Rock Drillability. George Exadaktylos. Professor of Rock Mechanics & Mining Engineering, School of Mineral Resources. Engineering, Technical University. The purpose of this paper is to provide a proper, practical and convenient drilling rate index (DRI) prediction model based on rock material. Quantifying the rock microfabric is one of the most important research interests and challenges in the field of rock drilling and excavation.
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Abstract Drillability of a rock is often expressed in terms of a large number of parameters; however, the industry hardly uses any. Quite often these are not well understood or communicated to the end users. As a compromise, the present work describes drillability in terms of eight simple physical, mechanical, and micro-structural properties, which are displayed visually and are available from either log data or from laboratory core testing.
The relevant rock properties are density, vrillability, compressional and shear wave velocities, unconfined compressive strength, Mohr friction angle, mineralogy, and grain sizes.
These are compiled and normalized in a scale of 1 to 8; value of 1 represents very soft rock and a value of 8 represents hard rock, ideally. The real rock is in between depending upon the rock type. Further, drillabillty gives an excellent tool to optimize the bit and drilling process for a given rock formation while depicting its physico-mechanical and micro-structural properties as a signature dgillability.
The drillability is assumed to be indicative of unconfined compressive strength UCS at atmospheric drilling [1,2] or confined compressive strength CCS together with a factor of efficiency [3,4,5].
Definition of drillability – glossary
It also indicates a co-efficient of energy transfer and the extent of vibration in an efficient drilling process. Drillability of rocks, by an expert, is defined by a large number of parameters [6,7,8,9,10,11,12,13,14,15,16], which are not well understood or communicated to the end user.
As a result, the industry uses hardly any. However, quite often, rock properties like UCS and abrasiveness have historically been used to define rock drillability and help decide bit selection, drive mechanism and drilling parameters.
These rock properties are typically estimated using electric log data [17,18,19,20,21] using a known calibration or mea in the laboratory in absence of logs. Further, it has been found that these estimates do not often provide the full picture of drillability of rocks, which have complicated mineralogy and microstructures that go through complex digenesis process.