JNTU B.Tech II DRILLING AND BLASTING Previous Question Paper (November/December - 2020 - R16)

Exploration drilling is the process of extracting rock samples from the subsurface to determine the geological structure, mineral composition, and physical properties of the ground. The main methods are categorized based on the drilling mechanism and the type of sample recovered:

1. Core Drilling (Diamond Drilling): This method uses a diamond-impregnated bit to cut a cylindrical core of rock. It is the most accurate method for geological mapping and mineral resource estimation because it provides a continuous, undisturbed sample of the subsurface strata.
2. Percussion Drilling (Cable Tool or Down-the-Hole): This method uses the impact force of a heavy bit to crush rock into small fragments, known as drill cuttings. While it does not provide a core sample, it is faster and more cost-effective for identifying lithological boundaries and water table levels.
3. Rotary Drilling: This technique employs a rotating bit to grind the rock. It is widely used in oil and gas exploration and deep mineral prospecting. The samples are typically retrieved as cuttings circulated to the surface by drilling mud.
4. Reverse Circulation (RC) Drilling: A specialized form of rotary drilling where the cuttings are returned to the surface through the inner tube of the drill string. This prevents sample contamination and provides high-quality representative samples at a lower cost than diamond core drilling.
5. Auger Drilling: This involves using a helical screw (auger) to penetrate soft, unconsolidated soil or overburden. It is primarily used for shallow exploration and environmental site investigations.
6. Sonic Drilling: This method utilizes high-frequency resonant energy to advance the drill string. It is highly effective in difficult ground conditions, such as boulders or mixed strata, providing high-quality continuous core samples with minimal waste.

The selection of the method depends on the required sample quality, depth of investigation, ground conditions, and the overall budget of the exploration program.

Horizontal drilling is a specialized drilling technique where the wellbore is drilled at an angle to reach a target reservoir that is located horizontally or at a significant distance from the surface location. Unlike conventional vertical drilling, this method involves deviating the drill string from the vertical path at a specific depth, known as the kick-off point, to reach a horizontal section within the target formation.

Key components and advantages of horizontal drilling include:

• Increased Reservoir Contact: By drilling horizontally through a thin or low-permeability reservoir, the wellbore exposes a much larger surface area of the formation compared to a vertical well, significantly enhancing production rates.
• Multiple Well Access: It allows for drilling multiple wells from a single surface pad, minimizing the surface footprint and environmental impact.
• Steering Mechanisms: The process utilizes specialized equipment such as Mud Motors, Measurement While Drilling (MWD) tools, and Rotary Steerable Systems (RSS) to maintain the desired trajectory and stay within the target pay zone.

Directional drilling is the broader practice of intentionally deviating a wellbore from the vertical axis to reach a specific subsurface target. While horizontal drilling is a specific type of directional drilling, the latter encompasses various profiles, including S-shaped, J-shaped, and extended-reach wells.

The fundamental aspects of directional drilling are:

• Trajectory Control: Engineers use directional surveys to monitor the well path in three-dimensional space, adjusting the inclination and azimuth to navigate around geological obstacles or to reach targets located under inaccessible surface areas like urban zones or water bodies.
• Drilling Assemblies: The use of bent subs, downhole motors, and stabilizers allows the driller to exert lateral force on the bit to change the well path.
• Applications: It is essential for sidetracking (re-entering an existing well to drill a new path), relief well drilling to control blowouts, and multi-lateral drilling where multiple branches are drilled from a single main wellbore.

In summary, while directional drilling provides the capability to steer the wellbore to a precise subsurface coordinate, horizontal drilling represents the application of this technology to maximize contact with a productive reservoir layer.