Shale has certain characteristic features that make them difficult to evaluate in a traditional laboratory setting. The unique characteristics of shale formations include low permeability, existence of micro fractures, and sensitivity to contacting fluids. Advances in the testing of shale has remained relatively stagnant, which has led to the fact that many current shale fracturing practices and technologies are mainly determined by fracture model simulations, and experiences drawn from the stimulation of conventional formations. Therefore, the objective of this study is to develop an experimental setup to measure the hydraulic breakdown pressure associated with the development of fractures in shale cores, and to use this setup to study the effect of different parameters on the breakdown pressure, fracture shape development, fracture direction, from various fluid types and characteristics, injection rates, shale bedding directions, acid injection, and other variables in different systems.
Shale cores from a Mancos formation outcrop were evaluated in this study. Based on the experimental results, breakdown pressures in shale cores have a strong exponential relationship to fluid viscosity, where increasing fluid viscosity increases the breakdown pressure. Increasing the injection rate will reduce the pressure needed to breakdown the shale formation. Fracture complexity will increase by reducing the viscosity of the fluid. A shale core drilled in the parallel bedding direction was fractured at a lower pressure with a faster propagation compared to a shale core drilled in the perpendicular bedding direction. Additionally, a relationship between closure stress and breakdown pressure has been experimentally established and verified with existing mathematical models. Closure stress increases the breakdown pressure by a factor of 2.8.