What type of drilling fluid is commonly referred to as "mud"?

The term "mud" in drilling contexts typically refers to water-based fluids that contain clay and polymers. These fluids are essential in the drilling process as they serve multiple functions, including cooling the drill bit, providing hydrostatic pressure to prevent the influx of formation fluids, and stabilizing the borehole walls. The clay component helps in the formation of a filter cake, which minimizes fluid loss to permeable formations, while polymers can improve the properties of the fluid, such as viscosity and stability.

In contrast, the other types of drilling fluids mentioned do not commonly carry the colloquial term "mud". Compressed air mixtures are typically used in dry drilling systems, which don’t involve the fluid dynamics typical of mud systems. Oil-based fluids, while used in certain drilling circumstances, are typically referred to by more specific labels like "oil-based mud" rather than just "mud". Solid substances like cement are used for different applications in the drilling process, such as well completion and abandonment, but again, do not align with the standard definition of drilling "mud".

Thus, the choice indicating water-based fluids with clay and polymers accurately reflects the industry standard terminology and its applications in drilling practices.