kinds (i.e., granite, marble, and limestone) were fractured under the Brazilian test and molded to manufacture 20X20 cm transparent replicas. Propping agents were injected at a constant rate into perfectly mating (joint) and sheared fractures in water and polymeric solutions representing typical rheological properties of hydraulic fracturing fluids. During these 2-D experiments, the inlet pressure was continuously monitored to quantify the permeability changes due to proppant distribution caused by the roughness of fracture surfaces. Simultaneously, corresponding images were collected to trace the transport of proppants and their behavior was correlated to the measured permeability change. For a better visualization of proppants, the injected fluid was dyed with a fluorescent material. The proppant behavior in joint and shear type fractures were different. In both cases, fracture closure areas existed, which controlled the proppant movement and permeability change significantly. The injection rate, proppant size, and fracture roughness controlled by lithological properties of the rocks were the other critical factors affecting the permeability and proppant transport. After quantifying the roughness characteristics through different fractal methods (e.g., variogram analysis, power spectral density, etc.), correlations between fracture permeability in the presence of proppant and rock types were presented. The quantitative and visual data collected for a wide range of rock types with original roughness characteristics are expected to be useful in fracking design and selection of proper proppants for different reservoirs.