Accurate assessment of rock drillability is crucial in improving the rate of penetration and drilling efficiency in drilling process. In this paper, experimental research on rock indentation fragmentation by single-indenter at varying confining pressures has been carried out. Moreover, measuring tool of three-dimensional digital microscope has been applied to observe and analyze the shape and volume of crushing craters. A new breakability index (BI) of rock considering the impact of both invasion difficulty of tool and characteristic of rock brittleness is defined from force-penetration curve of indentation test. The relationship of both specific energy (SE) and volume of crater (Vc) with BI has been investigated, and the influence of confining pressure (Cp) on BI has also been studied. The results indicate that both the SE and Vc have a good non-linear correlation with BI, with increasing of BI, Vc increases, while SE decreases. Besides, a good exponential relationship between Cp and BI is also found, along with Cp increasing, BI decreases. It can be concluded that the greater the BI is, the more prone to breakage the rock is. Therefore, the proposed breakability index could be used for quantitative evaluation of rock drillability.


Rock drillability is an important parameter to estimate the difficulty of rock crushing. An accurate assessment of the rock drillability plays a major role in improving rate of penetration and reducing drilling cost in the oil and gas drilling process. However, the resistance of rock crushing is a function of multiple variables, including the type of tool and rock mechanical properties, there is no agreement to describe or measure it. Currently, various researchers have evaluated the rock drillability by using different approaches. The drilling comminution was theoretically and experimentally analyzed based on a fractal approach by Carpinteri A. and Pugno N. [2]. They developed a model to evaluate rock drillability and energy dissipation with fractal dimensions of rock cuttings in comminution process. In addition, some rock mechanical properties and drilling parameters have long been widely used to represent rock drillability or to predict the drilling rate [3, 4, 5]. Yarali O. and Kahraman S. investigated the relation between the resistance of rock breaking and rock brittleness and concluded that the rock brittleness determined from compressive and tensile strength can be used for assessment of rock drillability [6]. Nevertheless, it can be found that the methods mentioned above do not consider the comprehensive influence of the tools, rock properties and confining pressure, thus they cannot truly reveal the rock drillability.

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