The concept of brittleness as defined by a rock’s ability to fail and maintain a fracture in a hydraulic fracture experiment plays a crucial role in the development of tight oil and gas. As such, various methodologies have been proposed in an attempt to quantify the degree of brittleness associated with a rock mass. These conventional methods have a few shortcomings. For example, failure criteria are not used and only the effective moduli are considered, leading to an incomplete description of the problem. In this paper, we address some of these issues by including failure criteria and rock physics models in the brittleness definition.

We propose that petrophysical parameters such as porosity and mineralogy play a more fundamental role in determining a rock’s brittleness. This is the result of fractures being supported by the rock frame and hence, the matrix composition must be considered. Furthermore, porosity variations result in a redistribution of stress concentrations, leading to areas that are more susceptible to failure and ultimately reducing the rock’s overall strength. A new measure to assess the rock quality for hydraulic fracturing was subsequently proposed that includes the use of physical bounds and regression variables that can be used for calibration to engineering parameters.

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