What can cause lost circulation in mud rotary drilling?

• A. Shallow water table
• B. Fractured formations, porous zones, or over-pressurized drilling fluids entering the formation
• C. Cold ambient temperature
• D. High viscosity mud

Answer: (B)

Lost circulation happens when the drilling fluid escapes the wellbore and flows into the surrounding rock instead of returning to the surface. The best explanation is that fractures in the rock, highly permeable or porous zones, or drilling fluids that are over-pressurized relative to the formation strength create ready-made pathways and a pressure gradient that pull mud into the formation. When there are cracks or abundant pore spaces, the mud can be drawn in easily; when the mud is pumped with enough pressure to exceed the rock’s fracture pressure, it can widen those pathways and push more fluid into the formation. That combination—pathways in the rock plus enough pressure to drive the fluid into them—explains why circulation is lost.

Surface or environmental factors like a shallow water table don’t directly create downhole losses, and cold ambient temperature isn’t a direct mechanism for mud loss. High viscosity mud can influence how easily the mud bridges or penetrates small fractures, but it isn’t the primary cause of lost circulation; the key issue is the rock’s ability to accept fluid and the pressure pushing it in.