What viscosity mud should you have for continuing drilling after you have drilled into rock and set the casing?

• A. 5 to 10 centipoise
• B. 30 to 50 centipoise
• C. 100 to 200 centipoise
• D. 300 to 500 centipoise

Answer: (B)

When you’ve drilled into rock and set the casing, the mud needs to do enough work to keep cuttings suspended and carried to the surface, but it shouldn’t be so thick that it makes circulation slow or energy-intensive. A moderate viscosity around 30–50 centipoise provides that balance: it’s viscous enough to hold and transport rock fragments from the drill bit, yet fluid enough to flow freely through the drill string and return up the wellbore without excessive pumping effort.

If the mud were much thinner, cuttings would settle and accumulate, leading to poor cleaning and possible sticking. If it were much thicker, circulation would be inefficient, you’d waste pump power, and it could complicate cementing and formation compatibility.