Quantitative and visual analysis of proppant transport in rough fractures

The consensus reached in the literature is that the roughness of fractures plays a crucial role on proppant transport affecting the aperture sustainability and conductivity of hydraulic fractures. To clarify this, an experimental scheme and analysis are presented in this paper. Propping agents were introduced into seven different transparent fracture replicas obtained from different origin rock samples (granite, marble, and limestone) at a high rate mimicking hydraulic fracturing process conditions. The inlet pressure was continuously monitored to quantify the change in hydraulic conductivity due to proppant distribution. Corresponding images were collected to trace the transport of proppants and their behavior was correlated to the measured pressure drop (representing conductivity) change. Experiments were repeated on joint (perfectly mating) and horizontally displaced (sheared-unmating) models using water and polymer solution. Existing closure areas controlled by roughness dictated proppant movement and the change in hydraulic conductivity significantly in both joint and displaced type fractures. To quantify this effect, pressure drop across the model and proppant distribution (area saturated with sands) were correlated to three fractal methods (variogram, power spectral density, and triangular prism) and the ratio of total and planar fracture surface areas. Correlation parameters and types differ for water and polymer solution. Joint and displaced type fractures also showed differences in hydraulic conductivity change and sand distribution for certain type of rocks, especially at the higher variogram fractal dimensions.