JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV ROCK FRAGMENTATION ENGINEERING Previous Question Paper (February/March - 2022 - R18)

The drilling rate and bit life are significantly governed by the geological characteristics of the rock mass. Key parameters include:

1. Rock Hardness and Compressive Strength: Higher compressive strength increases the resistance to indentation, reducing the penetration rate and accelerating bit wear.
2. Abrasivity: Rocks containing high quartz content are highly abrasive, causing rapid blunting of carbide inserts or diamond cutters, thereby shortening the bit life.
3. Rock Texture and Grain Size: Coarse-grained rocks often exhibit lower fracture toughness compared to fine-grained rocks, influencing the efficiency of chip formation.
4. Jointing and Fracturing: Pre-existing discontinuities can cause bit deviation, vibration, and energy loss, which negatively impact both the rate of penetration and the structural integrity of the bit.
5. Porosity and Permeability: These influence the flushing efficiency of the drilling fluid, which is critical for removing cuttings and cooling the bit.

A sketch would illustrate a drill bit face interacting with a rock surface, showing the zones of crushing (beneath the bit) and shearing (at the edges), with arrows indicating the stress distribution influenced by rock discontinuities.

Rotary drilling operates on the principle of applying simultaneous thrust (weight on bit) and torque (rotation) to induce rock failure. The mechanics involve three primary processes:

1. Indentation: The bit teeth or cutters are pressed into the rock surface by the axial thrust, creating a stress field that exceeds the compressive strength of the rock, leading to crushing beneath the bit.
2. Shearing and Chipping: As the drill string rotates, the cutters move laterally, creating shear stresses. When these stresses exceed the shear strength of the rock, chips are formed and detached from the rock face.
3. Flushing: The drilling fluid (air, water, or mud) circulates through the drill string to the bit face, effectively removing the generated rock cuttings from the hole bottom to prevent re-grinding and to provide cooling for the bit cutters.

The efficiency of this process is defined by the interaction between the bit geometry and the rock properties, where the penetration rate is a function of the weight on bit, rotational speed, and the effectiveness of the cleaning mechanism.