US Patent 7950472 Downhole local mud weight measurement near bit

A method for detecting a change in a wellbore fluid includes estimating at least two pressure differences in the wellbore fluid and estimating a change in a density of the fluid using the at least two pressure differences. The density change may be estimated by the equation, Δρ=(ΔPbefore—influx−ΔPafter—influx)/(g×ΔTVD), wherein ΔP is a fluid pressure difference between two points along the wellbore, ρ is a mean value of density of the fluid between the two points, g is gravity and ΔTVD is a vertical distance between the two points. The method may include estimating a density change using an estimated inclination of the wellbore. An apparatus for estimating density changes includes at least two axially spaced apart pressure sensors. The sensor positions may be switched to estimate a correction term to reduce a relative offset between the two pressure sensors.