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, porosity, 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. The plot is called a “spider plot,” which characterizes drillability fully in simple enough parameters for use in the industry, yet detailed enough to describe drillability issues to a great extent. Further, this 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 plot.
Drillability may be defined as an ease of drilling or rate of penetration (ROP), achieved using specific cutter-metallurgy type cutters or compacts with the given cutter-bit design parameters and operating parameters; an efficient cutting removal system for the drilling environment using a particular drill rig type; and an outcome of unchanged cutter /bit dullness or balling condition. 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]. The UCS is considered fundamental as it not only indicates atmospheric stress and strain at failure but also is related to elastic behavior (compressional and shear wave velocities, Young’s, bulk and shear modulii), which doesn’t change much under confinement. 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. In addition, it is also well known in the industry that different rocks such as limestone, anhydrite, shale, and sandstone with similar UCS values have very different drilling characteristics and need completely different sets of bit designs/drilling parameters.