Artificially created fracture networks with sufficient fracture conductivities are essential for economic production from shale reservoirs. Fracture conductivity can be significantly reduced in shale formations due to severe proppant embedment. In addition, proppant embedment induces shale flakes that migrate and clog fracture networks.
A laboratory investigation was performed to understand how excessive proppant embedment caused by the shale-water interaction impairs shale fracture conductivity. The experiments were conducted using Barnett shale samples with representative rock properties. The asperities on the fracture surface were carefully preserved. The damage process was simulated in the laboratory by flowing water through the shale fracture packed with proppants. The water used in the experiments had a similar chemical composition to flowback water in the field. The laboratory results were benchmarked with the results from an experimental study conducted with Berea sandstone samples. Post experimental analysis included microscopic imaging of the fracture surfaces and measurement of the proppant embedment depth.
A computational fluid dynamics study was conducted to quantify the conductivity loss due to proppant embedment on a theoretical basis. We developed pore-scale physical models of the proppant pack and calculated the fracture conductivity loss at different proppant embedment depths. The computation was repeated for a variety of proppant layers. The worst case assumed a 40% proppant grain volume embedment.
The experimental study showed up to 88% reduction in fracture conductivity after water flow under 4,000 psi closure stress. The conductivity loss was due to severe proppant embedment as the shale fracture face was softened after its exposure to water. Direct measurement of embedment depths indicated that for fractures that were exposed to water, the average embedment depth was about 50% of the proppant median diameter, while for fractures that were only exposed to gas, the average embedment depth was just 15% of the proppant median diameter. It was also observed that pore space of the sand grains at the outlet of the fracture was clogged by shale flakes and fragments. The computational fluid dynamics study proved that even a 10% proppant grain volume embedment can cause 45%~80% conductivity loss. With the same proppant volume loss due to embedment, the conductivity reduction was less in fractures containing multiple proppant layers than the fracture containing only one layer of proppants.