Displacement mud refers to the drilling fluid that is used to replace or “displace” another fluid already in the wellbore. This is often done while running in hole with tools, casing, or liners.

When Is It Used During RIH?
Running casing or liners: When casing is run into the well, especially to total depth (TD), a clean mud or spacer is pumped to displace cement or contaminated drilling mud.
Tool runs (e.g., MWD, LWD, or BHA): Before running in hole with sensitive tools, displacement mud might be used to ensure proper hole conditions, reduce solids content, and minimize wear or damage.
Completion operations: Clean brine or a completion fluid may displace drilling mud as part of well preparation.

Objectives of Displacing Mud While RIH
Hole cleaning: Remove cuttings, gelled mud, and debris before tools or casing are lowered.
Prevent differential sticking: Proper displacement ensures fluid compatibility and reduces the risk of sticking.
Pressure control: Maintain hydrostatic pressure in the wellbore while avoiding excessive ECD (Equivalent Circulating Density).
Fluid compatibility: Prevent mud contamination that could affect tool performance, cement bonding, or formation damage.

Challenges
Proper spacing and pill design: Using a spacer or viscous pill between fluids to prevent mixing.
Managing fluid losses: In loss zones, careful planning is needed to avoid losing expensive fluids or underbalancing the well.
Monitoring displacement efficiency: Returns are monitored to ensure the displaced mud is fully circulated out.

Field Practices
Displacement schedule: Pumping is done at calculated rates to ensure laminar-to-turbulent flow for effective displacement.
Use of spacers and flushes: Viscous pills and chemical washes are often used to clean the casing and formation face.
Monitoring parameters: Flow rates, pump pressure, returns, and mud properties are closely watched during RIH and displacement.