1. n. [Formation Evaluation]

The downward vertical movement of filtrate with time after invasion. In hydrocarbon zones, the filtrate is heavier than the formation fluid. Therefore, in a vertical well, gravity causes the filtrate to sink to the bottom of a permeable zone, while the hydrocarbons move back to the borehole at the top. In a horizontal well, the mud filtrate will sink below the well, leaving hydrocarbons above it. The amount of movement depends, among other factors, on the time since invasion, the fluid mobilities and the difference in fluid densities. In water zones, the direction of movement depends on the relative densities of filtrate and formation water. In the more usual case of fresh filtrate and salty formation water, the filtrate will move upwards.

See: depth of invasion,  formation fluid,  formation water,  step profile