Brittleness is an important mechanical property of rocks, and has great implication in well stability analysis, hydraulic fracture evaluation, sand production control, rate of penetration (ROP) prediction, and many other aspects of the exploration and production business. Although no standard or universally accepted formulation exists to date for brittleness quantification, many brittleness indices have been developed by various authors.

In this paper, five brittleness indices are studied, which are all derived from unconfined compressive and tensile strengths, but in different mathematical forms. The relationships between these indices and various petrophysical and geomechanical properties of rocks are investigated, and global correlations are built that relate the brittleness indices to Young's modulus, P-wave velocity, and porosity. The correlations are global in the sense that they represent a diverse dataset comprising various types of igneous, metamorphic, and sedimentary rocks from various locations across the world. Therefore, they can be applied to most rock types encountered in the petroleum industry, and are especially useful in the early exploration phase when few core data are available.

A workflow is further established for predicting the ROP of polycrystalline diamond compact (PDC) bits from gamma ray, neutron, density, and sonic log data based on the global correlations. A field example is presented to illustrate the workflow, in which one of the brittleness-porosity correlations is used in the ROP calculations. The results of the field example show a good correspondence between the measured and predicted ROP values, which demonstrates that the global correlations, together with the workflow, are feasible and useful in real-time analyses, e.g., during geosteering processes.

You can access this article if you purchase or spend a download.