Visualising Hydraulic Fractures in Bedded and Fractured Shales: A Series of Analogue Experiments

The rapidly growing shale gas industry, especially in the United States, has led to a growing number of research questions associated with understanding the controls on hydraulic fracturing. Having a better understanding of these controls will lead to an improved industry for both regulators and industry. Classic fracture mechanics states that hydraulic fractures will propagate in the direction of the maximum stress, which at these depths is often vertical. However, micro-seismic data from the industry shows that hydraulic fractures often have a considerable lateral extent. The natural heterogeneity of shales and there often heavily fractured nature has been hypothesised to explain this. To test this hypothesis a series of analogous hydraulic fracture visualisation experiments have been carried out using high speed photography of up to 1000 frames per second. Both layered arrangements of Bowland Shale and kaolinite clay discs and fractured arrangements have been examined. Hydraulic fractures were shown to become lithologically bound dependent on material properties. Furthermore, complex interactions between hydraulic fractures and natural fractures was observed. Further work is required to understand the way material properties govern fracture propagation. Results from this can be combined with well characterised shale formations to produce more accurate hydraulic fracture propagation models.