JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III DRILLING TECHNOLOGY Previous Question Paper (February - 2022 - R18)

Drilling fluids, commonly known as drilling mud, are circulating fluids used in rotary drilling operations to facilitate the drilling process by performing several critical mechanical and chemical functions.

The primary types of drilling fluids are:

1. Water-Based Muds (WBM): The most common type, where water is the continuous phase, containing additives like bentonite clay, barite, and various polymers.
2. Oil-Based Muds (OBM): Fluids where oil (diesel or mineral oil) is the continuous phase. These offer superior lubrication and thermal stability, often used in high-temperature or reactive shale formations.
3. Synthetic-Based Muds (SBM): Fluids using synthetic organic compounds as the base, providing the performance of oil-based systems with lower environmental toxicity.
4. Pneumatic Fluids: Air, mist, foam, or aerated muds used in low-pressure formations to improve penetration rates and prevent formation damage.

The main functions include: 1) Removing cuttings from the wellbore to the surface. 2) Controlling subsurface pressures by providing hydrostatic head. 3) Cooling and lubricating the drill bit and drill string. 4) Supporting the wellbore walls through the formation of a thin, low-permeability filter cake. 5) Suspending cuttings during circulation stops.

Surge pressure is the transient increase in bottom-hole pressure caused by the downward movement of the drill string or casing into the wellbore. As the pipe moves down, it displaces fluid, creating a pressure spike that can lead to lost circulation or formation fracturing if the pressure exceeds the fracture gradient.

Swab pressure is the transient decrease in bottom-hole pressure caused by the upward movement of the drill string or casing out of the wellbore. This movement acts like a piston, creating a suction effect that reduces the effective hydrostatic pressure at the bottom of the hole. If the resulting pressure drops below the formation pore pressure, it can lead to a well kick or influx of formation fluids.