How Hard Rocks Under High Stress Levels do Behave Upon Drilling
Authors: Goncalves Clemente Jose de Castro , Santos Erick Slis Raggio 
Published: June 2014

Rocks are frictional materials which tend to behave differently when subjected to tensile, shear or pure compressive stress states. Much work has been done on the characterization of rock reservoirs for the purposes of production problems such as fault reactivation, reservoir compaction, wellbore stability and solids production, but the drilling area still lacks of reliable rock failure mode characterization for drilling system design purposes.

The conventional drilling mechanisms on hard rocks are based on grinding, crack nucleation and fissure propagation. Diamond impregnated bits are the most clear examples of rock grinders since the rock is meant to be cut by replaceable sharp diamond edges being scratched onto the bottomhole surface. To some extent and depending on the rock stress state, the depth of cut and rock rheology, the scraping systems such as the PDC drillbits may present a grinding effect as well. On the other hand, the cracking and fissure propagation mechanisms presumably promoted by PDC and cone bits are known to be the most energy-efficient. The drilling systems deliver an effect in the form of a concentrated force to produce a free surface within the rock. Eventually some of these surfaces coalesce and detach a rock cutting from the formation. The energy spent on the creation or propagation of internal free surfaces when averaged by the volume of the detached rock cutting tends to be considerably smaller than the specific energy spent on rock grinding, leading to higher efficiency. The reader should notice there is a key effect of concentrated forces in promoting crack creation or propagation within the rock mass that may not behave as presumed.

Although some rocks present themselves at surface as hard and fragile materials, these might behave as ductile and plastic under in-situ conditions. On one side this ductility reduces stress concentration at the load application boundaries and within the material, preventing the internal microcracks to propagate further. On another side the ductile rock cut or smeared debris may accumulate on the bottomhole just under the drilling bitcones and ahead of cutters shielding the intact rock from the cutting structures and thus changing the dynamic cutter shape experienced by the rock. The combination of both may lead to a bit-balling alike behavior under dynamic conditions unnoticeable at surface. Unlike conventional bit balling in shally formations, there is no adherence between bit and rock debris that could evidence this phenomenum at surface once the bit is pulled out of the hole.

This work presents a series of unpublished rock mechanics tests resembling the in-situ stress conditions that support the above mentioned conclusions. A couple of tests with cement indentation under similar conditions illustrates the effects believed to happen below cone bits. A bibliography review combined with the ductility induced behavior results illustrates how the cutters shape may be affected dynamically and upon wear. Some evidences on the rock heterogeneity being amplified under in-situ conditions are exposed in detail to allow more robust PDC insert designs.

These results are to be regarded as novelty evidences in bit selection and design that could potentially bring drilling improvements in ductile rock environments.

Introduction

The mechanical structure of a rock presents several different appearances, depending upon the scale and the detail with which it is studied[1]. It thus adds up more complexity to the understanding of a mechanical behavior at any of these scales. Most rocks comprise an aggregate of crystals and amorphous particles joined by varying amounts of cementing materials. The size distribution and dispersion of these crystals, the bounding cement between crystals and pore amount and morphology affect the material behavior regarded as continuous or intact. On a larger scale the rock mass may present other levels of discontinuities, such as cracks, joints and bedding planes.