Hydraulic fracture propagation in layered geologic formations

Abstract

Hydraulic fracturing is widely used to enhance the hydrocarbon productivity in low-permeability reservoirs or to enhance heat recovery in deep geothermal systems. Predicting fracture propagation and geometry is an important but daunting task because of complexities associated with natural geologic rock conditions, including stress anisotropy, heterogeneities, natural fractures, and formation discontinuity. Therefore, this study explored the interactions between hydraulic fracture and interfaces in layered geologic formations. Hydraulic fracture propagation behaviors in gelatin plates were monitored via scaled laboratory experiments. Three kinds of bounding layers were tested with low-, medium-, and high-stiffness gelatins. Hydraulic fracturing processes were recorded using a digital camera while measuring fluid injection pressure, and the captured images were analyzed to extract the fracture geometry. When the bounding layer had lower stiffness, the fracture was observed to cross the interface and continued growing. Contrary to that, when the bounding layer had the same or higher stiffness, the fracture did not penetrate the bounding layer and propagated along the layer interface. Comparing with a simple geometry model, such as PKN model, it was found that a decrease in material stiffness increased the growth rate of fracture. The PKN model prediction was found to be quite different from the experiment results, possibly owing to the different shape of fracture cross-section and plastic deformation in gelatin. The photoelastic analysis was conducted to identify the variations in the stress intensity factor at the tip when the fracture met the interface. The stress intensity factor was fairly consistent in homogeneous media. However, when the fracture met a soft bounding layer, as it crossed the interface, the stress intensity factor was reduced significantly and became close to the toughness value of the soft bounding layer. When the fracture approached the bounding layer of the same or higher stiffness, the stress intensity factor increased for a short period of time. After the fracture followed along the interface, the stress distribution at the tip was different on the side of original medium and on the other side of bounding layer because of different stiffness. The presented results contribute to better understanding of interactions between hydraulic fracture and naturally existing discontinuities by visualizing the fracture propagation process and providing the experimental results on variations in fracture geometry and stress intensity factor with time.