By convention in the [[petroleum]] industry, ''overpressure'' refers to pressures higher than normal that require heavy drilling mud to keep formation fluids from entering the borehole. Pressures lower than normal are called ''subnormal''. | By convention in the [[petroleum]] industry, ''overpressure'' refers to pressures higher than normal that require heavy drilling mud to keep formation fluids from entering the borehole. Pressures lower than normal are called ''subnormal''. |