Shale has certain characteristic features that make it difficult to evaluate in a traditional laboratory setting. The unique characteristics of shale formations include low permeability, existence of microfractures, and sensitivity to contacting fluids. Advances in testing of shale remain relatively stagnant: Current shale fracturing practices and technology are mainly based on simulation models and extension of experience from conventional formations. The objective of this study is to develop an experimental setup to measure the hydraulic breakdown pressure for fractures in shale cores, and to use this setup to study the effect of different parameters (fluid types and characteristics, injection rate, shale bedding, acid injection, and different additives for different systems) on the breakdown pressure, fracture shape, and fracture direction. Shale cores from the Eagle Ford, Marcellus and Mancos, were evaluated in this study. Based on experimental results, breakdown pressures in shale formations have strong exponential relationship with the fluid viscosity, where increasing fluid viscosity increased the breakdown pressure. A linear relationship was observed between injection rate of fracture fluids and breakdown pressure. Additionally, a relationship between closure stress and breakdown pressure has been experimentally established and verified with existing mathematical model: Closure stress increases the breakdown pressure by a factor of 2.8. Even for shale with low HCl solubility (less than 2 wt%), the breakdown pressure of shale formations was reduced by injecting HCl acid, with further increase of the HCl acid contact time reducing the breakdown pressure more. Also, additives that improve the flow of fracturing fluid in microfractures tend to reduce the breakdown pressure and enhance fracture complexity. Finally, factors determining the fracture complexity have been identified, allowing enhanced complexity by optimizing fluid characteristics and treatment design. All of the above relationships and more experimental results will be detailed in the paper.
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