The density of drilling fluid (see mud), usually expressed in pounds per gallon. 9.0 lb/gallon mud produces hydrostatic or normal pressure (0.465 psi/ft or 1.05104 Pa/m); exact values depend on the fluid density. Equivalent mud weight (EMW) is the mud weight needed to balance formation-fluid pressure, i.e., it is the formation pressure divided by the depth below the Kelly bushing converted to pounds per gallon (the conversion factor is 0.0519 psi/ft=1 lb/gal). Generally mud weight must be kept between normal (hydrostatic) and fracture pressures to avoid risking a blowout; if it is smaller than the normal pressure, formation fluids are apt to flow into the borehole, if it is too large, the formation is apt to fracture (at some pressure lower than lithostatic pressure).
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