A Laboratory Investigation of the Effect of Bedding Plane Inclination Angle on Hydro-Fracturing Breakdown Pressure in Stratified Rocks

In this paper, the effect of layer inclination, thickness and physical and mechanical properties of layers on hydraulic breakdown pressure was investigated. To achieve this aim, the model materials composed of kaolinite, cement and water in distinct ratios were used to simulate the laminated blocks like sandstone-shale and limestone-shale interbedded rocks. The fabricated rock blocks were drilled at different dip angles of 0°, 30°, 45°, 60° and 90° with respect to their bedding planes. The prepared laminated cylindrical samples are 54 mm in diameter and 108 mm in length. Then, these specimens were drilled in the center to prepare the hollow cylinder samples. The prepared specimens have two different layer thicknesses designated as low-thickness (t=1cm) and high-thickness (t=2cm). The laboratory scaled hydraulic fracturing test was performed on thick-walled cylinder samples with different layer thicknesses and bedding plane inclination angles. The results of the study indicated the breakdown pressure is reduced for bedding plane angles ranging from 0° to 45° and increased for bedding plane angles 45° to 90°. The minimum breakdown pressure occurred at inclination angle of 45°. This phenomenon can be expressed as the anisotropy effect on the hydraulic fracturing breakdown pressure.