Measurements of the Evolution of the Fluid Lag in Laboratory Hydraulic Fracture Experiments in Rocks

At the early stage of hydraulic fracture growth, the fluid may lag behind the fracture front. We report three hydraulic fracture experiments performed in rocks (gabbro and marble) under conditions promoting an important fluid lag (low confining stress, large fluid viscosity). The spatio-temporal evolution of the fracture and fluid fronts is reconstructed from diffracted waves measured during repetitive acoustic scans. The attenuation of the acoustic transmitted energy for facing source-receiver transducers exhibits a decrease prior to the arrival of the fracture front, indicating the possible presence of a fracture process zone. The viscous effect is extremely strong in these experiments where we observe an increase of the fracture initiation pressure for more viscous fluids. The maximum (break-down) recorded pressure is also much larger than the initiation pressure as expected from the theory of hydraulic fracture mechanics. All the experimental data (pressures, acoustic scans) are provided for further investigations.