Quantitative and visual analysis of proppant transport in rough fractures and aperture stability

The consensus reached in the literature is that the roughness of fractures plays a crucial role on proppant transport affecting the aperture sustainability of hydraulic fractures. In this paper, an experimental scheme to visually and quantitatively investigate the hydraulic characteristics of rough fractures in the presence of proppants was presented. Rock samples of different kinds (i.e., granite, marble, and limestone) were fractured under the Brazilian test and molded to manufacture 20×20 cm transparent replicas. Propping agents were injected in a similar fashion and were introduced into the well with tracking fluid at a constant rate. Two types of fracture models were used: (1) perfectly mating (joint) and (2) sheared fractures in polymeric solutions. During the experiments, the inlet pressure was continuously monitored to quantify the permeability changes due to proppant distribution caused by the roughness of fracture surfaces. Meanwhile, 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. In both joint and shear type fractures, existing closure areas controlled the proppant movement and permeability change significantly. The fracture roughness controlled by the lithological properties of the rocks was a critical factor affecting the permeability and proppant transport. After quantifying the roughness characteristics through the variogram fractal dimension, relationship between fracture permeability in the presence of proppant and rock types were presented. Also provided was a semi-quantitative analysis of the stability (or settlement) of proppants during injection with respect to the roughness type (and lithology). The quantitative and visual data collected for a wide range of rock types with original roughness characteristics are expected to be useful in tracking design and selection of proper proppants for different reservoirs.