Well designed and properly controlled mud. Method requirements and subsurface characteristics can dictate the need to use a drilling fluid, and some drilling methods require specific types of fluid.
During drilling, a mud engineer should be present to monitor drilling-fluid viscosity and circulation pressure and to implement the procedures needed to minimize invasion of drilling fluid into the borehole wall and avoid causing fractures in the geologic material. With proper control, drilling mud should only penetrate the borehole to 0.25 inches, making subsequent well development relatively easy.