Estimation of rock strength using scratch test by a miniature disc cutter on rock cores or inside boreholes

Abstract

Accurate estimation of rock properties is crucial for performing realistic analyses in geomechanics, especially for assessment of rock mass strength. The conventional methods for strength measurement require testing on core samples, which are not always available or are time consuming/expensive to obtain. In addition, sample preparation can be an arduous task, and finally special equipment are required to perform the tests. This process offers the strength values with a time lag and not representing the in-situ conditions. Estimation of rock strength by scratching the surface of cores has been implemented with reasonable success in recent years. The need for application of this concept for assessment of rock strength in a borehole has led to development of a special probe that can scratch the borehole wall and offer an estimate of rock strength. This is accomplished by comparing the recorded forces with the strength of the known rock samples. To develop a relationship between cutting forces and rock strength, 27 different rocks covering a wide range of strengths, grain size, and origins have been tested by a miniature disc cutter and the results are discussed in this paper. The results show a promising correlation with R-square values of around 80% between the average normal force and the compressive (UCS) and tensile strengths (BTS) of the sedimentary/metamorphic rocks. However, no significant correlation was observed for igneous rocks. This could be attributed to the impacts of rock texture, grain orientations, and grain size. General equations are also introduced to estimate the UCS and BTS of sedimentary/metamorphic rocks by using average normal force and cutting depth with reasonable accuracy. These outcomes can pave the way for field application of the borehole strength measurement probe, which is currently under development for field trials.