Need for better knowledge of in-situ unconfined compressive strength of rock (UCS) to improve rock drillability prediction

Summarization: Rock - drill bit interaction while drilling has been modeled for many years but a complete understanding of the phenomena occurring has yet to materialize. Successful models will allow the prediction of rate of penetration in a given environment and optimal selection of drill bit and drilling parameters thus minimizing exploration costs. In most rock-drilling models the value of the unconfined compressive strength of the rock (UCS), one of the most important engineering properties of rocks, is used in the predictive equations, within the concept of specific energy, and the value of UCS is the percentage of the value of the stress applied on the drilling bit in order for the bit to advance. While the exact percentage depends on the model used and it is not known with certainty, good knowledge of UCS is never-the-less required before any decent prediction can be made on rate of penetration. Determination of UCS, normally done via destructive testing, requires not only availability of sound core samples, but also performance of expensive testing and significant time for the test, which many times is not available for routine drillability predictions. Hence, a multitude of methods and techniques has been proposed to estimate UCS from various indirect and/or notdestructive measurements, or combination of measurements using neural networks, such as point load index, block punch index, unit weight, apparent porosity, water absorption by weight, P-wave velocity and Schmidt hardness. The many proposed approaches are critically reviewed and the results are compared and what becomes apparent is that after many years, not only in mining but also in oil-well drilling, accurate indirect determination of UCS is still an elusive property. Various approaches are suggested to enable better indirect determination of UCS.Παρουσιάστηκε στο: 3rd International AMIREG Conferenc