Hydraulic fracturing has been used as a stimulation technique to improve the total oil and gas production from unconventional plays during the past decades. One of the key parameters affecting the performance of the hydraulic stimulation is the mechanical properties of the rocks i.e. fracture toughness. This parameter is a measure of material resistance to extension of a crack. For conventional reservoirs like sandstone, it is relatively straightforward to obtain cores or large samples to do the standard fracture toughness tests to measure this parameter. However, for the unconventional reservoirs like shales, due to the unstable chemical and mechanical properties, it is very expensive and sometimes impossible to get the cores and suitable sample sizes to perform the standard toughness test. Determining how to quantify the fracture toughness of shale formation remains a challenge. Nano-indentation, which can eliminate the need for large samples, shows a potential application in estimating the fracture toughness of shales. In this paper, samples from Upper Bakken Formation or Middle Bakken Formation were collected and analyzed. After sample preparation, a nano-indenter was applied to estimate Young’s Modulus and Hardness. Finally, based on the energy theory, the fracture toughness of each sample was calculated and compared.

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