The portion of formation surrounding a well bore into which drilling fluid has penetrated. displacing some of the formation fluids. This invasion takes place in porous, permeable zones when the pressure of the mud is greater than that of the formation fluids. A mud filter cake builds on the formation wall, limiting further invasion into the formation by mud filtrate. Directly behind the mud cake is a flushed zone from which almost all of the formation water and most of the hydrocarbons have been displaced by filtrate. The invasion process alters the distribution of saturations and other properties and, consequently, alters the values which are recorded on logs. The depth of invasion is the equivalent depth in an idealized model rather than the maximum depth reached by filtrate. In oil-bearing zones, the filtrate may push a bank of formation water ahead of it to produce what is referred to as an annulus.
