New Insights into Hydraulic Fracturing of Shale Formations
Authors: Gomaa Ahmed M. , Qu Qi , Maharidge Russell , Nelson Scott , Reed Ted 
Published: May 2016

Abstract
Shale has certain characteristic features that make it difficult to evaluate in a traditional laboratory setting. The unique characteristics of shale formations include very low permeability, the existence of microfractures, and sensitivity to contacting fluids. Advances in the testing of shale have remained relatively stagnant. As a result, current shale fracturing practices and technologies are mainly based on simulation models, and experience from conventional formations. Therefore, 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 the setup to study the effects of different parameters such as fluid types and characteristics, injection rate, shale bedding, acid injection, and different additives and systems on the breakdown pressure, fracture shape, and fracture direction.
Cores from a Mancos Shale outcrop were evaluated in this study. Based on experimental results, breakdown pressures in shale formations have a strong exponential relationship with the fluid viscosity, where increasing fluid viscosity increased the breakdown pressure. In addition, linear relationship was observed between the injection rate of fracture fluids and the breakdown pressure. Even for shale with low HCl solubility (less than 2 wt%), the breakdown pressure of the shale formation was reduced by injection of HCl acid, and a further increase in the acid contact time will reduce the breakdown pressure further. Also, additives designed to improve the flow of fracturing fluid in microfractures tend to reduce the breakdown pressure and enhance fracture complexity. Finally, fracture complexity increases with reduced viscosity and/or increased acidity of the fluid. Nitrogen gases will maximize the fracture complexity.

Introduction
Shales are fine-grained sedimentary rocks that can be rich sources of petroleum and natural gas. Over the past decade, the combination of horizontal drilling and hydraulic fracturing has allowed access to large volumes of shale gas/oil that were previously uneconomical to produce, Seale (2007). In North America, shale reservoirs such as the Bakken, Barnett, Montney, Haynesville, Marcellus, and most recently the Eagle Ford, Niobrara and Utica shales are drilled horizontally and then completed with multistage fracture stimulation (EERC 2013). These completion techniques may allow more than 30 stages to be pumped into the horizontal section of a single well, Mooney (2011). The production of oil and natural gas from shale formations has rejuvenated the natural gas industry and reversed the oil production decline in the United States. Of the natural gas consumed in the United States in 2009, 87% was produced domestically. Shale gas resource and production estimates increased significantly between the 2010 and 2011 and are likely to increase further in the future, International Energy Outlook (2013). The natural gas resources for gas shale in the USA are estimated to be around 500–1,000 TCF and oil around 25 billion barrels, EIA (2011).
Shales have certain characteristic features that make them difficult to evaluate in a traditional laboratory setting. The most important characteristics are low permeability, the existence of microfractures, and sensitivity to contacting fluids. Shales consist of minerals of variable composition mixed with organic matter commonly occurring finely dispersed in the matrix, or in thin laminae. Mechanical properties strongly depend on the volume fractions of minerals, kerogen and pore fill. Attempts have been made to relate acoustic velocity, and velocity anisotropy to the degree of kerogen maturity of the shales (Vernik and Nur, 1994; Vernik and Liu, 1997; and Prasad and Mukerji 2003). Prasad et al. (2009) showed that in low-porosity shales, velocity was directly correlated to kerogen content and textural heterogeneity; elastic impedance, velocity, and density increase with increasing shale maturity. Most studies on mechanical properties of shales were achieved using compressional or tensile loads (Jizba, 1991; Brown et al., 2002; Gurevich, 2002; Pham et al. 2005; Tran, 2009; Saenger and Steeb, 2011; and Lin and Lai 2013). Few studies using hydraulic fracturing fluids to break the shale cores have been reported.