An experimental investigation of hydraulic fracturing of stratified rocks

Hydraulic fracturing is the common method for increasing the production rate in reservoirs with low permeability. It is also used to raise the extraction rate of wellbores with low production due to the long-term utilization. The interfaces aligned with direction of the hydraulic fracture propagation, and different physical and mechanical properties of the adjacent layers are the most important factors in the performance of hydraulic fracturing treatments in laminated reservoirs. In this paper, the effects of layer interfaces, inclinations, thicknesses, and physical and mechanical properties on hydro-fracturing breakdown pressure and fracture propagation path were investigated. To achieve this aim, the laboratory-scaled hydraulic fracturing test was performed on thick-walled cylinder samples with different layer thicknesses and inclinations. The effect of the material property contrasts between different layers on the complex behavior of hydraulic fracture propagation in laminated specimens is also studied. The obtained results have indicated that with varying the bedding plane inclination angle from 0 to 45 degrees, the breakdown pressure would be decreased and then increase in breakdown pressure is observed from 45 to 90 degrees. The minimum breakdown pressure would occur at an inclination angle of 45 degrees. During the hydraulic fracture propagation from the hard layer to the soft layer, there is no deviation observed at the layer interface. However, the possible shear slippage along the layer interface and extending the fracture in the interface would occur, when the fracture propagates from the soft layer to the hard layer. The mechanism of fracture on the layer interface is assessed based on fracture energy in fracture tip. High fracture energy would significantly reduce the effect of the inclined interface on the hydraulic fracture propagation path.