Mud weight cannot be chosen randomly during drilling.

Why?
Because it must stay within a safe pressure window.

This window is defined by two limits:

• Pore (formation) pressure at the lower end
• Fracture pressure at the upper end

If mud pressure is lower than formation pressure
→ formation fluids enter the wellbore
→ kick risk

If mud pressure is higher than fracture pressure
→ the formation fractures
→ drilling fluid is lost into the rock (lost circulation)

The goal is simple:
Keep mud pressure between formation pressure and fracture pressure.

This is known as the safe mud weight window.

Finding and maintaining this balance is one of the most important responsibilities in drilling operations.

Food for Thought:
How do engineers estimate formation pressure before they even drill into it?