What condition leads to differential sticking?

• A. Mud viscosity too low.
• B. Pipe rotating too fast.
• C. Differential pressure causing pipe to stick to borehole wall.
• D. Drilling into hard rock.

Answer: (C)

Differential sticking happens when there is a pressure difference across the drill string that pushes the pipe against the borehole wall. If the mud outside the pipe in the annulus is at higher pressure than the mud inside the string, that differential pressure drives the pipe toward the wall. The contact, along with a mud cake that may form on the wall, creates friction so strong that the string cannot be pulled out or moved, even when surface equipment is trying to recover it. This isn't caused primarily by mud viscosity, how fast you rotate the pipe, or drilling into hard rock; those factors can influence other issues, but the sticking here is driven by the pressure imbalance pressing the pipe to the wall.