Toughness of rocks
Toughness of rocks: Another component of drillability is toughness - resistance to being pulled apart. There are some formation materials that are difficult to crush due to their plastic (non-brittle) nature. Tearing or cutting techniques are used to drill these materials.
Toughness depends on a variety of formation characteristics:
1. Grain Shape: The shape determines how well grains lock into one another. Platy grain such as you see in schist are more difficult to pull apart than round grains such as in sandstone.
2. Bonds between grains: The strength of the bonds between grains in consolidated rocks also determines toughness. The bonds act just like glues, with some bonds tougher than others. Silica bonding is tougher than carbonate, and far tougher than clays.
3. Fissility: This term describes the property of breaking into flakes. Rocks that break into flakes or thin plates are termed "fissile". Examples are phyllite, schist, slate, and some calcium-rich shales.
4. The tenacity of the rock matrix itself is determined by several physical characteristics:
a. Elasticity is the ability to spring back to the original shape after distortion, much like foam rubber. Micaceous schist is elastic.
b. Plasticity is the property of remoulding after distortion. Soft shales and clays are plastic.
c. Brittleness is the tendency to fracture under stress, like peanut brittle. Chert and high-purity carbonate rocks are brittle.
d. Stickiness, as the name suggests, describes a tendency to stick together. Cuttings tend to stick together or "ball up" rather than remaining separate. Examples are sticky shales and clay.
The knowledge of these properties comes into play after the material is crushed and we try to tear it apart to bring it up and out of the hole. For example, you may well have been able to crush the material, but if it is platy or sticky, the material tends to hang together. You then have to use a technique to pry the particles apart.